Chapter 8 Milling Operations

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Chap ter 8

MILLING OPERATIONS

Milling is the process of machining flat, curved, or Milling machines are basically classified as vertical or

irregular sur faces by feeding the workp iece against a rotating horizontal. These machines are also classified as knee-tycutter containing a nu mber of cutting edges. The m illing ram-type, manufacturing or bed type, and planer-type. Mmachine consists basically of a motor driven spindle, which milling machines have self-contained electric drive motomounts and revolves the milling cutter, and a reciprocating coolant systems, variable spind le speeds, and power-operaadjustable worktable , which mounts and feeds the workpiece. table feeds

TYPES OF MILLING MACHINES

KNEE-TYPE MILLING MACHINE

Knee-type milling machines are characterized by a vertically

adjustable worktable resting on a saddle which is supportedby a knee. The knee is a massive casting that rides verticallyon the milling machine column and can be clamped rigidly tothe column in a position where the milling head and millingmachine spindle are properly adjusted vertically for operation.

The plain vertical machines are characterized by a spindlelocated vertically, parallel to the column face, and mounted ina sliding head that can be fed up and down by hand or power.Modern vertical milling machines are designed so the entirehead can also swivel to permit working on angular surfaces,

The turret and swivel head assembly is designed for making

precision cuts and can be swung 360 on its base. Angularcuts to the horizontal plane may be made with precision bysetting the head at any required angle within a 180 arc.

The plain horizontal m illing machines column contains thedrive motor and gearing and a fixed position horizontalmilling m achine spindle. An adjustable overhead armcontaining one or more arbor supports projects forward fromthe top of the column. The arm and arbor supports are used tostabilize long arbors. Supports can be moved along theoverhead arm to supp ort the arbor where support is desireddepending on the position of the milling cutter or cutters.

The milling machines knee rides up or down the columnon a rigid track. A heavy, vertical positioning screw beneathpast the milling cutter. The milling machine is excellent forforming flat surfaces, cutting dovetails and keyways, formingand fluting milling cutters and reamers, cutting gears, and soforth. Many special operations can be performed with theattachments available for milling machine use.the knee isused for raising and lowering. The saddle rests upon the knee

and supports the worktable. The saddle moves in and out o

dovetail to control cross feed of the w orktable. The worktatraverses to the right or left up on the saddle for feeding workp iece past the milling cutter. The table may be manucontrolled or power fed.

UNIVERSAL HO RIZON TAL MILLINGMACHINE

The basic difference between a universal horizontal millmachine and a plain horizontal milling m achine is addition of a table swivel housing between the table and saddle of the universal machine. This permits the tableswing up to 45 in either direction for angular and heli

milling operations. The universal machine can be fitted wvarious attachments such as the indexing fixture, rotary tabslotting and rack cutting attachments, and various specfixtures.

RAM-TYPE MILLING MACHINE

The ram-type milling machine is characterized by a spinmoun ted to a movable housing on the column to permpositioning the milling cutter forward or rearward ihorizontal plane. Two pop ular ram-type milling machines the universal milling machine and the swivel cutter he

ram-type milling machine.UNIVERSAL RAM-TYPE MILLING

MACHINE

The universal ram-type milling machine is similar to tuniversal horizontal milling machine, the difference beinas its name implies, the spindle is mounted on a r am movable housing.


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SWIVEL CUTTER H EAD RAM -TYPEMILLING MACHINE

The cutter head containing the milling machine spindle isached to the ram. The cutter head can be swiveled from a

rtical spindle position to a horizontal spindle position orn be fixed at any desired angular position between verticald horizontal. The saddle and knee are hand driven forrtical and cross feed adjustment while the worktable can beher hand or power driven at the operators choice.

SAFETY RULES FOR M ILLING M ACH INES

Milling machines require special safety precautions whilebeing used. These are in addition to those safety precautionsdescribed in Chapter 1.

Do not make contact with the revolving cutter.

Place a wooden pad or suitable cover over the tablesurface to protect it from possible damage.

Use the buddy system when moving heavy attachments.Basic milling machine configurations are shown in Figure1.

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The pitch is determinedtooth face is the forwardforms the cutting edge.

by the number of teeth.facing surface of the tooth

Thethat

The cutting edge is the angle on each tooth that performsthe cutting.

The land is the narrow surface behind the cutting edge oneach tooth.

The rake angle is the angle formed between the face ofthe tooth and the centerline of the cutter. The rake angledefines the cutting edge and provides a path for chipsthat are cut from the workpiece.

The primary clearance angle is the angle of the land ofeach tooth measured from a line tangent to the centerlineof the cutter at the cutting edge. This angle prevents eachtooth from rubbing against the workpiece after it makesits cut.

This angle defines the land of each tooth and providesadditional clearance for passage of cutting oil and chips.

The hole diameter determines the size of the arbornecessary to mount the milling cutter.

Plain milling cutters that are more than 3/ 4 inch in widthare usu ally made w ith spiral or helical teeth. A plainspiral-tooth milling cutter produces a better and smootherfinish and requires less power to operate. A p lain helical-

tooth milling cutter is especially desirable when millingan uneven surface or one with holes in it.

Types of Teeth

The teeth of milling cutters may be mad e for right-hand orleft-hand rotation, and with either right-hand or left-hand

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helix. Determine the hand of the cutter by looking at the faceof the cutter when mou nted on the spindle. A right-handcutter must rotate counterclockwise; a left-hand cutter mustrotate clockwise. The right-hand helix is shown by the flutesleading to the right; a left-hand helix is shown by the flutes

leading to the left. The direction of the helix does not affectthe cutting ability of the cutter, but take care to see that thedirection of rotation is correct for the hand of the cutter

(Figure 8-4).

Saw Teeth

Saw teeth similar to those shown in Figure 8-3 are eitherstraight or helical in the smaller sizes of plain milling cutters,metal slitting saw milling cutters, and end milling cutters.The cutting edge is usually given about 5 d egrees primary

clearance. Sometimes the teeth are provided with off-setnicks which break up chips and make coarser feeds possible.

Helical Milling Cutters

The helical milling cutter is similar, to the plain m illingcutter, bu t the t eeth h ave a h elix angle of 45 to 60. Thesteep helix produces a shearing action that results in smooth,vibration-free cuts. They are available for arbor mounting, orwith an integral shank with or without a pilot. This type ofhelical cutter is particularly useful for milling elongated slotsand for light cuts on soft metal. See Figure 8-5.

Metal Slitting Saw Milling Cutter

The metal slitting saw milling cutter is essentially a verythin plain milling cutter. It is ground slightly thinner towardthe center to provide side clearance. These cutters are usedfor cutoff operations and for milling deep, nar row slots, andare made in widths from 1/ 32 to 3/ 16 inch.


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Side Milling Cutters

Side milling cutters are essentially plain milling cutterswith the addition of teeth on one or both sides. A plain sidemilling cutter has teeth on both sides and on the periphery.When teeth are added to one side only, the cutter is called ahalf-side milling cutter and is identified a s being either aright-hand or left-hand cutter. Side milling cutters aregenerally used for slotting and straddle milling.

Interlocking tooth side milling cutters and staggered toothside milling cutters are used for cutting relatively wide slots

with accuracy (Figure 8-6). Interlocking tooth side millingcutters can be repeatedly sharpened without changing thewidth of the slot they will machine.

After sharpening, a washer is placed between the two cutteto compensate for the ground off metal. The staggered toocutter is the most washer is placed between the two cutters compensate for efficient type for milling slots where the depexceeds the width.

End M illing Cutters

The end milling cutter, also called an end mill, has teeth othe end as well as the periphery. The smaller end millincutters have shanks for chuck mounting or d irect spindmounting. End milling cutters may have straight or spir

flutes. Spiral flute end milling cutters are classified as lefhand or right-hand cutters depend ing on the d irection rotation of the flutes. If they are small cutters, they may haveither a straight or tapered shank.

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The most common end milling cutter is the spiral flute cutter periphery and slightly concave sides to p rovide clearance.ontaining four flutes. Two-flute end milling cutters, These cutters are u sed for milling semicylindrical keyways inmetimes referred to as two-lip end mill cutters, are used for shafts.illing slots and keyways where no drilled hole is providedr starting the cut. These cutters drill their own starting holes.raight flute end milling cutters are generally used for milling Angle Milling Cutters

th soft or tough materials, while spiral flute cutters are usedostly for cutting steel. The angle milling cutter has p eripheral teeth which are

Large end milling cutters (normally over 2 inches inameter) (Figure 8-10) are called shell end mills and arecessed on the face to receive a screw or nut for mounting onseparate shank or mounting on an arbor, like plain milling

utters. The teeth ar e usu ally helical and th e cutter is usedrticularly for face milling operations requ iring the facing ofo surfaces at right angles to each other.

T-Slot Milling Cutter

The T-slot milling cutter is used to machine T-slot groovesworktables, fixtures, and other holding devices. The cutter

as a plain or side milling cutter mounted to the end of aarrow shank. The throat of the T-slot is first milled with ade or end milling cutter and the headspace is then milledth the T-slot milling cutter.

Woodruff Keyslot Milling Cutters

neither parallel nor perpendicular to the cutter axis. See Figure8-8. Common operations performed with an gle cutters arecutting V-notches and serrations. Angle cutters may besingle-angle milling cutters or d ouble-angle milling cutters.The single-angle cutter contains side-cutting teeth on the flatside of the cutter. The angle of the cutter edge is usually 30,45, or 60, both right and left. Double-angle cutters haveincluded angles of 45, 60, and 90 degrees.

Gear Hob

The gear hob is a formed tooth milling cutter with helicalteeth arranged like the thread on a screw. These teeth- arefluted to prod uce the required cutting edges. Hobs aregenerally used for such work as finishing spur gears, spiralgears, and worm gears. They may also be used to cut ratchetsand spline shafts.

Concave and Convex Milling Cutters

The Woodruff keyslot milling cutter is made in straight, Concave and convex milling cutters are formed tooth

pered-shank, and arbor-mounted types. See Figure 8-7. The cutters shaped to p rodu ce concave and convex contours of

ost common cutters of this type, under 1 1/ 2 inches in 1/ 2 circle or less. The size of the cutter is specified by the

ameter, are provided with a shank. They have teeth on thediameter of the circular form the cutter produces.

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Corner Rounding Milling Cutter

The corner-rounding milling cutter is a formed tooth cutterused for milling rounded corners on workplaces up to andincluding one-quarter of a circle. The size of the cutter isspecified by the radius of the circular form the cutterproduces, such as concave and convex cutters generally used

for such work as finishing spur gears, spiral gears, and wormwheels. They may also be used to cut ratchets and splineshafts.

Special Shap ed-Formed Milling Cu tter

Formed milling cutters have the ad vantage of beingadaptable to any specific shape for special operations. Thecutter is m ade specially for each specific job. In the field, afly cutter is formed by grinding a single point lathe cutter bitfor mounting in a bar, holder, or fly cutter arbor. The cuttercan be sharpened many times without destroying its shape.

Selection of Millin g CuttersConsider the following when choosing milling cutters:

45 angular cu ts may either be mad e with a 45 singleangle milling cutter while the workpiece is held in swivel vise, or with an end milling cutter while thworkpiece is set at the required angle in a universal vise.

The harder the material, the greater will be the heat thais generated in cutting. Cutters should be selected fotheir heat-resisting properties,

Use a coarse-tooth milling cutter for roughing cuts and finer-toothed milling cutter for light cuts and finishinoperations.

When milling stock to length, the choice of using a paiof side milling cutters to straddle the workpiece, a singleside milling cutter, or an end milling cutter will depenupon the number of pieces to be cut.

Some operations can be done with more than one type ocutter such as in milling the squar e end on a shaft o

High-speed steel, stellite, and cemented carbide cuttersreamer shan k. In this case, one or tw o side m illin

have a distinct advantage of being capable of rapidcutters, a fly cutter, or an end milling cutter may be usedHow ever, for th e m ajority of operations, cutters ar

production when used on a machine that can reach theproper speed.

specially designed and named for the operation they arto accomplish.

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The milling cutter should be small enough in diameter sothat the p ressure of the cut will not cause the workp ieceto be sprung or displaced while being milled.

Size of Millin g Cutter

In selecting a milling cutter for a particular job, chooseone large enough to span the entire work surface so the

job can be done with a single pass. If this cannot be done,remember that a small diameter cutter will pass over asurface in a shorter time than a large diameter cutterwhich is fed at the same speed. This fact is illustrated inFigure 8-9.

Care and M aintenance of Milling Cutters

The life of a milling cutter can be greatly prolonged byintelligent use and proper storage. General rules for thecare and maintenance of milling cutters are given below.

New cutters received from stock are usually wrapped inoil paper which should not be removed until the cutter isused.

Take care to operate the machine at the proper speed forthe cutter being used, as excessive speed will cause thecutter to wear rapidly from overheating.

Take care to pr event the cutter from striking the hardjaws of the vise, chuck, clamping bolts, or nuts.

Whenever practical, use the prop er cutting oil on thecutter and workpiece during operations, since lubricationhelps prevent overheating and cutter wear.

Keep cutters sharp. Dull cutters require more power todrive and this power, being transformed into heat, softensthe cutting edges. Dull cutters should be marked as suchand set aside for grind ing. For further information oncutter grinding, refer to Chapter 5, Grinding Machines.

Thoroughly clean and lightly coat milling cutters with oilbefore storing.

Place cutters in drawers or bins so that their cutting edgeswill not strike each other. H ang small cutters on hooks orpegs, and set large cutters on en d. Place taper an dstraight shank cutters in separate d rawers, bins, or racksprovided with suitable sized holes to receive the shanks.

Never operate a cutter backwards. Due to the clearanceangle, the cutter w ill rub, prod ucing a great d eal offriction. Operating the cutter backward may r esult incutter breakage.

ARBORS

Milling machine arbors are made in various lengths and instandard diameters of 7/ 8,1,1 1/ 4, and 1 1/ 2 inch. The shankis made to fit the taper hole in the spindle while the other endis threaded.

NOTE: The threaded end may have left or right-handedthreads.

The milling machine spindle may be self-holding or self-releasing. The self-holding taper is held in the spindle by thehigh wedging force. The spindle taper in most milling

machines is self-releasing; tooling must be held in place by adraw bolt extending through the center of the spindle.

Arbors are supp lied with one of three tapers to fit themilling machine spindle: the Standard Milling Machinetaper, the Brown a nd Sharpe tap er, and the Brown andSharpe taper with tang (Figure 8-10).

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The Standard Milling Machine Taper is used on mostmachines of recent m anu facture. See Figure 8-11. Thesetapers are identified by the number 30, 40, 50, or 60. Number50 is the most commonly used size on all modern machines.

The Brown an d Sharpe taper is found mostly on oldermachines. Adapters or collets are used to adapt these tapersto fit machines whose spindles have Standard MillingMachine tapers.

The Brown and Sharpe taper with tang is used on someolder machines. The tang engages a slot in the spindle toassist in d riving the arbor,

Standard Milling Machine Arbor

The standard milling machine arbor has a tapered ,cylindrical shaft with a standard milling taper on the d rivingend and a threaded portion on the opposite end to receive the

arbor nut. One or more milling cutters may be placed on thestraight cylindrical portion of the arbor and held in positionby sleeves and the arbor nut. The standard milling machinearbor is usually splined and keys are u sed to lock each cutterto the arbor shaft. These arbors are supplied in three styles,various lengths and, standard diameters.

The most common way to fasten the arbor in the millingmachine spindle is to use a draw bar. The bar threads into thetaper shank of the arbor to draw the taper into the spindle andhold it in place. Arbors secured in this manner are removed bybacking out the dr aw bar and tapping the end of the bar toloosen the taper.

The end of the arbor opposite the taper is supp orted by thearbor supp orts of the milling machine. One or more supportsreused depending on the length of the arbor and the degree ofrigidity required. The end may be supported by a lathe centerbearing against the arbor nut or by a bearing surface 0f thearbor fitting inside a bushing of the arbor support.

The arbor may also be firmly supported as it turns in thearbor support bearing suspended from the over-arm (Figure8-12).

Typical milling arbors are illustrated in Figure 8-13. Listedon the next page are several types of Style C arbors.

Style A has a cylindrical pilot on the end that runs in a

bronze bearing in the arbor support. This style is mostly usedon small milling machines or when maximum arbor supporclearance is required.

Style B is characterized by one or more bearing collars thacan be positioned to any part of the arbor. This allows thebearing supp ort to be positioned close to the cutter, to-obtainrigid setups in heavy duty milling operations).

Style C arbors are used to mount the smaller size millingcutters, such as end mills that cannot be bolted directly onthe spindle nose. Use the shortest arbor possible for thework.

Screw Arb or

Screw arbors are used to hold small cutters that havethreaded holes. See Figure 8-14. These arbors have a tapernext to the threaded portion to provide alignment and supportfor tools that require a nu t to hold them against a tapersurface. A right-hand threaded arbor must be used for right-hand cutters while a left-hand thread ed arbor is used tomount left-hand cutters.

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The slitting saw milling cutter arbor (Figure 8-14) is a shortarbor having tw o flanges between which the m illing cutter issecured by tightening a clamping nut. This arbor is used tohold m etal slitting saw milling cutters used for slotting,slitting, and sawing op erations.

The shell end milling cutter arbor has a bore in the end inwhich shell end milling cutters fit and are locked in place bymeans of a cap screw.

The fly cutter arbor is used to su pport a single-edge lathe,shaper, or p laner cutter bit for boring and gear cuttingoperations on th e milling m achine.

COLLETS, SPIND LE ADAPTERS, ANDQUICK-CHANG E TOO LING

Description

Screw arbors are used to h old small cutters that hav e Milling cutters that contain their own straight or taperedthread ed h oles. These arbors have a tap er next to thethreaded portion to provide alignment and support for tools

shanks are moun ted to th e milling machine spindle with

that require a nu t to hold them against a taper su rface. Acollets, spindle adapters, and quick-change tooling whichadapts the cutter shank to the spindle.

right-hand threaded arbor m ust be used for right-hand cutterswhile a left-hand threaded arbor is used to mount left-handcutters.

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Collets

A collet is a form of a sleeve bushing for reducing the sizeof the hole in the milling machine spindle so that small shanktools can be fitted into large spindle recesses (Figure 8-15).They are mad e in several forms, similar to d rilling machine

sockets and sleeves, except that their tapers are not alike.

Spindle Adapters

A spindle adapter is a form of a collet having a standardizedspindle end. They are available in a wide variety of sizes toaccept cutters that cannot be mou nted on arbors. They aremad e with either the Morse taper shank or the Brown an dSharpe taper w ith tang having a standard spindle end (Figure8-16).

Chuck Adapter

A chuck ad apter (Figure 8-17) is used to attach chuckmilling machines having a stand ard spind le end. The cholder is sometimes referred to as a collet chuck. Vari

forms of chucks can be fitted to milling machines spindlesholding drills, reamers, and small cutters for speoperations.

Quick-Change Tooling

The quick-change adapter mounted on the spindle noseused to speed u p tool changing. Tool changing with tsystem allows you to set up a num ber of milling operatiosuch as drilling, end milling, and boring without changing setup of the p art being machined. The tool holders amounted and removed from a master holder mounted to machine spindle by means of a clamping r ing (Figure 8-18)

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VISESINDEXING FIXTURE

Either a p lain or swivel-type vise is furnished with eachmilling machine. The plain vise, similar to the machine table

ise, is used for milling straight workplaces and is bolted to

he milling machine table either at right angles or parallel tohe machine arbor. The swivel vise can be rotated and containsscale graduated in degrees at its base to facilitate milling

workplaces at any angle on a horizontal plane. The universalise, which may be obtained as extra equipment, is d esignedo that it can be set at both horizontal and vertical angles. Thisype of vise maybe used for flat and angular m illing. The all-teel vise is the strongest setup because the w orkpiece islamped closer to the table. The vise can securely fastenastings, forgings, and rough-surfaced workplaces. The jawan be positioned in any n otch on the two bars toccomm odate d ifferent shap es and sizes. The air orydraulically operated vise is used more often in production

work. This type of vise eliminates tightening by striking therank with a lead hammer or other soft face hammer. See page-13 for examples ofvarious vises.

ADJUSTABLE ANG LE PLATE

The adjustable angle plate is a workpiece holding device,imilar to the universal vise in operation. Workpieces are

mounted to the angle plate with T-bolts and clamps in th eame manner u sed to fasten workplaces to the worktable ofhe milling machine. The angle plate can be adjusted to anyngle so that bevels and tapers can be cut without u sing apecial milling cutter or an adjustable cutter head.

The index fixture (Figure 8-19) consists of an ind ex head ,also called a dividing head, and footstock which is similar tothe tailstock of a lathe. The index head and footstock attach tothe worktable of the milling machine by T-slot bolts. An indexplate containing graduations is used to control the rotation ofthe index head spind le. The plate is fixed to the index head ,and an index crank, connected to the index head spindle by aworm gear and shaft. Workpieces are held between centers bythe index head spind le and footstock. Workpieces may also beheld in a chuck mounted to the index head spindle or may befitted directly into the taper spindle recess of some indexingfixtures. There are many variations of the indexing fixture.Universal index head is the name ap plied to an ind ex headdesigned to permit power drive of the spindle so that helixesmay be cut on the milling machine. Gear cutting attachment isanother nam e applied to an indexing fixture; in this case, onethat is primarily intended for cutting gears on the millingmachine.

HIGH-SPEED MILLING ATTACHMENT

The rate of spindle speed of the milling machine may beincreased from 1 1/ 2 to 6 times by using the high-speedmilling attachment. This attachment is essential when usingcutters and twist drills which must be driven at a high rate ofspeed in order to obtain an efficient surface speed. Theattachment is clamped to the column of the machine and isdriven by a set of gears from the milling machine spindle.

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VERTICAL SPIND LE ATTACHM ENT

This attachment converts the horizontal spindle of ahorizontal milling machine to a vertical spind le. It is clamped

to the colum n and driven from the h orizontal spindle. Itincorporates provisions for setting the head at any angle, fromthe vertical to the horizontal, in a plane at right angles to themachine spind le. End milling and face milling are more easilyaccomplished with this attachment, because the cutter and thesurface being cut are in plain view.

UNIVERSAL MILLING ATTACHM ENT

This device is similar to the vertical spindle attachment bu tis more versatile. The butterhead can be swiveled to any anglein any plane, whereas the vertical spindle attachment onlyrotates in one place from horizontal to vertical.

ROT ARY TABLE OR CIRCULAR M ILLINGATTACHMENT

This attachment consists of a circular worktable containingT-slots for mounting workplaces. The circular table revolveson a base attached to the milling machine worktable. Theattachment can be either hand or p ower d riven, beingconnected to the table drive shaft if power driven. It may beused for milling circles, angular indexing, arcs, segments,circular slots, grooves, and rad ii, as well as for slottinginternal and external gears. The table of the attachment is

divided in degrees (Figure 8-20). advances twice the amount shown on the micrometer dial.

MOUNTING AND INDEXING WORK

An efficient and positive method of holding workplaces

OFFSET BORING HEAD

Boring, an operation that is too often restricted to a lathe,can be done easily on a milling machine. The offset boringhead is an attachment that fits to the milling machine spindleand permits most drilled holes to have a better surface finishand greater diameter accuracy.

OFFSET BORING H EAD AN D TOOLS

Figure 8-21 shows an offset boring head. Note that thboring bar can be adjusted at a right angle to the spindle axiThis feature makes it possible to position the boring cutteaccurately to bore holes of varying diameters.

This adjustment is more convenient than adjusting the cuttin the boring bar holder or changing the boring bar. Anotheadvantage of the offset boring head is the fact that a graduatemicrometer collar allows the tool to be moved accurately specified am ount (usually in increments of 0.001) without thuse of a dial indicator or other measuring device.

NOTE: On some boring heads, the reading on the tool slid

is a direct reading. On other boring heads, the tool slid

the milling machine table is important if the machine tool is be used to its fullest advantage. The most common methods holding are clamping a workpiece to the table, clamping workpiece to the angle plate, clamping the workpiece ifixtures, holding a workpiece between centers, holding thworkpiece in a chuck, and h olding the workp iece in a visPage 4-13 of this manual shows a variety of moun ting an

holding devices. Regardless of the method used in holdingthere are certain factors that should be observed in every casThe workpiece must not be spru ng in clamping, it must bsecured to prevent it from springing or moving away from thcutter, and it m ust be so aligned th at it may be correctlmachined T-slots, Milling machine worktables are providewith several T-slots which are used either for clamping anlocating the workpiece itself or for

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mounting the various holding devices and attachments. TheseT-slots extend the length of the table and are parallel to its lineof travel. Most milling machine attachments, such as vises andindex fixtures, have keys or tongues on the underside of theirbases so that they may be located correctly in relation to theT-slots.

METHODS OF MO UNTING WORKPIECES

Clamping Workp ieces to the Table

When clamping a workpiece to the worktable of the millingmachine, the table and the workp iece should be free from dirtand burrs. Workpieces having smooth machined surfaces maybe camped directly to the table, provided the cutter does notcome in contact with the table surface during milling. Whenlamping workplaces with unfinished surfaces in this way, theable face should be protected from damage by using a shim

under the workpiece. Paper, plywood, and sheet metal arehim materials. Clamps shou ld be located on both sides of theworkpiece if possible to give a full bearing surface. Theselamps are h eld by T-slot bolts inserted in the T-slots of theable. Clamp sup ports mu st be the same height as the

workpiece. Never use clamp supports that are lower than theworkp iece. Adjustable step blocks are extremely u seful toaise the clamps, as the height of the clamp bar may bedjusted to ensure maximum clamping pressure. Clampingolts should be placed as near to the workp iece as possible sohat the full advantage of the fulcrum principle may bebtained. When it is necessary to place a clamp on anverhanging part, a support should be provided between the

verhang and the table to prevent springing or p ossiblereakage. A stop should be placed at the end of the workpiecewhere it will receive the thru st of the cutter when heavy cutsre being taken.

Clamping a Workpiece to the Angle Plate

Workpieces clamped to the angle plate may be machinedwith su rfaces parallel, perpendicular, or at an angle to a givenurface. When using this method of holding a workpiece,recautions shou ld be taken similar to those mentioned forlamp ing work d irectly to the table. Angle plates are eitherdjustable or nonad justable and are generally held in

lignment by keys or tongues that fit into the table T-slots.

Clamping Workpieces in Fixtures

Fixtures are generally used in prod uction work where aumber of identical pieces are to be machined. The design ofhe fixture depends upon the shape of the piece and theperations to be performed. Fixtures are always constructed

to secure maximum clamping surfaces and are built to use aminimum nu mber of clamps or bolts in order to reduce thesetup time required. Fixtures should always be provided w ithkeys to assure positive alignment with the table T-slots.

Holding Workpieces Between Centers

The indexing fixture is used to support workplaces whichare centered on both ends. When the p iece has been pre-viously reamed or bored, it may be pressed upon a mandreland then moun ted between the centers.

Two types of mandrels may be used for mountingworkplaces between centers. The solid mandrel is satisfactoryfor many operations, while one having a shank tapered to fitinto the index head spindle is preferred in certain cases.

A jackscrew is u sed to p revent sp ringing of long slenderworkp laces held between centers or workplaces that extendsome distance from the chuck.

Workpieces mounted between centers are fixed to the indexhead spind le by means of a lathe dog. The bent tail of the dogshould be fastened between the setscrews provided in thedr iving center clamp in such a m anner a s to avoid backlashand prevent springing the mandrel. When milling certain typesof workpieces, a milling machine dog is held in a flexible ball

joint wh ich eliminates shake or spring of the d og or theworkpiece. The flexible ball joint allows the tail of the dog tomove in a rad ius along the axis of the workp iece, making itparticularly useful in the rapid milling of tapers.

Holding Workpieces in a Ch uck

Before screwing the chuck to the ind ex head sp indle, itshould be cleaned and any burrs on th e spindle or chuckremoved. Burrs may be removed with a smooth-cut, threecornered file or scraper, while cleaning should beaccomp lished w ith a p iece of spring steel wire bent andformed to fit the angle of the threads. The chuck should not betightened on the spindle so tightly that a wrench or bar isrequired to remove it. Cylindrical workplaces held in theuniversal chuck may be checked for trueness by using a testindicator mou nted u pon a base resting upon the milling

machine table. The ind icator p oint should contact thecircumference of small diameter workp ieces, or the circum-ference and exp osed face of large d iameter p ieces. Whilechecking, the workp iece should be revolved by rotating theindex head spind le.

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Holding Workp ieces in the Vise

AS pr eviously men tioned, five typ es of vises aremanu factured in various sizes for holding milling machineworkplaces. These vises have locating keys or tongues on theunderside of their bases so they may be located correctly in

relation to the T-slots on the milling machine table (Figure 8-22).

The plain vise similar to the machine table vise is fastened tothe m illing machine table. Alignment with the m illingmachine table is provided by two slots at right angles to eachother on the underside of the vise. These slots are fitted withremovable keys that align the vise with the table T-slots eitherparallel to the machine arbor or perpendicular to the arbor.

The swivel vise can be rotated and contains a scale graduatedin degrees at its base which is fastened to the milling machinetable and located by means of keys placed in the T-slots. By

loosening the bolts which clamp the vise to its graduated base,the vise may be moved to hold the workpiece at any angle in ahorizontal plane. To set a swivel vise accura tely with themachine spindle, a test indicator should be clamped to th emachine arbor and a check made to determine the setting bymoving either th e transverse or the longitud inal feeds,depending upon the position of the vise jaws. Any deviationas shown by the test indicator should be corrected byswiveling the vise on its base.

The universal vise is used for work involving compoundangles, either horizontally or vertically. The base of the visecontains a scale graduated in degrees and can rotate 360 in

the horizontal plane and 90 in the vertical plane. Due to theflexibility of this vise, it is not adaptable for heavy milling.

The all-steel vise is the strongest setup where the workpieis clamped close to the table. This vise can securely fastecastings, forgings, and rough-surface workplaces. The jawcan be positioned in any n otch on th e two bars taccommodate different shapes and sizes.

The air or hyd raulically operated vise is used more often production work. This type of vise eliminates the tighteninby striking the crank with a lead hamm er or other soft fahammer.

When rough or unfinished workplaces are to be vismoun ted, a p iece of protecting m aterial should be p lacebetween the vise and the workpiece to eliminate marring bthe vise jaws.

When it is necessary to position a workpiece above the visjaws, par allels of the same size and of the proper heigh

should be u sed. These parallels should only be high enough tallow the required cut, as excessive raising reduces thholding ability of the jaws. When holding a workpiece oparallels, a soft hammer should be used to tap the top surfacof the piece after the vise jaws have been tightened. Thitapping should be continued until the parallels cannot bmoved by hand . After the workp iece is set, add itionatightening of the vise should not be attempted, as suchtightening has a tendency to raise the work off the parallelsCorrect selection of parallels is illustrated in Figure 8-23.

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Indexing

Indexing is the process of evenly dividing the circumferenceof a circular w orkpiece into equally spaced divisions, such asin cutting gear teeth, cutting splines, milling grooves inreamers and taps, and spacing holes on a circle. The index

head of the indexing fixture is used for this purpose.

Index Head

The index head of the indexing fixture (Figure 8-19)contains an indexing mechanism which is used to control therotation of the ind ex head spind le to space or divide aworkpiece accurately. A simple indexing m echanism consistsof a 40-tooth worm wheel fastened to the ind ex head spind le,a single-cut worm, a crank for turning the wormshaft, and anindex p late and sector. Since there are 40 teeth in the wormwheel, one turn of the index crank causes the worm, andconsequently, the index head spindle to make 1/ 40 of a turn;

Whenever possible, the workpiece should be clamped in so 40 turns of the index crank revolve the spindle one fullhe center of the vise jaws. However, when necessary to mill

a short workpiece which mu st be held at the end of the vise, apacing block of the same thickness as the piece should belaced at the opposite end of the jaws. This will avoid strain

on the movable jaw and prevent the piece from slipping. Ifhe w orkpiece is so thin that it is impossible to let it extendver the top of the vise, hold down straps are generally used.

See Figure 8-24. These straps are hardened pieces of steel,having one vertical side tapered to form an angle of about

2 with the bottom side and the other vertical side tapered tonarrow edge. By means of these tapered surfaces, the

turn.

Index Plate

The indexing plate (Figure 8-25) is a round plate with aseries of six or more circles of equally spaced holes; theindex pin on the crank can be inserted in any hole in anycircle. With the interchangeable plates regularly furnishedwith m ost index heads, the sp acing necessary for most gears,boltheads, milling cutters, splines, and so forth can beobtained . The following sets of plates are stand ard

workpiece is forced downward into the parallels, holding equipment:hem firmly and leaving th e top of the w orkpiece fullyxposed to the m illing cutter.

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Brow n an d Sharp e type consists of 3 plates of 6 circleseach drilled as follows:

Plate I -15, 16, 17, 18, 19, 20 holes

Plate 2-21, 23, 27, 29, 31, 33 holes

Plate 3-37, 39, 41, 43,47,49 holes

Cincinnati type consists of one plate drilled on both sideswith circles divided as follows:

The same principle applies whether or not the d ivisirequired divide equally into 40, For example, if it is desito index for 6 divisions, 6 divided into 40 equals 6 2/ 3 tursimilarly, to index for 14 spaces, 14 divid ed into 40 equa

6/ 7 turns. These examples may be multiplied indefinitely afrom them the following rule is derived: to determine num ber of turns of the index crank needed to obtain odivision of any number of equal divisions on the workpiedivide 40 by the number of equal divisions desired (providthe worm wheel has 40 teeth, which is standard practice).

Direct IndexingFirst side -24, 25, 28, 30, 34, 37,38, 39,41,42,43 holes

Second side -46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66holes

Sector

The sector (Figure 8-25) indicates the next hole in whichthe pin is to be inserted and m akes it unnecessary to countholes when moving the index crank after each cut. It consistsof two radial, beveled arms which can be set at any angle toeach other and then moved together around the center of theindex plate. Sup pose that, as show n in Figure 8-25, it isdesired to m ake a series of cuts, moving the index crank 11/ 4 turns after each cut. Since the circle illustrated has 20holes, turn the crank one full turn plus five spaces after eachcut, Set the sector arms to include the desired fractional partof a turn or five spaces between the beveled edges of itsarms, as shown. If the first cut is taken with the index pinagainst the left-hand arm, to take the next cut, move the pinonce against the r ight-hand arm of the sector. Before takingthe second cut, move the arms so that the left-hand arm isagain against the pin; this moves the right-hand arm anotherfive spaces ahead of the pin. Then take the second cut, andrepeat the operation until all the cuts have been completed.

NOTE: It is good practice always to index clockwise onthe plate to eliminate backlash.

Plain Indexing

The following principles apply to basic indexing ofworkpieces:

Suppose it is desired to mill a project with eight equallyspaced teeth. Since 40 turns of the index crank will turn thespindle one full turn, l/ 8th of 40 or 5 turns of the crank aftereach cut will space the gear for 8 teeth, If it is desired tospace equally for 10 teeth, 1/ 10 of 40 or 4 turn s wou ldproduce the correct spacing.

The construction of some index heads permits the wormbe disengaged from the w orm w heel, making possiblquicker method of indexing called direct indexing. The indhead is provided with a knob which, when turned throu

part of a revolution, operates an eccentric and disengages worm.

Direct indexing is accomplished by an ad ditional indplate fastened to the index head spindle. A stationary p lunin the index head fits the holes in this index plate. moving this plate by hand to index directly, the spindle athe workpiece rotate an equal distance. Direct index plausually have 24 holes and offer a quick means of millisquares, hexagons, taps, and so forth. Any numberdivisions which is a factor of 24 can be indexed quickly aconveniently by the direct indexing method.

Differential Indexing

Sometimes, a number of divisions is required which cannbe obtained by simple indexing with the index platregularly supplied. To obtain these divisions, a differentindex head is used. The index crank is connected to twormshaft by a train of gears instead of a direct couplingwith simple indexing. The selection of these gears involvcalculations similar to those used in calculating change gratio for lathe thread cutting.

Indexing in D egrees

Workpieces can be indexed in degrees as well as fractioof a turn with the usual index head. There are 360 degreesa complete circle and one turn of the index crank revolves spindle 1/ 40 or 9 degrees. Therefore, 1/ 9 turn of the crarotates the spindle 1 degree. Workpieces can therefore indexed in degrees by using a circle of holes divisible byFor example, moving the crank 2 spaces on an 18-hole circ3 spaces on a 27-hole circle, or 4 spaces on a 36-hole circle

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will rotate the spind le 1 degree, Smaller crank movemen tsfurther subdivide the circle: moving 1 space on an 18-holecircle turns the sp indle 1/ 2 degree (30 minutes), 1 space on a27-hole circle turns the spindle 1/ 3 degree (20 minutes), andso forth.

Indexing Operations

The following examples show how the index plate is used toobtain any desired par t of a whole spindle turn by p lainindexing,

Milling a hexagon. Using the rule previously given,divide 40 by 6 which equals 6 2/ 3 turns, or six full turnsplus 2/ 3 of a turn or any circle whose number is divisibleby 3. Take the denominator which is 3 into which of theavailable hole circles it can be even ly divided . In thiscase, 3 can be divided into the available 18-hole circle

exactly 6 times. Use this result 6 as a multiplier togenerate the proportional fraction required.

Example: 2 x 6 = 123x6 = 18-

Therefore, 6 full turns of the crank plus 12 spaces on an 18-hole circle is the correct indexing for 6 divisions.

Cutting a gear. To cut a gear of 52 teeth, using the ruleagain, divide 40 by 52. This means that less than onefull turn is required for each d ivision, 40/ 52 of a turn tobe exact. Since a 52-hole circle is not ava ilable, 40/ 52must be reduced to its lowest term wh ich is 10/ 13. Takethe denom inator of the lowest term 13, and d etermineinto which of the available hole circles it can be evenlydivided. In this case, 13 can be divided into a 39-holecircle exactly 3 times. Use this result 3 as a multiplier togenerate the proportional fraction required.

Examp le: 10 x 3 = 30

13 x 3 = 39

Therefore, 30 holes on a 39-hole circle is the correctindexing for 52 divisions. When counting holes, start withthe first hole ahead of the index pin.

GENERAL MILLING OPERATIONS

GENERAL

Setup

The success of any milling operation depends, Beforesetting up a job, be sure that the to a great extent, uponjudgment in setting up the job, workpiece, the table, the taperin the sp indle, selecting the p roper milling cutter, andholding the cutter by the best means under the circumstancesSome fundamental practices have been proved by experienceto be necessary for and the arbor or cutter shank are all cleanand good results on all jobs. Some of these practices arementioned be low...

Before setting up a job, be sure that the workpiece, table,the taper in the spindle, and the arbor or cutter shank arefree from chips, nicks, or bu rrs.

Do not select a m illing cutter of larger diameter than isnecessary.

Check the machine to see if it is in good running ord erand properly lubricated, and that it moves freely, but nottoo freely in all directions.

Consider direction of rotation. Many cutters can be

reversed on the arbor, so be sure you know whether thespindle is to rotate clockwise or counterclockwise.

Feed the workpiece in a d irection opposite the rotation ofthe milling cutter (conventional milling).

Do not change feeds or speeds wh ile the milling machineis in operation.

When using clamps to secure a workpiece, be sure thatthey are tight an d th at the p iece is held so it will notspring or vibrate under cut.

Use a recommended cutting oil liberally.

Use good judgment and common sense in planning every job, and profit from previous mistakes.

Set up every job as close to the milling machine spindleas circumstances will permit.

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Milling Operations

Milling operations may be classified u nder four generalheadings as follows:

Face milling. Machining flat surfaces which are at rightangles to the axis of the cutter,

Plain or slab milling. Machining flat surfaces which areparallel to the axis of the cutter.

Angular milling. Machining flat surfaces which are at aninclination to the axis of the cutter.

Form milling. Machining su rfaces having an irregu laroutline.

Special O perationsExplanatory names, such as sawing, slotting, gear cutting,

and so forth have been given to special operations. Routing isa term app lied to milling an irregular ou tline w hilecontrolling the workpiece movement by hand feed. Groovingreamers and taps is called fluting. Gang m illing is the termapplied to an operation in which two or more milling cuttersare used together on one arbor. Straddle milling is the termgiven to an operation in w hich two milling cutters are used tostraddle the workpiece and mill both sides at the same time.

SPEEDS FOR MILLING CUTTERS

The speed of milling is the distance in FPM at which thecircumference of the cutter passes over the work. The spindleRPM necessary to give a desired peripheral speed d epends onthe size of the milling cutter. The best speed is determined bythe kind of material being cut and the size and type of cutterused, width and depth of cut, finish required, type of cuttingfluid and method of application, and power and speedavailable are factors relating to cutter speed.

Factors Governing Speed

There are no hard and fast rules governing the speed ofmilling cutters; experience has show n that the followingfactors must be considered in regulating speed:

A metal slitting saw milling cutter can be rotated fasterthan a plain milling cutter having a broad face.

TC 9

Cutters having u ndercut teeth (positive rake) cut mfreely than those having rad ial teeth (without rhence, they may run at higher speeds.

Angle cutters must be run at slower speeds than pla

side cutters.

Cutters with inserted teeth generally will stand as mspeed as a solid cutter.

A sharp cutter may be operated at greater speeds thdu ll one.

A plentiful supply of cutting oil will permit the cuttrun at higher speeds than without cutting oil

Selecting Proper Cutting Sp eedsThe approximate values given in Table 8-1 in Append

may be used as a guide for selecting the proper cutting spIf the operator finds that th e machine, the milling cuttethe workpiece cannot be handled suitably at these speimmediate readjustments should be made.

Table 8-1 lists speeds for high-speed steel milling cuttecarbon steel cutters are used, the speed should be about half the recommended speed in the table. If carbide-tipcutters are used, the speed can be doubled.

If a plentiful supply of cutting oil is applied to the micutter and the workpiece, speeds can be increased 50 topercent. For roughing cuts, a moderate speed and coarse often give best resu lts; for finishing cuts, the best p ractito reverse these conditions, using a higher speed and ligfeed.

Speed Computation

The formula for calculating spindle speed in revolutionsminute is as follows:

RPM = CSx4D

Where RPM = Spindle speed (in revolutions per m inute).

CS = cutting speed of milling cutter (in SFPM)

D = diameter of milling cutter (in inches)
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For example, the spindle speed for machining a piece ofsteel at a speed of 35 SFPM with a cutter 2 inches in diameteris calculated as follows:

RPM= CSx4 = 35x4 = 140D 2 2 = 70 RPM

Therefore, the milling machine spindle would be set for asnear 70 RPM as possible.

Table 8-2 in App endix A is provided to facilitate spindlespeed computations for standard cutting speeds and standardmilling cutters.

FEEDS FOR M ILLIN G

The rate of feed, or the speed at w hich the workpiece passesthe cutter, determines the time required for cutting a job. In

selecting the feed. there are several factors which should beconsidered.

Forces are exerted against the workpiece, the cutter, andheir holding devices during the cutting process. The force

exerted varies directly with the amount of feed and depth ofcut. and in turn are dependent upon the rigidity and power ofhe machine. Milling machines are limited by the power they

can develop to turn the cutter and the amou nt of vibrationhey can resist when u sing coarse feeds and deep cuts. The

feed and depth of the cut also depend upon the type of millingcutter being used. For example. deep cuts or coarse feedsshould not be attempted when using a small diameter end

milling cutter. Coarse cutters with strong cutting teeth can beed at a faster rate because the chips maybe washed out moreasily by the cutting oil.

Coarse feeds and deep cuts should not be used on a frailworkp iece if the piece is moun ted in such a w ay that itsholding device is not able to prevent springing or bending.

Experience and judgment are extremely valuable inelecting the correct milling feeds. Even though suggestedate tables are given. remember that these are suggestions

only. Feeds are governed by many variable factors, such ashe degree of finish required. Using a coarse feed, the metal is

emoved more rap idly but the appearance and accuracy of theurface produced may not reach the standard desired for theinished product. Because of this fact. finer feeds andncreased speeds are used for finer. more accurate finishes.

while for roughing. to use a comparatively low speed andheavy feed. More mistakes are made on overspeeding and

nderfeeding than on underspeeding and overfeeding.

Overspeeding may be detected by the occurrence of asqueaking. scraping sound . If vibration (referred to aschattering) occurs in the milling machine during the cuttingprocess. the speed should be reduced and the feed increased.Too much cutter clearance. a poorly supported workpiece, ora badly worn machine gear are common causes of chattering.

Designation of Feed

The feed of the milling machine m ay be designated ininches per minute or millimeters per minute The milling feedis determined by m ultiplying the chip size (chip per tooth)desired (see Table 8-3 in Appendix A), the number of teethon the cutter, and the revolutions per minute of the cutter.

Example: the formula used to find the workfeed in inchesper minute.

IPM = CPTxNxRPMIPM = Feed rate in inches per minu te.CPT = Chip pertN = Num ber of teeth per minu te of the milling cutter.

The first step is to calculate the spindle speed before thefeed rate can be calculated,

RPM = CSD 4 = 300 x 4 = 1,200 =2,400D 1/2 0.5

The second step is to calculate the feed rate.

IPM = CPT x N x RPM= 0.005 x 2 x 2,400= 24

Therefore, the RPM for a l/ 2-inch-diameter end millmachining aluminum revolves at 2.400 RPM and the feedrate should be 24 inches per minute.

The formula used to find workfeed in millimeters per minuteis the same as the formu la used to find the feed in IPM,except that mm / min is substituted for IPM.

Direction of Feed

It is usually regarded as standard practice to feed theworkpicce against the milling cutter. When the workpiece isfed against the milling cutter. the teeth cut under any scale onthe workp iece surface and any backlash in the feed screw istaken up by the force of the cut. See Figure 8-26.

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As an exception to this recommendation. it is advisable tofeed with the milling cutter when cutting off stock or whenmilling comparatively deep or long slots.

The direction of cutter rotation is related to the manner inwhich the workplace is held. The cutter should rotate so thatthe piece springs away from the cutter; then there will be notendency for the force of the cut to loosen the piece. Nomilling cutter shou ld ever be r otated backward ; this willbreak the teeth. If it is necessary to stop the machine during afinishing cut, the power feed should never be thrown ou t, norshould the workpiece be fed back under the cutter unless thecutter is stopp ed or the w orkpiece lowered. Never changefeeds while the cutter is rotating.

CUTTING O ILS

The major advantage of using a coolant or cutting oil is thatit dissipates heat, giving longer life to the cutting edges of theteeth. The oil also lubricates the cutter face and flushes awaythe chips, consequently reducing the possibility of marringthe finish.

Types

Cutting oils are basically water-based soluble petroleum oils, and synthetic oils. Water-based coolants

excellent heat transfer qualities; other oils result in gsurface finishes. The cutting oil compounds for varmetals are given in Table 4-3 in Appendix A. In genersimple coolant is all that is required for roughing. Finisrequires a cutting oil with good lubricating properties to produce a good finish on the workp iece. Plastics and castare almost always machined dry.

Method of Use

The cutting oil or coolant should be directed by m eancoolant drip can, pump system, or coolant mist mix topoint where the cutter contacts the workpiece. Regardlemethod used, the cutting oil should be allowed to flow frover the workpiece and cutter.

PLAIN MILLING

General

Plain milling, also called surface milling or slab millingmilling flat surfaces with the milling cutter axis parallethe su rface being milled. Generally, plain milling is dwith the w orkpiece surface mounted parallel to the surfathe milling machine table and the milling cutter mounted

a standard milling machine arbor. The arbor is well suppoin a horizontal plane between the milling m achine spinand one or m ore arbor supports.

Mounting the Workpiece

The workpiece is generally clamped directly to the tablsupp orted in a vise for plain milling. The milling machtable should be checked for alignment before starting to cuthe workpiece surface to be milled is at an angle to the bplane of the piece, the workp iece should be m ounted universal vise or on an adjustable angle plate. The hold

device should be ad justed so that the workpiece surfacparallel to the table of the milling machine.

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Selecting the Cutter

A careful study of the drawing must be made to determinewhat cutter is best suited for the job. Flat surfaces may bemilled w ith a plain milling cutter moun ted on a n arbor.

Deeper cuts may generally be taken when using narrow cuttersthan with wide cutters. The choice of milling cutters should bebased on the size and shape of the workp iece. If a wide area isto be milled, fewer traverses will be required u sing a w idecutter. If large quantities of metal are to be removed, a coarsetooth cutter should be used for roughing and a finer toothcutter should be used for finishing. A relatively slow cuttingspeed and fast table feed should be used for roughing, and arelatively fast cutting speed and slow table feed used forfinishing. The su rface should be checked for accuracy after

ANG ULAR MILLING

General

Angu lar milling, or angle milling, is milling flat sur faceswhich are neither parallel nor perpendicular to the axis of themilling cutter. A single angle milling cutter is used for angularsurfaces, such as chamfers, serrations, and grooves.Milling dovetails (Figure 8-28) is a typical example of angularmilling.

each completed cut.

Setup

A typical setup for plain milling is illustrated in Figure 8-27.

Note that the milling cutter is positioned on th e arbor w ithleeves so that it is as close as practical to the milling machinepindle while maintaining sufficient clearance between the

vise and th e milling machine column. This practice reducesorque in the arbor and perm its more rigid supp ort for theutter.

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Milling D ovetails

When milling dovetails, the usual angle of the cutter is 45,50, 55, or 60 based on common dovetail designs.

When cutting d ovetails on the milling m achine, theworkpiece may be held in a vise, clamp ed to the table, orclamp ed to an angle plate. The tongu e or groove is firstroughed out u sing a side milling cutter, after w hich theangular sides and base are finished with an angle millingcutter.

In general practice, the d ovetail is laid out on the workpiecesurface before the milling operation is started. To do this, therequired outline should be inscribed and the line prick-punched. These lines and punch marks may then be used as aguide during the cutting operation.

STRADD LE MILLING

When two or more parallel vertical surfaces are machined ata single cut, the operation is called straddle milling. Straddlemilling is accomplished by mounting two side milling cutterson the same arbor, set apart at an exact spacing. Two sides ofthe workpiece are machined simultaneously and final widthdimensions are exactly controlled.


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MILLING A HEXAGO N

Stradd le milling has many useful app lications introductionmachining. Parallel slots of equal depth can be milled by usingstraddle mills of equal diameters. Figure 8-29 illustrates a

typical example of straddle milling. In this case a hexagon isbeing cut, but the same operation may be applied to cuttingsquares or splines on the end of a cylindrical workpiece. Theworkpiece is usually mounted between centers in the indexingfixture or mou nted vertically in a swivel vise. The two sidemilling cutters are separated by spacers, washers, and shims sothat the distance between the cutting teeth of each cutter isexactly equal to the width of the workp iece area required.When cutting a square by this method, two opposite sides ofthe square are cut, and then the spind le of the indexing fixtureor the swivel vise is rotated 90, and the other two sides of theworkpiece are straddle milled.

FACE MILLIN G

General

Face milling is the milling of surfaces that are perpendicularto the cutter axis, as shown in Figure 8-30. Face millingproduces flat surfaces and machines work to the requiredlength. In face milling, the feed can be either horizontal orvertical.

In face milling, the teeth on the periphery of the cutter dopractically all of the cutting. However, when the cutter isproperly ground , the face teeth actually remove a small

amount of stock wh ich is left as a resu lt of the springing of theworkpiece or cutter, thereby producing a finer finish.

It is important in face milling to have the cutter securelymoun ted and to see that all end p lay or sloppiness in themachine spindle is eliminated.

Mounting the Workpiece

When face milling, the workpiece may be clamped totable or angle plate or supported in a vise, fixture, or jig.

Large surfaces are generally face milled on a ver tical millmachine with the workpiece clamped directly to the millmachine table to simplify handling and clamping operations

Angular surfaces can also be face milled on a swivel cuhead milling m achine (Figure 8-31). In this case, tworkpiece is mounted parallel to the table and the cutter heis swiveled to bring the end milling cutter perpendicular to

surface to be produced.

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During face milling operations, the workpiece should be fedagainst the milling cutter so that the p ressure of the cut isdownward, thereby holding the piece against theable.Whenever possible, the edge of the workpiece should ben line with the center of the cutter. This position of the

workpiece in relation to the cutter will help eliminate slippage.

Depth of Cut

When setting the depth of cut, the workp iece should berought up to just touch the revolving cutter. After a cut haseen made from this setting, measurement of the workpiece isaken. At this point, the graduated dial on the traverse feed isocked and used as a guide in determining the depth of cut.

When starting the cut, the workpiece should be moved sohat the cutter is nearly in contact with its edge, after w hich

he automatic feed may be engaged.When a cut is started by hand, care must be taken to avoid

ushing the corner of the workpiece between the teeth of theutter too quickly, as this may result in cutter tooth breakage.n order to avoid wasting time during the operation, the feedrips should be adjusted to stop the table travel just as theutter clears the w orkpiece.

GAN G M ILLING

Gang milling is the term applied to an operation in whichtwo or more milling cutters are moun ted on the same arborand used w hen cutting horizontal surfaces. All cutters mayperform the same typ e of operation or each cutter mayperform a different type of operation. For example, severalworkplaces need a slot, a flat surface, and an angular groove.The best method to cut these wou ld be gang milling as shownin Figure 8-32. All the completed workp laces wou ld be th esame. Remember to check the cutters carefully for prop er size.

FORM MILLING

Form milling is the p rocess of machining special contour scomposed of curves and straight lines, or entirely of curves, ata single cut. This is done with formed milling cutters, shaped

to the contour to be cut. The more common form millingoperations involve milling half-round recesses and beads andquarter-round radii on w orkplaces (Figu re 8-33), Thisoperation is accomplished by using convex, concave, andcorner round ing milling cutters groun d to the desired circlediameter. Other jobs for formed milling cutters include millingintricate patterns on workplaces and milling several complexsurfaces in a single cut such as are produced by gang milling.

FLY CUTTING

General

Fly cutting, which is also called single point milling, is oneof the most versatile milling op erations. It is don e with asingle-point cutting tool shaped like a lathe tool bit. It is heldand rotated by a fly cutter arbor. You can grind this cutter toalmost any form that you need, as shown in Figure 8-34.Formed cutters are expensive. There are times when you needa special form cutter for a very limited number of parts. It ismore economical to grind the desired form on a lathe-type toolbit than to buy a preground form cutter, which is veryexpensive and usually suitable only for one particular job.

Gear Cutting

The single-point or fly cutter can be used to great ad vantagein gear cutting. A II that is needed is enough of the broken gearto grind the cutting tool to the proper shape. It can also beused in the cutting of splines and standard and special forms.

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Flat Surfaces

Another typ e of fly cutter, wh ich d iffers mainly in the

design of the arbor, can be u sed to mill flat su rfaces as inplain or face milling (Figure 8-34). The arbor can easily bemanufactured in the shop using common lathe tool bits. Thistype of fly cutter is especially useful for milling flat surfaceson aluminum and other soft nonferrous metals, since a highquality finish can be easily obtained. Boring holes with thistype of fly cutter is not recommended. The arbor is so shortthat only very shallow holes can be bored.

KEYWAY M ILLIN G

Keyways are grooves of different shapes cut along taxis of the cylindrical surface of shafts, into which keys fitted to provide a positive method of locating and drivimembers on the shafts. A keyway is also machined in tmounted member to receive the key.

The type of key and corresponding keyway to be usdepends upon the class of work for which it is intended. Tmost commonly used types of keys are the Woodruff key, square-ends machine key, and the round -end machine k(Figure 8-35).

Woodruff Key

The Woodru ff keys are semicylindrical in shape an d manufactured in various diameters and widths. The circuside of the key is seated into a keyway which is milled in shaft. The upper portion fits into a slot in a mating part, suas a pulley or gear. The Wood ruff key slot milling cut(Figure 8-36) must have the same diameter as that of the k

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Woodruff key sizes are designated by a code number in the keyw ay of the bore. This clearance may be from awhich the last two d igits indicate the diameter of the key in minimum of 0.002 inch to a maximum of 0.005 inch.ighths of an inch, and the digits preceding the last two digits Positive fitting of the key in the shaft keyway is provided by

give the width of the key in thirty-seconds of an inch. Thus, a making the key 0.0005 to 0.001 inch wider than the keyway.num ber 204 Woodruff key would be 4/ 8 or 1/ 2 inch indiameter and 2/ 32 or 1/ 16 inch wide, while a number 1012

Square-End Machine KeyWoodruff key would be 12/ 8 or 1 1/ 2 inches in d iameter and10/ 32 or 5/ 16 inch wide. Table 8-4 in Appendix A lists Square-ends m achine keys are square or rectangular inWoodruff keys commonly u sed and pertinent information section and several times as long as they are wide. For thepplicable to their machining. purpose of interchangeability and standardization, these keys

For proper assembly of the keyed members to be made, aare usually proportioned with relation to the shaft diameter inthe following method:

learance is required between the top surface of the key and

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Key width equals approximately one-quarter of the shaftdiameter.

Key thickness for rectangu lar section keys (flat keys)equals appr oximately 1/ 6 of the shaft diameter.

Minimum length of the key equals 1 1/ 2 times the shaftdiameter.

Depth of the keyway for square section keyswidth of the key.

Depth of the keyway for rectangular section

is 1/ 2 the

keys (flatkeys) is 1/ 2 the thickness of the key,

Table 8-5 in Appendix A lists common sizes for square-endmachine keys. The length of each key is not included because

the key may be of any length as long as it equals at least 1 1/ 2times the shaft diameter.

Round-end machine keys (Figure 8-35). The round-endsmachine keys are square in section with either one or bothends rounded off. These keys are the same as square-endsmachine keys in measur ements (see Table 8-5 in AppendixA).

Milling Cutters Used for Milling Keyways

Shaft keyways for Woodruff keys are milled with Woodruffkeyslot milling cutters (Figure 8-35). The Woodruff keyslot

milling cutters are numbered by the same system employedfor identifying Woodruff keys, Thus, a number 204 Woodruffkeyslot cutter has the proper diameter and width for milling akeyway to fit a number 204 Woodruff key.

Square-end keyways can be cut with a plain milling cutteror side milling cutter of the proper width for the key

Round-end keyways must be milled with end milling cutters(Figure 8-37) so that the rounded end or ends of the key mayfit the ends of the keyway. The cutter should be equal indiameter to the width of the key.

Alignment of Milling Cutters

When milling keyways. the shaft may be supported in thevise or chuck, moun ted between centers. or clamp ed to themilling machine table. The cutter must be set centrally withthe axis of the workpiece. This alignment is accomplished byusing one of the following methods:

When u sing a Woodruff keyslot milling cutter, the sshould be positioned so that the side of the cutter is tangeto the circumference of the shaft. This is done by movingshaft transversely to a point that permits the workpiectouch the cutter side teeth. At this point the graduated dia

the cross feed is locked and the milling machine tablowered. Then, using the cross feed graduated dial as a guthe shaft is moved transversely a distance equal to the raof the shaft plus 1/ 2 the width of the cutter.

End mills may be aligned centrally by first causingworkpiece to contact the periphery of the cutter, tproceeding as in the paragraph above.

Milling Woodru ff Key Slot

The milling of a Woodruff keyslot is relatively simple sithe proper sized cutter has the same diameter and thickn

as the key. With the milling cutter located over the positionwhich the keyway is to be cut, the workpiece should be moup into the cutter until you obtain the desired keyseat depRefer to Table 8-4 in Appendix A for correct depth of keycut for standard Woodruff key sizes. The work may be helda vise. chuck. between centers. or clamp ed to th e millmachine table. Depending on its size, the cutter is held inarbor or in a spring collet or drill chuck that h as bemounted in the spindle of the milling machine.

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Milling Keyslot for Square-End Machine Key

The workpiece should be properly mounted, the cuttercentrally located, and the workpiece raised until the millingcutter teeth come in contact with the workp iece. At this point,

the graduated dial on the vertical feed is locked and theworkp iece moved longitudinally to allow the cutter to clearthe workpiece. The vertical hand feed screw is then used toraise the workpiece until the cutter obtains the total depth ofcut. After this adjustment. the vertical adjustment controlshould be locked and the cut mad e by feeding the tablelongitudinally.

Milling Keyway for Round -End Machine Key

Rounded keyways are milled with an end milling cutter Ofthe prop er diameter. As in the case of square-ends m achinekey keyways, the workpiece should be properly mounted and

the cutter centrally located with respect to the shaft. The shaftor cutter is then positioned to permit the end of the cutter totear a piece of thin paper held between the cutter and theworkp iece. At this point the grad uated feed d ial should belocked and used as a guide for setting the cutter d epth. Theends of the keyway should be well marked and the workpiecemoved back and forth making several passes to eliminateerror due to spring of the cutter.

T-SLOT MILLING

Cutting T-slots in a workpiece holding device is a typical

milling op eration. The size of the T-slots depends up on thesize of the T-slot bolts which will be used. Dimensions of T-slots and T-slot bolts are standa rdized for specific boltdiameters. The dimensions for bolt diameters commonly usedare given in Table 8-6 (Append ix A).

Selection of Millin g Cutters

Two milling cutters are required for milling T-slots, a T-slot milling cutter and either a side milling cutter or an endmilling cutter. The side milling cutter (preferably of the stag-gered tooth type) or the end milling cutter is used to cut a slotin the workpiece equal in width to the throat width of the T-

slot and equal in depth to slightly less than the head spacedepth plus the throat depth). The T-slot milling cutter is thenused to cut the head space to the prescribed dimensions.

Millin g the T-Slot

The position of the T-slot is laid out on the workpiece. Thethroat depth is determined by considering the thickness of theworkpiece and the maximum and minimum dimensionsallowable (Table 8-6. Appendix A).

A side m illing cutter or an end milling cutter is thenselected. The cutter should be of proper size to mill a slotequal in width to the throat w idth p rescribed for the T-slotsize desired. Cut a plain groove equal to about 1/ 16 inch lessthan the combined throat d epth and head space depth.

Select a T-slot milling cutter for the size T-slot to be cut. T-slot milling cutters are identified by the T-Slot bolt diameterand remanufactured with the proper diameter and width tocut the head space to the dimensions given in Table 8-6 inAppendix A. Position the T-slot milling cutter over the edgeof the workpiece and align it with the previously cut groove.Feed the table longitudinally to make the cut. Flood the cutterand workpiece with cutting oil during this operation. Figure8-38 shows a T-slot milling cutter and dimension locationsfor T-slots.

SAWING AND PARTING

Metal slitting saw milling cutters are used to part stock on amilling machine. Figure 8-39 illustrates p arting solid stock.

The workpiece is being fed against the rotation of the cutter.For greater rigidity while parting thin material such as sheetmetal, the vvorkpiece may be clamped directly to the tablewith the line of cut over one of the table T-slots. In this case,the workpiece should be fed with the rotation of the millingcutter (climb milling) to prevent it from being raised off thetable. Every p recaution should be taken to eliminate backlashand spring in order to prevent climbing or gouging theworkpiece.

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HELICAL M ILLING

A helix may be defined as a regular curved path. such as isformed by w inding a cord arou nd the surface of a cylinder.Helical par ts most commonly cut on the milling machineinclude helical gears. spiral flute milling cutters, twist drills.and helical cam grooves. When m illing a helix. a universalindex head is used to rotate the workpiece at the p roper rate

of speed w hile the piece is fed against the cutter. A train ofgears between the table feed screw and the index head servesto rotate the workp iece the required amou nt for a givenlongitud inal movem ent of the table. Milling h elical partsrequires the use of special formed milling cutters and double-angle m illing cutters, The calculations and formulasnecessary to compu te prop er work table angles, gearadjustments. and cutter angles and positions for helicalmilling are beyond the scope of this manual,

GEAR CUTTING

Gear teeth are cut on the m illing machine using formedmilling cutters called involute gear cutters. These cutters aremanufactured in m any pitch sizes and shapes for differentnumbers of teeth per gear (Table 8-7, Appendix A).

If involute gear cutters are not available and teeth must berestored on gears that cannot be r eplaced. a lathe cutter bitground to the shape of the gear tooth spaces may be mountedin a fly cutter for the operation. The gear is m illed in thefollowing manner:

NOTE: This method of gear cutting is not as accurausing an involute gear cutter and should be used onlyemergency cutting of teeth which have been built upwelding,

Fasten the indexing fixture to the milling machine taUse a mandrel to mount the gear between the index headfootstock centers. Adjust the ind exing fixture on the m imachine table or adjust the position of the cutter to makegear axis perpendicular to the milling machine spindle Fasten the cutter bit that has been ground to the shape ogear tooth spaces in the fly cutter arbor. Adjust the cucentrally with the axis of the gear. Rotate the milling macspind le to position the cutter bit in the fly cutter so thacutting edge is downw ard.

Align the tooth space to be cut with the fly cutter arbor

cutter bit by turning the index crank on the index head.

Proceed to mill the tooth in the same m anner as m illikeyway.

SPLINE MILLING

Splines are often u sed instead of keys to transmit pofrom a shaft to a hub or from a hub to a shaft. Splines areeffect. a series of parallel keys formed integrally withshaft. mating with corresponding grooves in the hub or fit(Figure 8-40). They are particularly useful where the

must slide axially on the shaft, either und er load or freTypical applications for splines are found in geatransmissions, machine tool drives. and in autommechanisms.

Splined Shafts and Fittings

Splined shafts and fittings are generally cut by bobbing broaching on special machines. However. when spline shm ust be cut for a repair job. the operation m ayaccomplished on the milling machine in a manner similathat described for cutting keyways. Standard spline shafts splint fittings have 4, 6, 10, or 16 splines, a

theirdimensions dep end u pon the class of tit for the desapp lication: a perm anen t fit, a sliding fit when not unload, and a sliding fit under load. Table 8-8 in Appendilists the stand ard dimensions for 4, 6, 10, and 16-spline sha

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Milling Splines

Spline shafts can be milled on the milling machine in amanner similar to the cutting of keyways.

The shaft to be splined is set up between centers in theindexing fixture.

Two side milling cutters are mounted to an a rbor with aspacer and shims inserted between them. The spacer andshims are chosen to make space between the inner teeth of thecutters equal to the width of the spline to be cut (Table 8-8,Appendix A).

The arbor and cutters are mounted to the milling machinespindle. and the milling machine is adjusted so that thecutters are centered over the shaft.

The splines are cut by stradd le milling each spline to therequired depth (Table 8-8. Appendix A) and using the indexhead of the indexing fixture to rotate the workp iece thecorrect distance between each spline position.

After the sp lines are milled to the correct depth, mou nt anarrow plain milling cutter in the arbor and m ill the spacesbetween the splines to the proper depth. It will be necessary tomake several passes to cut the groove un iformly so that thespline fitting will not interfere with the grooves. A formedspline milling cutter, if available, can be used for thisoperation.

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DRILLING

The milling machine may be used effectively for drilling,since accurate location of the hole may be secured by meansof the feed screw gradu ations. Spacing holes in a circular

path , such as the holes in an index plate, may beaccomplished by ind exing w ith the index head positionedvertically.

Twist drills may be supported in drill chucks fastened in themilling machine spindle or moun ted d irectly in millingmachine collets or adapters. The workpiece to be drilled isfastened to the milling machine table by clamps, vises, orangle plates.

BORING

Various types of boring tool holders may be used for boring

on the milling machine. the boring tools being provided witheither straight shanks to be held in chucks and holders ortaper shanks to fit collets and adapters. The two attachmentsmost commonly used for boring are the fly cutter arbor andthe offset boring head.

The single-edge cutting tool used for boring on the millingmachine is the same as a lathe cutter bit. Cutting speeds,feeds, and depth of cut should be the same as that prescribedfor lathe operations.

Documents

Chapter 8 Milling Operations