2026-04-04
0.15 mm (0.006") of clearance per side. That is the gap between a part and its soft jaw pocket — enough for the part to slide in by hand, tight enough that it does not shift under a finishing pass.
Clearance is the gap between the part surface and the pocket wall. A pocket with 0.15 mm clearance per side has a total gap of 0.30 mm across any dimension — the pocket is 0.30 mm wider than the part.
The clearance is applied as a uniform outward offset of the 2D pocket profile. Every wall moves outward by the clearance amount. The pocket is always larger than the part by exactly the clearance on every wall, regardless of the profile shape.
This is distinct from the tool radius compensation offset, which accounts for corner geometry. The clearance offset determines fit. The tool radius offset determines machinability. Both are applied to the same profile, in sequence.
Fit Type Per-Side (mm) Per-Side (in) Use Case
Press fit 0.05 0.002 Interference, part must be pressed in
Location fit 0.10 0.004 Precision location, hand-push fit
Slip fit 0.15 0.006 Standard workholding, drop-in
Free fit 0.25 0.010 Easy load/unload, chip clearanceSoft jaws default to 0.15 mm (slip fit). Vise clamping force holds the part laterally — the jaws squeeze inward on both sides, so the pocket only needs to locate the part, not restrain it. A slip fit lets the operator drop the part in by hand and close the vise.
Fixture plates and zero-point plates default to 0.20 mm. Gravity plus top clamps provide less lateral restraint than a vise. The extra clearance allows chip clearance in the pocket floor and easier part seating when the operator is working from above.
Thermal expansion. Aluminum 6061-T6 has a coefficient of thermal expansion (CTE) of 23.6 μm/m/°C. A 100 mm (3.937") part that arrives 10°C above ambient has grown 0.024 mm (0.001") — eating 16% of a 0.15 mm clearance budget. A hot part from a previous operation may not fit a 0.05 mm press-fit pocket at all.
Chip trapping. Below 0.10 mm (0.004") of clearance, swarf trapped between the part and the pocket wall prevents the part from seating flat. The part rocks on a chip, producing a surface finish defect on the next operation and potentially shifting under cutting forces. On a production run where the operator is loading parts every 90 seconds, clearing sub-0.10 mm gaps with an air gun is not realistic.
Cutting forces shift the part laterally within the pocket. On a vise, clamping force counteracts this — the part is pinched between the jaws, and friction prevents lateral movement. The pocket clearance is less critical because the vise is doing the restraining.
On a fixture plate, the pocket wall is the only lateral restraint. A 0.25 mm (0.010") clearance means the part can shift 0.25 mm before contacting the wall. That is 0.25 mm of positioning uncertainty on every feature cut in that setup. For a finishing pass targeting ±0.05 mm (0.002"), that clearance is the dominant error source.
The tradeoff: tighter clearance improves positional accuracy but makes loading harder and risks chip interference. Looser clearance makes loading easy but introduces positional uncertainty. The defaults — 0.15 mm for vise jaws, 0.20 mm for fixture plates — are the midpoints that work for most parts.
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