Soft jaws

In the worked soft-jaw example, three jaws each apply 6 kN to a 20 mm wide contact over 10 mm of grip length. Assuming uniform pressure, that is 200 mm² and 30 MPa per jaw. Halving grip length doubles the nominal pressure. Actual contact pressure and deformation depend on the bored profile, jaw loading and part stiffness.

Use Choosing workholding to compare location, support and restraint across fixture types.

What soft jaws do

Soft jaws can be bored for a round chuck grip or pocketed to match an irregular part in a vise. SCHUNK offers steel and aluminum top jaws. SCHUNK soft top jaws, Kurt soft-jaw applications

Chuck jaws add rotating mass. A jaw pocket can provide both a gripping surface and a seating shoulder.

The mounting interface is part of the jaw

A bored diameter does not make a chuck jaw compatible with another chuck. SCHUNK's range includes 1.5 mm × 60°, 3 mm × 60°, 1/16 inch × 90° and tongue-and-groove interfaces, alongside different slots and dimensions. These are alternatives, not interchangeable descriptions. SCHUNK interface range

Kitagawa's SJ catalog separates serration pitch, jaw height, jaw count and bolt-spacing variants; it even distinguishes bolt spacings within the same chuck diameter. Its model-specific compatibility table is more informative than nominal chuck size alone. Kitagawa SJ catalog, p. 222

Vise interfaces differ again. Kurt's CarveSmart SMARTstop jaws use dovetailed jaws with an internal slot-and-pin locating system. Kurt specifies relocation of ±0.0003 inch in the same ground steel master jaw after one use; this is a conditional jaw-relocation claim, not a general workpiece machining tolerance. Its machinable options include aluminum and 1018 cold-rolled steel. Kurt SMARTstopped products

Machining under load

TAIKI describes a boring ring as a means of simulating clamping force while machining chuck jaws, maintaining their loaded position. The ring supplies a reaction while the intended gripping surface remains accessible to the boring tool. TAIKI boring rings

Haas's lathe-chatter guidance, updated November 13, 2025, places a support slug or adjustable boring ring near the center of jaw travel and calls for a bore matching nominal part size. It notes that jaw deflection can require a compensating taper for full contact. Chips, burrs and screw bottoming can undermine the result. Thus a circular bore in unloaded jaws alone does not establish concentric gripping under production load. Haas TG0092

Loaded machining does not imply unlimited ring pressure. Haas's HJB instructions recommend no more than 113 psi chuck pressure and list maximum boring speeds from 800 rpm for HJB-05/-06 down to 400 rpm for HJB-15. Those limits concern these boring rings, not the finished workpiece setup. Haas HJB operating specifications

In its February 6, 2026 manual revision, Kurt specifies for PFW-series vise carvable jaws precision blocks just below the cutting surface and the same vise torque during jaw machining as during production. Scribed machining boundaries protect internal jaw features. This is a vise-specific example, not a transferable chuck pressure rule. Kurt PFW460 manual, p. 12

Contact, grip length and rotating loads

Kitagawa's HRS08 manual, version 1.05 dated September 18, 2024, states that excessive gripping force can deform pipe-shaped parts. Higher gripping centers reduce static gripping force and increase mounting loads; heavier or taller replacement jaws require operating-condition calculations. Its external-gripping specifications distinguish 38.3 kN maximum static force from 23.4 kN dynamic force at 3,000 rpm. These values depend on the specified standard jaws, pressure and measurement geometry. Dirty serrations impair accuracy; incorrect serration pitch or faulty engagement can weaken mounting. Kitagawa HRS08 manual, pp. 7, 11–18

Original contact-area example: assume three jaws each contact a 20 mm-wide surface over 10 mm axial grip length, with 6 kN normal force per jaw. Nominal area is 200 mm² per jaw and average pressure is 6,000/200 = 30 MPa. Halving grip length to 5 mm gives 100 mm² and 60 MPa at unchanged force. This simplified flat-area calculation excludes curved contact, uneven seating and elastic deformation; it predicts neither tube ovality nor permissible cutting load.

Original rotating-mass example: one 0.20 kg jaw with its center of mass 0.08 m from the axis at 3,000 rpm has an outward centrifugal-force magnitude of m r (2πn/60)² ≈ 1.58 kN. Doubling speed quadruples that force. This is a free-body illustration, not a chuck derating formula: the complete chuck mechanism and all jaws determine the actual change in grip.

Grip length, axial overhang, shoulder seating and thin-wall stiffness therefore belong to the workpiece–jaw combination. A deeper pocket increases nominal contact length but also removes jaw stock and can restrict tool access. No universal minimum grip length, allowable remaining jaw section or safe rpm follows from these examples.

Coverage limits

The cited interfaces and limits apply to named products. Repeatable location also depends on the machine, mounting cleanliness, loaded jaw geometry and workpiece condition. The undated web pages and HJB/SJ documents establish no verified issue date here; the PFW manual prints its revision date on the back cover. This reference does not establish a generic runout guarantee, friction coefficient or production clamping setting.