Machining Acetal (POM): Properties and Tips

This article is part of the CNC Machining Materials Guide: Metals, Plastics & Superalloys on CNX Precision.

Machining acetal is a proven route to durable, low-friction parts that hold their shape over time. Acetal, also called POM, is one of the most popular engineering plastics for CNC milling and turning. Designers choose it for gears, bushings, rollers, and precision sliding components. This guide covers the properties of POM and the practical tips you need for successful machining acetal.

POM combines metal-like stiffness with self-lubricating behavior. It absorbs very little moisture, so dimensions stay stable in humid environments. Parts run quietly and wear slowly. For these reasons, machining acetal remains a default choice across many industries.

What Makes Acetal (POM) Special

Acetal delivers a balanced set of mechanical properties. It offers low friction against steel and other polymers. It resists wear even in continuous sliding contact. Its friction coefficient is often below 0.2 against steel. As a result, POM parts need no external lubrication in many applications.

The material is also stiff and strong. It holds its shape under load better than many other plastics. This stiffness makes it ideal for gears, cam followers, and precision spacers. It remains stable from about -40 degrees C to 100 degrees C in continuous service.

Acetal also resists common solvents, fuels, and weak acids. This broad chemical tolerance extends its use in pumps, valves, and fuel system components. Its low moisture pick-up keeps electrical properties stable too. For these reasons, POM appears in precision mechanisms across many sectors.

Moisture resistance is another key advantage. Nylon absorbs water and swells. Acetal absorbs far less, so machined dimensions stay consistent. This makes machining acetal ideal for parts that must hold tolerance in changing humidity. The same stability makes machining acetal repeatable across production batches.

• Low friction coefficient
• High stiffness and creep resistance
• Low moisture absorption
• Good wear resistance
• Excellent machinability
• Wide operating temperature range

Homopolymer vs Copolymer Acetal

Acetal comes in two main families: homopolymer and copolymer. Both machine well, but they differ in service behavior. The table below summarizes the difference.

Homopolymer offers higher strength and stiffness. Copolymer handles hot water and aggressive chemicals better. Both are available in unfilled, glass-filled, and oil-filled versions. Glass-filled POM increases stiffness but wears tools faster. Oil-filled POM reduces friction further for sliding applications.

Stock form matters during machining. Acetal bar, plate, and tube all cut well. Larger cross-sections carry more internal stress, so thick blanks should be stress-relieved before final cuts. This simple step prevents parts from moving after they leave the machine. It also improves the consistency of tight tolerances.

Food-grade POM meets FDA requirements for many contact applications. It resists cleaning agents and can be sterilized by common methods. When machining acetal for food processing equipment, confirm the correct grade and any needed certificates with your supplier.

Practical Tips for Machining Acetal

When machining acetal, sharp tools and clean cuts make the process forgiving. Still, a few rules improve results and protect the material.

1. Use sharp carbide tools with positive rake angles.
2. Keep feeds and speeds moderate to prevent melting at the cut zone.
3. Use coolant or air blast to control heat. Excessive heat causes expansion and poor tolerances.
4. Support thin sections during clamping to avoid distortion.
5. Deburr edges after machining. POM leaves sharp burrs that can crack.
6. Allow the part to stabilize before final inspection if tolerances are tight.

Thermal expansion deserves special attention. POM expands about 5 to 10 times more than metal. A part machined at 20 degrees C changes size if it operates at 60 degrees C. Designers should allow for this change or choose a glass-filled grade for hot environments.

Inspection conditions matter for precision work. Measure the part at the same temperature where it will be used. A difference of a few degrees changes a POM part by a few microns. Document the inspection temperature with the first article report. This practice avoids disputes and keeps tolerances meaningful.

Clamping pressure is another common source of error. Plastics are softer than metal. If you clamp too hard, the part deflects, and the machined dimension becomes wrong once released. Use low clamping force and support the part across its full area.

Surface finish is generally excellent. For sliding surfaces, a smooth finish lowers friction further and reduces wear. For gears, tooth profiles require careful tool path control. Experienced shops hold plus or minus 0.05 mm or better on POM parts. Our experience with machining acetal spans thousands of parts.

Tool geometry deserves attention too. Positive rake angles slice the material instead of pushing it. Two-flute end mills clear chips well in pockets and slots. For deep cavities, peck drilling and reduced step-overs keep the cut zone cool. These details separate a clean part from a melted one. When machining acetal, test the first piece before running the full batch. Small adjustments to speed pay off in surface quality.

Common Applications of Machined Acetal

Machining acetal serves nearly every industry. In mechanical design, it replaces metal parts where weight, noise, or corrosion is a problem.

• Gears and gear assemblies
• Bushings and bearing sleeves
• Rollers and guide rails
• Pump impellers and valve parts
• Food processing components
• Electrical insulators and spacers

In the food industry, POM parts contact ingredients without contaminating them. In automotive plants, acetal rollers and guides keep production lines moving. In medical devices, small POM components provide precise motion with low wear. Each application benefits from the same properties: low friction, stability, and machinability.

Acetal also works well for prototypes. Because it machines quickly and predicts accurately, designers validate gear ratios and linkage motion before committing to molded production. A machined POM prototype behaves close to the final part. This reduces development risk and shortens time to market. Many successful products start as machined acetal samples.

Frequently Asked Questions

Can you machine acetal to tight tolerances?

Yes. Skilled shops hold plus or minus 0.05 mm on most acetal parts. Because POM absorbs little moisture, dimensions remain stable after machining. Many customers prefer machining acetal when tolerances matter. Control heat and clamping during the process to protect accuracy.

Is acetal safe for food contact?

Food-grade acetal meets FDA requirements for many applications. It resists cleaning agents and does not release harmful compounds. Always confirm the grade and certification with your supplier.

Should I choose homopolymer or copolymer acetal?

Choose homopolymer for maximum strength and stiffness. Choose copolymer for hot water and chemical resistance. Your application and environment decide the best fit.

Conclusion

Acetal delivers low friction, high stiffness, and stable dimensions. Machining acetal is straightforward when you use sharp tools, control heat, and clamp gently. These practices produce reliable parts for gears, bushings, and precision mechanisms.

For reference, machining standards such as the ASTM material standards define the quality and tolerance requirements we follow.

CNX Precision has deep experience with POM machining under ISO 9001 quality systems. We help you select the right grade and deliver parts within tolerance. Send us your drawing and specifications for a prompt quote and DFM feedback.