MILING

www.toolingandproduction.com Chapter 12/Tooling & Production 1

CHAPTER 12Milling Cuttersand OperationsMetal Removal

Cutting-Tool MaterialsMetal Removal MethodsMachinability of Metals

Single Point MachiningTurning Tools and Operations

Turning Methods and MachinesGrooving and Threading

Shaping and Planing

Hole Making ProcessesDrills and Drilling Operations

Drilling Methods and MachinesBoring Operations and Machines

Reaming and Tapping

Multi Point MachiningMilling Cutters and OperationsMilling Methods and Machines

Broaches and BroachingSaws and Sawing

Abrasive ProcessesGrinding Wheels and OperationsGrinding Methods and Machines

Lapping and Honing

George Schneider, Jr. CMfgEProfessor Emeritus

Engineering TechnologyLawrence Technological University

Former ChairmanDetroit Chapter ONESociety of Manufacturing Engineers

Former PresidentInternational Excutive BoardSociety of Carbide & Tool Engineers

Lawrence Tech.- www.ltu.edu

Prentice Hall- www.prenhall.com

12.1 IntroductionThe two basic cutting tool types used in the metal working industry are of thesingle point and multi-point design, although they may differ in appearance andin their methods of application. Fundamentally, they are similar in that theaction of metal cutting is the same regardless of the type of operation. Bygrouping a number of single point tools in a circular holder, the familiar millingcutter is created.

Milling is a process of generating machined surfaces by progressively remov-ing a predetermined amount of material or stock from the workpiece witch isadvanced at a relatively slow rate of movement or feed to a milling cutter rotatingat a comparatively high speed. The characteristic feature of the milling process isthat each milling cutter tooth removes its share of the stock in the form of smallindividual chips. A typical face milling operation is shown in Figure 12.1.

12.2 Types of Milling CuttersThe variety of milling cutters available for all types of milling machines helpsmake milling a very versatile machining process. Cutters are made in a largerange of sizes and of several different cutting tool materials. Milling cutters aremade from High Speed Steel (HSS), others are carbide tipped and many arereplaceable or indexable inserts. The three basic milling operations are shown inFigure 12.2. Peripheral and end milling cutters will be discussed below. Face

FIGURE 12.1: A typical milling operation; the on-edge insert design is being used.(Courtesy Ingersoll Cutting Tool Co.)

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Chap. 12: Milling Cutters & Operations

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milling cutters are usually indexableand will be discussed later in thischapter.

A high speed steel (HSS) shellend milling cutter is shown in Fig-ure 12.3 and other common HSScutters are shown in Figure 12.4and briefly described below:

12.2.1 Periphery Milling CuttersPeriphery milling cutters are usu-ally arbor mounted to performvarious operations.

Light Duty Plain Mill: Thiscutter is a general purpose cutterfor peripheral milling operations.Narrow cutters have straight teeth,while wide ones have helical teeth(Fig. 12.4c).

Heavy Duty Plain Mill: Aheavy duty plain mill is similar tothe light duty mill except that it is

used for higher rates of metal removal.To aid it in this function, the teeth aremore widely spaced and the helix angle

is increased to about 45degrees.

Side Milling Cutter:The side milling cutterhas a cutting edge on thesides as well as on theperiphery. This allows thecutter to mill slots (Fig.12.4b).

Half-Side Milling Cut-ter: This tool is the sameas the one previously de-scribed except that cuttingedges are provided on asingle side. It is used for

milling shoulders. Two cutters of thistype are often mounted on a singlearbor for straddle milling.

Stagger-tooth Side Mill: This cut-ter is the same as the side millingcutter except that the teeth are stag-gered so that every other tooth cuts ona given side of the slot. This allowsdeep, heavy-duty cuts to be taken(12.4a).

Angle Cutters: On angle cutters,the peripheral cutting edges lie on acone rather than on a cylinder. Asingle or double angle may be provided(Fig. 12.4d and Fig. 12.4e).

Shell End Mill: The shell end millhas peripheral cutting edges plus facecutting edges on one end. It has a holethrough it for a bolt to secure it to thespindle (Fig. 12.3).

Form Mill: A form mill is a periph-eral cutter whose edge is shaped toproduce a special configuration on thesurface. One example of his class oftool is the gear tooth cutter. The exactcontour of the cutting edge of a formmill is reproduced on the surface of theworkpiece (Fig.12.4f, Fig.12.4g, andFig.12.4h).

12.2.2 End Milling CuttersEnd mills can be used on vertical andhorizontal milling machines for a vari-ety of facing, slotting, and profilingoperations. Solid end mills are madefrom high speed steel or sintered car-bide. Other types, such as shell endmills and fly cutters, consist of cuttingtools that are bolted or otherwise fas-tened to adapters.

Solid End Mills: Solid end millshave two, three, four, or more flutesand cutting edges on the end and theperiphery. Two flute end mills can befed directly along their longitudinalaxis into solid material because thecutting faces on the end meet. Three

Arbor

End mill

Spindle

ShankSpindle

Millingcutter

(a) (b) (c)

FIGURE 12.2: The three basic milling operations: (a) milling, (b) face milling, (c) end milling

FIGURE 12.3: High-speed steel (HSS) shellend milling cutter. (Courtesy Morse CuttingTools)

(a) (b) (c) (d)

(e) (f) (g) (h)

FIGURE 12.4: Common HSS milling cutters: (a) staggered-tooth cutter, (b) sidemilling cutter, (c) plain milling cutter, (d) single-angle milling cutter, (e) double-angle milling cutter, (f) convex milling cutter, (g) concave milling cutter, (h) cornerrounded milling cutter.





and four fluted cutters with oneend cutting edge that extends pastthe center of the cutter can also befed directly into solid material.

Solid end mills are double orsingle ended, with straight or ta-pered shanks. The end mill can be

of the stub type, with short cut-ting flutes, or of the extra longtype for reaching into deep cavi-ties. On end mills designed foreffective cutting of aluminum,the helix angle is increased forimproved shearing action andchip removal, and the flutes maybe polished. Various single anddouble-ended end mills areshown in Figure 12.5a. Varioustapered end mills are shown inFigure 12.5b.

Special End Mills: Ball endmills (Fig. 12.6a) are availablein diameters ranging from 1/32to 2 1/2 inches in single anddouble ended types. Single pur-pose end mills such as Woodruffkey-seat cutters, corner roundingcutters, and dovetail cutters

(Fig.12.6b) are usedon both vertical andhorizontal millingmachines. They areusually made of highspeed steel and mayhave straight or ta-pered shanks.

12.3 MillingCutter NomenclatureAs far as metalcutting action isconcerned, the per-tinent angles onthe tooth are thosethat define the con-figuration of thecutting edge, the

orientation of the tooth face, and therelief to prevent rubbing on the land.

The terms defined below and illus-trated in Figures 12.7a and 12.7b areimportant and fundamental to millingcutter configuration.

Outside Diameter: The outside di-ameter of a milling cutter is the diam-eter of a circle passing through theperipheral cutting edges. It is thedimension used in conjunction with thespindle speed to find the cutting speed(SFPM).

Root Diameter: This diameter ismeasured on a circle passing throughthe bottom of the fillets of the teeth.

Tooth: The tooth is the part of thecutter starting at the body and endingwith the peripheral cutting edge. Re-placeable teeth are also called inserts.

Tooth Face: The tooth face is thesurface of the tooth between the filletand the cutting edge, where the chipslides during its formation.

Land: The area behind the cuttingedge on the tooth that is relieved toavoid interference is called the land.

Flute: The flute is the space pro-vided for chip flow between the teeth.

Gash Angle: The gash angle ismeasured between the tooth face andthe back of the tooth immediatelyahead.

Fillet: The fillet is the radius at thebottom of the flute, provided to allowchip flow and chip curling.

The terms defined above apply pri-marily to milling cutters, particularlyto plain milling cutters. In definingthe configuration of the teeth on thecutter, the following terms are impor-tant.

Peripheral Cutting Edge: The cut-ting edge aligned principally in thedirection of the cutter axis is called theperipheral cutting edge. In peripheralmilling, it is this edge that removes themetal.

FIGURE 12.5a: Various single- and double-ended HSS end mills. (Courtesy The WeldonTool Co.)

FIGURE 12.5b: Various tapered HSS end mills.(Courtesy The Weldon Tool Co.)

(a) (b)

FIGURE 12.6: (a) Ball-nose end-milling cutters areavailable in diameter ranging from 1/32 to 2 ½inches. (Courtesy The Weldon Tool Co.) (b) HSSdovetail cutters can be used on both vertical andhorizontal milling machines. (Courtesy MorseCutting Tools)

ToothTooth face

Gash angle

LandFlute

Fillet

Outside d

iamete

r

Root diameterRadial

rake angle

Peripheralcutting edge

Secondary clearance

Primary clearance

(a)(b)

Relief

FIGURE 12.7: Milling cutter configuration: (a) plain milling cutternomenclature; (b) plain milling cutter tooth geometry.





Face Cutting Edge: The face cut-ting edge is the metal removing edgealigned primarily in a radial direction.In side milling and face milling, thisedge actually forms the new surface,although the peripheral cutting edgemay still be removing most of themetal. It corresponds to the end cut-ting edge on single point tools.

Relief Angle: This angle is mea-sured between the land and a tangentto the cutting edge at the periphery.

Clearance Angle: The clearanceangle is provided to make room forchips, thus forming the flute. Nor-mally two clearance angles are pro-vided to maintain the strength of thetooth and still provide sufficient chipspace.

Radial Rake Angle: The radialrake angle is the angle between thetooth face and a cutter radius, mea-sured in a plane normal to the cutter

axis.Axial Rake Angle: The axial rake

angle is measured between the periph-eral cutting edge and the axis of thecutter, when looking radially at thepoint of intersection.

Blade Setting Angle: When a slotis provided in the cutter body for ablade, the angle between the base of theslot and the cutter axis is called theblade setting angle.

12.4 Indexable Milling CuttersThe three basic types of milling opera-tions were introduced earlier. Figure12.8 shows a variety of indexable mill-ing cutters used in all three of the basictypes of milling operations (Fig. 12.2).

There are a variety of clamping sys-tems for indexable inserts in millingcutter bodies. The examples showncover the most popular methods now inuse:

12.4.1 Wedge ClampingMilling inserts havebeen clamped usingwedges for many yearsin the cutting tool in-dustry. This principleis generally applied inone of the followingways: either the wedgeis designed and ori-ented to support the in-sert as it is clamped, orthe wedge clamps onthe cutting face of theinsert, forcing the insertagainst the millingbody. When the wedge

is used to support the insert, the wedgemust absorb all of the force generatedduring the cut. This is why wedgeclamping on the cutting face of theinsert is preferred, since this methodtransfers the loads generated by the cutthrough the insert and into the cutterbody. Both of the wedges clampingmethods are shown in Figure 12.9.

The wedge clamp system however,has two distinct disadvantages. First,the wedge covers almost half of theinsert cutting face, thus obstructingnormal chip flow while producing pre-mature cutter body wear, and secondly,high clamping forces causing clamp-ing element and cutter body deforma-tion can and often will result. Theexcessive clamping forces can causeenough cutter body distortion that insome cases when loading inserts into amilling body, the last insert slot willhave narrowed to a point where the lastinsert will not fit into the body. Whenthis occurs, several of the other insertsalready loaded in the milling cutter areremoved an reset. Wedge clampingcan be used to clamp individual inserts(Fig. 12.10a) or indexable and replace-able milling cutter cartridges as shownin Figure 25.10b.

12.4.2 Screw ClampingThis method of clamping is used inconjunction with an insert that has apressed countersink or counterbore. Atorque screw is often used to eccentri-cally mount and force the insertagainst the insert pocket walls. Thisclamping action is a result of eitheroffsetting the centerline of the screwtoward the back walls of the insert

FIGURE 12.8: A variety of indexablemilling cutters. (Courtesy IngersollCutting Tool Co.)

Insert

Support andclamping

wedge

Clampingwedge

FIGURE 12.9: Two methods of wedge clamping indexablemilling cutter inserts.

(a) (b)

FIGURE 12.10: (a) Face milling cutter with wedge clamped indexable inserts.(Courtesy Iscar Metals, Inc.) (b) Face milling cutters with indexable inserts andwedge clamped milling cartridges. (Courtesy Greenleaf Corp.)





pocket, or by drilling and tapping themounting hole at a slight angle,thereby bending the screw to attain thesame type of clamping action.

The Screw clamping method forindexable inserts is shown in Figure12.11.

Screw clamping is excellent forsmall diameter end mills where spaceis at a premium. It also provides anopen unhampered path for chips toflow free of wedges or any other ob-structive hardware. Screw clampingproduces lower clamping forces thanthose attained with the wedge clamp-ing system. However, when the cuttingedge temperature rises significantly,the insert frequently expands and

c a u s e san unde-s i r a b l er e t i gh t -ening ef-fect, in-creasingthe torque required to unlock the insertscrew. The screw clamping method canbe used on indexable ball milling cut-ters (Fig. 12.12a) or on indexable in-sert slotting and face milling cutters asshown in Figure 12.12b.

12.5 Milling Cutter GeometryThere are three industry standard mill-ing cutter geometries: double negative,double positive, and positive/negative.Each cutter geometry type has certainadvantages and disadvantages thatmust be considered when selecting theright milling cutter for the job. Posi-tive rake and negative rake milling

cutter geometries are shown in Figure12.13.

Double Negative Geometry: Adouble negative milling cutter usesonly negative inserts held in a negativepocket. This provides cutting edgestrength for roughing and severe inter-rupted cuts. When choosing a cuttergeometry it is important to rememberthat a negative insert tends to push thecutter away, exerting considerableforce against the workpiece. Thiscould be a problem when machiningflimsy or lightly held workpieces, orwhen using light machines. However,this tendency to push the work down,or push the cutter away from the

workpiece may be benefi-cial in some cases becausethe force tends to ‘load’the system, which often re-duces chatter.

Double Positive Geom-etry: Double positive cut-ters use positive insertsheld in positive pockets.This is to provide theproper clearance for cut-ting. Double positive cut-ter geometry provides forlow force cutting, but theinserts contact theworkpiece at their weakestpoint, the cutting edge. Inpositive rake milling, the

cutting forces tend to lift theworkpiece or pull the cutterinto the work. The greatestadvantage of double posi-

Insert

Insertscrew

FIGURE 12.11: Screw clamping method forindexable inserts.

FIGURE 12.12: (a) Indexable-insert ball-nosed milling cutters using the screw clamping method.(Courtesy Ingersoll Cutting Tool Co.) (b) Slotting cutters and face milling with screw-on-typeindexable inserts. (Courtesy Duramet Corp.)

(b)(a)

Lead angle or corner angle orperipheral cutting edge angle

Face or endcutting edgeangle

Effective diameter

Side View

2-45°

Axial relief angle

Chamferor radius

Axial rakeangle (positive)

FCEA2-4° Effective diameter

Side View

Lead angle2-4°

Axial relief angle

Chamfer 45°

Axial rakeangle (negative)

Wedge lock

Radial rakeangle (positive)

Bottom View

Peripheral orradial relief angle Wedge lock

Radial rakeangle (positive)

Bottom View

Peripheralrelief angle

FIGURE 12.13: Positive-rake and negative-rake face milling cutternomenclature.





tive milling is free cutting. Less forceis exerted against the workpiece, soless power is required. This can beespecially helpful with machining ma-terials that tend to work harden.

Positive / Negative Geometry:Positive/negative cutter geometry com-bines positive inserts held in negativepockets. This provides a positive axialrake and a negative radial rake and aswith double positive inserts, this pro-vides the proper clearance for cutting.In the case of positive/negative cutters,the workpiece is contacted away fromthe cutting edge in the radial directionand on the cutting edge in the axialdirection. The positive/negative cuttercan be considered a low force cutterbecause it uses a free cutting positiveinsert. On the other hand, the positive/negative cutter provides contact awayfrom the cutting edge in the radialdirection, the feed direction of a facemill.

In positive/negative milling, some ofthe advantages of both positive andnegative milling are available. Posi-tive/negative milling combines the freecutting or shearing away of the chip ofa positive cutter with some of the edgestrength of a negative cutter.

Lead Angle: The lead angle (Fig.12.14) is the angle between the insertand the axis of the cutter. Severalfactors must be considered to deter-mine which lead angle is best for aspecific operation. First, the lead anglemust be small enough to cover thedepth of cut. The greater the leadangle, the less the depth of cut that canbe taken for a given size insert. Inaddition, the part being machined mayrequire a small lead angle in order toclear a portion or form a certain shapeon the part. As the lead angle in-creases, the forces change toward thedirection of the workpiece. This couldcause deflections when machining thin

sections of thepart.

The lead anglealso determinesthe thickness ofthe chip. Thegreater the leadangle for the samefeed rate or chipload per tooth, thethinner the chipbecomes. As insingle point tool-ing, the depth of cut is distributed overa longer surface of contact. Therefore,lead angle cutters are recommendedwhen maximum material removal isthe objective. Thinning the chip al-lows the feed rate to be increased ormaximized.

Lead angles can range from zero to85 degrees. The most common leadangles available on standard cutters are0, 15, 30 and 45 degrees. Lead angleslarger than 45 degrees are usually con-sidered special, and are used for veryshallow cuts for fine finishing, or forcutting very hard work materials.

Milling cutters with large leadangles also have greater heat dissipat-ing capacity. Extremely high tempera-tures are generated at the insert cuttingedge while the insert is in the cut.Carbide, as well as other tool materi-als, often softens when heated, andwhen a cutting edge is softened it willwear away more easily. However, ifmore of the tool can be employed in thecut, as in the case of larger lead angles,the tool’s heat dissipating capacity willbe improved which, in turn, improvestool life. In addition, as lead angle isincreased, axial force is increased andradial force is reduced, an importantfactor in controlling chatter.

T h euse ofl a r g e

lead angle cutters is especially benefi-cial when machining materials withscaly or work hardened surfaces. Witha large lead angle, the surface is spreadover a larger area of the cutting edge.This reduces the detrimental effect onthe inserts, extending tool life. Largelead angles will also reduce burringand breakout at the workpiece edge.

The most obvious limitation on leadangle cutters is part configuration. If asquare shoulder must be machined on apart, a zero degree lead angle is re-quired. It is impossible to produce azero degree lead angle milling cutterwith square inserts because of the needto provide face clearance. Often a nearsquare shoulder is permissible. In thiscase a three degree lead angle cuttermay be used.

12.5.1 Milling Insert CornerGeometryIndexable insert shape and size werediscussed in Chapter 2. Selecting theproper corner geometry is probably themost complex element of insert selec-tion. A wide variety of corner stylesare available. The corner style chosenwill have a major effect on surfacefinish and insert cost. Figure 12.15ashows various sizes and shapes of

indexable millingcutter inserts.

Nose Radius: Aninsert with a nose ra-dius is generally lessexpensive than asimilar insert withany other corner ge-ometry. A nose ra-dius is also the stron-gest possible cornergeometry because ithas no sharp cornerswhere two flats cometogether, as in thecase of a chamferedcorner. For these two

Lead angle

FIGURE 12.14: Drawing of a positive lead angle on anindexable-insert face milling cutter.

Cutter Cutter

Chipflowdirection

A

Chipflowdirection

(b)

Workpiece Workpiece

FIGURE 12.15: (a) Various sizes and shapes of indexable milling cutter inserts. (Courtesy AmericanNational Carbide Co.) (b) indexable milling cutter insert chip flow directions are shown.

(a) (b)





reasons alone, a nose radius insertshould be the first choice for any appli-cation where it can be used.

Inserts with nose radii can offer toollife improvement when they are usedin 0 to 15 degree lead angle cutters, asshown in Figure 12.15b. When achamfer is used, as in the left drawing,the section of the chip formed aboveand below point A, will converge atpoint A, generating a large amount ofheat at that point, which will promotefaster than normal tool wear. When aradius insert is used, as shown in theright drawing, the chip is still com-pressed, but the heat is spread moreevenly along the cutting edge, result-ing in longer tool life.

The major disadvantage of an insertwith a nose radius is that the surfacefinish it produces is generally not asgood as other common corner geom-etries. For this reason, inserts withnose radii are generally limited toroughing applications and applicationswhere a sweep wiper insert is used forthe surface. A sweep wiper is an insertwith a very wide flat edge or a verylarge radiused edge that appears to beflat. There is usually only one wiperblade used in a cutter and this bladegets its name from its sweeping actionthat blends the workpiece surface to avery smooth finish.

Inserts with nose radii are not avail-able on many double positive and posi-tive/negative cutters because the clear-ance required under the nose radius isdifferent from that needed under theedge. This clearance difference wouldrequire expensive grinding proceduresthat would more than offset the otheradvantages of nose radius inserts.

Chamfer: There are two basic waysin which inserts with a corner chamfer

can be applied. Depending both on thechamfer angle and the lead angle of thecutter body in which the insert is used,the land of the chamfer will be eitherparallel or angular (tilted) to the direc-tion of feed, as shown in Figure12.16a.

Inserts that are applied with thechamfer angular to the direction offeed normally have only a single cham-fer. These inserts are generally not asstrong and the cost is usually higherthan inserts that have a large noseradius. Angular-land chamfer insertsare frequently used for general purposemachining with double negative cut-ters.

Inserts designed to be used with thechamfer parallel to the direction offeed may have a single chamfer, asingle chamfer and corner break, adouble chamfer, or a double chamferand corner break. The larger lands arereferred to as primary facets and thesmaller lands as secondary facets. Thecost of chamfers, in relation to othertypes of corner geometries, dependsupon the number of facets. A singlefacet insert is the least expensive,while multiple facet inserts cost morebecause of the additional grinding ex-pense. Figure 12.16b shows two preci-sion ground indexable milling cutterinserts. A face milling cutter with sixsquare precision ground indexablemilling cutter inserts was shown inFigure 12.10a.

The greatest advantage of using in-serts with the land parallel to the direc-tion of feed is that, when used cor-rectly, they generate an excellent sur-face finish. When the land width isgreater than the advance per revolu-tion, one insert forms the surface. Thismeans that an excellent surface finish

normally will be produced regardlessof the insert face runout. Parallel-landinserts also make excellent roughingand general purpose inserts for posi-tive/negative and double positive cut-ters. When a parallel land chamferinsert is used for roughing, the landwidth should be as small as possible toreduce friction.

Sweep Wipers: Sweep wipers areunique in both appearance and applica-tion. These inserts have only one ortwo very long wiping lands. A singlesweep wiper is used in a cutter bodyfilled with other inserts (usually rough-ing inserts) and is set approximately0.003 to 0.005 inches higher than theother inserts, so that the sweep wiperalone forms the finished surface.

The finish obtained with a sweepwiper is even better than the excellentfinish attained with a parallel landchamfer insert. In addition, since theedge of the sweep wiper insert is excep-tionally long, a greater advance perrevolution may be used. The sweepwiper also offers the same easy set-upas the parallel-land insert.

Sweep wiper inserts are availablewith both flat and crowned wipingsurfaces. The crowned cutting edge isground to a very large radius, usuallyfrom three to ten inches. The crownedcutting edges eliminate the possibilityof saw-tooth profiles being producedon the machined surface because theland is not exactly parallel to the direc-tion of feed, a condition normallycaused by spindle tilt. On the otherhand, sweep wipers with flat cuttingedges produce a somewhat better finishif the land is perfectly aligned with thedirection of feed.

Cutter Cutter

(a)

Workpiece Workpiece

Parallel-landchamber

Angular-landchamber

(b) (c)FIGURE 12.16: (a) indexable milling cutter inserts with angular-land chamfer and parallel-land chamfer. (b and c) Twoprecision ground indexable milling cutter inserts. (Courtesy Iscar Metals, Inc.)





12.6 Basic Milling OperationsBefore any milling job is attempted,several decisions must be made. Inaddition to selecting the best means ofholding the work and the most appro-priate cutters to be used, the cuttingspeed and feed rate must be establishedto provide good balance between rapidmetal removal and long tool life.

Proper determination of a cuttingspeed and feed rate can be made onlywhen the following six factors areknown:• Type of material to be machined• Rigidity of the set-up• Physical strength of the cutter• Cutting tool material• Power available at the spindle• Type of finish desired

Several of these factors affect cuttingspeed only, and some affect both cut-ting speed and the feed rate. The tablesin reference handbooks provide ap-proximate figures that can be used asstarting points. After the cutting speedis chosen, the spindle speed must becomputed and the machine adjusted.

Cutting Speed: Cutting speed isdefined as the distance in feet that istraveled by a point on the cutter pe-riphery in one minute. Since a cutter’speriphery is its circumference:

Circumference = Pi × d

in case of a cutter, thecircumference is:

Cutter circumference = Pi/12 × d = .262 × d

Since cutting speed is expressed insurface feet per minute (SFPM)

SFPM = Cutter circumference × RPM

by substituting for the cutter circum-ference, the cutting speed can be ex-pressed as:

SFPM = .262 × d × RPM

The concept of cutting speed(SFPM) was introduced in Chapter 4(Turning Tools and Operations) andexplained again in Chapter 8 (Drillsand Drilling Operations). It has againbeen reviewed here without giving ad-ditional examples. However, sincemilling is a multi-point operation, feedneeds to be explained in more detailthan in previous chapters.

Feed Rate: Once the cutting speedis established for a particularworkpiece material, the appropriatefeed rate must be selected. Feed rate isdefined in metal cutting as the lineardistance the tool moves at a constantrate relative to the workpiece in aspecified amount of time. Feed rate isnormally measured in units of inchesper minute or IPM. In turning anddrilling operations the feed rate is ex-pressed in IPR or inches per revolu-tion.

When establishing the feed rates formilling cutters, the goal is to attain thefastest feed per insert possible, toachieve an optimum level of productiv-ity and tool life, consistent with effi-cient manufacturing practices. Theultimate feed rate is a function of thecutting edge strength and the rigidityof the workpiece, machine andfixturing. To calculate the appropriatefeed rate for a specific milling applica-tion, the RPM, number of effectiveinserts (N) and feed per insert in inches(IPT or apt) should be supplied.

The milling cutter shown in Figure12.17 on the left (one insert cutter) willadvance .006 inches at the cuttercenterline every time it rotates one fullrevolution. In this case, the cutter issaid to have a feed per insert or an IPT(inches per tooth), apt (advance pertooth) and an apr (advance per revolu-tion) of .006 inches. The same style ofcutter with 4 inserts is shown in theright hand drawing. However, tomaintain an equal load on each insert,the milling cutter will now advance.024 inches at he centerline every timeit rotates one full revolution. Themilling cutter on the right is said tohave and IPT and apt of .006 inches,but and apr (advance per revolution) of.024 inches (.006 inch for each insert).

These concepts are used to deter-

mine the actual feed rate of a millingcutter in IPM (inches per minute) us-ing one of the following formulas:

IPM = (IPT) × (N) × (RPM)or

IPM = (apt) × (N) × (RPM)

where:IPM = inches per minuteN = number of effective insertsIPT = inches per toothapt = advance per toothRPM = revolutions per minute

For Example: When milling automo-tive gray cast iron using a 4 inchdiameter face mill with 8 inserts at 400SFPM and 30.5 IPM, what apr and aptwould this be?

Answer: = .080 in. apr= .010 in. apt

When milling a 300M steel landinggear with a 6 inch diameter 45 degreelead face mill (containing 10 inserts)at 380 SFPM and a .006 inch advanceper tooth, what feed rate should be runin IPM?

Answer: = 14.5 IPMThe following basic list of formulas

can be used to determine IPM, RPM,apt, apr, or N depending on what

++

FeedFeed

apr = .024πapr = apt = .006π apt = .006π

FIGURE 12.17: Drawing of a milling cutter showing the difference between advanceper revolution (apr) and advance per tooth (apt).

SFPM 400.262 × d .262 × 4RPM = = = 382

IPM 30.5 RPM 382apr = = = .080 in.

apr .080 N 8apt = = = .010 in.

SFPM 380.262 × d .262 × 6RPM = = = 242

IPM=apt×N×RPM = .006×10×242=14.5





information is supplied for a specificmilling application:

IPM = inches per minuteN = number of effective insertsapr = inches of cutter advance every revolutionapt = inches of cutter advance

for each effective insert every revolution

RPM = revolutions per minute

Find Given Using

IPM apr, RPM IPM = apr × RPM

IPM RPM, N, apt IPM =apt × N × RPM

apr IPM, RPM apr = IPM/RPM

RPM IPM, apr RPM = IPM/apr

RPM IPM, N, apt RPM = IPM N × apt

N IPM, RPM, apt N = IPM RPM × apt

apt IPM, N, RPM apt = IPM RPM × N

Note: In the formulas shown above IPTcan be substituted for apt and IPR canbe substituted for apr.

Horsepower Requirements: Inmetal cutting, the horsepower con-sumed is directly proportional to thevolume (Q) of material machined perunit of time (cubic inches / minute).Metals have distinct unit power factorsthat indicate the average amount ofhorsepower required to remove onecubic inch of material in a minute.The power factor (k*) can be usedeither to determine the machine size interms of horsepower required to makea specific machining pass or the feedrate that can be attained once a depthand width of cut are established on aparticular part feature. To determinethe metal removal rate (Q) use thefollowing:

Q = D.O.C. × W.O.C × IPMwhere:

D.O.C. = depth of cut in inchesW.O.C. = width of cut in inchesIPM = feed rate, in inches/minute

The average spindle horsepower re-quired for machining metal workpiecesis as follows:

HP = Q × k*where:

HP = horsepower required at themachine spindle

Q = the metal removal rate incubic inches/minute

k* = the unit power factor inHP/cubic inch/minute

*k factors are available from refer-ence books

For example: What feed should beselected to mill a 2 inch wide by .25inch depth of cut on aircraft aluminum,utilizing all the available horsepoweron a 20 HP machine using a 3 inchdiameter face mill?

HP = Q × k*k* = .25 H.P./in.3/min. for aluminum

The maximum possible metal re-moval rate (Q), for a 20 H.P. machinerunning an aluminum part is:

Answer: Q = 80 in.3/min.

To remove 80 in3/min., what feed ratewill be needed?

Answer:= 160 IPM

12.6.1 Direction of Milling FeedThe application of the milling tool interms of its machining direction iscritical to the performance and tool life

of the entire operation. The two op-tions in milling direction are describedas either conventional or climb mill-ing. Conventional and climb millingalso affects chip formation and tool lifeas explained below. Figure 12.18shows drawings of both conventionaland climb milling.

Conventional Milling: The termoften associated with this milling tech-nique is ‘up-cut’ milling. The cutterrotates against the direction of feed asthe workpiece advances toward it fromthe side where the teeth are movingupward. The separating forces pro-duced between cutter and workpieceoppose the motion of the work. Thethickness of the chip at the beginningof the cut is at a minimum, graduallyincreasing in thickness to a maximumat the end of the cut.

Climb Milling: The term oftenassociated with this milling techniqueis ‘down-cut’ milling. The cutter ro-tates in the direction of the feed and theworkpiece, therefore advances towardsthe cutter from the side where the teethare moving downward. As the cutterteeth begin to cut, forces of consider-able intensity are produced which favorthe motion of the workpiece and tendto pull the work under the cutter. Thechip is at a maximum thickness at thebeginning of the cut, reducing to aminimum at the exit. Generally climbmilling is recommended wherever pos-sible. With climb milling a betterfinish is produced and longer cutter lifeis obtained. As each tooth enters thework, it immediately takes a cut and isnot dulled while building up pressureto dig into the work.

Advantages and Disadvantages: Ifthe workpiece has a highly abrasivesurface, conventional milling will usu-

noitatoR

Workpiece

Cutter

Feedup-cut milling

noitatoR

Workpiece

Cutter

Feeddown-cut milling

FIGURE 12.18: Conventional or up-milling as compared to climb or down-milling.

Q = (D.O.C.) × (W.O.C.) × IPMQ 80

(D.O.C.)×(W.O.C.) .25×2IPM= = =160

Q = = = 80 in3/min.HP 20k .25





ally produce better cutter life since thecutting edge engages the work belowthe abrasive surface. Conventionalmilling also protects the edge by chip-ping off the surface ahead of the cut-ting edge.

Limitations on the use of climb mill-ing are mainly affected by the condi-tion of the machine and the rigiditywith which the work is clamped andsupported. Since there is a tendency

for the cutter to climb up on the work,the milling machine arbor and arborsupport must be rigid enough to over-come this tendency. The feed must beuniform and if the machine does nothave a backlash eliminator drive, thetable gibs should be tightened to pre-vent the workpiece from being pulledinto the cutter. Most present-day ma-chines are built rigidly enough. Oldermachines can usually be tightened to

permit use of climb milling.The downward pressure caused by

climb milling has an inherent advan-tage in that it tends to hold the workand fixture against the table, and thetable against the ways. In conventionalmilling, the reverse is true and theworkpiece tends to be lifted from thetable.



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