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How to Use a Rotary Broach
Guide · Updated October 28, 2026
A rotary (wobble) broach holder carries the broach at 1° to the work centerline. The broach spins with the work and its edge rocks in and out of the cut like a wobbling coin, so only one point cuts at a time. That lets a lathe or mill cut hex, square and Torx shapes in seconds.
Check the pilot hole
Measure the pilot hole. The free Gage Block Calculator builds the stack.
Step by step
- Drill the pilot: About 3% over across-flats for hex (Polygon Solutions); free-cutting materials can be closer.
- Drill deep: At least 1.3–1.5× the profile length (rotarybroaching.net) or 2× the broach depth (Tormach) for chips.
- Chamfer: 60–90° chamfer slightly larger than the broach across-corners.
- Set center: Holder centered on the hole.
- Feed: Feed steadily per maker; do not dwell.
- Retract: Retract without stopping the spindle.
- Check: Check across flats with a pin or gage.
Worked example: 1/4 in hex
Polygon Solutions: a 1/4 in hex rotary broach has 0.2525 in across flats and a typical 0.260 in pilot (0.2525 × 1.03 = 0.260). For 0.250 in of hex depth, drill the pilot at least 0.375 in deep (1.5×) or 0.500 in (2×, Tormach). Chamfer larger than the across-corners size: for a 0.2525 in hex, across-corners is 0.2525 ÷ cos 30° = 0.2916 in, so chamfer to about 0.30 in.
How the wobble works
The 1° offset makes the tool cut one point at a time; the broach itself has about 1-1/2° clearance (rotarybroaching.net).
Lathe vs mill
On a lathe, the work spins and the holder is stationary; on a mill, the holder spins and the work is stationary.
Common problems
Spiraled form: holder off center or broach dull. Broken broach: pilot too small or no chip room. Rough walls: feed too low.
Worked example: depth for the hex
For a 1/4 in hex with a 0.5 in long socket, at 1.3–1.5× the across-flats size the practical broach depth is 0.33–0.38 in (1.3 × 0.2525 = 0.328 in, 1.5 × 0.2525 = 0.379 in) without extra relief. For deeper forms, drill a deeper relief past the broached depth so chips have somewhere to go.
More common problems
Form spirals or twists: the broach is not on center; indicate the holder. Rough corners: pilot too small. Broken tip: pilot too small or feed too heavy.
Tips
- Broach: keyway broach.
- Drill: spot drill.
- Hex: cut a hex on a mill.
FAQ
What is a rotary broach?
A broach held at 1° that wobbles to cut shapes.
Pilot size?
About 3% over across-flats for hex.
How deep?
1.3–2× the broach depth.
Why chamfer?
To start the broach.
Lathe or mill?
Both.
Related guides and calculators
- How to Use a Keyway Broach
- How to Center Drill and Spot Drill
- How to Cut a Hex on a Mill
- How to Use an Arbor Press
- How to Ream a Hole
- Taper, Plug and Bottoming Taps: How to Use a Hand Tap Set
- Spiral Point vs Spiral Flute Taps
- All free machinist calculators
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General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.