Machining PEEK Plastic — Speeds, Feeds, Tooling & Tips
Machining PEEK is straightforward for any shop experienced with engineering plastics — it cuts cleanly, holds tight tolerances, and produces good surface finishes with standard carbide tooling. The challenges are specific to grade and geometry: filled grades (GF30, CF30) accelerate tool wear dramatically, thick stock benefits from pre-annealing, and thin-wall geometry requires careful fixture design to avoid heat buildup and distortion. This guide covers the complete workflow from stock preparation through threading and finishing.
At a glance:
- PEEK machines dry or wet; coolant is recommended for filled grades and deep cuts
- Surface speed: 400–600 SFM (turning, virgin); reduce 20–30% for GF30/CF30
- Carbide tooling standard; PCD inserts for highest volume work on filled grades
- Pre-anneal thick stock (>1.5 in) at 150°C for 4–8 hours before finish machining
- Avoid dwelling — continuous feed prevents localized heat and surface glazing
- Tolerances of ±0.001–0.002 in achievable on CNC lathe and mill
Why PEEK Machining Differs from Other Plastics
PEEK is semicrystalline, not amorphous. Amorphous plastics like Ultem or polycarbonate tend to smear under tool pressure; PEEK's crystalline structure produces a cleaner chip break and more predictable cutting force. The same crystallinity that aids chip formation also makes PEEK susceptible to residual stress relief under heat — which is why thermal management during machining matters.
Grade is the second variable. Virgin PEEK machines like a medium-hardness engineering plastic. GF30 is abrasive — glass fiber at 30% loading dulls carbide inserts 3–5× faster than virgin. CF30 is harder on tooling still and generates conductive carbon dust requiring dust collection.
Pre-Machining Preparation
Stress Relief Annealing
Extruded PEEK rod and compression-molded sheet carry residual stress from the manufacturing process. In rod diameters above 1.5 inches and sheet thicknesses above 1.0 inch, this stress can be significant enough to cause warping during or after machining when the surface layers are removed. Pre-annealing eliminates most of this risk.
Standard anneal protocol:
- Ramp to 150°C (302°F) at no more than 2°C/min
- Hold at 150°C for 4–8 hours (longer for thicker stock; rule of thumb: 1 hour per inch of cross-section up to 8 hours)
- Cool slowly in oven to 60°C before removing (do not quench)
- Machine within 24 hours of anneal for best dimensional stability
Stock Inspection
Inspect rod for eccentricity and sheet for flatness before setup. Large-diameter rod (>4 in) can have a crystallinity gradient from surface to core; for high-precision parts, discuss this with your supplier and consider whether core material requires different finish allowances.
Turning Parameters
Recommended Speeds and Feeds (Lathe/Turning)
Tooling for Turning
Use sharp, positive-rake carbide inserts with polished chip-breaker geometry. C-2 uncoated carbide works for virgin and GF30; TiAlN-coated or PCD for CF30 high-volume work. Relief angle 6–8°; high-speed steel is not recommended.
Turning tips:
- Maintain continuous chip — interrupted cuts spike heat
- Support slender parts (L/D > 6) with steady rest or live center
- Flood coolant preferred for filled grades; dry acceptable for virgin at moderate SFM
- Clear chips frequently to prevent re-cutting
Milling Parameters
Recommended Speeds and Feeds (Milling)
End Mill Selection
Use 2- or 3-flute end mills with sharp edges and a helix angle of 30–40° for good chip evacuation. Uncoated carbide for virgin PEEK; TiAlN-coated for GF30 and CF30.
Milling tips:
- Either conventional or climb milling works; conventional gives slightly better finish
- Machine thin walls in progressive passes, not full-depth single cuts
- Flood coolant or through-coolant for pockets/slots deeper than 3× tool diameter
- Dust collection required for CF30 — carbon fiber dust is conductive and mildly irritating
Drilling
Drilling is common for tapped holes, clearance holes, and through-bores. PEEK drills cleanly but requires attention to heat management and chip clearance in deep holes.
Recommended Drill Parameters
| Hole Depth | SFM | Feed (IPR) | Tool |
|---|---|---|---|
| <3× diameter | 100–200 | 0.003–0.006 | Parabolic flute carbide |
| 3–8× diameter | 80–150 | 0.002–0.005 | Parabolic flute, through-coolant preferred |
| >8× diameter | 60–100 | 0.001–0.003 | Through-coolant carbide gun drill |
Use a spot drill or center drill first to locate the hole accurately — PEEK offers little tactile resistance so the drill can walk on entry without a positive start. Peck drilling (0.5–1× diameter pecks) is recommended for deep holes even with through-coolant to prevent chip packing. Use cutting fluid or compressed air for dry drilling.
Drill 0.005–0.010 in undersize and ream to final tolerance for the most consistent bore diameter and finish.
Tapping and Threading
PEEK taps well with standard and spiral-flute taps. Thread quality is good, and PEEK holds thread strength sufficient for most fastener-retention applications.
Tapping guidelines:
- Use spiral flute (gun) taps for blind holes; spiral point (gun tap) for through holes
- Drill to 75% thread engagement — PEEK's modulus is high enough that full 100% engagement is rarely needed and increases tap breakage risk
- Cutting fluid or tapping oil recommended; dry tapping works for coarse threads in virgin PEEK but risks tap seizure in fine-pitch or filled grades
- For threads ≤ #10–32 in CF30 or GF30, carbide taps reduce breakage risk significantly
- Standard tool coatings (TiN, TiAlN) are acceptable on taps for moderate production
For helical inserts (Heli-Coil, Keensert): PEEK accepts standard insert installation. For applications with repetitive fastener removal cycles, inserts prevent thread wear and are recommended in CF30, which has lower thread shear strength than virgin despite its higher tensile strength.
Surface Finishing
PEEK machines to a good as-machined finish (Ra 32–63 µin with standard cuts; Ra 8–16 µin with finishing passes). If optical-quality or sealing-surface finish is needed, additional steps are available:
- Diamond turning: Ra < 4 µin achievable on virgin PEEK with PCD tool on a precision lathe
- Lapping: appropriate for flat sealing surfaces; use silicon carbide lapping compound, then aluminum oxide for final pass
- Bead blasting: produces a uniform matte finish (Ra 100–200 µin) useful for reducing reflectivity or grip enhancement
- Solvent vapor polishing: not applicable — PEEK is resistant to virtually all common solvents
Thermal Management Summary
| Scenario | Risk | Solution |
|---|---|---|
| High SFM in virgin PEEK | Surface glazing, subsurface stress | Reduce speed; use coolant |
| Low feed rate | Heat buildup, rubbing | Increase chip load; keep cutter moving |
| Dry CF30 milling | Rapid tool wear, heat buildup | Add flood coolant or mist |
| Thick-wall turning without anneal | Post-machine warping | Pre-anneal per protocol above |
| Re-cut PEEK chips | Poor surface finish, dimensional error | Clear chips continuously |
For a comparison of machining PEEK against other high-performance plastics, see how it compares with Ultem PEI machining and Delrin/acetal machining.
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