Delrin CNC machining guide

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  Key takeaways
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  What Delrin actually is and how it cuts
• 3
  Heat, chip evacuation and the cutting parameters that work
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  Why the bore moves after you unclamp it
• 5
  Features Delrin handles well and features it does not
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  A shop sequence that keeps acetal on size
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  Moisture, creep and the tolerances you can hold
• 8
  Step by step: cutting a Delrin part to size
• 9
  Delrin vs. common alternatives for machined parts

Delrin is DuPont's brand name for homopolymer acetal (POM-H). The generic shop-floor equivalent is acetal, or POM-C when the copolymer grade is supplied. Both are semi-crystalline thermoplastics with a tight, ordered molecular structure. That structure is why the material feels stiff and slippery, keeps its shape under load, and machines to a clean finish rather than a gummy edge.

The crystalline phase gives acetal its useful properties: tensile strength around 65–70 MPa, good fatigue resistance, low friction, and very low water absorption compared with nylon. It also behaves differently from metal under a cutter. There is no long chip. Instead, the tool shears material away in a semi-continuous stream, and almost all the energy that does not go into the chip stays in the workpiece as heat.

POM is a poor conductor of heat. Heat generated at the cutting edge has nowhere to go, so the surface layer softens quickly. Once the surface passes roughly 160 °C, the material starts to smear instead of shear. You see it as a dull, torn finish, a built-up edge welded to the tool, and a dimension that no longer repeats.

That single fact drives most of the rules in this Delrin CNC machining guide. Every decision about tool geometry, spindle speed, feed rate and depth of cut is really a decision about how much heat you leave in the part.

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  POM-H (homopolymer)Higher stiffness and better creep resistance; slightly harder to machine without stress.
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  POM-C (copolymer)Better chemical resistance and easier on the tool; the usual choice for complex cavities.
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  Both are semi-crystallineSharp transitions on heating, no gradual softening like PVC or ABS.

Use sharp, polished carbide or high-speed steel tooling with a positive rake. A tool that has already been used on steel will feel blunt in acetal and will rub instead of cut. Rake angles of 10–20° and generous clearance let the chip clear without dragging on the finished wall. For deep pockets and small bores, two-flute cutters clear chips better than four-flute tools.

Running speeds and feeds higher than you would use on aluminum is normal here. A typical starting point for carbide end mills in POM is 200–400 m/min surface speed and 0.1–0.25 mm per tooth feed, adjusted by rigidity. The goal is a thick enough chip to carry heat away. Very light finishing passes at high speed are the fastest way to burnish and melt the surface.

Chip evacuation matters as much as the numbers. Acetal swarf is light and springy, and it packs into pockets and around the tool. Air blast is the first choice. Flood coolant works, but it can push chips back into the cut and, if left on the part, some coolants attack POM over time. Water-soluble coolant should be rinsed and the part dried before measurement.

Cooling the part is not the same as cooling the tool. A cold air gun aimed at the cut zone removes heat before it soaks into the workpiece. If you see strings or a fuzzy edge, the feed is too light or the tool is dull, not the coolant flow too low.

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  Surface speed200–400 m/min with carbide for roughing and finishing.
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  Feed per tooth0.1–0.25 mm; raise it until the chip is continuous, not powdery.
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  Depth of cutKeep radial engagement modest; shallow passes at high speed make heat, not parts.
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  Tool materialPolished carbide or HSS. Never a tool that has cut steel.

Acetal is viscoelastic. Under a vise or chuck it deforms, and the material surrounding a bore is compressed during the cut. When the clamp opens, the part springs back and the bore is no longer round or on size. The effect is worst on thin walls, long slots and parts held in a three-jaw chuck.

Stress relief also comes from the extrusion process. Extruded rod and plate carry residual stress from cooling. Machining removes material unevenly, and the remaining stress rebalances. A plate that was flat on the machine can bow overnight. A bore that measured Ø12.02 mm at 4 pm can read Ø11.96 mm the next morning.

The practical answer is to measure twice. Take dimensions when the part comes off the machine, then again after 24 hours in the same temperature and humidity. If the two readings differ by more than your tolerance, the process needs an anneal, not a tighter inspection plan.

For tight work, rough machine with 0.5 mm of stock left, anneal, then finish. Announcing the target tolerance before cutting is what separates a repeatable acetal job from one that needs three setups and a rework note.

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  Clamp lightlyUse soft jaws or a fixture that spreads load; avoid point contact.
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  Rough, stress, finishLeave 0.5 mm stock, anneal, then take the finishing pass.
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  Measure after 24 hoursCompare on-machine and post-relaxation numbers before you accept the setup.

Acetal is at its best in parts that need low friction and moderate loads: bushings, wear pads, gears, rollers, insulators, valve bodies, and sliding components that run dry. It also suits fluid-handling parts because it resists many solvents, fuels and weak acids. Machined manifolds, pump housings and impellers with modest pressure are common work.

Sharp internal corners are the first thing to soften. A cutter has a radius, and a square internal corner simply cannot be produced by milling. Add a corner radius of at least 0.5 mm, larger on deep pockets, or accept a drilled relief at the corner. Threads below M3 or 4-40 in acetal strip easily, so use a coarser pitch or a metal insert where the joint will be opened more than a few times.

Thin walls under 1 mm flex during cutting and never quite settle. Plan for 1.5 mm minimum for unsupported walls and more if the part sees load. Very long, unsupported bores tend to bell-mouth; keep the depth-to-diameter ratio under about 4:1 for a single pass, or drill from both ends.

Delrin is not a high-temperature material. Continuous service above about 80 °C causes creep, and the part will relax under load. It also expands noticeably with heat, so a part that must hold a clearance at 60 °C needs that clearance designed in, not machined in at 20 °C.

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  Good fitBushings, gears, rollers, insulators, dry-running slides, fluid parts.
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  Poor fitThin walls under 1 mm, sharp internal corners, fine threads, hot service.
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  Add radius0.5 mm minimum at internal corners; more in deep pockets.

The sequence below is the one we use for acetal parts with tight bores or flatness requirements. It is not the fastest route, but it is the one that holds a ±0.005 mm tolerance without chasing dimensions on the bench.

Order of operations matters more here than in aluminum. Facing both sides first gives you a stable reference. Drilling before profiling keeps the part rigid. Finishing after annealing lets the material settle before the final cut, which is the only point where the tolerance is actually created.

Inspection closes the loop. We check raw material certificates, monitor dimensions during the run, and inspect 100% of parts before shipment. Reports are available on request. If the drawing calls for a fit that depends on the part staying dry, note it, because the end user's humidity will move the part more than the machine ever will.

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  Face firstEstablish parallel faces before any profile cut.
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  Drill before profileKeep the blank rigid for as long as possible.
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  Anneal before finish140–150 °C, slow ramp, slow cool; then take the final pass.
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  Deburr by handA light scrape beats a powered brush, which rounds edges and leaves fuzz.

Acetal absorbs less water than nylon, but it is not inert. In humid air or in contact with water, POM picks up moisture and swells. The change is small, roughly 0.2–0.5% depending on grade and environment, but on a Ø50 mm bore that is 0.1–0.25 mm of movement. If your fit has 0.05 mm of clearance, the environment has already used it up.

Creep is the second slow effect. A part under constant load at room temperature will keep deforming for months, and the rate climbs quickly once service temperature passes 60–70 °C. Springs, snap fits and preloaded bearings in acetal lose their preload over time. Design the load path so the plastic is not the only thing holding the assembly together.

On a rigid setup, with sharp tooling and a stable room, we hold ±0.005 mm on acetal features and finish critical faces to Ra 0.2–0.8 μm. That is a process capability, not a promise. The realistic limit is set by the part geometry and the environment it will live in, not by the machine.

The honest engineering position is this: use acetal when low friction and moderate strength matter more than dimensional stability. If your tolerance is tight and your environment is wet or hot, either design more clearance or choose a material that does not move. Aluminum and PEEK both hold size better; they cost more and lose the dry-running wear behavior.

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  Humid serviceAdd clearance for 0.2–0.5% swell, or specify a low-absorption grade.
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  Hot serviceAbove 80 °C, expect creep. Change material rather than tighten the drawing.
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  Tight fitIf clearance is under 0.05 mm, acetal is probably the wrong choice.