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| New Developments in Broaching |
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| High Speed Broaching : |
This is the first outcome of the research, which was carried on different type
of materials with different cutting speeds. The components were broached
beyond the conventional barrier of 6 meters/min. cutting speed. The
experiments were done with different cutting geometries to get maximum
possible output. The variations of the coolants also helped in achieving the
maximum possible production rate at higher cutting speeds. The acceptance
of a higher cutting speed was basically due to the better surface finish
obtained in the components.
It was observed that suitable finish is poorer at lower cutting speeds and
gradually gets improved at higher cutting speeds. This however, remains not
much affected for broaches with lower rise per tooth. It is now quite clear
that higher cutting speeds can be recommended however this requires good
rigidity of the broaching machine and the fixture holding the component
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| Helical Broaching : |
The development of automatic Gearboxes can be regarded as the
development of helical broaches. Gears with helical teeth are still popular
today which can he produced by helical broaches only. By virtue of the
components, helical broaches are divided into two types:
Those which have helical teeth but produces spur type splines in the
Components and
Which have helical teeth and also produces helical splines in the
components.
Helical broaches are most preferred for components having smaller
broaching length and requiring high accuracy in splines. Helical broaches
can be designed with plain. combination and interspaced cutting teeth
similar to broaches with spur teeth.
Helical broaches, which require producing helical splines, can be used only
on special broaching machines having attachments/fixtures to provide
helical movement. Helical broaching is carried out by rotating the
component either by self rotating support or by the helical nut in the
component support which guides the broach through it's lead. The use of
lead bar, which is provided either at bottom side or the topside, can also give
rotary motion directly to the broaching tool. There can be provision of
separate lead bar coupled with the broaching tool with gears at the bottom.
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| Hard Broaching : |
The desire to control the transmission losses in the transmission units and
other units of automotives can be considered the reason for development of
hard broaching. The transmission losses can be controlled by providing the
best attention to the dimensions at gear shaft connections, the better
concentricity between outer and inner profiles of components etc. Earlier,
these were controlled by modifying the dimensions at green stage and more
attention was paid to the method of hardening. This, however. did not
improve the quality required in the components. The concept of hard
broaching at this stage was then later developed
The application of hard broaching is possible in following areas:
Components where bearing area is important such as gears with shafts.
Components, which require automatic joining in assembly lines for transmissions of torque.
Components requiring precision concentricity of internal splines.
Components having very delicate shapes.
Gears with shafts meant for transferring the power usually have press
fitment. It is necessary to have their bearing area in 1OO% contact within
limited dimensions. This requires control over dimensions in a narrow
range, which is very difficult to achieve due to heat treatment deviations. It
is, therefore, Hard Broaching by which the final dimensions in the gears can
be controlled to a very tolerance and also with better quality. In hard
broaching major dia., minor dia., and flank all can be broached to a very
close tolerance.
Components requiring automatic joining require very precise inner profile.
Hard broaching helps in achieving this and therefore can be considered a
process of very high reliability. The biggest advantage of this process is that
it is simple to control it by simpler statistical technique on the broached
splines.
Gears, which are engaged between external and internal teeth, require very
high degree of concentricity of profile with respect to minor/root dia. The
distortions due to hardening cannot permit such close accuracies and
therefore, it is only hard broaching which can broach with accurate profile
and within close tolerance the internal and external splines.
Some components have very delicate and indifferent shape, which also
contribute difference in profile dimensions. Such components when
hardened cause large amount of distortion, poor concentricity and run out.
In such components, by trial and errors the effect of contractions is
minimized by adjusting a portion of error at soft stage or by adjusting the
procedure of hardening. Hard broaching in such components permits only
required portion of deviation in the components as required for easy
assembly
with mating parts.
Hard broaching was uneconomical in the beginning however, today with
improved coating process and the modified coatings, this operation is very
successful and economical too. There are several advantages to the user of
hard broaching, these are as below: |
a) Process Reliability:
By Hard broaching, it is possible to have
consistency in achieving the tightened tolerances and reliable rate of
production with the components.
b) Short processing time:
The processing time is very less (< 15 sacs)
due to very high cutting speeds. It also offers broaching more than one
piece, which can affect cost effectiveness.
c) Better Surface Finish:
Surface finish obtained is far better than the
finish obtained by green stage broaching. The normal surface finish
obtained is Rz <2.5/ Mu and Ra <0.6 Mu.
d) Improved quality:
The Components produced are with very small
portion of distortion than the distortion at green stage. Thus there is
significant Improvement in the quality of component. The rejection rate
is almost zero.
e) Cost effective:
The life of 100,000 components without sharpening the
broach indicates the cost of hard broaching per component. |
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| Full Form Mono Block Broaching : |
In order to reduce the profile errors, which are caused by drifting of the
broach, high precision components such as "Ring Gear" are broached with a
special broaching shell, which is attached to the main broach at the end of
finishing teeth. This shell calibrates the rough profile produced by the
portion before the shell .The procedure for profile cutting is quite complicate.
This shell with special design is known as Full Form Monoblock broach.
It is to be noted that in this type of broaching, each tooth has a flank rise in
Microns in addition to the increase in core dia. which cuts the bore. The
flanks have the original profile and therefore, Full Form relieved. Here, since
roughing and finishing full form cutting teeth are provided, the errors in the
profile are most minimum. Due to this, the component has problem free
entry into mating parts. Further, in this case, since each tooth is cutting on
form, there is no chance of premature wear, which happens due to drifting. |
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