Almost any metal alloy is broachable, including soft materials like brass, bronze, and copper. Nonmetallic materials – graphite, hard rubber, wood, composites, and certain plastics – also can be successfully broached.

Free-machining materials are easier to broach than tough, hard ones. In steels, machinability is closely related to hardness. Steels with hardnesses of Rc 16 to 24 range having the "ideal" hardness. Usually, steels harder than Rc 35 dull broach teeth too quickly, forcing frequent tool changes and regrinds.

Cast and malleable irons allow more stock to be removed per tooth than steels, as do brass and bronze.

Caution should be exercised when determining stock-removal rates. Too heavy a cut will cause the broach to overload.

Broach hook angles vary between 0 degrees and 20 degrees +, depending on work piece ductility. Brittle materials such as cast iron require a smaller hook angle, usually 5 degrees to 10 degrees. The softer alloys of aluminum and rolled steel pose special problems because they may adhere to the broach teeth during cutting. This can be controlled with proper sharpening, the right cutting fluid, adjustment of the cutting speed, and altering the step per tooth and tool geometry.

Each broaching operation is different. But there are five areas where broaching excels: on parallel, multiple surfaces; when fast cutting is needed; automated operations; large production runs; and tough materials. The examples of typical broaching applications that follow illustrate broaching’s versatility.

Figure 1 depicts a firing-pin locking bolt for a large-caliber gun. The part was broached on all sides with a combination flat/contour broach fitted to a 48"-stroke, dual-ram surface-broaching machine. With hand loading and automatic clamping, the machine completed 60 parts per hour.

Figure 2 shows another example of multiple-surface broaching. It involved machining four surfaces on a aluminum forging. Employing a common fixture, only the adapters had to be changed prior to broaching each side of the part. A 48"-stroke, vertical machine was loaded and clamped by hand. Two hundred forty parts per hour were made.

The openings in the combination wrenches shown in Figure 3 were broached; size varied from ¼" to 1". The wrenches first were loaded by hand into one of two adjustable magazines. The operation ran completely automated afterwards, with parts being fed into position, clamped, broached, unclamped, and then ejected. Production varied from 800 to 1000 parts per hour, depending upon the wrench’s size.

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Slot-cutting is another common broaching application. The clutch shown in Figure 4 is a rough-turned pearlitic casting. The fixture automatically indexed the part, permitting 12 slots to be broached on the part’s OD. Ninety of these complex work pieces were produced per hour.

To cut the teeth in the three brass styling combs pictured in Figure 5, the parts were hand-loaded into a magazine, automatically positioned, clamped, broached, and then ejected. The slots measure 0.047" wide and 0.687" deep. Depending on the part size and style, between 800 and 1000 parts were broached an hour.

Considerations

Broaching is not suitable for every situation. Like any machining operation, it has limitations Several of these must be taken into account before determining whether broaching is right for a certain application.

The first consideration is the work piece surface. If the surface to broached isn’t parallel to the direction the work piece or tool travels, broaching may be inappropriate. Also, if the broach’s passage over or through the work piece is obstructed, another method should be sought.

Second, complex, contoured surfaces having curves in two or more planes can’t be formed in a single broaching pass, except in the case of surface broaching helical gear teeth. In helical surface broaching, the broach and gear teeth are uniformly rotated in relationship to each other. The gear and broach must rotate at the desired helix angle as the broach is pulled through the piece to obtain the proper relationship.

Finally, the broached part must have sufficient strength to resist the forces exerted by the broach. Likewise, these forces also demand that machines and fixtures be rigid. Parts with thin walls or cross sections may prove to fragile for broaching. This is true for narrow slots as well, because the broach tools would have to have excessively thin cross sections.

Broaching won’t solve every metal-working problem. But if the work piece design and production volume lend themselves to it, broaching could prove to be the most efficient, least costly way to make top-quality parts.

About the Author

Chris Van De Motter is manager of engineering for The Ohio Broach & Machine Company, Willoughby, OH., U.S.A.

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