PEEK (polyether ether ketone) is one of the most capable engineering plastics you can put on a mill or lathe — and one of the easiest to get wrong if you treat it like a metal or like a commodity plastic. It holds tight tolerances, survives continuous service near 250 °C, and shrugs off most chemicals. But it also costs many times what Delrin or nylon do, and it moves with temperature and clamping in ways that punish a careless setup.
Here's how we approach PEEK parts on the shop floor, and how to decide whether your part actually needs it.
What tolerances you can realistically hold
On well-planned setups, machined PEEK holds tolerances comparable to soft metals — roughly ±0.002" on general features, and tighter on critical dimensions when the part is designed for it. What limits you isn't the tool; it's heat and internal stress.
Two things matter most:
- Stress-relieve before final cuts. Extruded and molded PEEK carries internal stress. Rough the part, let it normalize (an annealing step for tight-tolerance work), then finish. Skipping this is the single most common reason "in-spec at the machine" becomes "out-of-spec on the CMM an hour later."
- Keep it cool and lightly clamped. PEEK's thermal expansion is several times that of steel. Aggressive clamping distorts thin walls, and a hot cut grows the dimension you just measured. Sharp tooling, modest depths of cut, and air or coolant to carry heat away keep parts stable.
Tooling and speeds — the short version
- Sharp, polished, positive-rake tooling. Dull edges rub and generate heat instead of cutting. Carbide holds up well; polished flutes keep chips from welding.
- High spindle speed, moderate feed. PEEK likes surface speed. Let the tool cut cleanly rather than pushing feed.
- Clear chips aggressively. Recut chips mar the finish and add heat.
- Watch inside corners and thin walls. Generous radii and supporting features prevent the flex and chatter that ruin an otherwise good part.
Glass- and carbon-filled PEEK grades are more dimensionally stable and stiffer, but they're abrasive — expect faster tool wear and plan tooling accordingly.
When PEEK is worth the price (and when it isn't)
PEEK earns its cost when the application genuinely needs it:
- High continuous temperature — sustained service where acetal or nylon would soften or creep.
- Aggressive chemical exposure — solvents, acids, and steam that degrade cheaper plastics.
- Strength-to-weight in demanding sectors — aerospace, medical, and semiconductor parts where a metal is heavier or a lesser plastic won't survive.
- Wear and dimensional stability under load — bearings, bushings, and seals that must stay in spec.
If your part lives at room temperature, sees mild chemistry, and isn't weight- or wear-critical, a material like Delrin, nylon, or UHMW will do the job for a fraction of the cost. The engineering win is matching the material to the duty — not defaulting to the most exotic option.
The takeaway
PEEK rewards a disciplined process: relieve the stress, keep the part cool, finish with sharp tooling, and design in radii and support. Do that, and you get precise, stable parts in a material that performs where almost nothing else will. The place most PEEK jobs go sideways is the setup, not the material.
If you have a PEEK part — or you're weighing PEEK against a cheaper plastic and want a straight answer — our team has been CNC machining engineering plastics since 1968. Send us the print and we'll tell you what's realistic. Request a quote and we'll take it from there.