Here's a few tips to help with your rotary broaching application:
1. Rotary broaching requires a chamfer as large (or larger) than the major diameter. This helps the broach get started and minimizes skid marks. To be safe, make the chamfer .005" larger per side than the sharp corner dimension.
2. Broaching depth may be limited to 1-2x the across flat or minor dimension. If the chips are too long, they may bind on each other in the hole and create resistance to the broaching tools.
3. Pre-drilled pilot holes must be larger than the across flat or minor dimension. Undersized pilot holes increase resistance due to large chip sizes. Check to see if your predrilled hole is at least 3% larger for hexagons, and even larger for squares. A good pilot hole is critical to successful rotary broaching, be sure to use sharp drills and avoid work hardening in the hole.
4. Check to make sure the broach is on center with the hole.
5. Check to make sure the hole is deep enough for chips to flow into. If the hole is not deep enough, excessive force may damage the broaching tools, work piece, or machine.
6. If the hole appears undersize, it may be due to a spiral condition. This twisting is caused by the backtaper and free turning spindle allowing the broach to drift if there is too much pressure. Try to reduce the amount of material removal or thrust required if possible. Also, try reversing spindle direction half-way through the hole. Reversing the spindle will reverse the direction of the spiraling and often correct the condition.
7. Finally, check the bearings in the rotary broach holder. If the bearings are not allowing the spindle to rotate smoothly, they may need to be replaced.