Guides / Process
Six Defects That Show Up on Machined Acetal Parts and What Causes Them
September 21, 2026
Most acetal defects are diagnosable from the part itself. This is a field guide for reading one, whether you are the buyer looking at a first article or the engineer deciding whether to reject a batch.
1. Smeared or melted surface
Looks like: a glossy band or a dragged, slightly raised film on an otherwise clean surface, often on a wall left by a finishing pass.
Cause: the cutting edge is rubbing instead of cutting. A dull tool, too little feed per tooth or too small a depth of cut all do it. Heat builds in a thin surface layer, melts it and the tool drags it along.
Fix: sharper tooling and more feed. Counterintuitively, slowing the spindle usually makes it worse, because the edge spends longer in contact per unit of material removed.
2. Chatter marks on a turned diameter
Looks like: regular fine bands around a diameter, often worse near the free end of a long part.
Cause: the part or the tool is deflecting. Acetal is not stiff, so a long slender turned part flexes away from the tool and springs back.
Fix: tailstock support or a steady, shorter tool overhang, lighter depth of cut on the finish pass. On the design side, this is an argument for not specifying a long unsupported slender feature when a stepped one would do.
3. Packed chips in a deep bore or blind hole
Looks like: a scored bore, an oversized hole, or a drill that suddenly loads up and leaves a rough exit.
Cause: acetal chips are springy and they do not always clear on their own. In a deep or blind hole they compact, and once they compact they act as an abrasive and a heat source at the same time.
Fix: peck cycles, through tool air or coolant, and clearing between operations. On the design side, avoid blind holes deeper than about five diameters where a through hole would work.
4. Oversized bore after reaming
Looks like: a bore that measures above the reamer size, sometimes by a surprising amount.
Cause: a mix of thermal expansion during cutting, elastic recovery after the tool passes and chips riding between the reamer and the wall.
Fix: reaming at low speed with a steady feed, leaving the right amount of stock for the ream, and measuring after the part has cooled. Some shops prefer to bore acetal rather than ream it for exactly this reason.
5. Stripped or weak tapped threads
Looks like: a thread that feels fine on assembly and strips at a torque a metal thread would take easily.
Cause: too little thread engagement, too fine a thread pitch or a tapping drill sized for metal.
Fix: coarse threads, engagement of at least one and a half to two times the diameter, and thread inserts where the joint will be assembled more than a few times. Threads in plastic are a design decision, not a machining one.
6. Voids on the centreline of a thick part
Looks like: a dull porous band on a face that sits on the axis of the original rod, usually on a thick homopolymer part.
Cause: large diameter acetal homopolymer rod cools from the outside in, which can leave the core less dense than the skin. Machinists call it centreline porosity. It is a characteristic of the stock, not of the cutting.
Fix: copolymer for thick sections, or design so that the finished part does not put a functional surface on the rod axis. A shop that machines acetal regularly will rough a thick part, look at the core and flag it before finishing rather than after.
How to use this on a first article
When a first article comes back with a mark you do not like, ask two questions before rejecting:
- Is it on a functional surface? If the drawing does not say which faces matter, the shop had to guess. Marking functional faces is the cheapest quality improvement available on an acetal drawing.
- Is it a process fault or a stock characteristic? Smearing, chatter and packed chips are process. Centreline voids in thick homopolymer are stock. The first is fixable by the shop. The second needs a material or a design change.
A shop that answers those questions clearly is the one worth keeping.
Getting parts back with a defect you cannot place? Send your RFQ with photos and the drawing. The shops we match will tell you plainly what caused it. Reply within 24 business hours.
Frequently asked questions
Why is the bore on my acetal part bigger than the reamer?
Usually heat, chips or both. The part expands while it is cut, the reamer pushes packed chips against the wall, and the finished bore reads oversize once it cools. Peck drilling, chip clearance and a finishing pass usually close the gap.
What causes a glossy smeared band on a machined POM surface?
A dull edge rubbing rather than cutting. It generates local heat, melts a thin layer and drags it. Sharper tooling and more feed per tooth is the standard fix, not a slower spindle.
Is a small void on a turned face a rejectable defect?
It depends on where it is. A void on the centreline of a thick homopolymer part is a known characteristic of large rod, not necessarily a process fault. On a sealing face it is a reject. Say on the drawing which faces are functional.
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