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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

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