Guides / Materials

5-Axis Machining PEEK and Engineering Plastics

September 26, 2026

Engineering plastics like PEEK show up on 5-axis drawings for the same geometric reasons metal parts do: thin walls, compound angles, or features on faces that would otherwise need multiple setups. But plastic does not behave like metal under a cutting tool, and a shop that only runs metal well can still struggle with a plastic job.

Why plastic cuts differently

PEEK and similar engineering plastics conduct heat poorly, so cutting heat stays concentrated at the tool instead of spreading through the part the way it does in metal. Run too fast or too slow, and that heat can soften or even melt the material at the cut, leaving a poor finish or a dimensional error that only shows up after the part cools. Plastic also flexes more than metal under the same clamping and cutting force, which matters more on thin-walled 5-axis geometry than on a simple block.

Chip evacuation and coolant

Plastic chips can melt and re-adhere to the tool or the part if they are not cleared quickly, unlike metal chips that break away cleanly at the right parameters. Many shops run PEEK dry or with air blast rather than flood coolant, since some plastics absorb moisture or react poorly with certain cutting fluids. This is a setup detail worth asking about if your part has tight moisture or contamination requirements, common in medical and semiconductor applications.

Fixturing without marking or deforming the part

Metal fixturing clamped the way it would be for aluminum can mark or deform a plastic part, especially a thin-walled one. Shops experienced with plastics use lighter clamping, softer fixture surfaces, or vacuum fixturing to hold the part without damaging it, which matters more the thinner and more complex the geometry gets.

Tolerance and finish

±0.05 mm is a reasonable target for most PEEK features, looser than what the same shop might hold in aluminum, because plastic moves more under cutting force and with temperature. Thin, unsupported walls should carry a looser tolerance than thicker sections of the same part. Surface finish is achievable to a fine level, but glossy or optical finishes need a specific process, not just a fine cutting pass, and should be called out explicitly.

Typical parts

• Medical instrument components and implantable device parts
• Semiconductor processing components
• Insulators and low-friction bearing components
• Lightweight structural parts for aerospace and robotics

What to confirm on your RFQ

Name the exact grade, since PEEK itself comes in filled and unfilled variants with different machining behavior and cost. Flag any thin-wall or unsupported features so the shop can plan fixturing and tolerance accordingly, and state any cleanliness or contamination requirements up front if the part is going into a medical or semiconductor application.

The short version

Plastic is not simply an easier version of metal to machine. Heat management, fixturing and tolerance all need different handling than a metal part on the same 5-axis machine. A shop experienced with PEEK and engineering plastics, not just metal, is worth confirming before you send a plastic part out for quote.

Frequently asked questions

Keep reading

• 5-Axis Machining Copper and Brass: Burrs, Heat and Finish
• Hard Milling on 5-Axis: Machining Tool Steel Above 50 HRC