Rotary Broaching is a popular method for creating hexagon and square holes in metal parts. The same tooling can be used for created serration forms, such SAE, Rosan style, J500, and ANSI standard involute splines. When considering rotary broaching as an option for producing serrated forms consider these factors.
1. Tooth Height. Half of the difference between the major and minor diameters will give you a pretty good idea of the amount of material to be removed on each side of the hole, or tooth height. Rotary Broaching is typically only suitable for small forms under one inch in diameter. For example, a standard 1/4″ hexagon broach has a minor flat-to-flat dimension of .2525”. The major sharp corner dimension is .2915”. The difference between the major and the minor is .039”. Divide the difference in half, and the result is a cut less than .020” wide per side. This is a suitable amount of material to be removed with a rotary broach. Broaching larger forms successfully typically requires a shallow depth, over-sized pilot hole or soft material to be successful.
2. Corner radii. Rotary broaches are precision ground cutting tools and any serrations are likely to be considered special or custom order broaches. Nevertheless, it is not always possible to create sharp corners in the broach form. Due to the back taper required for the broaching process, and the limitations of modern grinding equipment, small radii from .002″-.005″ should be expected in the corners of any serrated forms. The result of the radius on the broach is a matching radius in the workpiece. Often these radii are insignificant due to chamfers and radii employed on mating parts. However, it is a limitation if the rotary broaching process and should be considered.
Any custom order serrated rotary broaches will require dimensions for production including pitch diameter, number of teeth, major and minor diemeters, etc. Providing as much data as possible during the quoting process should help eliminate any problems mentioned above.