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Milling Operations - Backlash
Broaching Home

Broaching Principles

Broaching is a machining process that pushes or pulls a cutting tool (called a broach) over or through the surface being machined. Its high-production, metal-removal process is sometimes required to make one-of-a-kind parts. The concept of broaching as a legitimate machining process can be traced back to the early 1850s. Early broaching applications were cutting keyways in pulleys and gears. After World War 1, broaching contributed to the rifling of gun barrels. Advances in broaching machines and form grinding during the 1920s and 30s enabled tolerances to be tightened and broaching costs to become competitive with other machining processes. Today, almost every conceivable type of form and material can be broached. It represents a machining operation that, while known for many years, is still in its infancy. New uses for broaching are being devised every day.

Figure 1. Cutting action of a broaching tool.
Broaching is similar to planing, turning, milling, and other metal cutting operations in that each tooth removes a small amount of material (Figure 1 ).

Figure 2. Typical push keyway broaching tools and a shim.

The broaching tool has a series of teeth so arranged that they cut metal when the broach is given a linear movement as indicated in figure 1. The broach cuts away the material since its teeth are progressively increasing in height.

Properly used, broaching can greatly increase productivity, hold tight tolerance, and produce precision finishes. Tooling is the heart of broaching. The broach tool's construction is unique for it combines rough, semi-finish, and finish teeth in one tool (Figure 3).

                   Figure 3. Parts of a broaching tool.

There are two types of broaching procedures: internal broaching and external broaching. For exterior broaching, the broach tool may be pulled or pushed across a workpiece surface, or the surface may move across the tool. Internal broaching requires a starting hole or opening in the workpiece so the tool can be inserted. The tool, or workpiece, is then pushed or pulled to force the tool through the starter hole.

Almost any irregular cross-section can be broached as long as all surfaces of the section remain parallel to the direction of broach travel (Figure 4). Helical cuts can also be produced by twisting the broach tool as it passes the workpiece surface.

Figure 4. Different types of broaches.

In conclusion, it may be said that the broach tool and the broaching process are versatile and important and that anyone who works in the field of metals, woods, or plastics should be familiar with them.

Hints for successfully broaching a keyway

Maintain a rigid set-up at all times. The workpiece must be solidly fixed or nested perfectly square with the baseplate and ram face. Check to make sure that all square and parallel surfaces on the face of the ram and the baseplate remain true.

Proper alignment of the broach, workpiece, and ram is the most important factor in all broaching operations. Misalignment can cause drifting, deflection, and even breakage. Alignment Tips--If a keyway broach drifts and cuts a taper, try the following:

1. Reverse workpiece or turn broach so teeth face toward the back of the press.
2. Let the bushing protrude above the workpiece to give more support to the back of the broach
thereby helping to keep it aligned. If a collared bushing is used, place it upside down under
the workpiece.
3. Make sure the broach is centered under the ram at the beginning of the cut. If the
broach moves out of alignment after starting to cut, back off the pressure on the ram and align the broach itself. Repeat during successive cuts to ensure perfectly straight cuts.