Polyformaldehyde CNC Machining Service

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  Key takeaways
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  When POM is the right call, and when it is not
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  The same POM part from three suppliers: what the quote should tell you
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  What POM does on a machine tool
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  Machining parameters that actually hold tolerance
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  Stress relief and why POM parts move after machining
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  What to verify before you place a POM order
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  When POM machining stops making sense
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  Five mistakes that scrap POM parts
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  Step by step: from drawing to POM parts

Polyformaldehyde, also called POM, acetal or Delrin, is a semi-crystalline thermoplastic. It cuts to a clean chip at the right speed, which is why it is popular for gears, bushings and sliding parts. The same semi-crystalline structure is why it fights back. Heat builds at the cutting edge, the chip softens, and the surface tears instead of shearing. Cutter geometry and feed rate matter more here than on aluminium.

The material is not brittle like acrylic, and it is not soft like HDPE. It sits in a middle band that needs a positive rake and a sharp edge. A cutter that has already run aluminium will rub POM, not cut it. In practice we keep a separate set of POM tooling and replace it at the first sign of edge wear.

Unfilled POM is the standard grade. Glass-filled and PTFE-modified grades machine differently: the filler wears tools faster, and the finish tends to be duller. Tell the shop which grade you are specifying, because feeds and cutter life change with it.

POM is often chosen for low friction and good dimensional stability. It also has a low coefficient of thermal expansion compared with many plastics, so a part machined at 20 °C stays close to size at 30 °C. That is the reason it holds tolerance better than PA or PP on the same drawing.

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  Unfilled POMBest finish and tool life. Use for gears, bushings, rollers.
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  Glass-filled POMStiffer, but abrasive. Expect more tool wear and a duller surface.
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  PTFE-modified POMLower friction, slightly softer. Good for sliding contacts.

Spindle speed for POM usually lands between 2,000 and 6,000 rpm on small cutters, with a feed per tooth around 0.05–0.15 mm. The goal is a thick enough chip to carry heat away. Too light a chip and the edge rubs, which raises temperature and glazes the surface. Depth of cut around 0.5–1.0 mm per pass works for roughing on most features.

Cooling is a decision point. Flood coolant keeps the part cool and flushes chips, but POM can absorb a small amount of moisture on the cut surface. Many shops run dry with high-pressure air, which avoids moisture entirely and keeps chips clear. For deep pockets and long runs, flood coolant is usually the safer choice for size control.

Clamping is where thin parts fail. POM deflects under vise pressure, springs back after unclamping, and the measured size moves. Soft jaws bored to the part profile spread the load. For thin plates, vacuum fixturing or a light double-sided tape on a flat plate often beats a vise.

Chip evacuation matters more than it looks. POM produces a stringy chip that can wrap the cutter and recut. Air blast or through-tool coolant breaks the string and keeps the finish consistent across a long run.

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  Cutting speed2,000–6,000 rpm on small diameter cutters.
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  Feed per tooth0.05–0.15 mm. Keep the chip thick, not dusty.
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  Depth of cut0.5–1.0 mm roughing; 0.1–0.2 mm finishing.
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  CoolingAir blast for open features, flood for deep pockets.

Extruded POM bar carries internal stress from the extrusion process. When you remove material from one side, the balance changes and the part bows. This is not a machining defect, it is a material behavior. The fix is process, not tighter tolerance. Rough machine with 0.3–0.5 mm stock left, let the blank relax, then finish.

How long to rest depends on the part. Thin plates and long slender parts need more time than a compact bushing. A common shop rule is to rough in the morning and finish after the part has sat at room temperature. For parts where flatness is critical, an annealing cycle before final finishing reduces the residual stress further.

Heat from machining does the same thing on a shorter time scale. A part measured hot off the machine will be smaller than the same part measured the next morning. This is why final inspection should happen after the part reaches room temperature, not at the machine door.

If the drawing calls for ±0.005 mm, ask how the shop plans to reach it. The answer should mention roughing stock, a rest period, and a controlled finish pass. A shop that only talks about machine accuracy is not answering the question.

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  Rough with stockLeave 0.3–0.5 mm for the finish pass.
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  Rest the partLet it return to room temperature before finishing.
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  Anneal when flatness mattersReduces residual stress before final cuts.

Start with material traceability. Ask for the POM grade, the supplier, and the batch. Unfilled, glass-filled and PTFE-modified grades machine differently, and a substitution changes the part. A shop that cannot name the grade it will cut is a risk on any tolerance-critical job.

Next, ask for the inspection plan. For POM, that means which features are measured, with what instrument, and at what temperature. A CMM report on a part still warm from the machine is not the same as one taken after the part has settled. We run 100% inspection before shipment and issue reports on request.

Check the machine mix. POM parts often need a second operation on a different face, and 5-axis or mill-turn capability reduces the number of setups. Fewer setups means less clamping stress and better feature-to-feature alignment. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers.

Finally, look at the certifications that match your industry. ISO 9001:2015 covers general quality systems. IATF 16949:2016 matters for automotive, ISO 13485:2016 for medical, and ISO 27001:2022 for data handling on confidential drawings. The right certificate depends on where the part ends up.

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  Grade and batchConfirm unfilled, glass-filled or PTFE-modified before quoting.
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  Inspection planFeatures, instruments, temperature, report format.
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  SetupsFewer setups means less clamping stress and better alignment.
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  CertificatesMatch the certificate to the end industry, not the brochure.

CNC machining is the right process from one prototype to a few thousand parts. Above that, injection molding usually wins on unit cost, but only if the geometry is stable and the volume is committed. The crossover depends on part size, wall thickness and how many features need secondary operations.

For a simple bushing at 10,000 parts a year, molding is almost always cheaper. For a bracket with five faces and a tolerance on two of them, machining stays competitive much longer because the mold cost is high and the design may still change.

POM also has a practical size limit on machined bar stock. Large plates are available, but the thicker the stock, the more internal stress it carries. A 100 mm thick POM plate will move more after machining than a 20 mm plate of the same footprint. If the part is large and flat, plan the roughing and resting steps carefully.

No minimum order quantity applies here, so a single prototype and a 10,000-part run go through the same process. That is useful when you want to prove the design in POM before committing to a mold.

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  Prototype to low volumeMachining is the default. No tooling cost.
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  High volume, simple shapeMolding wins above roughly 10,000 parts.
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  Large flat partsThicker stock carries more stress. Plan the rest step.

The first mistake is measuring hot. A part taken off the machine and checked immediately will read small. By the next morning it has grown. If the inspection happens at the machine door, good parts get rejected and bad ones get shipped.

The second is over-clamping. POM is elastic enough to deflect under a vise and spring back. Soft jaws and light clamping pressure solve most of it, but the operator has to know the part is compliant. This is a training issue as much as a tooling issue.

The third is reusing cutters that ran metal. A worn edge rubs POM, raises the temperature, and leaves a glazed or torn surface. Keep POM tooling separate and change it on a schedule, not on failure.

The fourth is ignoring the rest period. Roughing releases stress and the part moves. Finishing immediately locks in the distortion. The fix costs one extra handling step and saves the whole run.

The fifth is quoting from the model alone. A quote that does not mention grade, stock form or inspection is not a quote you can compare. Ask what is included before you compare prices.

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  Measuring hotInspect after the part returns to room temperature.
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  Over-clampingUse soft jaws and light pressure on thin walls.
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  Dull cuttersKeep POM tooling separate from metal tooling.
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  Skipping the rest stepRough, rest, then finish.