Machining PEEK or PTFE is nothing like cutting aluminum. The heat doesn't leave with the chip — it stays in the part. The material moves while you're cutting it. And sometimes the part you measure at 2pm is a different size at 3pm because the shop warmed up.
We machine engineering plastics every day — PEEK, PEI, PI, PPS, PTFE, PFA, PCTFE, PAI, PVDF. The full range. Here's what I've learned about doing it well.
Why plastics cut so differently from metals
Three things make plastic machining its own discipline:
Heat stays in the part. Metals conduct heat away through the chip. Plastics don't. Your cutting zone gets hot and stays hot, which softens the material ahead of the tool, causing smearing, melting, or dimensional drift. Sharp tools and the right coolant matter ten times more than they do in metal.
The material moves. Plastics have 5-10x the thermal expansion of metals. Internal stresses from extrusion or molding relax as you remove material. A part fixtured tightly will warp when you unclamp it. Stress-relieved stock helps. Roughing passes followed by finish passes help more.
It's soft, not weak. PEEK at 260°C still carries structural load. PTFE can't hold a tight tolerance at room temperature because it creeps under its own weight. Every plastic has its quirks, and you need to know them.
What your machine needs
A standard 3-axis VMC is fine for most engineering plastic work. You don't need 20,000 RPM spindles — 8,000-12,000 is plenty for carbide tooling in the common plastic families. What you need more than speed is sharp tools and good chip evacuation.
Coolant: compressed air or mist for most plastics. Flood coolant is fine for PEEK and PEI but can cause thermal shock in PI and some filled grades. For PTFE, dry cutting works best — the chips are waxy and coolant makes a slurry.
Tools
Carbide, sharp. Not "reasonably sharp" — brand new sharp. The edge radius on a worn tool that still cuts aluminum fine will smear plastic.
For PEEK and PEI, uncoated polished carbide works best. Coatings add friction and heat. For glass-filled grades, diamond-coated or PCD tooling handles the abrasive fillers.
Two-flute end mills give better chip clearance than four-flute in plastics. For finishing, a single-flute or "O-flute" tool running at higher feed per tooth produces a cleaner surface.
Holding the part
This is where most first-time plastic jobs go wrong. Over-tighten a vise on PTFE and the part springs back 0.05mm when you release it. Under-clamp PEEK and it chatters because the material is stiff but not dense.
Vacuum fixturing works beautifully for flat plastic parts. Double-sided tape works for very thin sheets (under 3mm). For irregular shapes, soft jaws machined to match the part profile distribute clamping force.
Tolerances
±0.05mm is achievable in PEEK, PEI, PI, and PAI with careful process control. PTFE and softer fluoropolymers top out around ±0.05mm at best, and that takes experience. The limiting factor isn't the machine — it's thermal expansion. A 100mm PEEK part grows about 0.05mm per 1°C temperature change.
For tight plastic tolerances: climate-controlled inspection (20°C), sharp tools, light finish passes, and let the part thermally stabilize before final measurement.
Material cheat sheet
- PEEK: The easiest of the high-temp plastics to machine. Cuts clean, holds tolerance, predictable. Watch for burrs on exit.
- PEI (Ultem): Stiffer, more brittle. Chips at sharp corners if tools are dull. Lighter cuts than PEEK.
- PI (Vespel/DuPont): Expensive. Cuts beautifully but can micro-crack if thermal-shocked. Mist coolant only.
- PPS: Similar to PEEK but slightly more abrasive. Glass-filled grades eat tools — use diamond coating.
- PTFE: The wildcard. So soft it deforms under cutting pressure. Dead-sharp tools, light cuts, stress-relieved stock.
- PFA/PCTFE: Similar challenges to PTFE but slightly stiffer. Still creeps. Still needs sharp tools.
- PAI (Torlon): Cuts like a dream when tools are sharp. Brittle failure if you push too hard. Respect the feed rate.
- PVDF: Softer side. Machines OK but burrs are a constant battle. Light deburring pass helps.
Common problems
Burrs on PEEK. Too much heat. Reduce speed, increase feed, or switch to a sharper tool. A light chamfer pass at the end catches most exit burrs.
Melted surface on PTFE. Tool is rubbing instead of cutting. Increase feed per tooth. PTFE needs a proper chip load — light rubbing generates heat without removing material.
Dimensional drift. The part was measured warm. Let it sit at inspection temperature for 30 minutes minimum before measuring. For precision parts, rough, let stabilize, then finish.
Chatter on thin walls. Reduce stick-out, use a smaller step-over, or switch to a single-flute tool that applies less radial force. Sometimes changing the toolpath direction (conventional vs climb) fixes it.
Send us your drawing and material requirements if you want a DFM review specific to your plastic part. The right feed and speed for PEEK is not the same as for PTFE, and getting it wrong costs real money.