broaching prs[1]
TRANSCRIPT
-
7/28/2019 Broaching Prs[1]
1/41
Broaching
-
7/28/2019 Broaching Prs[1]
2/41
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.
-
7/28/2019 Broaching Prs[1]
3/41
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.
-
7/28/2019 Broaching Prs[1]
4/41
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
-
7/28/2019 Broaching Prs[1]
5/41
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.
-
7/28/2019 Broaching Prs[1]
6/41
-
7/28/2019 Broaching Prs[1]
7/41
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
-
7/28/2019 Broaching Prs[1]
8/41
Internal and external broaching (tool)
-
7/28/2019 Broaching Prs[1]
9/41
External broaching :
-
7/28/2019 Broaching Prs[1]
10/41
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.
-
7/28/2019 Broaching Prs[1]
11/41
.
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
-
7/28/2019 Broaching Prs[1]
12/41
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.
-
7/28/2019 Broaching Prs[1]
13/41
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
-
7/28/2019 Broaching Prs[1]
14/41
Parts of a broaching tool
-
7/28/2019 Broaching Prs[1]
15/41
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.
-
7/28/2019 Broaching Prs[1]
16/41
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.
-
7/28/2019 Broaching Prs[1]
17/41
A typical broach is shown in Figure
along with its nomenclature
-
7/28/2019 Broaching Prs[1]
18/41
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.
-
7/28/2019 Broaching Prs[1]
19/41
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.
-
7/28/2019 Broaching Prs[1]
20/41
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
-
7/28/2019 Broaching Prs[1]
21/41
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.
-
7/28/2019 Broaching Prs[1]
22/41
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.
-
7/28/2019 Broaching Prs[1]
23/41
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
-
7/28/2019 Broaching Prs[1]
24/41
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.
-
7/28/2019 Broaching Prs[1]
25/41
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.
-
7/28/2019 Broaching Prs[1]
26/41
Table 1
-
7/28/2019 Broaching Prs[1]
27/41
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.
-
7/28/2019 Broaching Prs[1]
28/41
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.
-
7/28/2019 Broaching Prs[1]
29/41
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
-
7/28/2019 Broaching Prs[1]
30/41
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
-
7/28/2019 Broaching Prs[1]
31/41
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.
-
7/28/2019 Broaching Prs[1]
32/41
The design and
dimensions of the
pull ends havebeen standardized.
Table 2
-
7/28/2019 Broaching Prs[1]
33/41
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.
-
7/28/2019 Broaching Prs[1]
34/41
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
-
7/28/2019 Broaching Prs[1]
35/41
Table 3
-
7/28/2019 Broaching Prs[1]
36/41
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
-
7/28/2019 Broaching Prs[1]
37/41
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
-
7/28/2019 Broaching Prs[1]
38/41
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)
-
7/28/2019 Broaching Prs[1]
39/41
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.
-
7/28/2019 Broaching Prs[1]
40/41
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.
-
7/28/2019 Broaching Prs[1]
41/41
THANK YOU