Capabilities · Materials
Delrin and acetal machining
Delrin and acetal machining is the CNC milling of polyoxymethylene, sold as Delrin, Acetron and Celcon, into precision components. BELL Machine Works machines Delrin and acetal to ±0.001" from our shop in Gilroy, California, and it is the material we run more than any other.
Why acetal is the default engineering plastic
Acetal machines better than any other thermoplastic. It shears cleanly instead of tearing, holds a sharp edge, takes threads without stripping, and produces a good finish straight off the cutter.
Dimensional stability. Acetal absorbs roughly 0.2% moisture, against 2% or more for nylon. It doesn't grow on the shelf, which is why it's specified for fixtures and gauges that have to stay correct.
Stiffness and fatigue resistance. High crystallinity gives acetal a modulus most plastics can't match, and it survives repeated flexing.
Low friction and good wear resistance, which puts it in guides, bearings, saddles and anything that slides.
Electrical insulation and chemical resistance to most solvents, fuels and neutral cleaning agents.
Where it fails: strong acids and oxidizers attack it, it isn't UV stable outdoors, and it will burn. It can't be solvent-bonded, so assemblies are mechanically fastened.
Homopolymer vs copolymer
Worth deciding before the drawing is finished, because it changes what we can hold.
Homopolymer (Delrin) is stiffer, stronger, and more resistant to creep and fatigue, with a higher crystalline melting point. The tradeoff is centerline porosity in larger-diameter rod, a real risk if your part's critical feature sits on the axis of the stock.
Copolymer (Acetron, Celcon) trades a little stiffness for better resistance to hot water and hydrolysis, better chemical resistance in alkaline environments, and no centerline porosity.
Most of what we run is Delrin 150, an unfilled medium-viscosity homopolymer, in white and black. If you haven't specified, tell us the service environment and we'll recommend.
What actually goes wrong
Acetal is forgiving relative to PTFE or UHMW, but three things still bite.
Thermal expansion is roughly ten times steel's. A part cut warm measures differently cold. On a ±0.001" tolerance, that isn't a rounding error.
Extruded stock carries internal stress. Machining releases it. A plate that was flat in the vise can bow on the bench an hour later, and nothing about the machining was wrong.
Heat has nowhere to go. Acetal doesn't conduct heat away from the cut. Let a dull tool rub and the surface gums, the finish goes, and the dimension goes with it.
Finish
Acetal shears more cleanly than any other engineering thermoplastic, which is exactly why a poor finish on it is a process failure rather than a material limitation. Let a tool go slightly past its best and the surface gums, hazes and picks up drag marks, and on acetal that is permanent.
We finish Delrin and acetal straight off the machine, with no polishing or secondary operation. Against a standard callout of 125 μin Ra, we measure 32 μin Ra on a profilometer.
Parameters we hold in acetal
| Standard tolerance | ±0.001" |
| Tightest held | ±0.0002" where the part calls for it, a tightest-held figure, not a standard |
| Grades | Delrin 150 homopolymer in white and black · FDA-compliant grades for fluid and food contact · copolymer on request |
| Governing difficulty | Thermal expansion ~10× steel · stress release from extruded stock · no heat conduction away from the cut |
Part classes we machine in acetal
Pipe, tube and cable clamping. Saddle-style clamps and inserts with contoured profiles that cradle cylindrical tubing, matching tube OD closely enough to distribute load and damp vibration without crushing the line, and holding that fit through thermal cycling against glycol coolants. Also cable saddles for routing and strain relief, and tube raceway brackets with structural ribs between saddles.
Assembly, trim and inspection fixtures. Complex curved geometry, counterbored holes, stepped surfaces and radiused contours, in a material that resists sterilisation chemistry and will not mark the product it holds.
Stacked sealing assemblies. Gasket plates with sealing grooves, perforated distribution plates that spread flow evenly across multiple outlets, and base manifolds with internal channel geometry, sealing on O-rings between layers.
Contour-matched, non-marring workholding. Mirrored left- and right-hand fixture pairs with cavities machined to match the profile of a finished assembly, holding a polished, cosmetically critical part securely without distorting the fixture under clamping load or scratching what it holds.
Thin-wall guide and funnel plates, where the part deflects as you cut it and relaxes again when the clamps come off.
Test fixture guides and nests that position units under test and align connectors through thousands of insertion cycles.
Precision spacers and stack-up hardware, to four inches in diameter and beyond.