CNC Machining Delrin Guide

• 1
  Why Delrin moves during and after cutting
• 2
  Tool geometry and edge preparation for acetal
• 3
  Speeds, feeds and depth of cut that work
• 4
  Warpage, stress relief and fixturing
• 5
  Where Delrin fits and where it does not
• 6
  Acetal grades and alternatives compared

Delrin is a homopolymer acetal. Its chain structure is highly crystalline, roughly 75–85% in the molded or extruded bar, which is what gives it stiffness, fatigue life and low friction. That same crystallinity is the source of most machining problems. The material has a linear thermal expansion coefficient around 100–120 × 10⁻⁶ /°C, several times that of aluminum and nearly ten times that of steel. A 100 mm Delrin block that warms 10 °C during roughing grows about 0.1 mm before the cutter touches it.

Heat leaves the cut in two places: the chip and the part. Acetal has poor thermal conductivity, so the part keeps a large share of it. The surface layer expands, the tool pushes into softer, hotter material, and the finished wall cools back to a different size than the one the control thought it was cutting. That is why a Delrin part can pass inspection warm and fail it cold the next morning.

The second mechanism is slower. POM absorbs moisture from air, on the order of 0.2–0.3% at 50% relative humidity for the homopolymer. Absorption takes days to weeks to equilibrate in a thick section, so a part machined from dry stock will creep slightly larger as it takes on water. For fits tighter than ±0.05 mm, this is often the dominant error, not the machine.

Both effects are predictable, and both are manageable. The rest of this guide covers the tool geometry, parameters, fixturing and stress relief that keep them inside tolerance.

Delrin is soft enough to cut at high surface speed but abrasive enough to dull an edge quickly, especially filled or regrind-containing stock. Sharp, polished carbide is the default. Uncoated micro-grain carbide with a mirror-polished rake face works well; titanium nitride or alumina coatings help on long runs, mostly by reducing friction and chip welding rather than by adding hardness.

Rake angle matters more here than in aluminum. A positive rake of 10–20° lowers cutting force and lets the chip curl away instead of rubbing. Zero or negative rake smears the material, raises temperature at the edge and produces a rough, gummy wall. Relief angle should be generous, 10–15°, so the flank does not drag on the springy material.

Use two or three flutes for most work. Single-flute cutters clear chips best and are the right answer for deep pockets and small diameters, but they deflect and limit feed rate. Four flutes and above leave too little chip room in acetal; the flutes pack, friction climbs, and the part starts to melt at the cutter. For finishing passes, a larger nose radius spreads the load and improves surface finish, but it also increases radial force on thin walls.

Keep the edge keen. A dull tool on Delrin does not simply cut worse; it generates heat, and heat is what causes the dimensional drift described above. Change tools on a schedule, not on failure.

• 1
  Positive rake 10–20°Lowers force and stops smearing.
• 2
  2–3 flutesEnough chip room without losing rigidity.
• 3
  Polished carbideCoating is optional, sharpness is not.
• 4
  Large nose radius for finishingBetter finish, higher wall force.

Surface speed for acetal typically runs 150–400 m/min with carbide. That is fast enough to throw a clean chip and slow enough to limit frictional heating. Spindle speed is then set by diameter: a Ø6 mm cutter at 300 m/min is roughly 16,000 rpm, which many machines cannot reach, so in practice small tools run at the top of the spindle range and feed per tooth is adjusted instead.

Feed per tooth is the parameter that decides whether you get a chip or a smear. Aim for 0.05–0.15 mm per tooth on a Ø6–12 mm cutter. Below about 0.03 mm per tooth the edge rubs, heat builds and the finish degrades. Feed rate and spindle speed should be raised or lowered together, keeping the chip load in range, rather than reducing feed alone to chase a better finish.

Radial depth of cut of 25–50% of tool diameter and axial depth of 0.5–2 × diameter suit roughing in most setups. Delrin is forgiving on depth; it is not forgiving on dwell. Any pause with the tool in contact, including a hesitation in the control or a slow axis reversal, transfers heat into one spot and leaves a mark that cannot be polished out.

Cooling is a choice, not a default. Dry machining with strong air blast is usually best because it keeps chips clear and avoids the thermal shock of intermittent flood coolant. Compressed air at 4–6 bar aimed at the cut works well. Flood coolant is acceptable on deep pockets where chip evacuation is otherwise impossible, but expect a wet, slightly different surface and dry the part before measuring.

Extruded and molded acetal bar carries internal stress from the manufacturing process. Machining removes material unevenly, and the remaining stress redistributes. A long slot milled down the center of a bar will bow toward the cut side as the balance changes. This is not a tool problem and no parameter change fixes it. The answer is stock preparation.

For parts with tight flatness or parallelism, rough machine with 0.5–1.0 mm of stock left, then anneal. A typical cycle for POM is a slow ramp to 140–150 °C, hold for one hour per 25 mm of section thickness, then cool at no more than 20 °C per hour. Slow cooling is the part people skip, and it is the part that matters; fast cooling re-introduces the stress you just removed. Finish machine after the part has cooled to room temperature.

Fixturing should hold the part without squeezing it. Acetal deflects under clamp pressure and springs back after the vise opens, so a part machined under heavy clamping can be out of tolerance with no visible cause. Use soft jaws machined to the part profile, apply minimum pressure, and support thin floors from below. Vacuum fixturing works well for flat plates.

When walls are thinner than about 2 mm, reduce radial engagement and increase the number of passes. The material has low stiffness, so cutting force deflects the wall away from the cutter; the tool then cuts less than programmed and the wall finishes oversize. A spring pass at full depth with a light radial step corrects most of it.

Delrin earns its place in parts that slide, roll or index: gears, cams, bushings, wear pads, conveyor components, pump internals, valve bodies and electrical insulators. Its fatigue resistance under repeated load is better than most unfilled thermoplastics, and its friction against steel is low and consistent without lubrication. It also machines roughly three to five times faster than PEEK or polyimide, which matters when you are making 500 parts.

It is a poor choice when the service temperature is high. Continuous use above about 90 °C is not realistic for the homopolymer, and short excursions above 120 °C will relax molded-in stress and change dimensions. PEEK, PEI or PPSU are the alternatives. Steam and strong acids also rule it out; acetal hydrolyzes in hot water and is attacked by strong mineral acids.

UV exposure is a slow problem. Unstabilized acetal chalks and embrittles outdoors. If the part sees sunlight, specify a UV-stabilized grade, or accept a shorter service life. For food-contact and medical work, grade selection is separate from machining: the same geometry in a compliant grade may need different stress relief, and documentation has to follow the resin lot.

Nothing in this list is exotic. The design decisions that matter are wall thickness, temperature, moisture and whether the part needs to hold a fit over years or only over a few cycles.