Broaching Prs[1]

7/28/2019 Broaching Prs[1]

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Broaching


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Introduction

Broaching is a machining process in which a cutting

tool, having multiple traverse cutting edges, is pushedor pulled through a hole or a surface to remove metal

by axial force.

Multiple traverse cutting edges of the tool are

arranged in a specific manner. Each cutting edgeoffers successively amount of interference for

satisfactory removal of metal.

Broaching has a wide range of applications and

several advantages over other machininh processes.It is rapid and efficient because roughing and finishing

can be done in a single pass.

Close tolerances, Smooth surface finish, and higher

accuracies are attainable.


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Major limitations : It deviates from the fundamental

characteristics of the process itself. Broaches move

forward in a straight line which emphasized that allelements of the broached surface must be parallel to

the axis of travel.

tapered hole ---

Length of stroke and heavy tonnage required makesthis process unsuitable for heavy stock removal.

Thus the first few sets of teeth remove most of the

material, while the last few provide a finishing cut with

very small amount of material removal. The geometricshape of the last set of teeth is identical to the

required geometry of the designed part.


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Process

A broach is usually a tapered bar on which teeth

(cutting edges) are cut in order to produce desired

contour in a work-piece with a single pass of the tool.

Fig.

Three cutting zones :o Roughing Teeth

o Semi- finishing Teeth

o Finishing Teeth


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A work piece, to be broached should be providedwith a hole through which the broach is pushed orpulled. This hole should be just large enough to

permit the front section (pilot) of the broach toenter freely.

Progressive and gradual machining withadvancement of broach.

Broachability :

Similar to machinabilty - All machinablematerials can be broached.

Suitable range 12 to 22 HRC Below 12 HRCmaterial tend to tear --- poor surface finish.

Increased hardness pose difficulty broach lifepower but better surface finish.


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Types of Broaches

Broaching is widely used machining technique, if feasible,

for high productivity as well as good product quality.

Broaches can be broadly classified in several aspects such

as,

Internal broaching and External broaching

Pull type and Push type

Ordinary cut or Progressive type

Solid, Sectional or Modular type

Profile sharpened or form relieved type


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Internal and external broaching (tool)


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External broaching :


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Pull type and push type broaches

Pull type broaches are generally made as a long single piece

and are more widely used, for internal broaching in particular.

During operation a pull type broach is subjected to tensile

force, which helps in maintaining alignment and prevents

buckling.

Push type broaches are essentially shorter in length (to avoid

buckling) and may be made in segments. Push type broachesare generally used for external broaching, preferably, requiring

light cuts and small depth of material removal.


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.

Ordinary cut and Progressive type broach

Most of the broaches fall under the category of Ordinary cut type where the teeth

increase in height or protrusion gradually from tooth to tooth along the length of thebroach. By such broaches, work material is removed in thin layers over the

complete form. Whereas, Progressive cut type broaches have their teeth

increasing in width instead of height. Fig. shows the working principle and

configuration of such broach


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Solid, Sectional and module type broachesBroaches are mostly made in single pieces specially those used for pull type

internal broaching. But some broaches called sectional broaches, are made by

assembling several sections or cutter-pieces in series for convenience in

manufacturing and resharpening and also for having little flexibility required byproduction in batches having inter batch slight job variation. External broaches are

often made by combining a number of modules or segments for ease of

manufacturing and handling. Fig. typically shows solid, sectional and segmented

(module) type broaches.


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Profile sharpened and form relieved type

broaches

Like milling cutters, broaches can also be classified as

Profile sharpened type broaches;

Such cutters have teeth of simple geometry with same rake and clearance

angles all over the cutting edge. These broaches are generally designed

and used for machining flat surface(s) or circular holes.

Form relieved type broaches

These broaches, being used for non-uniform profiles like gear teeth etc.,

have teeth where the cutting edge geometry is more complex and varies

point to point along the cutting edges. Here the job profile becomes the

replica of the tool form. Such broaches are sharpened and resharpened by


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Parts of a broaching tool


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Pull End

Pull end is made to attach the broach to the

broaching machine through the puller head. Front Pilot

This centers the broach in the hole to be finished just

before start of processing.

Roughing Teeth

These are the cutting edges which remove larger

amount of stocks during cutting. Larger amount

removal generates poor quality of surface finish butmakes the operation faster.


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Finishing Teeth

These are cutting edges removing smaller stocks of

material. These are used for final finishing of thesurfaces and their accurate sizing

Rear Pilot and Follower Rest

This is a supporting device to the broach when it is

likely to complete its operation of broaching.


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A typical broach is shown in Figure

along with its nomenclature


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Geometry of broaching teeth and their cuttingedges

Fig. shows the general configuration of the broaching teeth

and their geometry. The cutting teeth of HSS broaches are

provided with positive radial or orthogonal rake (5o to 15o) and

sufficient primary and secondary clearance angles (2o to 5o

and 5o to 20o respectively) as indicated in fig

Small in-built chip breakers are alternately provided on the

roughing teeth of the broach as can be seen in Fig. to break

up the wide curling chips and thus preventing them from

clogging the chip spaces and increasing forces and tool wear.More ductile materials need wider and frequent angle.


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Land

It is the width of flank face of the broach normally it

is kept slightly inclined to give relief angle to the flankface of broach.

Pitch

It is the distance between two corresponding pointson two successive teeth of a broach. Normally pitchof finishing teeth of a broach is kept comparativelysmaller than the rough cutting teeth.

Height of the Teeth

Height of the roughing and finishing teeth graduallyincreases from the shank to the finishing teeth. Thisincrement is called the cut per tooth, it depends onthe material being machined. Normally the cut pertooth is taken from 0.01 or 0.2 mm for the finishing

teeth and it may go up 0.2 mm for the cutting teeth.


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Pitch:

The pitch of a broach may be defined as the linear

distance between the successive edges and is oneof the important parts to be considered in the design

of the broach.

The pitch decides the length of the broach and the

thickness of the chip the broach has to handle. Italso decides the tooth construction, strength and the

number of teeth in engagement at any instant and

the ability of broach to handle its alignment during

the cutting stroke.The pitch depends on the length of the cut and is

decided by the following formulae:

For internal broaches,

Pitch = (1.25 to 1.5) x length of the cut in mm


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For rotary cut broaches,

Pitch = (1.45 to 2) x (length of the cut in mm)

For surface broaches,Pitch = 3 x (length of cut x rise per tooth x ratio of

gullet area to chip cross section)

Another rule governing the pitch is that at least three

cutting teeth should be in engagement with the workduring the cutting operation so that proper alignment

is achieved.

In general a pitch may be maintained constant

through out the length of the broach, but from afunctional point of view, the roughing teeth may be

provided with a pitch coarser than the semi-finishing

ones as the chip volume removed by the former is

more as compared to the latter.


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Finishing teeth which do not remove any

considerable material are made with a close pitch tokeep the length of the broach short.

Another important thing to be considered is providing

a non uniform pitch. This is due to the fact that a

uniform pitch has disadvantage in that it leads tochatter.


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Chip space

The space between each two teeth, also called the

gullet, plays a critical role in the design of thebroach. With the most internal broaching tools, once

the tooth is engaged, there is no way out for the

chips till the tooth comes out of the part.

The chip carrying capacity should be assessed inthe initial stages of the broach design, which would

help in determining the maximum depth of the cut

per tooth and also the number of teeth needed to

perform the operation. The design of the chip space is regulated by all the

elements of the geometry surrounding it. The radii

(face angle and back of tooth) within the gullet are

designed to reduce friction and the curling of the


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As a general rule, the tooth depth should be between

one-fourth and one-half of the pitch. If the depth is

made larger than this, the tooth form will becomethinner and weaker and number of possible regrinds

will be reduced. In case of smaller diameter

broaches, the core diameter is reduced excessively,

resulting in a broken broach. For this reason, thetooth depth should never exceed the one-sixth of the

broach diameter.


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Face Angle:

The face angle or the hook angle is equivalent

to the rake angle of single point cutting tools and

depends on the material to be cut. The face angle(or rake angle) is selected to suit the machinability of

the work material. Table (1) gives the recommended

values for the face angles for various materials. It is

also an important factor in determining the chipthickness. Generally, the face angle is higher

for ductile materials and is proportional to it.

Soft steel workpieces usually require large face

angles. Hard brittle materials such as cast iron andbrass need smaller face angles. The face angle

should not exceed 20 degrees as this may weaken

the tool.


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Table 1


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Land and Back off angle:

The land width determines the structural

strength of the tooth and also limits the number ofregrinds before the tool becomes undersized. The

lands are backed off to provide a cutting edge as

well as to reduce the friction on the tooth flank.

The back of angles thus formed are defined inthe plane lying in the direction of the cutting motion

and may vary from 1/4 degree to 3 degrees for

internal broaches and up to about 3 1/2 for surface

broaches. A part of the land of finishing teeth isstraight (parallel to the broach axis) whose width

may vary from 0.1 to 0.4 mm.


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Cut per tooth:

Cut per tooth also known as the rise per tooth is the

amount of material removed by each tooth. Thedepth of the cut is based on the amount of total stock

removal, work material used, the pulling force

permissible for the broach cross section and the

power capacity of the machine.The cut per tooth should neither be too big to

overload and result in breakage, nor too small to

cause rubbing rather than cutting. The rubbing of

teeth through the cutting stroke damages the cuttingedges and the work piece will be galled and teared

due to the heat caused by the friction.


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For round hole broaches the chip per tooth is

equivalent to 1/2 of the cut per tooth.

The standard chip per tooth is .00125 inches. The

theoretical maximum chip per tooth is given by Cpt = % of (CA/LC)

Where,

Cpt = Chip per tooth.CA = Circle area.

LC = Length of cut.

The percentage of circle area is dependent on the

workpiece material type and is as follows [1]For round hole internal broaches, 120/0 of the CA

for broaching ductile materials and 100/0 of the CA

for broaching cast iron or bronze type


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For spline-type internal broaches, 25% of the CA for

broaching ductile materials and 200/0 of the CA for

cast iron or bronze. For flat surface broaches making cuts wider than

0.375 inches (9,52 mm), 30 % of the CA for

broaching ductile metals and 20% for cast iron or

bronze. For flat surface broaches making cuts narrower than

0.375 in 35% of CA for broaching ductile metals and

25% of CA for cast iron or bronze.

More percentage of circle area is used for ductilemetals and less for bronze or iron because of their

chip forming nature. Bronze and iron form flakes

rather than continuous chips and hence they need to

have smaller percentage. The theoretical maximum


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Pull end:

The pull end of the broach serves to engage the

broach with the pulling head of the machine.

One of the most common type is the key-type

puller which has a slot that has a corresponding slot

on the puller head and both are locked by dropping a

key through the slot. If the slot for a key weakens the

shank, a pin type pull end can be used, in which the

broach end is engaged with a pin through an off set

hole. Pin type pullers can also be automatic.


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The design and

dimensions of the

pull ends havebeen standardized.

Table 2


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Follower End:

The follower end dimensions depend on the design

of the follower rest with which the machine isequipped.

It also depends on whether it is used to retrieve the

broach at the end of the stroke.

Push broaches do not have follower ends.


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Pilots:

The front pilot aligns the broach and the work, serves toslide the tool through the work. It acts as a gauge tocheck if the starting hole is not undersized. It is designedslightly smaller than the starting hole diameter so that itcan pass through the hole unobstructed. Thus thediameters for the front pilots are determined bysubtracting .003 inch from starting hole diameter and

their length is determined by adding .125 inch to thelength of cut. The length is designed longer than thelength of the cut so that the broach teeth will not beengaged in the work piece prematurely.

The rear pilot, which is at the end of the cut, should be a

sliding fit for the finished hole. It engages with the bushprovided in the fixture and protects the broach fromsagging and also helps in maintaining the alignment forthe return travel. The rear pilot is made slightly smallerthan the finished teeth through which a closer tolerance

can be obtained. The rear pilots length is same as the


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Table 3


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Force Required:

For round broaches, P = KsDZCK

For spline broaches, P = KsSWZCK

For surface broaches, P = KsLZCK

P broaching force, kgf

Z number of teeth cutting at a time

D finish dia of hole, mmC rise/tooth, mm

L width of contact of each tooth, mm

S number of splinesW width of spline,mm

Ks specific cutting force, kgf/mm2

K blunt broach factor (1.25 to 1.4)

St th f B h


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Strength of Broach:

Pull broach:-

P = area of critical section * safe tensile

stress of broach materialPush broach:-

for L/D > 25, load is calculated according to

Eulers formula ( P = (2EI)/FL2 )

for L/D


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Material of broach

Being a cutting tool, broaches are also made of materials

having the usual cutting tool material properties, i.e., high

strength, hardness, toughness and good heat and wear

resistance. For ease of manufacture and resharpening the

complex shape and cutting edges, broaches are mostly

made of HSS (high speed steel). To enhance cutting speed,

productivity and product quality, now-a-days cemented

carbide segments (assembled) or replaceable inserts arealso used specially for stronger and harder work materials

like cast irons and steels. TiN coated carbides provide much

longer tool life in broaching. Since broaching speed (velocity)


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Advantages

Very high production rate (much higher than

milling, planing, boring etc.)

High dimensional and form accuracy and surface

finish of the product.

Roughing and finishing in single stroke of thesame cutter.

Needs only one motion (cutting), so design,

construction, operation and control are simpler.

Extremely suitable and economic for massproduction.


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Limitations Only through holes and surfaces can be

machined. Usable only for light cuts, i.e. low chip load and

unhard materials.

Cutting speed cannot be high.

Defects or damages in the broach (cutting edges)severely affect product quality.

Design, manufacture and restoration of thebroaches are difficult and expensive.

Separate broach has to be procured and usedwhenever size, shape and geometry of the jobchanges.

Economic only when the production volume is

large.


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Documents

Broaching Prs[1]