Rotary broaching is but one technique that is used to cut shapes into metal. The shapes are usually square or hexagonal and they can be cut inside or outside using annular cutters. The cutting tool is fixed into a tool holder, this arrangement then advances into the part like it was a bolt, cutting as it goes in. The tool holder is a rotating component, thus the cutter rotates and cuts the desired shape. Many different symmetrical shapes can be cut and as the process is fast this technique is used for volume production.
The tool head is placed at a very small angle and the work gives the appearance that it is wobbling. This design allows the cutter head to only come into contact with and place pressure on the surface being worked on. This “wobble broaching” technique allows each surface to be cut individually which gives reliable and high precision shapes.
Rotary broaching is often done by hand but for high volume the preferred method is to do it in a CNC machine (Computer numerically controlled). The operator can set the CNC machine to cut the desired shape and the required quantity. In this way hexagonal head bolts and other components can be shaped quickly and accurately. Each step of the manufacturing process is controlled by the computer; the only thing done by the operator is to make periodic checks to ensure that quality is maintained.
There are other ways to broach but the method that is chosen is the one which will perform the best based on the material of the part. Soft metals are ideal for broaching because the metal is less likely to clog the annular cutters as they move through the material. Regardless of the material, the broaching process must be done in a lubricant bath to reduce friction and overheating. The lubricant also aids in flushing the metal cuttings from the work area to a repository on the machine where they are later cleaned out.
Rotary broaching machines are expensive but the cost is dependent on the size and capacity. For companies that only have broaching demands periodically, there are specialty machine shops than can do the work under contract where the work can be done accurately and at the lowest cost.
