CNC Material Polyetheretherketone PEEK: How It Machines and Where It Fits

• 1
  Why PEEK Machines Differently From POM and PA
• 2
  Choose the Grade Before You Choose the Toolpath
• 3
  Annealing, Stress Relief, and the Sequence That Holds Size
• 4
  Tooling, Feeds, and Coolant for PEEK on a CNC
• 5
  Where PEEK Makes Sense and Where It Does Not
• 6
  Measuring and Inspecting Machined PEEK Parts
• 7
  PEEK Grade Selection at a Glance
• 8
  PEEK Compared With Other CNC Plastics and Metals

PEEK is a semi-crystalline aromatic polymer. Its melting point sits near 343 °C and its glass transition is around 143 °C, so a chip that gets hot does not soften the way a POM chip does. That sounds helpful, and partly it is. The same stiffness that holds a part at 250 °C also means the cutting edge has to shear a hard, abrasive material at every pass.

The practical consequence is heat. Because PEEK conducts heat poorly, the energy from the cut stays at the tool tip instead of leaving with the chip. Tool edges run hot even at moderate surface speed. If you keep the same feeds and speeds you would use on unfilled POM, you will see edge rounding, a shiny rubbed surface, and dimensional drift as the part cools.

Unfilled PEEK is the easier grade. Add 30% carbon fiber or glass fiber and the material becomes noticeably abrasive. Carbide will still cut it, but tool life drops sharply and you should plan on more frequent edge changes. The fiber also changes chip formation: instead of a continuous string, you get a powder and short chips that do not clear themselves.

There is a second difference that catches people late in the job. PEEK absorbs a small amount of moisture from air, and it also holds internal stress from the extrusion or injection process. Machine a thin wall from a stressed blank and the wall moves after the fixture releases it. The fix is not a tighter tolerance. It is the right blank, the right annealing cycle, and a rough-then-finish sequence with a pause in between.

Grade selection decides most of the machining difficulty. Unfilled natural PEEK (often called 450G or a medical grade) gives the best combination of toughness and machinability. It is the default for seals, insulators, and wear pads where the load is moderate and no fiber reinforcement is needed.

Carbon-fiber-reinforced PEEK, usually 30% CF, raises stiffness and creep resistance and lowers thermal expansion. It is the grade for structural brackets, drone airframe fittings, and pump components that must hold shape at temperature. Expect faster tool wear and a higher risk of delamination at sharp internal corners. Radius those corners whenever the design allows.

Glass-filled PEEK, typically 30% GF, is cheaper than carbon-filled and has better electrical insulation, but it is more abrasive to tooling and more notch-sensitive. Use it when you need stiffness and dielectric strength together, not when the part sees impact.

Medical grades matter in a different way. Implant-grade PEEK is a controlled material with a documented traceability chain, and mixing it with general-purpose stock is not acceptable. If the part touches the body, tell us the grade and the standard at the quoting stage so we can keep the material lot separate and document it.

• 1
  UnfilledBest machinability, highest elongation, natural color.
• 2
  30% carbon fiberStiff, low creep, abrasive; watch corner delamination.
• 3
  30% glass fiberGood dielectric strength, more abrasive, notch-sensitive.
• 4
  Medical / implant gradeRequires lot traceability and separate handling.

PEEK stock arrives with residual stress from extrusion. Cut a deep pocket into that blank and the stress relieves itself as a bow or a taper. Annealing before machining is the standard answer. A controlled ramp to a temperature below the melting point, a hold long enough to let the chains relax, and a slow cool back to room temperature will remove most of the built-in stress.

The exact cycle depends on the cross-section. Thin plates need less hold time than a thick billet because heat has less distance to travel. We size the cycle to the blank. What matters to the drawing is that the part is machined after annealing, not before, and that the annealed blank is allowed to stabilize before the first cut.

Rough and finish in separate operations. Take the bulk of the material in the roughing pass, leave 0.3–0.5 mm on the walls, then let the part rest before finishing. For tight bores or long thin walls, a second stress-relief anneal between roughing and finishing is cheap insurance. It costs one extra setup day and it saves the whole lot.

Cooling matters as much as cutting. A finished PEEK part that is measured hot will read large. Let it come back to 20 °C in the inspection room before you accept or reject a dimension. If the print calls for ±0.005 mm, that temperature step is not optional.

Use sharp, polished carbide with a high rake angle and as few flutes as the tool can tolerate. Two or three flutes give chip room on unfilled PEEK. For fiber-filled grades, more flutes help finish but demand a rigid setup. Coatings are not a cure for abrasive filler; a fresh edge is.

Keep the surface speed moderate and the feed per tooth high enough to cut rather than rub. Rubbing is the main cause of a glazed, melted-looking surface on PEEK. If the chip comes off as dust and the part feels warm, you are rubbing. Increase chip load and reduce spindle speed before you change anything else.

Compressed air or a light mist is usually enough for chip clearing. Flood coolant brings its own problem: PEEK can absorb moisture, and a part that soaks coolant may swell slightly and then shrink after drying. If the job has tight tolerances, dry machining with air blast keeps the material stable.

Reamers and boring bars work well on PEEK, but leave stock for the finishing pass. Drilling straight to size in one shot tends to produce a hole that closes slightly as the material cools. Drill 0.05–0.1 mm undersize, then ream or bore to the final dimension after the part has settled.

• 1
  RoughingAggressive depth of cut, air blast, leave 0.3–0.5 mm on walls.
• 2
  FinishingSharp polished carbide, high chip load, light radial engagement.
• 3
  Boring and reamingLeave 0.05–0.1 mm for the final sizing pass.

PEEK earns its cost in three situations. The part runs continuously above 150 °C and must keep its shape. The part needs chemical resistance to fuels, solvents, or sterilizing agents. Or the part must be light, non-conductive, and stiff at the same time, which rules out aluminium and steel.

It is a poor choice when the load is high and the duty cycle is long at room temperature. Aluminium, stainless, or POM will do the same job for less money. PEEK is also the wrong material for thin, unsupported flat sections; it is stiff but it is not rigid like metal, and a 0.5 mm free-standing wall will flex under load.

Impact is another boundary. Unfilled PEEK has reasonable toughness, but glass-filled grades are notch-sensitive and can crack at a sharp corner under a shock load. If the part sees impact, either use unfilled PEEK or design generous radii and avoid sharp internal corners entirely.

Watch the coefficient of thermal expansion as well. PEEK expands far more than steel for the same temperature rise. A PEEK part running against a steel shaft will change clearance as it warms. Design the fit for the operating temperature, not the inspection room temperature, and say so on the drawing.

PEEK is soft enough that a contact probe can leave a mark on a finished surface. Use low contact force on a CMM, or switch to optical measurement for critical features. On a polished sealing face, a probe mark is a leak path, not just a cosmetic flaw.

Dimensional inspection has to wait for the part to reach equilibrium. Measure after the part has cooled to room temperature and, for moisture-sensitive jobs, after it has stabilized in the inspection area. A reading taken one minute off the machine is a reading of a different part.

For medical and aerospace work, we keep the material certificate, the annealing record, and the inspection report together so the lot can be traced. If your quality system requires first-article inspection, say so at the quote stage. The inspection plan is built before the first chip, not after.

Visual criteria matter too. A cloudy or glazed surface usually means the tool was rubbing. A ragged edge on a fiber-filled part usually means the fiber is pulling out at the exit. Both are process signals, and both are correctable on the next run rather than by reworking the finished part.