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36. BROACHED PARTS
36.1. THE PROCESS
Broaching is the cutting of a machinable material by passing a cutter with a
series of progressively stepped teeth over or through it. These teeth travel in
a plane parallel to the surface being cut and, by removing a predetermined
amount of stock, produce precision contours and finishes.
Most often, all the cutting teeth will be contained in one tool, known as a
broach, to rough-out and finish-cut the part completely in a single machine
stroke. (See Fig. 4.9.1.) When excessive stock prevents a one-stroke
application, additional strokes can be employed, utilizing the same tool, if
possible, or a series of tools. The location of the tool in relation to the
workpiece must be changed by an amount equal to the stock removed on
each stroke if the same tool is used for multiple strokes. The broach can be
pulled or pushed and can be vertical or horizontal.
36.1.1. External Broaching
This method involves machining an external surface of the part. The
workpiece is usually clamped in a holding fixture and the tool secured in a
broach holder. Either the broach holder or the fixture is attached to the
powered slide of a broaching machine, and the other is held in a fixed
position relative to the surface to be broached. Pot broaching and straddle
broachingare two processes whereby the workpiece is surrounded by
broaches exerting balanced cutting pressures, which eliminate the need for
clamping the workpiece.
BROACHED PARTS
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candidate for broaching. Some parts have no practical alternative method of
manufacturing and must be broached. Typical parts are those with square,
circular, or irregular holes, the key slot in lock cylinders (Fig. 4.9.2), splines
and matching holes with straight-sided, involute, cycloidal, or specially
shaped teeth, cam forms, gears, ratchets, and other complex forms requiring
tight tolerances and precision finishes. Both helical and straight shapes are
feasible. Sizes range from very small parts to parts weighing several tons,
such as the stationary steam-turbine-rotor forms illustrated in Fig. 4.9.3.
Two exceptional characteristics, extremely high speed of production and
outstand-
Figure 4.9.2. Key slots in lock cylinders are normally broached.
Figure 4.9.3. Mounting openings for steam-turbine blades are
commonly broached.
ing repetitive accuracy, promote broaching over conventional machining
processes. Broaching is often chosen to replace milling, planing, shaping,hobbing, slotting, boring, reaming, and grinding. A fine surface finish is
produced because of the shaving action of the final teeth. Usually, no
additional surface-refining operations are required. Tool marks are axial
rather than circumferential.
36.3. ECONOMIC PRODUCTION QUANTITIES
Broaching usually requires high-volume production. Small production
quantities often cannot justify the initial cost of the broaching tool and other
tooling and the need for a special machine. The exceptions are cases in which
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there is no practical alternative method of machining or when standard
broaching tools, already on hand, can be employed.
Broaches range in cost from the low hundreds of dollars for a simpler tool to
several thousands of dollars for complex form-cutting tools, such as that
required for the turbine-rotor form shown in Fig. 4.9.3. Production rates
usually will range from 15 to more than 100 times higher than with
alternative machining methods. For example, pot broaching 24 gear teeth in
an SAE 1144 steel blank, 22 mm (7/8 in) thick, at 1000 pieces per hour, using
fully automated tooling on one broaching machine, replaced 16 hobbing
machines and four operators. A minimum production requirement of 1 million
parts was needed to justify the tooling and machine costs.
Tooling a machine for more than one part or for a group of similar parts with
the same machined surface can often make broaching an economical
operation on small-lot quantities.
36.4. SUITABLE MATERIALS FOR BROACHING
Most of the known metals and alloys, especially steels, cast irons, bronze,
brass, and aluminum, some plastics, hard rubber, wood graphite, asbestos,
and other composites, have been broached. In all cases, machinability of the
material is the key factor. Broaching of porous forgings and castings having
nonuniform densities creates the problems of unacceptable surface finish
and poor size control and should be avoided.
Controlling material hardness is important to prevent excessive part-to-part
varia-
Table 4.9.1. Typical Machining Results with Commonly Broached
Materials
Material
Brinell
hardness
number
Tolerance
Common
designation
UNS
designation
Finish mm/
surface
i
surm in
SAE
1008/1010
G10080/G10100 6070 R 1.1/1.6 45
65
0.025 0.
SAE G10200/G 7074 R 1.1/1.5 45 0.025 0.
b
b
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1020/1023 10230 60
SAE 1040 G 10400 8086 R 0.8/1.1 30
45
0.025 0.
SAE
1064/1065
G10640/G
10650
1820 R 0.6/1.3 25
50
0.038 0.0
SAE
1069/1070
G10690/G
10700
1820 R 0.6/1.3 25
50
0.038 0.0
SAE 1095 G10950 2325 R 0.9/1.5 35
60
0.046 0.0
SAE 1144 G11440 9397 R 0.8/1.5 30
45
0.020 0.0
SAE 4027 G40270 9195 R 0.6/1.3 25
50
0.013 0.0
SAE
4140/4142
G41400/G41420 9294 R 1.3/2.0 50
80
0.038 0.0
SAE 4145 G41450 9294 R 1.3/2.0 50
80
0.038 0.0
SAE 4340
cast
G43400 3133 R 2.0/3.0 80
120
0.064 0.0
SAE 5140 G5140 1216 R 1.5/2.0 60
80
0.051 0.
SAE 52100 G52986 1825 R 1.1/1.5 45
60
0.020 0.0
SAE 6150 G61500 1822R 1.1/1.5 45
60
0.020 0.0
SAE 8620 G86200 1827 R 1.0/1.5 40
60
0.020 0.0
SAE 8640 G86400 1825 R 1.3/2.0 50
80
0.025 0.
SAE 8642 G86420 1825 R 1.3/2.0 50
80
0.025 0.
b
c
c
c
b
b
b
b
c
c
c
c
c
c
c
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SAE 8645 G86450 1825 R 1.3/2.0 50
80
0.025 0.
Gray cast
iron
8894 R 2.0/2.5 80
100
0.063 0.0
Pearlitic
malleable
iron
9096 R 1.1/1.5 45
60
0.013 0.0
303
stainless
steel
S30300 8590 R 0.8/1.1 30
45
0.038 0.0
17-4PH
stainless
steel
S 17700 3440 R 0.5/0.8 20
30
0.030 0.0
403
stainless
steel
S40300 2732 R 0.6/0.9 25
35
0.030 0.0
410
stainless
steel
S41000 8492 R 0.4/0.8 15
30
0.025 0.
416
stainless
steel
S41000 1822 R 0.4/0.8 15
30
0.025 0.
M-2 high-
speed steel
T11303 2428 R 1.1/1.5 45
60
0.038 0.0
Inconel 750X N07750 2932 R 0.8/1.1 30
45
0.025 0.
Greek
Ascoloy
G41800 3238 R 0.9/1.1 35
45
0.025 0.
Rene 41 N07041 4042 R 0.8/1.0 30
40
0.051 0.
Stellite 31
2014-T6
R30031 3032 R 2.0/3.0 80
120
0.051 0.
c
b
b
b
c
c
b
c
c
c
c
c
c
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tions. Wide variations can result in poor or inconsistent surface finish and
are a major factor in tool life and size control. The ideal hardness range of
ferrous parts is between R 25 and R 32 with a tolerance of 3 to 5 points.
Table 4.9.1 lists some of the more commonly broached materials and the
surface finishes and tolerances that may be achieved under normal
conditions.
36.5. DESIGN RECOMMENDATIONS
36.5.1. Entrance and Exit Surfaces
A part to be broached should be designed so that it can be easily located and
held in the proper attitude. Surfaces contiguous to the area to be cut should
be square and relatively flat. Care in selecting the location of parting lines
and gates to prevent poor support during machining is important. The
designer should visualize how the part is to be retained and supported and
Aluminum A92014 7080 R 0.8/1.1 32
45
0.058 0.0
Copper C 12200 4585 R 1.1/1.5 45
60
0.038 0.0
Tellurium
copper
C 14500 4550 R 0.8/1.1 30
45
0.025 0.
Free-cutting
brass
C36000 7075 R 0.5/0.9 20
35
0.025 0.
Navy M
bronze
C92200 6570 R 0.8/1.1 30
45
0.038 0.0
Magnesium 6075 R 0.2/0.4 815 0.038 0.0
Aluminum
bronze (8%) C95600 7583 R 1.4/2.0 55
80
0.063 0.0
b
f
f
b
b
b
b
c c
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avoid the possibility of uneven or inconsistent surfaces in these areas.
This is especially true in internal broaching, in which the tool is not retained
or guided by the machine or fixturing. Uneven or inclined surfaces can cause
side-thrust pressures to the tool, which can result in inaccuracies in the
finished hole and possible tool failure.
External broaching is not so demanding, provided the part is so designed
that the holding fixture can retain and support it rigidly during the cutting
stroke. However, it is still advisable to design the part with square
supporting faces whenever possible.
36.5.2. Stock Allowances
Figure 4.9.4. Stock allowances for broaching a typical forged section.
When forgings are planned for broaching, they should be held to as close
dimensions as possible, allowing only minimum stock for finishing. Figure
4.9.4 shows a typical forged section with stock allowances recommended to
avoid overloading the broach tool during production runs.
Castings require a greater stock allowance to ensure that inclusions, scale,
and hard spots are removed and clean surfaces are produced in machining.Cold-punched or pierced holes present much the same problems as castings,
and stock allowances should be ample enough to allow for blanking breakout.
36.5.3. Wall Sections
It is advisable to avoid frail or thin wall sections and to maintain a uniform
thickness for any wall that will be subjected to machining forces. Sections
should be at least sufficient to withstand fixture-retaining pressures and to
minimize deflections caused by cutting forces.
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36.5.4. Families of Parts
If all of a group of parts will require a similar broaching operation, the
designer should attempt to design the parts so that all use the same
broaching tool and, if possi-
ble, the same holding fixture. For example, a number of levers of differentsizes or shapes could be designed with the same square hole in one end.
36.5.5. Round Holes
Starting holes may be cored, punched, bored, drilled, flame-cut, or hot-
pierced. When the starting hole is drilled or bored, 0.8-mm (1/32-in) stock on
the diameter of holes up to 38 mm (1 1/2 in) in diameter and 1.6-mm (1/16-in)
stock on larger holes are usually sufficient for cleanup. When cored holes are
planned, draft angles, surface texture, and size variations must be taken into
consideration in determining core size so as to assure cleanup.
Long holes should be chambered as shown in Fig. 4.9.5 to improve accuracy
as well as to reduce costs. Table 4.9.2 shows the recommended maximum
total length of the total hole surface for various diameters.
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Figure 4.9.5. Long holes should be chambered.
36.5.6. Internal Forms
Symmetrically shaped internal forms are usually broached by starting from
round holes, and the guidelines under Round Holes apply to them.
Irregularly shaped internal forms may be started from round holes as shown
in Fig. 4.9.6 or from cored, punched, pierced, or machined irregular holes.
Sometimes the cost of removing excess stock prior to broaching may prove to
be higher than that of broaching from the round hole, and the product
designer should investigate this cost before finalizing the part-blank design.
If this is not practicable, it is suggested that optional constructions for the
part-blank starting hole be specified.
Whenever stock allowance for broaching can be controlled by the method
used to form the blank, e.g., casting, stamping, or forging, it is always
advisable to leave a minimum amount of stock for cleanup plus draft,
mismatch, or out-of-round tolerances.
Table 4.9.2. Recommended Maximum Total Length of Hole for Various
Diameters
Hole diameter Maximum total length
mm in mm in
1.41.5 0.0550.060 3 2 0.125
1.51.9 0.0600.075 4 8 0.188
1.92.3 0.0750.090 5 5 0.218
2.32.8 0.0900.110 6.3 0.250
2.83.8 0.1100.150 9.5 0.375
3.85.1 0.1500.200 12.7 0.500
5.16.3 0.2000.250 15.9 0.625
6.37.6 0.2500.300 19.0 0.750
7.68.9 0.3000.350 22.2 0.875
8.910.2 0.3500.400 25.4 1.000
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Figure 4.9.6. Irregularly shaped broached holes are started from
round holes.
36.5.7. Internal Keyways
Whenever possible, it is advisable to design keyways to ASA specifications as
shown in Table 4.9.3. In doing so, standard keyway broaches, available from
some manufacturers as off-the-shelf tools, can be used. Many subcontract
10.212.1 0.4000.475 31.8 1.250
12.114.0 0.4750.550 34.9 1.375
14.016.5 0.5500.650 41.3 1.625
16.520.3 0.6500.800 50.8 2.000
20.325.4 0.8001.000 63 .5 2.500
25.431.8 1.0001.250 82.5 3.250
31.838.1 1.2501.500 102 4.000
38.141.3 1.5001.625 121 4.750
41.346.0 1.6251.812 131 5.500
46.050.8 1.8122.000 152 6.000
50.853.9 2.0002.125 178 7.000
53.957.2 2.1252.250 203 8.000
57.263.5 2.2502.500 267 10.500
63.576.2 2.5003.000 305 12.000
76.288.9 3.0003.500 457 18.000
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broaching sources stock these standard keyway broaches and solicit the
broaching of any quantity of parts.
Table 4.9.3. Standard Keyway Sizes, in
36.5.8. Internal Ke s
*Minimum length of part recommended to prevent the part from dropping between the
teeth of the broach.
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clearance for the upset burr of a cold-rolled spline shaft. This undercut
procedure does increase the cost of the tool, but it can economically
eliminate assembly problems.
Figure 4.9.9. This shape allows room for the upset burr on a cold-
rolled spline.
Figure 4.9.10. Avoid dovetail or inverted-angle splines.
5. Dovetail or inverted-angle splines should be avoided whenever possible
(see Fig. 4.9.10).
36.5.10. Spiral Splines
The guidelines for straight splined holes also apply to spiral splines:
1. Spiral splines with helix angles greater than 40 cannot be broached by
using conventional methods. It is advisable to use the lowest helix angle
possible.
2. Splines with helix angles greater than 10 usually will require the broach
to be driven rotationally during its travel through the part. The designer
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should provide some means of retaining the part to prevent rotation. This
can be accomplished by an irregular contour of some prominence such as a
projection, notch, hole, or indentation in the face of the part.
36.5.11. Tapered Splines
Tapered splines should be avoided (see Fig. 4.9.11):
1. Splines that taper on tooth thickness usually cannot be broached.
2. Splines that taper with the bore should be avoided. Single keyways are an
excep tion, provided the bore is large enough to permit insertion of a
mandrel having a tapered slot to guide the broach.
Figure 4.9.11. Avoid tapered splines.
3. Shallow tapers, such as those used for steering (Pitman) arms to provide a
solid lock-up, are often broached to the smallest through-hole size and
swaged to size in a secondary operation. In rare instances, a combination
push broach and swage tool is inserted to depth and retracted.
36.5.12. Square and Hexagonal Holes
It is advantageous to use a slightly oversized starting hole, particularly for
square holes. This method is optional, but it will reduce the cost of broaching
considerably (see Fig. 4.9.12).
Avoiding sharp corners at the major diameter is recommended to reduce
broach costs. This is best accomplished by specifying a slightly smaller major
diameter. If corner radii are a design requirement, they will add to the cost of
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the broach (see Fig. 4.9.13).
Figure 4.9.12. Use a slightly oversize starting hole.
Figure 4.9.13. Avoid sharp corners on a major diameter.
36.5.13. Saw-Cut or Split Splined Holes
When the part will have an intersecting cut into the splined hole, such as is
used to provide a clamping method or to allow expansion for the mating part,
the splined hole should be designed with an omitted space as shown in Fig.
4.9.14. This allows room for the burr produced by the saw cut.
Figure 4.9.14. Allow room for the burr produced by the saw cut by
eliminating one tooth.
36.5.14. Blind Holes
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Broaching blind holes should be avoided if at all possible. If splines or similar
shapes are necessary in blind holes, there should be a relief at the bottom of
the broached area to allow the chip to break off. This relief area should be as
wide as possible to retain the material removed by broaching and to allow for
the fact that the broach will be shorter as it is sharpened. Grooves that
produce a tapered heel for the broached area are recommended (see Fig.
4.9.15).
Figure 4.9.15. If blind holes are necessary, they should have a relief
at the bottom of the spline major diameter as shown.
36.5.15. Gear Teeth
Internal gear teeth should be given the same consideration as internal
involute splines
36.5.16. Chamfers and Corner Radii
In all situations in which corners must be broken by machining, chamfers are
preferred over radii.
1. Sharp internal corners should be avoided to eliminate stress points and
minimize tooth-edge wear. Chamfers are preferred to simplify manufacture,
but radii may be specified (see Fig. 4.9.16).
Figure 4.9.16. Internal-corner design.
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Figure 4.9.17. Chamfer outer corners rather than rounding them.
2. Outer corners or edges that must be machined should be chamfered
rather than rounded, also for ease in manufacturing (see Fig. 4.9.17).
3. Sharp corners or edges of intersecting outer broached surfaces should be
avoided whenever possible. Castings, forgings, and extrusions should be
designed with a corner break that does not require machining.
36.5.17. External Surfaces
Whenever possible, external machined surfaces should be relieved to reduce
the area that must be broached.
1. Reliefs of undercuts in the corners will simplify the broaching operation
(see Fig. 4.9.18).
2. Large surfaces should be broken into a series of bosses whenever possible
(see Fig. 4.9.19).
Figure 4.9.18. Reliefs or undercuts in the corners simplify broaching
of external surfaces.
Figure 4.9.19. Break large surfaces into series of bosses.
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36.7. RECOMMENDED TOLERANCES
36.7.1. Surface Finish
The surface finish produced by broaching is generally of high quality. While it
does not match a grinding finish, it will be superior to the finish produced by
most other manufacturing methods. By employing good tool design and
proper coolant oils, finishes of a burnished quality can be obtained in good-
machinability-rated materials.
Table 4.9.1 shows the surface finish that may be expected under normal
conditions. Smoother surface finishes can be obtained in most materials but
should not be specified unless absolutely necessary.
36.7.2. Flatness
Parts of uniform section and sufficient strength to withstand cutting
pressures can be expected to be broached within 0.013 mm (0.0005 in) TIR
(total indicator reading). An exception may be found on the exit edge of soft
or gummy materials such as aluminum or stainless steels, where the metal
extrudes during the cut and snaps back. A flatness of 0.025 mm (0.001 in) is a
safe assumption for most broached parts.
36.7.3. Parallelism
Parallelism of surfaces machined in the same cutting stroke should be within
0.025 mm (0.001 in) TIR on good- to fair-machinability-rated materials.
36.7.4. Squareness
For parts that can be fixtured and retained on true surfaces, a squareness of
0.025 mm (0.001 in) TIR is possible, and tolerances of 0.08 mm (0.003 in) can
be obtained consistently under controlled conditions in good-machinability-
rated materials.
36.7.5. Concentricity
Broaches usually will follow the pilot hole, and concentricity errors due to
broach drift should not exceed 0.025 to 0.05 mm (0.001 to 0.002 in) for round
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or similarly shaped holes in good- to fair-machinability-rated materials. Free-
cutting materials such as brass allow the broach greater freedom for drifting
during the cut. A special broach design and selection of the proper
broaching machine often can solve this problem for the product designer.
36.7.6. Chamfers and Radii
Tolerances on chamfers and radii should be as liberal as possible. Radii
under 0.8 mm (0.030 in) should have a minimum tolerance of 0.13 mm (0.005
in); 0.25 mm (0.010 in) should be allowable on larger sizes. Generous
tolerances reduce broach manufacturing and maintenance costs.
36.7.7. Basic Dimensions
Apply the values in Table 4.9.1.
Citation
James G.Bralla: Design for Manufacturability Handbook, Second Edition. BROACHED
PARTS, Chapter (McGraw-Hill Professional, 1999, 1986), AccessEngineering
EXPORT
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