Acetal CNC Machining Guide for Engineers

Acetal is a semi-crystalline thermoplastic built from polyoxymethylene, usually shortened to POM. In the machine shop the two grades you will see quoted are POM-C, a copolymer, and POM-H, a homopolymer often sold under the Delrin name. Both cut well. They do not behave the same after the part leaves the machine.

POM-H has the higher crystallinity, so it brings slightly better stiffness, tensile strength, and creep resistance. It also machines to a harder, glossier surface. POM-C has a wider processing window, among others

The grade matters most when a part sees load over time. A bushing that holds a preload for months, a latch that stays engaged, a gear tooth running warm: those favor POM-H. Prototype brackets, housings, and parts with thick sections or tight internal corners are easier to source and machine in POM-C, and the cost per kilogram is usually lower.

Neither grade is a metal substitute. If the part needs to carry a sharp thread under load, take impact without cracking, or hold a thin wall below 1 mm, a filled grade, PA, or PEEK is worth a second look before you commit. For typical acetal CNC machining work, the practical ceiling on unsupported walls sits near 1.5 mm.

Acetal machines like free-cutting brass in some ways and like a soft wax in others. Chips break cleanly at moderate feeds, and surface finish straight off the tool is often good enough to skip polishing. The trap is heat. POM has a low thermal conductivity, so the heat stays at the edge instead of flowing into the chip.

That heat shows up as a gummy, smeared surface, a burr that will not break off, or a whitish stress ring around a drilled hole. If you see any of these, the tool is dull or the spindle is running too slow for the feed. Sharp, uncoated carbide with polished flutes is the standard choice. Two-flute end mills clear chips well in pockets.

Coolant is optional, but air blast is not. Compressed air moves chips out and keeps the cutter from re-cutting them. Flood coolant works too, and helps on deep pockets. What you want to avoid is running dry in a closed pocket with no evacuation at all.

A tight tolerance on a plastic part is a statement about the environment as much as the machine. POM moves with temperature and moisture. A bore measured at 20 °C will not measure the same at 40 °C, and the shift is larger than the tolerance band on small features. Design for the service condition, then tell us what that condition is.

For most acetal parts, ±0.05 mm on a mating diameter is routine and ±0.005 mm is achievable on a controlled feature with the right setup and inspection plan. The second number only means something if the drawing names a temperature and the part is measured there. We inspect 100% before shipment and can supply reports on request.

Wall thickness drives distortion more than tolerance does. Uneven walls cool and release stress unevenly, so a thick hub next to a thin web will move after machining. Where the design allows it, keep sections uniform and add a radius instead of a sharp internal corner.

POM is soft enough that a vise will leave marks and stiff enough that over-clamping will bow a thin part. Soft jaws machined to the part profile solve most of this. For thin plates and long parts, vacuum fixturing or a sacrificial backing plate holds the part flat without crushing it.

Stress relief matters on parts with a lot of material removed. A roughing pass, an anneal, then a finishing pass is the usual sequence for a tight flatness callout. Annealing also helps parts that will see elevated service temperature, because it removes the internal stress that would otherwise drive creep.

Some features are simply hard in plastic. Threads below M3 strip easily unless they are formed rather than cut. Sharp corners concentrate stress and crack. Deep, narrow slots deflect the cutter. A 5-axis setup helps when a part has features on several faces, because fewer setups mean fewer chances to lose position. We run 16 simultaneous 5-axis centers alongside 12 four-axis mills and 27 three-axis machines, so the setup can be matched to the geometry rather than forced.