Driverless Future Shock

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.The rotary broaching process takes just seconds and is capable of producing forms with an accuracy in excess of 0.0005-inch. The process can be performed at the same time as other operations, increasing the speed and profitability of production.

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 [inches per rotation (IPR)] = (diameter of tool [inches]) × sin(Angle of misalignment [degrees])

Types of Rotary Broach ToolsThere are many types of rotary broach tools available. These include adjustment-free Swiss types that are designed for smaller European machines, where clearance is a concern. The standard, adjustment-free type is designed to accept a wide variety of forms and is easy to use while the heavy-duty type is built for speed and designed for larger forms. There is also a large form type designed for accuracy on larger forms.
Internal Hexagonal Rotary Broaches

The internal hexagonal rotary broaches from Polygon Solutions have a shank diameter of 0.315 inch (8 mm) and an overall length of 1.25 inches. These broaches range in dimension, from 0.0510-inch across the flats and a broach depth of 0.102-inch, to 0.4775-inch across flats and a depth of 0.650-inch.