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Plastics Speeds and Feeds
Guide · Updated October 28, 2026
Most plastics machine easily, but they melt, spring back and grow with heat. The goal is a big chip that carries heat away, a very sharp tool, and holding the part gently enough that it does not distort.
Measure after it cools
Plastics move with temperature. Let the part settle, then check your mic against gage blocks before the final size. The free Gage Block Calculator builds the stack.
Step by step
- Know the material: Acetal (Delrin) and PEEK cut cleanly; nylon and UHMW are gummy; acrylic and polycarbonate chip or craze if heated.
- Use sharp, polished tools: One- or two-flute uncoated carbide or HSS with high rake and polished flutes clears chips best.
- Pick a starting SFM: Acetal and nylon: roughly 500–1,000 SFM. Acrylic: 400–800 SFM. UHMW tolerates high speeds if chip load is high.
- Feed hard enough: Chip load around 0.002–0.006 in per tooth for small cutters; too little feed melts the plastic.
- Cool with air: Air blast clears chips; mist or water-soluble coolant works on most plastics, but avoid coolants that stress-crack acrylic and polycarbonate.
- Hold gently: Light clamping and full support; plastics deflect and spring back after release.
- Finish last: Leave 0.010–0.020 in for a light finish pass after the part relaxes.
Worked example
Pocketing acetal with a 1/4 in 2-flute carbide end mill at 800 SFM: RPM = 3.82 × 800 ÷ 0.250 = 12,224 RPM. At 0.003 in per tooth, feed = 12,224 × 2 × 0.003 = 73.3 IPM. If your spindle tops out at 6,000 RPM, keep the same chip load: 6,000 × 2 × 0.003 = 36 IPM.
Size and temperature
Plastics expand roughly 5–10 times as much as steel per degree. A 2.000 in acetal part warmed 20 °F can grow about 0.002–0.003 in, so measure cool parts and keep the shop temperature in mind on tight fits.
Common problems
Melted, stringy chips mean too little feed or a dull tool. Burrs and fuzz mean a dull edge or too low a speed. Cracks or crazing in acrylic usually come from heat or an incompatible coolant.
Tips
- Compare: aluminum speeds and feeds.
- Basics: feeds and speeds basics.
- Edges: deburring and breaking edges.
FAQ
What end mill is best for plastic?
A sharp one- or two-flute uncoated carbide with polished flutes.
Why does my Delrin melt?
Feed is too low or the tool is dull; raise chip load.
Can you use coolant on acrylic?
Air or plain water is safest; some coolants cause crazing.
Why is my plastic part undersize later?
It was measured warm or released stress after unclamping.
Is PEEK hard to machine?
It cuts cleanly but is abrasive when filled; use carbide.
Related guides and calculators
- Feeds & Speeds Basics
- Milling Speeds and Feeds
- Aluminum Speeds and Feeds
- How to Deburr and Break Edges
- Turning Speeds and Feeds
- Titanium Speeds and Feeds
- All free machinist calculators
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