Workholding · Released

Soft-Jaw Step Calculator

Calculate soft jaw boring dimensions for 3-jaw and 4-jaw chucks. When you bore soft jaws to hold a part, the "step" (back-cut) provides positive axial location. This tool computes the bore diameter, step diameter, and engagement depth from the part dimensions.

Part diameter to clamp — mm

Step height (axial engagement) — mm

Number of jaws

Jaw bore depth — mm

Main bore diameter

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Step (relief) diameter

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Step radial depth

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Contact arc per jaw

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How soft jaw boring works

Soft jaws are bored in the chuck they will be used in (to run true). Two diameters are machined:

Main bore: sized to the part OD with a small clearance (typically +0.02–0.05 mm).
Step (back-cut / relief): a smaller diameter behind the main bore. The part butts against this shoulder for axial location. The radial step depth is typically 0.5–1.5 mm.

The step height (axial engagement) should be at least 1× the radial depth, and 2–3 mm is typical for gripping strength. Bore the soft jaws at the same clamping pressure and chuck position they will run at in production.

Important: Always bore soft jaws at the approximate clamping diameter you will use in production. If you bore them at a different jaw opening, they will not be concentric. Use a boring bar or the existing chuck bore tool.