Guides / Process

Machining Tolerances on Delrin and POM: What Is Realistic

September 16, 2026

Acetal machines beautifully. Chips break, surfaces come out clean and tool life is long. The hard part is not cutting it. The hard part is that the part you measured in the machine is not the same size an hour later, or in a different room, or after a week on a shelf in a humid plant.

Three effects cause almost all of it.

Thermal expansion

POM expands roughly 110 micrometres per metre per kelvin. Steel sits near 12. So acetal moves about nine or ten times as much for the same temperature change.

Work it through on a real part. A 100 mm long plate, measured at 20 degrees C and again at 28 degrees C, grows about 0.088 mm. That is nearly two full steps of a plus or minus 0.05 mm tolerance, from nothing but the room being warm.

This is why acetal drawings should carry an inspection temperature. Twenty degrees C is the usual choice. Without one, you and the shop can both measure correctly and still disagree.

Moisture

Acetal takes up roughly 0.2 percent water in 24 hours of immersion and something under one percent at saturation. That is far less than nylon, which is one of the reasons POM gets picked in the first place. It is not zero.

A part machined dry and then put into service in water grows slightly. On a bushing with a 20 mm bore, that is small but measurable, and it is worth knowing if the bore is a running fit rather than a clearance hole.

Internal stress

Extruded rod and plate carry stress from how they cooled. Machining releases it unevenly. Take 8 mm off one face of a 25 mm plate and the plate bows, sometimes while it is still clamped, more often an hour after it comes off the machine.

Stress is the effect that catches people out, because it appears after inspection passes.

What shops actually hold

These are typical ranges from machining acetal in a shop with a stable temperature. They are not promises, and the shop quoting your part confirms them against your geometry.

Feature	Routine	Achievable with care
General dimensions under 100 mm	plus or minus 0.05 mm	plus or minus 0.025 mm
Turned outside diameter, under 50 mm	plus or minus 0.05 mm	plus or minus 0.02 mm
Bore diameter, under 25 mm	plus or minus 0.05 mm	plus or minus 0.02 mm
Flatness on a 100 mm plate	0.1 to 0.2 mm	0.05 mm after stress relief
Length over 300 mm	plus or minus 0.15 mm	Depends on inspection temperature

Anything tighter than plus or minus 0.02 mm in acetal is a conversation, not a drawing note. It can be done on a small feature, on a part that is allowed to settle, in a temperature controlled room. It cannot be done on every dimension of a large plate at any price.

When annealing earns its cost

A stress relief cycle means holding the part at an elevated temperature for a period that scales with wall thickness, then cooling it slowly. Shops run it in an oven or in a heated oil or salt bath. It costs a day or more of lead time and some oven time.

It is worth it when:

• The part is thick and a lot of material comes off one side
• Flatness or parallelism is tight over a long face
• The part is machined from plate rather than rod
• The part will see a temperature cycle in service and must not move

It is usually not worth it for a small turned bushing out of rod, where the geometry is symmetric and little stress is released.

The sequence that works is rough, anneal, finish. Annealing after finishing moves the part you just finished.

How to tolerance the drawing

The most useful thing you can do is tolerance selectively. On a typical acetal part, two or three dimensions matter and the rest do not. If the whole drawing carries a tight general tolerance, the shop has to treat every dimension as critical, and you pay for inspection on features nobody cares about.

Practical notes to put on an acetal drawing:

• Inspection temperature, for example 20 degrees C
• Which dimensions are critical, with everything else on a loose general tolerance
• Whether the part runs wet, so the shop knows the bore may grow
• Edge break, because POM holds a fine burr on sharp corners
• The grade, and whether substitution is allowed

What to expect in a quote

A shop reading a well toleranced acetal drawing will usually quote one of three routes. Turning from rod for round parts, milling from plate for flat parts, or roughing, annealing and finishing when the tolerances demand it. The third route costs more and takes longer, and a good quote will say which one it assumed.

If you get quotes that differ by a lot, this is often the reason. One shop assumed a stress relief cycle and another did not.

Frequently asked questions

Keep reading

• Deburring and Edge Finishing Acetal Without Leaving Fuzz
• Cutting Delrin Sheet to Blanks: Waterjet, Router or Saw