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.

    The sample of broaching's surface finish and tolerance capabilities given in Table 1, does not define the limits of broaching technology; it simply shows what can be achieved in common place.

Table 1: Commonly broached materials and typical results ASM No. Metal Heat Treat Hardness (Rc or Rb) Tolerance (in.) Finish (mu-in.) 4132 2618-T61 AL G 70 Rb 0.0023 32 4135H 2014-T6AL G 70 Rb 0.0023 32 4928 Ti-6AI-4V E 36-38 0.00075 24-32 5382B Stellite 31 B 32 0.002 80 5613C SAE 51410 (410SS) H 32-36 0.002 63 5616C Greek Ascoloy I 32-38 0.001 35-42 5665C Inconel A 85 Rb 0.005 80 5668D Inconel X H 29 0.001 32 5727B Timken 16-25-6 F 20-28 0.001 32-63 5735D A-286 Q 28-30 0.0024 32 30-35 0.001 35 32-38 0.0006 32 5765A S-816 G 23-30 0.001 32-40 62500 SAE 3310 E 20 0.010 63 6260E SAE 9310 I 36-38 0.002 63 6302 17-22A(S) H 29-34 0.001 60 6304 17-22A H 35-40 0.003   6342B SAE 9840 I 32-36 0.001 50 6370D SAE 4130 I 32 0.0005 63 6382D SAE 4140 I 25-29 0.002 32-63 6415E SAE 4340 I 38 0.002 45-63   M-2 (tool) A 24-28 0.0008 32   EMS544 - 40-47 0.001 30   Inconel 901 I 32-36 0.0015 63   Rene 41 G 40-42 0.0024 32   WAD7823A   28 0.0003 40-60   D-979 I 38-40 0.0005 60   EMS 73030   32-36 0.0028 63   M-308   36-38 0.0024 32   Chromoloy   31-32 0.004 32   PWA-682 (Ti)   34-36 0.001 32   Lapelloy J 30-37 0.008 32   303 stainless A 85 Rb 0.001 63   304 stainless A 80-85 Rb 0.002 63   403 stainless | 37-40 0.0006 63   SAE 1010 D 60 Rb 0.001 30   SAE 1020 D 3-12 0.002 60-80   SAE 1037 I 15-20 0.0003 30   SAE 1045 I 24-31 0.0005   SAE 1063 E 12-18 0.004 25-60   SAE 1070 E 5-10 0.002 28-60   SAE 1112 87 Rb 0.001 40-45   SAE 1145 C 13-18   50-100   SAE 1340 C 15-20 0.003   SAE 4047 C 8-15 0.002 60-80   SAE 5140 C 8-15 0.002 60-80   SAE 52100 D 25 0.0005 30 Gray cast iron B 90 Rb 0.003 80-100 KP-7 cast Iron B 0.0005 125 Note: Treatment or Condition

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. (See Table 2 below)

Table 2: Typical broach hook and backoff angles Material Hook angle, deg Back  off angle, deg Aluminum 6 to 10   Babbitt 8 to 10   Brass -5 to 5 2 to 3 Bronze 0 1/2 to 2 Cast iron 6to10 2 to 5 Copper 15 2 to 3 Zinc 6   Aluminum     bronze 15 2 to 3 SAE 1037 15 1 to 2         1112 15 2 1/2         B-1113 15 2 to 3         1340 12 1 to 2         4140 8 to 15 1 to 3         4337 8 to 15 1 to 3         5140 15 1 to 2         5140             (type410SS) 18 (roughing) 2   20 (finishing) 2         9310 18 (roughing) 2   20 (finishing) 1 to 2 303 stainless 15 1/2 to 2 304 Stainless 15 1/2 to 2 403 Stainless 15 to 20 (roughing) 3   30 (finishing) 5 431 Stainless up to 28   M-308 15 3 N-155 20 2 Greek Ascoloy 15 2 to 3 Chromalloy 15 2 Lapelloy 12-15 2 A-286 10 to 15 (roughing) 2 to 3   15 to 18 (finishing)   Rene 41 15 3 Incoloy 901 15 (roughing) 3   18 (finishing)   Titanium 140A 5 to 15 2 to 4 Titanium 150A 5 to 9 2 to 5 Titanium PWA A68 12 to 15 (roughing) 3   15 (finishing) 3