EN
Engineering plastics such as PEEK, Delrin (POM), Nylon, and PTFE have become standard materials for precision components in medical, aerospace, and automation equipment. They are light, corrosion-resistant, and easy to cut — at least in theory. In practice, machining plastic parts on a CNC machine is nothing like cutting aluminum or steel, and shops that treat them the same way quickly run into scrapped parts and missed tolerances. Below are the most common CNC machining problems seen with plastic parts like PEEK, why they happen, and how an experienced machinist prevents them.
This is the single most frequent complaint in PEEK CNC machining. Extruded or compression-molded PEEK rods and plates carry residual internal stress from the forming process. As the cutter removes material, that stress is released unevenly, and the part slowly bows, twists, or drifts out of tolerance — sometimes hours or days after it left the machine.
How to prevent it: anneal the raw stock in an oven before machining (a typical PEEK cycle soaks at around 200°C with slow, controlled heating and cooling ramps), rough-machine with generous allowance, then anneal again before the finishing pass. For tight-tolerance parts, this rough → anneal → finish sequence is not optional — it is the difference between a part that holds its tolerance in service and one that quietly drifts out of spec.
PEEK conducts heat very poorly — its thermal conductivity is only about 0.25 W/(m·K), a tiny fraction of any metal. Cutting heat stays concentrated at the tool tip instead of being carried away by the chip and workpiece. The result is localized softening, smeared surfaces, and melted chips welding themselves onto the cutting edge, which then tears the next surface it touches.
How to prevent it: use sharp tools with a positive rake angle, keep cutting speeds moderate rather than aggressive, maintain a steady feed so the tool never dwells in one spot, and clear chips with compressed air. Water-soluble coolant at low concentration also works for unfilled PEEK, but strongly alkaline fluids should be avoided because they degrade the surface.
Where a dull tool pushes a metal chip aside, it drags and tears a plastic surface. Symptoms include a matte or fuzzy finish, visible drag marks, raised burrs around edges and hole exits, and surfaces that look smeared rather than cut. Because plastics are soft, the defect is often blamed on the material when the real cause is tooling.
How to prevent it: run sharp, dedicated tools (a tool that has already cut fiberglass-filled material is finished), increase cutting speed within the limits of cooling, and use tool geometry designed for plastics — generous rake and clearance angles so the edge shears instead of plows. Finishing passes should be light and continuous, never stopped mid-surface.
Virgin PEEK is gentle on tools, but the moment you switch to a reinforced grade such as GF30 (30% glass fiber) or CF30 (30% carbon fiber), the equation changes. The embedded fibers act as hard abrasive particles that grind the cutting edge down part after part. A worn edge then generates more heat, which circles back to the melting and surface-finish problems above.
How to prevent it: carbide tooling is the baseline for filled grades, and PCD (polycrystalline diamond) or diamond-coated tools are the right choice for production runs. High-speed steel simply does not survive. Cutting speeds should be lowered, since abrasive wear accelerates with speed, and tool changes should be scheduled by part count rather than waiting for visible failure.
Plastic stock compresses under vise pressure in ways metal never does. Over-clamping leaves the part distorted while restrained — it measures fine in the fixture and springs back wrong the moment it is released. Thin walls and sharp internal corners are also prone to stress cracking when depth of cut is too aggressive or a dull tool wedges instead of cuts. Certain coolants and cleaning solvents can trigger environmental stress cracking in amorphous plastics like polycarbonate and acrylic weeks after machining.
How to prevent it: use soft jaws or dedicated fixtures that support the part instead of squeezing it, reduce clamping force to the minimum that holds the workpiece securely, take lighter finishing passes on thin features, and verify that every coolant and cleaning agent in the process is chemically compatible with the plastic being machined.
Drilling is one of the most failure-prone operations in plastic machining. A standard drill can “grab” and dive into soft material, oversizing the hole or cracking the surrounding wall. Packed chips overheat deep holes and melt the bore surface. Tapped threads strip easily, and small threads in unannealed stock can crack during tapping.
How to prevent it: start every hole with a spot drill, use peck drilling to clear chips before heat builds, and finish precision bores with reaming or boring rather than relying on the drill alone. Threads in PEEK should be tapped rather than thread-milled at small diameters, with cutting oil to reduce friction — and after annealing, so the material is stable enough to take the tap without splitting.
Plastics expand and absorb moisture far more than metals, so a PEEK part measured straight off the machine — still warm from cutting — will read differently after it cools and acclimates. Parts rejected at inspection are sometimes perfectly good parts measured too early or in an uncontrolled environment.
How to prevent it: let precision plastic parts equilibrate in a temperature-controlled inspection room before measurement, and record the measurement conditions alongside the results. Consistent measurement practice is part of the machining process, not an afterthought.
Nearly every common problem in plastic CNC machining traces back to four root causes: residual stress in the stock, heat that has nowhere to go, the wrong tooling for the grade, and clamping that fights the material instead of supporting it. Control those four things — anneal the stock, manage heat, match the tool to the material, and fixture gently — and plastics like PEEK machine predictably and repeatably.
At ANOK Precision Manufacturing, we have machined PEEK, PTFE, Delrin, Nylon, and other engineering plastics since 2011, holding tolerances down to ±0.002 mm under an ISO 9001:2015 quality system. If your project involves demanding plastic CNC machining work, send us your drawings — our engineers will review the design, flag any machining risks, and quote precision CNC machined plastic parts built to hold tolerance from the first piece to the last.