Broach Materials
Almost all broaches are made of high-speed
tool steels in monolithic construction.
Brazed carbide or disposable inserts are sometimes
used for cutting edges, most often on tools used
for broaching cast irons.
Here is a list of tool steels and the materials
that are commonly broached with these steels. (The
list is only a sampling.)
M-2 steel:
Part hardness should be held under Rc 28. General
use, including brass, aluminum, magnesium, and the
following steels: 1018, 1020, 1063, 1112, 1340,
1345, B-1113, 4140, 4340, 5140, 8620, (RC26), 347
stainless steel (annealed)
M-3
Part hardness should be held under Rc 28. Aluminum
castings, cast irons, A-286 and the following steels:
4140, 4337, 8617, 8620, 9840, 403 stainless, Greek
Ascoloy, M-252, D-279, 4140, 4337, 4340, 8617, 8620,
9310, 9840, 403 stainless
PM-4 (Powdered Metal):
Part hardness should be held under Rc 30. An increasingly
popular tool steel used on a wide variety of applications.
Has a very high wear resistance. High Silicon Steels,
Silicon Bronze, Aluminum Die Casting, Armature Grade
Irons, 9250, 9260, All materials listed under M-2
& M-3 above.
T-15 (Powdered Metal):
One of the best and most expensive tool steels.,
Aluminum 2219, A-286 (Rc 32-36), Stellite, 17-22A(S)(Rc
29-34), N-155 (Rc 30-40), WASPOLOY, INCOLOY SOL
(Rc 32-36), 4340 (Rc 30-40), 52100, 931- (Rc 26-30),
17-4 PH stainless steel, 416 stainless steel (Rc
35-40), 403 stainless steel (Rc 37-40), Custom 450,
High Nickel, 4337 (Rc 29-34), 9310 (Rc 36-38), 9840
(Rc 32-36), Greek Ascoloy
Carbides
Most of the carbide cutters used to broach cast
iron are used in flat surface broaching applications,
although contoured cast-iron surfaces have been
broached successfully. Surface broaching of pine
tree slots has been tried with carbides on high-temperature
alloy turbine wheels, but with little success. The
carbide edges tend to chip on the first stroke.
Carbide-Tipped Broaches
Carbide tips are seldom used on conventional steel
parts and forgings. One reason is that good performance
is obtained from high-speed-steel tools; another
is the low cutting speeds of most broaching operations
(from 12 to 30 fpm) do not lend themselves to the
advantages of carbide tooling. The success of carbide
tooling on cast irons is due to carbide's resistance
to abrasion on the tool flank below the cutting
edge.
Another problem with carbide-tipped
tools is that a broaching machine work fixture must
be exceptionally rigid to prevent chipping of the
cutting edge. Experimental work with extra-rigid
tools and workpiece fixtures, however, has shown
that tool life and surface finish can be greatly
improved with carbide tipped tools, even when used
on alloysteel forgings.
Cast high-speed tool steels are almost
never used in broaches. One property of the cast
tool materials that prohibits their use in monolithic
internal pull broaches is low tensile strength.
Most cast alloys that can attain a hardness of Rockwell
C 60 or higher do not have ultimate tensile strengths
much in excess of 85,000 psi.
SURFACE TREATMENT
There are several practical ways of
extending the life of a broach tool. One can be
the use of surface treatment, such as nitriding,
TICN, TIN, oxidation, or hard chrome plating, to
increase the surface hardness and wear resistance
of the broaching tool workpiece. The return on the
investment of coatings must be evaluated on a case
by case basis.
COMMONLY BROACHED MATERIALS
Broaches have been used on almost
every material at one time or another - most of
the known metals and alloys, some plastics, hard
rubber, wood, composites, graphite, and so on. Metals
and alloys are, by far, the most commonly broached
materials. The products made from the other materials
are not usually made to the stringent dimensional
tolerances, or in the quantities, that make broaching
economical.
In general, any material that can be
machined can be broached. And the higher the machinability
of the material, the easier it is to broach. In
steels, machinability correlates closely with hardness.
That is why workpieces with a high surface hardness,
such as produced by previous work-hardening or scale,
require that the first broach tooth cut beneath
the scale or hard surface is possible.
The hardness of the workpiece material
also influences the allowable cut per tooth. On
harder metals, it is customary to take a relatively
fine finishing cut; on softer nonferrous metals,
a fine surface finish can be achieved w3ith a heavier
finishing cut.
Too heavy a cut, however, will tend
to overload the broach tool - no matter what material
is being broached. Too fine a cut, on the other
hand, tends to interfere with free-cutting action
and increases the tendency of the material to glaze,
gall, or tear. Smaller steps can be used for finishing
than for roughing.
Stainless Steels
Stainless steels with hardnesses above Rockwell
C 35 can be broached. Stainless harder than this,
however, tends to dull broach teeth fairly fast,
reducing the number of pieces produced between grinds.
The approximate rise per tooth (round
broaches) runs from 0.001 to 0.005 in. This range
will cover practically all types of stainless steel.
Broaches with hook angles between 12 and 18 usually
give the best results. Backoff should be held to
a minimum; a 2 angle is preferable, but in no case
should it exceed 5. Chipbreakers should be used.
Free-Cutting Steel
Free-cutting steel will allow a greater cut per
tooth, or step, than will a hard or tough steel.
However, a step of 0.0005 in. on a broach diameter
is practical minimum. Hook angles also vary with
the material being cut as was mentioned previously.
They range between 15 and 20 for the soft steels
and between 8 and 12 for the hard steels. Backoff
angles of 2 to 3 on the roughing teeth, 1 on the
semi-finishing teeth, and 0.5 on the finishing teeth
give good results when broaching steel. Chipbreakers
should be used.
Cast and Malleable Irons
Cast and malleable irons permit a greater rise per
tooth than even the free-machining steel. Brittle
materials such as cast iron call for small hook
angles, usually around 6 degrees to 8 degrees. Backoff
angles are the same as for the general run of steels.
Usually, a shorter pitch is permissible in broaching
cast irons than in broaching steels because less
chip room is required for the irons.
Brasses and Bronzes
Brasses and bronzes allow a slightly heavier step,
or rise per tooth, than steel. Too heavy a rise,
however, will tend to overload the broach. Hook
angles usually range from 0 degrees up to 10 degrees
and even higher, increasing with ductility of the
metal being broached. Brittle brasses call for smaller
angles, from +5 degrees to -5 degrees. Backoff angles
are usually 2 degrees on the roughing teeth, 1 degree
on the semi-finishing teeth, and 0.5 degrees on
the finishing teeth. Some form of chipbreaker is
required.
Aluminum and Magnesium
Aluminum and magnesium can be broached with standard
tool design, although special broaches give even
better results. A hook angle of 10 degrees to 15
degrees and a backoff angle between 1 degree and
3 degrees are recommended. Heavier cuts can be taken;
even the finishing teeth can remove as much as 0.002
in. each. If trouble is experienced in maintaining
proper tolerances, the size of the finishing cut
can be increased, rather than decreased, to correct
the situation.
Ductility of a Metal
The ductility of a metal has a considerable influence
on the selection of an optimum hook angle for the
broach teeth. In general, this angles decreases
with decreasing ductility. Brittle materials, therefore,
call for very small hook angles
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