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Delrin and Acetal Machining Processes, from Rod to Finished Part
Acetal is one of the easiest plastics to cut. Holding size on it is another matter. This is how shops that machine POM every week route a part across the lathe, the mill and the annealing oven.
Process, typical parts and what to expect
Typical ranges for unfilled acetal, not promises. Part size, wall thickness and the temperature at inspection move them. The shop that quotes you confirms what it can hold on your geometry.
| Process | Typical parts | Typical tolerance | Typical finish |
|---|---|---|---|
| CNC turning | Bushings, rollers, pulleys, valve seats, spacers | ±0.05 mm, ±0.025 mm on small diameters | Ra 0.8 to 1.6 µm |
| Swiss-type turning | Small pins, shafts and insulators under about 32 mm | ±0.025 mm | Ra 0.4 to 1.6 µm |
| 3-axis and 5-axis milling | Manifold blocks, housings, cams, wear pads, gears | ±0.05 mm | Ra 0.8 to 1.6 µm |
| Drilling and reaming | Bearing bores, dowel holes, fluid passages | H8 to H9 on reamed holes | Ra 0.8 to 1.6 µm |
| Threading | Tapped holes, turned or milled threads, insert bosses | Class 6H / 2B | As machined |
| CNC routing of sheet | Guide rails, wear strips, star wheels, plates | ±0.1 mm on profile | Ra 1.6 to 3.2 µm |
| Gear hobbing or milling | Spur and helical gears, sprockets | Gear quality per drawing, ISO 1328 or AGMA | As machined |
How each step handles acetal
What shops that cut POM regularly do differently from a shop that mostly cuts metal.
- TurningSharp, polished carbide or HSS inserts with a positive rake and a small nose radius. Typical speeds run 200 to 500 m/min, with light finishing cuts so the part does not push away from the tool. Speeds, feeds and tooling
- Swiss-type turningThe guide bushing supports slender parts close to the cut, so long pins and small insulators stay straight. Bar feeding from rod suits runs of hundreds or thousands.
- MillingTwo or three flute high-helix end mills with polished flutes clear chips before they re-weld. Shops avoid recutting chips, which leaves a smeared, fuzzy surface.
- Drilling and reamingDrills with modified points reduce grabbing. Deep holes are pecked to clear chips and limit heat. Holes often close up slightly after drilling, so critical bores are bored or reamed to size.
- ThreadingAcetal threads cut cleanly but wear with repeated assembly. Shops cut threads with single-point or thread mills and recommend inserts for frequent reassembly. Threads in Delrin
- WorkholdingPOM springs under clamping. Soft jaws, vacuum plates and light clamping force keep thin parts round and flat after release.
- AnnealingA controlled stress-relief cycle between roughing and finishing for thick or tight-tolerance parts. It adds a day or more to lead time. When annealing is worth it
- DeburringAcetal holds a fine, flexible burr on sharp edges. Hand deburring with a blade or a light edge break specified on the drawing is the usual answer.
Background reading: polyoxymethylene on Wikipedia.
Which route for which feature
If your drawing has one of these, expect the quote to reflect the route on the right. It explains most of the price difference between two similar-looking parts.
| Feature on the drawing | Usual route |
|---|---|
| Wall under 1 mm | Light finishing cuts with support. Thin acetal walls flex under the tool and spring back after. |
| Flatness under 0.05 mm on plate | Stress-relieved stock, roughing, rest or anneal, then finishing from both faces. |
| Bore at ±0.025 mm | Bore or ream after the part settles, inspect at a stated temperature. |
| Many small threads | Cut threads for low-cycle use; threaded metal inserts where the thread is reassembled often. |
| Long slender shaft | Swiss-type turning, or a tailstock and steady rest on a conventional lathe. |
| Large flat part from sheet | Routing from stress-relieved plate, with vacuum or tab workholding. |
Design notes that save machine time
Most acetal quotes are decided by setups, workholding and inspection, not cutting time. These drawing choices change all three.
- Tolerance by feature±0.1 mm on general dimensions and ±0.05 mm or tighter only on fits. A blanket tight tolerance forces annealing and temperature-controlled inspection on the whole part.
- Inspection temperatureFor tolerances under ±0.05 mm, state the temperature the part is measured at, usually 20°C or 23°C.
- Wall thicknessKeep walls at 1.5 mm or more where possible. Thinner walls need support fixtures and slower finishing.
- Edge breaksSpecify a light chamfer or edge break. It removes the burr question and the hand time that comes with it.
- Surface finishAs-machined acetal is already around Ra 0.8 to 1.6 µm. Call out a finer finish only on sealing or sliding faces. Surface finish on acetal
Questions about machining acetal
Do shops use coolant when machining Delrin?
Most cut acetal dry or with compressed air, which keeps parts clean and avoids absorbing anything. Water-soluble coolant is used on deep holes and long runs to control heat. Flood coolant on acetal is less about the tool and more about keeping the part from warming and growing.
Why do machined POM parts change size after machining?
Two reasons: internal stress in the stock is released as material is removed, and acetal expands about 110 µm per metre per kelvin. Shops rough, let the part rest or anneal it, finish, then inspect at a stated temperature.
Is annealing needed for every Delrin part?
No. Small parts with ordinary tolerances usually do not need it. Thick parts, flat plates and parts with tolerances under about ±0.05 mm benefit from a stress-relief cycle between roughing and finishing.
Can Delrin be laser marked or engraved?
Engraving on a mill works on any grade. Laser marking natural acetal gives weak contrast; laser-markable grades or black stock give a cleaner mark. State the marking method and content on the drawing.