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How to Set Up Soft Jaws on a Lathe Chuck
Guide · Updated October 28, 2026
Soft jaws are unhardened steel or aluminum jaws you machine in place, so they grip a finished diameter with very low runout and without marring it. The key is to bore them while they are clamped under the same pressure they will see in use.
Size the jaw bore
Check the part diameter with a mic proved on gage blocks before boring the jaws. The free Gage Block Calculator builds the stack.
Step by step
- Mount the jaws: Clean the serrations and mount each jaw in the same position, number them.
- Clamp a spacer: Clamp a ring or plug behind the gripping area so the jaws are loaded as in use.
- Bore the jaws: Bore the gripping diameter with a boring bar at the part size.
- Face a locating step: Face a shoulder so parts seat to a repeatable depth.
- Remove the spacer: Open, remove the spacer, deburr.
- Test with a part: Clamp a part and indicate it.
- Mark the jaws: Mark the chuck and jaw position so they can be reused.
Worked example
Parts are 1.5000 in diameter. Bore the jaws to the part size or a hair under, about 1.4990–1.5000 in, while clamped on the spacer. A 0.001 in under bore gives three-line contact near the jaw centers. Indicate the part: 0.0003 in TIR is typical for well-bored soft jaws, compared with 0.002–0.004 in on many hard 3-jaw chucks.
Why clamp a spacer
Jaws move in their slots under pressure. Boring them under clamping force means they land in the same place when gripping the part.
Common problems
Part rocks: jaws bored too big, contact only at the tips. Runout: jaws not loaded when bored or remounted in the wrong slot. Marks: grip too tight.
Tips
- Mill: soft jaw basics for vises.
- Boring: use a boring bar.
- Runout: runout basics.
Quick shop check
Bore soft jaws at the same grip pressure you will use in production, clamping on a ring or plug just inside the cutting area. Bore to the part diameter or about 0.001–0.002 in larger, depending on your fit preference. Mark jaw position numbers; swapping jaws between slots can add 0.001 in or more of runout.
FAQ
What are soft jaws?
Unhardened jaws machined in place to fit a part.
Why clamp a spacer?
So the jaws are loaded as in use.
What size to bore?
The part size or slightly under.
How accurate?
Often under 0.0005 in TIR.
Can I reuse them?
Yes, mark positions and rebore as needed.
Related guides and calculators
- Soft Jaw Basics for Vises
- How to Use a Boring Bar
- Runout and TIR Basics
- How to Indicate a Part in a 4-Jaw Chuck
- How to Use a Lathe Collet Closer
- How to Cut Internal Acme Threads
- How to Cut Left-Hand Threads on a Lathe
- How to True a 3-Jaw Chuck
- All free machinist calculators
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