PEEK is one of the most useful high-performance plastics a design engineer can choose, but it is not a metal substitute that happens to be lighter. It is a semi-crystalline thermoplastic with its own rules: heat moves through it slowly, stock shape stress matters, filled grades are abrasive, and dimensional behavior changes with temperature.
That is where many expensive CNC machining problems begin. A drawing that works well in aluminum, acetal, or even PEI may become difficult, slow, or unstable when the material changes to PEEK. The part may still be machinable, but the cost, lead time, scrap risk, and inspection uncertainty all move in the wrong direction.
1. Designing Zero-Radius Internal Corners
Perfectly square internal pockets often come from sheet metal, EDM, or molded-part thinking. CNC end mills are round, so a true zero-radius internal corner cannot be produced with standard milling. Trying to approach it forces the shop into very small tools, low feed rates, longer cycle time, and higher tool breakage risk.
Sharp internal corners can also concentrate stress, especially in parts exposed to thermal cycling or clamping load.
DFM guidance: Add the largest practical internal corner radius. As a starting point, avoid radii below 0.5 mm unless the feature is function-critical. For deep pockets, increase the radius to allow a stiffer cutter and reduce tool deflection.
2. Pushing Wall Thickness Too Far
Thin walls that are straightforward in aluminum can become unstable in PEEK. The material may flex away from the cutter, chatter, or move under clamping pressure. Filled grades such as glass-filled or carbon-filled PEEK are stiffer, but they can also be more abrasive and less forgiving around thin edges.
DFM guidance: For many CNC machined PEEK parts, walls above 1.5 mm are easier to control. For filled grades, 2.0 mm or more is a safer starting point. If thinner walls are unavoidable, use support ribs, reduce unsupported height, and discuss machining sequence early.
3. Ignoring Stock Shape Stress and Annealing
Extruded or compression-molded PEEK stock can contain residual stress. If a design removes most of the material from one side while leaving a heavy base on the other, the part may move after roughing, after unclamping, or during final inspection.
DFM guidance: Keep material removal as balanced as possible. For tight-tolerance parts, consider a staged process: rough machining, stress relief based on the stock supplier’s guidance, and finish machining after the part stabilizes. Do not specify a universal annealing temperature without considering grade, size, stock form, and supplier recommendations.
4. Applying Tight Tolerances Everywhere
A blanket ±0.01 mm tolerance may look precise on a drawing, but it can make a PEEK part expensive without improving function. PEEK expands more than aluminum, and the exact coefficient depends on grade, fiber orientation, temperature range, and stock form. A part inspected at one temperature may measure differently in another environment.
DFM guidance: Reserve tight tolerances for functional features: sealing faces, bearing bores, locating surfaces, and critical mating geometry. Use a general tolerance for non-critical features, and specify the inspection temperature when dimensional stability is important. This makes quotation and inspection much more realistic.
5. Relying on Fine Tapped Threads in Plastic
PEEK is strong for a thermoplastic, but it should not be treated like aluminum or steel when threads carry repeated assembly load. Fine threads can strip more easily, and threads placed too close to an outer edge increase chipping or breakout risk during machining.
DFM guidance: Use coarse threads where possible. For repeated assembly, higher clamp load, or service access panels, consider threaded inserts. Keep enough edge distance around tapped holes and avoid placing threads in very thin wall sections.
6. Creating Deep, Narrow Pockets
Deep narrow cavities require long tools. Long tools deflect, chatter, and struggle to remove chips. In PEEK, poor chip evacuation increases heat and can leave smeared material on the wall. In filled grades, rubbing can accelerate tool wear.
DFM guidance: Keep pocket proportions friendly to rigid tooling. A depth-to-width ratio below about 4:1 is a practical target for many 3-axis parts. If the pocket must be deeper, increase corner radii, add access relief, split the component, or review whether 5-axis access can reduce tool length.
7. Forgetting How the Part Will Be Held
PEEK parts still need to be clamped, supported, and referenced. Organic shapes, thin edges, and fully contoured outer surfaces may require custom soft jaws or vacuum fixturing. That is not always a problem, but it should be known before the drawing is released.
DFM guidance: Add temporary clamping pads, non-functional reference lands, or sacrificial stock where possible. If the finished geometry cannot provide stable holding, involve the machining supplier before finalizing the CAD model.
8. Calling Out Only “PEEK” on the Drawing
“PEEK” is not a complete material specification. Virgin PEEK, glass-filled PEEK, carbon-filled PEEK, bearing grades, and medical or semiconductor-oriented grades can behave very differently in machining and in service. Filled grades may improve stiffness or wear behavior, but they are also more abrasive and can have different dimensional behavior.
DFM guidance: Specify the exact grade, stock form, manufacturer if required, color, and any compliance documentation needed for the project. If the grade is still open, describe the operating environment and let the supplier recommend candidates. For material selection context, see our PEEK CNC machining and engineering plastic materials pages.
9. Not Stating Coolant or Cleanliness Restrictions
Some PEEK components are used in semiconductor equipment, laboratory devices, or fluid-handling systems where coolant residue, oil contamination, or cleaning requirements matter. If the drawing does not mention this, the supplier may quote a standard process and later discover that the part must be machined dry, with air blast, or with a controlled cleaning procedure.
DFM guidance: State restrictions clearly: no oil-based coolant, DI water cleaning, packaging requirement, particle sensitivity, or documentation needs. The earlier these requirements are known, the easier it is to choose the right cutting strategy and inspection flow.
10. Designing Assembly Clearances Only at Room Temperature
PEEK’s dimensional behavior changes with temperature. Its glass transition temperature is typically around 143°C, but clearance problems can appear well before or above that point depending on load, mating material, geometry, and service conditions. A shaft, bearing, or metal insert that fits at 20–25°C may behave differently at operating temperature.
DFM guidance: Calculate assembly clearances using the maximum and minimum operating temperatures, not only the inspection condition. Consider the expansion of both the PEEK part and the mating metal component. For semiconductor use cases, our guide to semiconductor PEEK parts may help define typical application requirements.
PEEK DFM Checklist Before RFQ
- Use realistic internal radii instead of square pockets.
- Keep thin walls supported and avoid fragile edge features.
- Review whether roughing and stress relief are needed before finishing.
- Apply tight tolerances only where the function requires them.
- Choose thread design based on assembly load and service frequency.
- Specify exact PEEK grade, stock form, and documentation needs.
- State coolant, cleaning, and packaging restrictions on the drawing.
- Check assembly clearances at actual operating temperature.
How Micrylix Reviews PEEK Machining Projects
For drawing-based PEEK components, Micrylix reviews material grade, wall thickness, tool access, tolerance zones, hole and thread design, fixturing, surface finish, and inspection requirements before quotation. The goal is not to make every design conservative. The goal is to identify which features truly affect function and which features only add cost or risk.
If you are designing plastic insulating components, precision fixtures, semiconductor equipment parts, or small-batch engineering plastic components, send the drawing and application notes early. A short DFM review can often prevent avoidable cost before the part reaches production.