Guides / Design

Designing Acetal Parts for Machining: Walls, Radii, Fits and Features

September 21, 2026

Acetal is forgiving to machine and unforgiving of a few specific design habits carried over from metal. These rules cover most of what separates a part that quotes cheaply from one that does not.

Wall thickness

Plan on 1.5 to 2 mm as a general minimum for an unsupported wall, and treat 1 mm as achievable only on short supported sections.

Thin acetal walls deflect under the cutter, which costs passes, and they deflect under clamping, which costs a fixture. Both show up in setup time rather than machine time, so the penalty is worst on small quantities.

Keep wall thickness reasonably uniform. A part that goes from 20 mm to 3 mm releases stress unevenly when machined and is a candidate for annealing, which adds a day or more to lead time.

Internal radii

Give every internal corner a radius. Acetal is notch sensitive and a sharp internal corner is both a crack starter and a machining problem.

A practical rule is a radius of at least a third of the pocket depth, and never smaller than 1 mm unless the function demands it. A corner radius that matches a standard cutter size costs nothing. A corner radius of 0.4 mm on a 15 mm deep pocket forces a small tool, many shallow passes and a price that surprises people.

Pockets and bosses

• Keep pocket depth under about four times the corner radius where you can
• Avoid blind holes deeper than roughly five diameters, chips pack and bores go oversize
• Support tall bosses with a fillet at the base
• Avoid knife edges where a thin wall meets a face at a shallow angle, they burr badly and break in handling

Fits and clearances

This is where acetal differs most from metal.

Thermal expansion runs near 110 micrometres per metre per kelvin, roughly ten times steel. So a steel shaft in an acetal bore has a clearance that changes with temperature, and in the direction most people do not expect: the acetal grows faster, so the clearance opens as it warms.

Work out the clearance at both ends of the service temperature range, not just at 20 degrees C. For a running fit over a wide range, that calculation often drives the design more than the load does.

Press fits in acetal need care. Acetal creeps, so an interference fit relaxes over time and a press fit that was tight on day one may not hold a year later. For anything that must not move, use a mechanical feature such as a keyway, a flat or a pin rather than relying on interference alone.

Tolerancing

Tolerance selectively. On a typical acetal part two or three dimensions matter and the rest do not.

A workable pattern:

General tolerance: ISO 2768 medium unless otherwise stated. Critical dimensions as marked. Inspect at 20 degrees C.

Routine machined tolerances in acetal land around plus or minus 0.05 mm on general dimensions, with plus or minus 0.025 mm realistic on small features when the shop controls temperature and lets the part settle. Anything tighter than plus or minus 0.02 mm is a conversation about a specific feature, not a drawing note.

Features that cost more than they look

• Long slender turned features. They deflect, they chatter and they need support.
• Deep narrow slots. Small tool, many passes, poor chip clearance.
• Many intersecting edges. Every intersection is hand deburring.
• Text and engraving. Fine in acetal, but it is a separate operation with its own setup.
• Tight flatness over a long face. This is the classic annealing trigger.

Five changes that lower a quote without changing function

1. Open the general tolerance and mark only the critical features
2. Increase internal corner radii to a standard cutter size
3. Convert a blind hole to a through hole where you can
4. Increase a thin wall by half a millimetre
5. Say which faces are functional so the shop knows where to spend effort

None of these change what the part does. Together they often move a quote more than a change of supplier would.

Before you release the drawing

Check that the drawing answers these five questions without anyone having to ask:

1. Which grade, and is substitution allowed?
2. Which dimensions are critical?
3. At what temperature is it inspected?
4. What does the edge condition need to be?
5. What environment does the part live in?

A drawing that answers all five gets comparable quotes from every shop that sees it. One that answers none gets five different assumptions and five different prices.

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