Chuck Jaw Selection Calculator

Mounting a blank securely in a scroll chuck comes down to matching your tenon or recess diameter to a jaw set that actually closes (or expands) onto it within range, and this calculator works that match out from the workpiece diameter alone if you haven't cut a tenon yet. Absent a specified tenon diameter, it recommends one at roughly one-third of the blank's diameter — a proportion that balances enough gripping surface against not wasting excess wood on the mounting foot. It then searches a set of standard jaw sizes (1.5" up to 4.5") for the closest one whose grip range actually covers your tenon or recess diameter, using a wider ±0.5" tolerance for compression (external, gripping a tenon) mounting and a tighter ±0.375" for expansion (internal, gripping inside a recess) mounting, since expansion jaws generally have a narrower effective range. If nothing fits within tolerance, it falls back to whichever standard size is numerically closest, which is a signal you should recut your tenon rather than force an out-of-range fit.

The dovetail angle output — a standard 10° for compression mounting, 0° for expansion — reflects that compression tenons are typically cut with an angled dovetail profile to lock into the jaws' matching dovetail, while expansion recesses don't need that angle. It also checks your tenon depth against a safe 3/8" to 3/4" range: too shallow and the jaws bottom out without gripping; too deep and they may not seat against the tenon's shoulder at all. The safe-RPM figure applies a conservative diameter-based speed cap so the mounted blank stays controllable while turning.