PEEK Machining: Grades, Tooling, and Design Rules

PEEK earns its price — the stock is expensive by plastic standards — through a combination no other machinable plastic quite matches. It is rated for continuous service at about 250 °C, keeps its mechanical properties in hot water and steam, resists nearly all common solvents, and absorbs almost no water (about 0.07% in 24 hours per ISO 62 testing on VICTREX 450G). At 1.30 g/cm³ unfilled, it is less than half the density of aluminum and about one-sixth that of steel, and it is inherently radiolucent, flame-resistant (UL 94 V-0), and a good electrical insulator.

The metal-replacement logic: where an aluminum or stainless part carries corrosion allowance, needs insulating bushings, or adds weight the assembly cannot afford, a PEEK part often drops in with less mass, no corrosion, and built-in insulation. Unfilled PEEK starts at a tensile strength around 100 MPa (~14,500 psi, at yield, 23 °C); glass transition sits at 143 °C and the melt point at 343 °C. Unfilled PEEK holds a heat-deflection temperature around 160 °C, and 30% glass or carbon reinforcement lifts it to roughly 315 °C.

Ask a machinist and the answer is consistent: unfilled PEEK machines like brass. It shears instead of smearing, holds a machined edge without crumbling, produces manageable chips, and leaves a clean surface straight off a sharp tool — a different universe from gummy materials like UHMW, and the reason PEEK parts carry real machined detail: threads, grooves, thin ribs, flat seats.

Two caveats keep the scrap bin honest. First, heat: PEEK is a thermal insulator, so cutting heat stays in the cut zone instead of leaving with the chip. Push parameters carelessly and the surface softens and smears — and the part that measures correctly while warm shrinks off-dimension as it cools. Air blast or mist cooling, sharp tools, and honest feeds keep the zone cool. Second, filled grades are abrasive: 30% glass or carbon turns the stock into a grinding compound. Carbide dulls quickly, a dull edge plows instead of cutting, and exit edges suffer fiber breakout. PCD tooling is the standard answer. Burrs are the quieter issue — PEEK edges roll rather than snap clean, so deburring belongs in the process plan, not discovered at inspection.

PEEK rewards the same discipline as any heat-sensitive material: sharp tools, decisive feeds, controlled temperature at the cut. For unfilled PEEK, sharp polished carbide is sufficient. For any fiber-filled grade, use PCD or diamond-coated tooling — the filler abrades carbide fast enough that the cheaper tool becomes the expensive one. And keep the tool cutting, not rubbing: light, hesitant passes generate heat without removing material, which is exactly backwards for PEEK.

A note on coolant: many PEEK shops run compressed air or mist rather than flood coolant. PEEK itself shrugs off water-based coolant, but parts headed for medical, semiconductor, or vacuum service often carry cleanliness requirements that make avoiding contamination simpler than removing it. Our CNC machining services team runs these parameters on PEEK daily; for background on the process itself, see our CNC machining overview.

Not every drawing needs PEEK, and honest material selection saves real money. Delrin (acetal homopolymer) is the workhorse for general mechanical parts — gears, bushings, fixtures — with tensile strength around 10,000 psi, superb machinability, and a small fraction of PEEK's price; it bows out when temperatures climb or steam sterilization enters the picture. Polycarbonate machines well, adds transparency and outstanding impact toughness, and covers guards and sight components where PEEK's temperature rating would be wasted money. Nylon fills the wear-and-bearing niche at low cost, with the caveat that moisture absorption moves dimensions. Each has its own machining personality, covered in our Delrin vs nylon and nylon CNC machining guides.