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PEEK machining processes
How PEEK is milled, turned, drilled and threaded, which tools and coolant suit each grade, where annealing goes in the sequence and what tolerance is realistic. Typical practice in shops that cut PEEK weekly, so you can read a quote and write a drawing that matches it.
Why PEEK needs its own process
PEEK is not hard to cut. It is easy to cut badly. Four material properties decide the setup more than horsepower does. Values are typical published figures for unfilled PEEK, with every grade on the data page.
- Low conductivityAbout 0.25 W/m·K unfilled, roughly one five hundredth of aluminium. The heat of the cut stays at the cut, so a worn tool or a packed chip shows up as a glazed surface and a part that grew while it was cut.
- High expansionAbout 45 to 50 µm/m·K unfilled below the glass transition, four times steel. A 100 mm feature grows about 0.06 mm between 20 and 33 °C.
- Low stiffnessA modulus of 3.5 to 4 GPa unfilled, about one fiftieth of steel. Clamping pressure that would not mark aluminium squeezes a thin PEEK wall out of round.
- Locked in stressExtruded and moulded stock cools from the outside in. Removing material unevenly releases that stress and the part moves, sometimes hours later.
Process against part feature
Which operation makes which PEEK feature, the tolerance shops typically quote and what PEEK changes in the setup. Typical values, never a promise. The shop quoting your geometry confirms what it can hold.
| Process | Typical parts | Typical tolerance | What PEEK changes |
|---|---|---|---|
| CNC milling, 3 axis | Manifolds, wafer handling combs, insulator plates, fixture blocks | ±0.05 mm general, ±0.025 mm on small annealed features | Two or three flute cutters so the chip has room. Light finishing passes, support under thin sections. |
| CNC milling, 4 and 5 axis | Valve bodies, pump housings, complex medical device components | As above | Fewer setups means fewer clamping cycles on a part that yields under the clamp. |
| CNC turning | Bushings, seal rings, thrust washers, valve seats | ±0.05 mm on diameters, roundness ±0.03 mm after stress relief on thin walls | Rod stock follows the part axis. Light chuck pressure or collets, and a finishing cut after the part has rested. |
| Swiss type turning | Small pins, connector bodies, instrument and medical device parts | Short features slightly under ±0.025 mm in a temperature controlled shop | Guide bushing supports long slender parts. Small diameters magnify any stress in the bar. |
| Drilling and boring | Fluid passages, bolt holes, precision bores | Bores ±0.025 to ±0.05 mm, roughed and finished | Slow helix drills, peck every one to two diameters. Holes deeper than about 20 mm are drilled then bored. |
| Threading | Fittings, mounting threads | Standard thread classes, slightly generous pitch diameter | Cut the thread with a polished two flute tap or thread mill. Metal inserts for repeated assembly. |
| Finishing and polishing | Seal faces, bearing surfaces, optical or cosmetic faces | Ra 0.8 µm finishing pass, Ra 0.4 µm with polished tools | Sharp tool and light depth. Polishing compounds and cleaning have to suit the end use. |
Tool choice and coolant
The grade decides the tool. The depth of the cut decides the coolant. Full detail in PEEK tooling, speeds and feeds.
| Operation | Starting cutting data |
|---|---|
| Turning, unfilled | 200 to 500 m/min, feed 0.05 to 0.25 mm/rev |
| Turning, filled | Lower end of that range. Tool wear sets the limit |
| Milling, unfilled | 200 to 400 m/min, 0.05 to 0.15 mm per tooth |
| Drilling | 50 to 150 m/min, peck at 1 to 2 diameters |
| Finishing pass | Light depth, sharp tool, air or coolant on the cut |
A starting point, not a recipe. A clean chip in its original colour is right. A brown or gummy chip means heat.
- Unfilled and implant gradeSharp uncoated fine grain carbide, positive rake typically 5 to 15 degrees, polished flute. Tools kept for plastics, never a tool that has been in steel.
- GF30Diamond coated carbide or PCD. Glass fibre wears plain carbide quickly and leaves a fuzzy edge once the tool is round.
- CF30Diamond coated carbide or PCD. Carbon fibre is abrasive and the dust is fine and electrically conductive, so extraction and machine protection matter.
- Bearing gradeDiamond coated or PCD for runs. Carbon fibre with PTFE and graphite: abrasive like CF30, but the lubricants make the chip softer and the surface less prone to glazing.
- CoolantAir blast for light cuts and open profiles. Water soluble coolant, flooded, for drilling, deep pockets and long finishing passes. For medical and semiconductor parts, agree the coolant and the cleaning step on the RFQ.
Annealing before and after machining
Annealing takes the part above the glass transition of 143 °C, holds it while the stress relaxes and cools it slowly. Typical schedules from stock suppliers ramp slowly to about 200 °C, hold roughly two hours per 25 mm of thickness and cool at 20 to 25 °C per hour. The melting point of 343 °C is far above the soak, so the part keeps its shape. Estimate a cycle with the annealing calculator.
| Stage | What happens | Why it is there |
|---|---|---|
| Before machining: stock | Rod and plate delivered stress relieved by the stock supplier | The cheapest stress relief there is. Ask for it with the material certificate. |
| Between roughing and finishing | Rough with finishing stock left on critical features, anneal, then finish | Whatever the roughing released is machined away. This is the step that saves thin walls and tight bores. |
| After machining | A final cycle on the finished part, then a check of critical features | For parts that will run hot or be thermally cycled, so shrinkage and crystallinity change happen in the oven rather than in service. Allow for movement in the tolerance. |
The cycle adds oven hours, a second setup and a day or two of lead time. Shops without an oven send parts out. Flag thin sections on the RFQ so the cycle is in the quote rather than in a surprise. More in annealing PEEK parts.
Tolerance and stress relief
Values shops quote without a long conversation, on parts made from stress relieved stock with an annealing step where the geometry needs one. Tighter is possible on some features and not on others, and the difference is geometry. Why in PEEK machining tolerances.
Measure after the part has rested and cooled, with light or fixed force. A ratchet micrometer reads PEEK small because the material yields.
| Feature | Typical on PEEK |
|---|---|
| General machined features | ±0.05 mm |
| Small annealed features, carbon filled stock | ±0.025 mm |
| Very short features, temperature controlled shop | A little under ±0.025 mm |
| Bore roundness, thin walled part, after stress relief | ±0.03 mm |
| Flatness, thin plate part | 0.1 mm over 100 mm as a starting point |
| Outside profiles that touch nothing | ±0.1 mm |
Glass and carbon filled grades in the shop
Fillers buy stiffness and stability and cost tool life. Typical published values. Each grade has its own page: GF30, CF30, bearing grade.
| Grade | Expansion µm/m·K | Modulus GPa | Tolerance quoted | What changes in the shop |
|---|---|---|---|---|
| Unfilled | 45 - 50 | 3.5 - 4.0 | ±0.05 | Machines cleanly with sharp carbide. Moves most with temperature and stress, so annealing and inspection temperature matter most here. |
| GF30 | 20 - 24 | 9 - 11 | ±0.05 | Abrasive. Diamond tooling, a tooling line in the quote, and edges checked for exposed fibre. |
| CF30 | 14 - 18 | 12 - 14 | ±0.025 | The most stable grade to hold size on. Abrasive, conductive dust. Often the answer when unfilled cannot hold the tolerance. |
| Bearing grade | 18 - 24 | 7 - 9 | ±0.05 | Carbon fibre, PTFE and graphite. Diamond tooling. Keep the wear surface as machined unless the drawing says otherwise. |
In extruded filled rod and plate the fibres lie mostly along the extrusion direction, so stiffness and shrinkage differ along and across the stock. Orient critical features with that in mind, or ask the shop how it will cut the blank. Implant grade is unfilled PEEK sold to ASTM F2026 with documented testing. It machines like unfilled, with tighter control of tooling, coolant, cleaning and lot traceability.
Design for manufacture
Each line removes a decision a shop would otherwise make for you. The RFQ checklist turns them into the ten lines a PEEK request needs.
- Name the grade and the stock form, and ask for stress relieved stock with the certificate
- Mark thin walls and deep pockets so the shop prices the annealing cycle in
- Tolerance only what fits something. ±0.1 mm on profiles that touch nothing
- Write the service temperature next to any fit that must work hot
- State the inspection temperature for anything tighter than ±0.05 mm
- Radius internal corners. A sharp corner is a stress raiser and a slow toolpath
- Keep walls even. Use rod rather than plate for round parts
- Specify metal inserts where a thread is assembled repeatedly
Questions about PEEK processes
More in the full question list.
Does every PEEK part need annealing?
No. A chunky bushing with open tolerances usually does not. Thin walls under about 3 mm, deep pockets, long slender parts, bores held tighter than ±0.05 mm and parts that run continuously above about 150 °C usually do. Stress relieved stock plus a rough, anneal, finish sequence covers most of those.
Why do glass and carbon filled PEEK need different tools?
Glass and carbon fibre are abrasive. They wear a plain carbide edge round within a short run, and a round edge rubs, heats the cut and pulls fibres instead of cutting them. Diamond coated or PCD tools hold the edge, which is why shops quote filled grades with a tooling line that unfilled PEEK does not need.
Can PEEK be machined dry?
Light cuts and open profiles run on air blast. Drilling, deep pockets and long finishing passes need water soluble coolant, because PEEK conducts heat about one five hundredth as well as aluminium and the heat stays at the cut. Dry cutting to protect a cosmetic surface is what produces a glazed, stressed surface.
Should I specify cut threads or a metal insert?
Cut threads are normal practice in PEEK and survive assembly. If the joint is taken apart and reassembled many times, or carries a high clamp load, specify a metal insert. Forming taps are not used on PEEK the way they are on ductile metals.
Have a PEEK part to quote?
We are a matching service, not a machine shop. Send the drawing and we match it with up to 3 shops that cut PEEK, with a reply within 24 business hours.