Broaching is a
machining process that uses a toothed tool, called a
broach, to remove material. There are two main types of broaching:
linear and
rotary.
In linear broaching, which is the more common process, the broach is
run linearly against a surface of the workpiece to effect the cut.
Linear broaches are used in a
broaching machine, which is also sometimes shortened to
broach.
In rotary broaching, the broach is rotated and pressed into the
workpiece to cut an axis symmetric shape. A rotary broach is used in a
lathe or
screw machine. In both processes the cut is performed in one pass of the broach, which makes it very efficient.
Broaching is used when precision machining is required, especially
for odd shapes. Commonly machined surfaces include circular and
non-circular holes,
splines,
keyways, and flat surfaces. Typical workpieces include small to medium sized
castings,
forgings, screw machine parts, and
stampings.
Even though broaches can be expensive, broaching is usually favored
over other processes when used for high-quantity production runs.
Broaches are shaped similar to a saw, except the teeth height
increases over the length of the tool. Moreover, the broach contains
three distinct sections: one for roughing, another for semi-finishing,
and the final one for finishing. Broaching is an unusual machining
process because it has the
feed
built into the tool. The profile of the machined surface is always the
inverse of the profile of the broach. The rise per tooth (RPT), also
known as the
step or feed per tooth, determines the amount of
material removed and the size of the chip. The broach can be moved
relative to the workpiece or vice-versa. Because all of the features are
built into the broach no complex motion or skilled labor is required to
use it
. A broach is effectively a collection of
single-point cutting tools arrayed in sequence, cutting one after the other; its cut is analogous to multiple passes of a
shaper.
Process
The process depends on the type of broaching being performed. Surface
broaching is very simple as either the workpiece is moved against a
stationary surface broach, or the workpiece is held stationary while the
broach is moved against it. Internal broaching is more involved. The
process begins by clamping the workpiece into a special holding
fixture, called a
workholder, which mounts in the broaching machine. The broaching machine
elevator,
which is the part of the machine that moves the broach above the
workholder, then lowers the broach through the workpiece. Once through,
the broaching machine's
puller, essentially a hook, grabs the
pilot
of the broach. The elevator then releases the top of the pilot and the
puller pulls the broach through the workpiece completely. The workpiece
is then removed from the machine and the broach is raised back up to
reengage with the elevator.
The broach usually only moves linearly, but sometimes it is also rotated to create a spiral spline or gun-barrel
rifling.
Cutting fluids
are used for three reasons. First, to cool the workpiece and broach.
Second, to lubricate cutting surfaces. Third, to flush the chips from
the teeth. Fortified petroleum cutting fluids are the most common,
however heavy duty water soluble cutting fluids are being used because
of their superior cooling, cleanliness, and non-flammability.
Usage
An example of a broached workpiece. Here the broaching profile is a spline.
Broaching was originally developed for machining internal keyways.
However, it was soon discovered that broaching is very useful for
machining other surfaces and shapes for high volume workpieces. Because
each broach is specialized to cut just one shape either the broach must
be specially designed for the geometry of the workpiece or the workpiece
must be designed around a standard broach geometry. A customized broach
is usually only viable with high volume workpieces, because the broach
can cost $15,000 USD to $30,000 USD to produce
.
Broaching speeds vary from 20 to 120
surface feet per minute
(SFPM). This results in a complete cycle time of 5 to 30 seconds. Most
of the time is consumed by the return stroke, broach handling, and
workpiece loading and unloading.
The only limitations on broaching are that there are no obstructions
over the length of the surface to be machined, the geometry to be cut
does not have curves in multiple planes,
and that the workpiece is strong enough to withstand the forces
involved. Specifically for internal broaching a hole must first exist in
the workpiece so the broach can enter.
Also, there are limits on the size of internal cuts. Common internal
holes can range from 0.125 to 6 in (3.2 to 150 mm) in diameter but it is
possible to achieve a range of 0.05 to 13 in (1.3 to 330 mm). Surface
broaches' range is usually 0.075 to 10 in (1.9 to 250 mm), although the
feasible range is 0.02 to 20 in (0.51 to 510 mm).
Tolerances
are usually ±0.002 in (±0.05 mm), but in precise applications a
tolerance of ±0.0005 in (±0.01 mm) can be held. Surface finishes are
usually between 16 and 63 microinches (μin), but can range from 8 to
125 μin.
There may be minimal burrs on the exit side of the cut.
Broaching works best on softer materials, such as
brass,
bronze,
copper alloys,
aluminium,
graphite, hard
rubbers,
wood,
composites, and
plastic. However, it still has a good
machinability rating on
mild steels and
free machining steels. When broaching, the machinability rating is closely related to the
hardness of the material. For steels the ideal hardness range is between 16 and 24
Rockwell C (HRC); a hardness greater than HRC 35 will dull the broach quickly. Broaching is more difficult on harder materials,
stainless steel and
titanium,
but is still possible.
Types
Broaches can be categorized by many means:
- Use: internal, or surface
- Purpose: single, or combination
- Motion: push, pull, or stationary
- Construction: solid, built-up, hollow or shell
- Function: roughing, sizing, or burnishing
If the broach is large enough the costs can be reduced by using a
built-up or
modular
construction. This involves producing the broach in pieces and
assembling it. If any portion wears out only that section has to be
replaced, instead of the entire broach.
Most broaches are made from
high speed steel (HSS) or an
alloy steel;
TiN coatings are common on HSS to prolong life. Except when broaching
cast iron,
tungsten carbide is rarely used as a tooth material because the cutting edge will crack on the first pass.
Surface broaches
The
slab broach is the simplest surface broach. It is a general purpose tool for cutting flat surfaces.
Slot broaches (G & H) are for cutting slots of various dimensions at high production rates. Slot broaching is much quicker than
milling
when more than one slot needs to be machined, because multiple broaches
can be run through the part at the same time on the same broaching
machine.
Contour broaches are designed to cut concave, convex, cam-, contoured, and irregular shaped surfaces.
Pot broaches are cut the inverse of an internal broach; they
cut the outside diameter of a cylindrical workpiece. They are named
after the pot looking fixture in which the broaches are mounted; the
fixture is often referred to as a "pot". The pot is designed to hold
multiple broaching tools concentrically over its entire length. The
broach is held stationary while the workpiece is pushed or pulled
through it.
This has replaced
hobbing for some
involute gears and cutting external splines and slots.
Straddle broaches use two slab broaches to cut parallel
surfaces on opposite sides of a workpiece in one pass. This type of
broaching holds closer tolerances than if the two cuts were done
independently.
It is named after the fact that the broaches "straddle" the workpiece on multiple sides.
Internal broaches
Solid broaches are the most common type; they are made from one solid piece of material. For broaches that wear out quickly
shell
broaches are used; these broaches are similar to a solid broach, except
there is a hole through the center where it mounts on an
arbor.
Shell broaches cost more initially, but save cost overall if the broach
must be replaced often because the pilots are on the mandrel and do not
have to be reproduced with each replacement.
Modular broaches are commonly used for large internal
broaching applications. They are similar to shell broaches in that they
are a multi-piece construction. This design is used because it is
cheaper to build and resharpen and is more flexible than a solid design.
A common type of internal broach is the
keyway broach (C & D). It uses a special fixture called a
horn to support the broach and properly locate the part with relations to the broach.
A
concentricity broach is a special type of spline cutting broach which cuts both the
minor diameter and the spline form to ensure precise concentricity.
The
cut-and-recut broach is used to cut thin-walled
workpieces. Thin-walled workpieces have a tendency to expand during
cutting and then shrink afterward. This broach overcomes that problem by
first broaching with the standard roughing teeth, followed by a
"breathing" section, which serves as a pilot as the workpiece shrinks.
The teeth after the "breathing" section then include roughing,
semi-finishing, and finishing teeth.
-
An internal broach for cutting splines
-
-
-
-
-
The slot in the tip of the broach where the broaching machine latches on to the broach to pull it through the workpiece
Design
For defining the geometry of a broach an internal type is shown below. Note that the geometry of other broaches are similar.
where:
- P = pitch
- RPT = rise per tooth
- nr = number of roughing teeth
- ns = number of semi-finishing teeth
- nf = number of finishing teeth
- tr = RPT for the roughing teeth
- ts = RPT for the semi-finishing teeth
- tf = RPT for the finishing teeth
- Ls = Shank length
- LRP = Rear pilot length
- D1 = Diameter of the tooth tip
- D2 = Diameter of the tooth root
- D = Depth of a tooth (0.4P)
- L = Land (behind the cutting edge) (0.25P)
- R = Radius of the gullet (0.25P)
- α = Hook angle or rake angle
- γ = Back-off angle or clearance angle
- Lw = Length of the workpiece (not shown)
A progressive surface broach
The most important characteristic of a broach is the rise per tooth
(RPT), which is how much material is removed by each tooth. The RPT
varies for each section of the broach, which are the roughing section (
tr), semi-finishing section (
ts), and finishing section (
tf). The roughing teeth remove most of the material so the number of roughing teeth required dictates how long the broach is.
The semi-finishing teeth provide surface finish and the finishing teeth
provide the final finishing. The finishing section's RPT (t
f) is usually zero so that as the first finishing teeth wear the later ones continue the sizing function. For
free-machining steels
the RPT ranges from 0.006 to 0.001 in (0.15 to 0.025 mm). For surface
broaching the RPT is usually between 0.003 to 0.006 in (0.076 to 0.15
mm) and for diameter broaching is usually between 0.0012 to 0.0025 in
(0.030 to 0.064 mm). The exact value depends on many factors. If the cut
is too big it will impart too much stress into the teeth and the
workpiece; if the cut is too small the teeth rub instead of cutting. One
way to increase the RPT while keeping the stresses down is with
chip breakers.
They are notches in the teeth designed to break the chip and decrease
the overall amount of material being removed by any given tooth (see the
drawing above).
For broaching to be effective, the workpiece should have 0.020 to 0.025
in (0.51 to 0.64 mm) more material than the final dimension of the cut.
The
hook (
α) angle is a parameter of the material being
cut. For steel, it is between 15 and 20° and for cast iron it is
between 6 and 8°. The
back-off (
γ) provides clearance for the teeth so that they don't rub on the workpiece; it is usually between 1 and 3°.
When radially broaching a workpieces that require a deep cut per tooth, such as forgings or
castings, a
rotor-cut or
jump-cut design can be used; these broaches are also known as
free egress or
nibbling broaches.
In this design the RPT is designated to two or three rows of teeth. For
the broach to work the first tooth of that cluster has a wide notch, or
undercut, and then the next tooth has a smaller notch (in a three tooth
design) and the final tooth has no notch. This allows for a deep cut
while keeping stresses, forces, and power requirements low.
There are two different options for achieving the same goal when
broaching a flat surface. The first is similar to the rotor-cut design,
which is known as a
double-cut design. Here four teeth in a row
have the same RPT, but each progressive tooth takes only a portion of
the cut due to notches in the teeth (see the image gallery below). The
other option is known as a
progressive broach, which completely
machines the center of the workpiece and then the rest of the broach
machines outward from there. All of these designs require a broach that
is longer than if a standard design were used.
For some circular broaches,
burnishing teeth are provided
instead of finishing teeth. They are not really teeth as they are just
rounded discs that are 0.001 to 0.003 in (0.025 to 0.076 mm) over-sized.
This results in
burnishing the hole to the proper size. This is primarily used on non-ferrous and cast iron workpieces.
The pitch defines the tooth construction, strength, and number of
teeth in contact with the workpiece. The pitch is usually calculated
from workpiece length, so that the broach can be designed to have at
least two teeth in contact with the workpiece at any time; the pitch
remains constant for all teeth of the broach. One way to calculate the
pitch is:
-
Example of a double-cut surface broach
-
Top view of a double-cut surface broach
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Side view of a double-cut surface broach
Sumber:
http://en.wikipedia.org/wiki/Broaching_%28metalworking%29
