PEEK is our bread and butter. We machine more PEEK than any other material — probably 60% of the parts that come through our shop. Over thousands of hours of production, we've learned what works, what doesn't, and what the generic feeds-and-speeds calculators get wrong.
Here's the practical machining knowledge that took us years to accumulate, distilled into one place.
Material prep matters
PEEK stock shapes carry residual stress from extrusion or molding. Skip this step and your tolerances will wander. Before any precision machining:
- Anneal the stock. Heat to 180-200°C, hold for 2-4 hours depending on thickness, slow cool. This relieves internal stress and stabilizes dimensions.
- Let it normalize. After annealing, let the material sit at room temperature for at least a few hours before machining.
- For tight-tolerance parts, rough machine, let the part stabilize overnight, then finish machine.
We've had parts that moved 0.05mm between roughing and finishing because the stress was still releasing. Annealing eliminates that.
Tooling
Carbide, uncoated, polished flutes. Coatings like TiAlN or AlTiN add friction and heat — counterproductive in PEEK. The edge needs to be sharp, not just new. A tool that's machined 50 aluminum parts might still cut PEEK, but the surface finish will suffer.
For roughing: 2-3 flute end mills with 30-35° helix angle. Chip evacuation matters more than edge count.
For finishing: single-flute (O-flute) end mills produce the cleanest surface. Higher feed per tooth = actual cutting, not rubbing.
For glass-filled or carbon-filled PEEK: diamond-coated or PCD tooling. The fillers are abrasive and will eat carbide quickly at production volumes.
Speeds and feeds (6mm carbide end mill)
Unfilled PEEK:
- Roughing: 200-350 m/min surface speed, 0.08-0.15 mm/tooth
- Finishing: 250-400 m/min, 0.05-0.10 mm/tooth
30% glass-filled PEEK:
- Roughing: 150-250 m/min, 0.05-0.10 mm/tooth
- Finishing: 200-300 m/min, 0.03-0.07 mm/tooth
Increase feed if you're seeing melting or smearing. The tool is rubbing — it needs more chip load to actually cut.
Depth of cut: 0.5-1.5mm roughing, 0.1-0.3mm finishing. PEEK's low thermal conductivity means heat stays in the cutting zone. Lighter cuts manage the heat better.
Coolant
Compressed air is usually enough for unfilled PEEK. Clear chips, cool the tool, no thermal shock.
Mist coolant helps for filled grades or deep pockets where chip evacuation is a problem. Flood coolant works but can cause thermal shock if the coolant temperature isn't controlled — sudden cooling creates surface micro-cracks in some PEEK grades.
The one thing to avoid: intermittent coolant. If coolant hits the tool on some passes but not others, the thermal cycling creates stress. Either run dry or run wet — not somewhere in between.
Workholding
PEEK is stiff and tough, so clamping pressure isn't as critical as with PTFE. But it does expand with heat, and the heat from machining can change clamping pressure mid-operation.
Vacuum fixturing is excellent for flat PEEK parts. For irregular shapes, soft jaws distribute pressure without point-loading. Avoid clamping thin sections directly — they'll deflect and spring back.
For thin-wall parts (under 2mm), support the wall from both sides if possible. If not, take very light finishing passes and let the part cool between passes.
Tolerances
±0.025mm is routine in PEEK with proper process control. We hit this daily. ±0.01mm is achievable but requires temperature control, sharp tools, and process discipline.
The biggest variable: thermal expansion. PEEK's CTE is roughly 47 μm/m·°C below Tg. A 100mm feature changes 0.047mm per 10°C. Measure at controlled temperature (20°C), not when the part is warm from machining.
Common problems and fixes
Exit burrs. Too much heat at the cutting edge. Reduce speed, increase feed, or switch to a sharper tool. A light chamfer pass before finishing catches most burrs.
Surface haze or cloudiness. The tool is dull or the feed is too low, causing rubbing instead of cutting. Replace the tool. Increase feed per tooth.
Dimensional variation across a batch. Temperature variation in the shop. Parts machined at 9am (cooler) are slightly larger than parts machined at 3pm (warmer shop). Climate control solves this. If you don't have it, rough all parts, then finish all parts at the same time of day.
Chatter on internal pockets. Chip packing. Use a two-flute tool with better chip clearance, increase air blast pressure, or reduce depth of cut.
PEEK is forgiving compared to PI or PAI — it's the easiest of the high-temperature plastics to machine well. But the margin between "good enough" and "excellent" is process control, not machine capability. Send us your drawing if you want a DFM review specific to your PEEK part.