Do rotary broach tools cut undersize or oversize?

Many customers ask if the broaches cut oversize or undersize. However, rotary broaches are precision ground to very tight tolerances. The holes that are cut will be nearly the exact same size as the broach. For this reason, most broaches are made to the high side of tolerance, so that as they wear, they will stay within the tolerance as long as possible.

Some imagine that if the broach is slightly off-center from the hole, each corner will be cut slightly larger than the intended form. However, this is not the case. The rotary broach will typically follow the pilot hole. If the hole is too far off-center, the tooling might break.

Others believe, or may have experienced, the rotary broach may cut an undersized hole. This rare situation is usually one of two problems. The first problem has to do with broaching thin wall parts. The walls expand while broaching and then shrink back to form resulting in an undersized hole.

The second reason for an undersized hole has to do with too much pressure in the broaching operation. Because the broach is not driven by live tooling, and has a back taper, it is possible for excessive pressure to cause the broach to push back, and result in a slight twist in the form. This is usually found when the operator puts a gage into the hole and finds that it will not fit, and he thus assumes it has cut undersize.

In the rare instance a twist is developing in the form, multiple solutions are available to remove it. First, the pilot hole can opened or other changes made to reduce the pressure. Second, the machine spindle can be reversed half-way through the hole, causing the twist to reverse direction, usually solving the problem. Finally, a guide post or fixture can be set-up to keep the broach aligned while broaching.