Accurate PEEK CNC machining solutions

Unfilled PEEK melts around 343 °C. That number matters less at the spindle than at the tip of the tool. PEEK conducts heat poorly, so friction at the cutting edge has nowhere to go. The chip carries some away, the tool body takes the rest, and the workpiece keeps a share it cannot shed. A 0.5 mm depth of cut at the wrong feed can push the surface past its glass transition long before the bulk part feels warm.

Once the surface softens, three failures follow. The edge rubs instead of shearing, which smears material across the flank. Chips weld to the flute and recut the wall on the next pass. And the finished surface pulls away from the tool with a torn, cloudy look that no deburring operation will fix. All three show up as dimensional drift on the next measurement, not as an obvious crash.

Carbon-fibre and glass-filled grades behave differently again. The fibre raises stiffness and lowers elongation, so the material cuts cleaner but wears tools fast. Carbide edges dull in a predictable curve, and a dull edge in filled PEEK is a heat source. We track tool life by part count rather than by spindle hours for that reason.

Sharp, polished carbide is the baseline. Two or three flutes, high helix, and a positive rake angle that shears rather than presses. Uncoated carbide works for unfilled PEEK because there is no abrasive wear to fight. Filled grades want a diamond-like coating, and we budget for more frequent edge changes. HSS is not worth the setup time on a machine this size.

Feeds sit in a narrow band. Too slow and the edge rubs, generating heat with no chip to carry it away. Too fast and the tooth load climbs until the wall deflects. A workable starting point for unfilled PEEK is 200–400 m/min surface speed with 0.05–0.15 mm per tooth, then adjust from the chip shape. Long stringy chips mean the feed is too light.

Cooling is where most shops get it wrong. Flood coolant creates thermal shock on a material with low conductivity, and the part can craze. Compressed air or a cold-air gun clears chips and removes heat without a quench. For deep pockets we add a vacuum shoe. If a part does need coolant for chip evacuation, we use it warm and consistent, never a cold jet on a hot surface.

• 1
  Rough with air, finish dryAir blast for roughing, no liquid on the finish pass.
• 2
  Climb mill the wallsReduces rubbing and gives a cleaner finish on thin sections.
• 3
  Keep the tool engagedConstant radial engagement avoids dwell marks and local heat.
• 4
  Change edges earlyFilled grades dull fast; a worn edge is a heat source.

PEEK is soft relative to aluminium and it remembers force. A vise tightened like it is holding steel will bow a 3 mm wall, and the part springs back after unclamping. We use soft jaws machined to the blank profile, torque-limited clamping, and vacuum chucks on flat plates. For a thin disc or a long thin section, support underneath matters more than clamping pressure on top.

A 5-axis setup removes a large share of the risk. One fixturing holds the part through five faces, so we avoid re-datuming a material that moves when it is released. Features that would need a second op on a 3-axis machine can be cut while the blank is still supported. That is usually the difference between a PEEK part that holds ±0.005 mm and one that drifts out of tolerance after the last clamp comes off.

Not every part needs five axes. A simple bushing, a flat seal plate or a small manifold with one working face runs faster on a 3-axis machine, and the setup is cheaper. We route those jobs to 3-axis and save the 5-axis centers for contoured seals, impellers, and housings with features on several faces. The right answer is the one that holds tolerance without adding operations.

Extruded PEEK bar arrives with locked-in stress from the extrusion process. Machine it away and the part relieves that stress on its own schedule, which is usually the day after it leaves the shop. A stress-relief anneal before machining, and often a second one between roughing and finishing, removes most of that movement. Skipping it shows up as a part that measures correctly on the bench and fails a day later.

PEEK absorbs a small amount of moisture, and the amount changes with the environment. For close-tolerance work we condition the blank in a controlled environment before the finish pass so the part and the inspection room agree. A part measured wet and shipped dry is a tolerance claim waiting to happen.

Wall thickness sets the ceiling on what is achievable. A 1 mm wall in unfilled PEEK can hold ±0.02 mm across a short span with careful fixturing. A 5 mm section on the same part holds ±0.005 mm without drama. When a drawing calls for tight tolerance on a thin wall, the honest answer is often to thicken the wall or accept a looser number, and we will say so at the DFM stage.