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How to True a 3-Jaw Chuck
Guide · Updated October 28, 2026
A 3-jaw scroll chuck centers itself, but rarely perfectly: 0.002–0.005 in runout is common on used chucks. You can reduce it with cleaning, an adjustable-back chuck, shims, regrinding or soft jaws.
Use a known pin
Measure runout on a ground pin you trust. Check it on gage blocks. The free Gage Block Calculator builds the stack.
Step by step
- Clean everything: Clean the spindle nose, backplate, scroll and jaws.
- Check jaw numbers: Jaws must be in their numbered slots.
- Chuck a ground pin: Grip a ground pin with the master pinion (often marked) and read TIR near the jaws.
- Try other pinions: Tighten from each pinion; use the one with least runout.
- Adjust a Set-Tru: On an adjustable chuck, loosen the mounting bolts and tap or screw the body by half the TIR toward the low side.
- Shim for one job: For a single job, shim one jaw with foil.
- Recheck: Recheck at the size you use most.
Worked example
A 1.000 in ground pin shows 0.004 in TIR. The center is off by 0.002 in. On a Set-Tru chuck, push the body 0.002 in toward the low reading; recheck to under 0.0005 in. With a plain chuck, a common shop approximation is a shim about 1.5 × the wanted offset under one jaw (it holds when the jaw contact is wider than about 1.73 × the offset, which is true for small corrections like this): 1.5 × 0.002 = 0.003 in. Treat it as a first try, then indicate and adjust; for exact centering use a 4-jaw or an adjustable chuck.
Why only at one size
The scroll is not perfect, so runout changes with diameter. A Set-Tru setting is right only near the size you trued it at.
Regrinding jaws
Grip a ring at the back of the jaws to preload them as in use, then grind the jaw faces with a tool post grinder. This fixes bell-mouth and worn jaws.
Common problems
Runout changes each time: chips in the scroll or worn scroll. Bell-mouth grip: worn jaws. Runout returns: loose backplate.
Tips
- 4-jaw: 4-jaw chuck for exact centering.
- Soft jaws: soft jaws bore true.
- Grinder: tool post grinder.
FAQ
How much runout is normal?
0.002–0.005 in on a used chuck.
What is a Set-Tru chuck?
An adjustable chuck you can shift to zero runout.
Does it hold at all sizes?
No, only near the size you set.
How do I shim a 3-jaw?
A common approximation is about 1.5 times the offset under one jaw, then indicate and adjust.
When use soft jaws?
When you need repeatable concentricity.
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- How to Use a Tool Post Grinder
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