Broaching (metalworking) - Rotary Broaching

Rotary Broaching

A somewhat different design of cutting tool that can achieve the irregular hole or outer profile of a broach is called a rotary broach or wobble broach. One of the biggest advantages to this type of broaching is that it does not require a broaching machine, but instead is used on lathes, milling machines, screw machines or Swiss lathes.

Rotary broaching requires two tooling components: a tool holder and a broach. The leading (cutting) edge of the broach has a contour matching the desired final shape. The broach is mounted in a special tool holder that that allows it to freely rotate. The tool holder is special because it holds the tool so that its axis of rotation is inclined slightly to the axis of rotation of the work. A typical value for this misalignment is 1 degree. This angle is what produces a rotating edge for the broach to cut the workpiece. Either the workpiece or the tool holder is rotated. If the tool holder is rotated, the misalignment causes the broach to appear as though it is "wobbling", which is the origin of the term "wobble broach".

For internal broaching the sides of the broach are drafted inward so it becomes thinner; for external broaching the sides are drafted outward, to make the pocket bigger. This draft keeps the broach from jamming; the draft must be larger than the angle of misalignment. If the work piece rotates, the broach is pressed against it, is driven by it, and rotates synchronously with it. If the tool holder rotates, the broach is pressed against the workpiece, but is driven by the tool holder.

Ideally the tool advances at the same rate that it cuts. The ideal rate of cut is defined as:

Rate of cut = (diameter of tool ) × sin(Angle of misalignment )

If it advances much faster, then the tool becomes choked; conversely, if it advances much slower, then an interrupted or zig-zag cut occurs. In practice the rate of cut is slightly less than the ideal rate so that the load is released on the non-cutting edge of the tool.

There is some spiraling of the tool as it cuts, so the form at the bottom of the workpiece may be rotated with respect to the form at the top of the hole or profile. Spiraling may be undesirable because it binds the body of the tool and prevents it from cutting sharply. One solution to this is to reverse the rotation in mid cut, causing the tool to spiral in the opposite direction. If reversing the machine is not practical, then interrupting the cut is another possible solution.

In general, a rotary broach will not cut as accurately as a push or pull broach. However, the ability to use this type of cutting tool on common machine tools is highly advantageous.