Machining PEEK and PTFE: A Guide to CNC Turning High-Performance Plastics

PEEK and PTFE are both high-performance technical polymers, yet they behave in almost opposite ways under the tool. A setter who applies metal-turning parameters — or worse, the parameters of a different plastic — ends up with either cracked parts or stringy burrs that are impossible to clear in series production. In an iso 13485 machining shop, these two materials are often behind the least understood non-conformances on the shop floor.

This article covers the mechanical and thermal behaviour of PEEK (reference grade Ketron 1000 / Tecapeek) and virgin PTFE, the mechanisms governing thermal expansion and tolerances, then the cutting parameters and tool geometry needed to hold a clean, burr-free surface finish.

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1. PEEK vs Teflon: Two Opposite Behaviours on the CNC Lathe

1.1 PEEK — Rigid, but Sensitive to Internal Stress

Polyetheretherketone (PEEK) is a semi-crystalline thermoplastic. For the unfilled extruded grade Ketron 1000 (Mitsubishi Chemical Advanced Materials), the ASTM datasheet gives:

• Tensile strength Rm = 110 MPa, elongation at break 40%, tensile modulus 4.34 GPa (ASTM D638)
• Flexural strength 172 MPa, flexural modulus 4.14 GPa (ASTM D790)
• Hardness Rockwell M100 / R126, Shore D 85 (ASTM D785 / D2240)
• Crystalline melting point 340 °C, continuous service temperature in air 249 °C, HDT at 1.8 MPa = 160 °C (ASTM D648)
• Dynamic coefficient of friction against steel, dry: 0.32

This rigidity (tensile modulus close to 4.3 GPa — roughly half that of aluminium but far above other technical polymers) is precisely what makes PEEK autoclave-sterilisable at 134 °C without permanent deformation — hence its widespread use in instrumentation and implants subject to medical CNC turning subcontractor requirements. But that same rigidity, combined with very low thermal conductivity (see Section 2), means that heat generated during cutting does not dissipate through the material: it concentrates locally and builds up residual internal stress. On an asymmetric geometry — a shoulder, a deep groove, a significant section change — these stresses can release after machining as delayed cracking, sometimes hours after the part leaves the machine. This is PEEK's number-one risk in CNC turning, far more critical than tool wear.

1.2 PTFE — Ductile, Slippery, the Material Flows Ahead of the Tool

Virgin Polytetrafluoroethylene (PTFE) belongs to a radically different mechanical family. A typical datasheet (data consistent with ASTM D638 / D785 / D2240) shows:

• Ultimate tensile strength 10–43 MPa (typically 20–35 MPa for virgin extruded PTFE) — up to 10 times lower than PEEK
• Elongation at break 50–650% — versus 40% for PEEK
• Tensile modulus 0.4–1.8 GPa — 2.4 to 10 times less rigid than PEEK (4.34 GPa)
• Hardness Shore D 50–59, Rockwell R58 (well below PEEK's Shore D 85)
• Dynamic coefficient of friction 0.06–0.10 — the lowest of any known solid, versus 0.32 for PEEK

This combination — very low tensile modulus and very high elongation at break — explains PTFE's characteristic behaviour on the lathe: instead of shearing cleanly ahead of the cutting edge like a metal or PEEK, the material deforms elastically and plastically under tool pressure before it yields. In practice, PTFE "flows" ahead of the edge instead of being cut — a direct consequence of its low stiffness and high ductility. This behaviour demands a specific tool geometry (Section 3.2) and explains why PTFE, despite its reputation as an "easy" material, actually produces more dimensional scrap than PEEK in an unprepared shop.

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2. Managing Thermal Expansion and Tolerances on Polymers

2.1 Why the Heat Stays in the Part

On a metal, most of the cutting heat leaves in the chip. On a plastic, this mechanism barely works: the thermal conductivity of PEEK (λ = 0.252 W/m·K, Ketron 1000) and PTFE (λ = 0.24–0.35 W/m·K) is 20 to 25 times lower than titanium TA6V (λ = 6.7 W/m·K) — a metal already described as "catastrophic" on thermal grounds in our article on turning company in france for titanium.

Material	λ (W/m·K)	Where the cutting heat goes
C45 structural steel	50	Mostly into the chip
Stainless 316L	16	Chip + workpiece
Titanium TA6V	6.7	~80% stays on the cutting edge
PEEK (Ketron 1000)	0.252	Stays concentrated in the part
Virgin PTFE	0.24–0.35	Stays concentrated in the part

With no evacuation through the chip or the surrounding material, heat generated by tool-material friction accumulates locally at the cutting interface. Beyond the heat deflection temperature (PEEK HDT = 160 °C at 1.8 MPa), the surface softens, sticks to the tool and loses its geometry; beyond the melting point (340 °C PEEK, 330 °C PTFE), it melts locally. Hence the requirement for abundant flood coolant — not a light air blast suitable for light roughing, but a substantial water-soluble coolant flow that removes heat by convection directly at the cutting point, since internal conduction through the material cannot be relied upon.

2.2 Thermal Expansion: The PEEK/PTFE Gap That Traps Tight Tolerances

PEEK's linear coefficient of thermal expansion (CTE) is 46.8 µm/m·°C (measured between −40 and 149 °C, ASTM E831). PTFE's ranges from 79 to 150 µm/m·°C at 20 °C, climbing to 170–220 µm/m·°C at 250 °C — up to 4.7 times PEEK's CTE. This non-linearity is not incidental: PTFE passes through crystalline phase transitions around 19 °C and 30 °C that cause abrupt dimensional jumps, independent of any machining-related thermal effect.

In practice, on a Ø20 mm part, a mere 10 °C gap between cutting temperature and inspection temperature produces:

• ≈ 9.4 µm of radial variation on PEEK (20 mm × 46.8×10⁻⁶ × 10 °C)
• ≈ 20 µm of radial variation on PTFE (20 mm × 100×10⁻⁶ × 10 °C, median value)

On IT7 tolerances or tighter, this gap alone consumes a significant share of the tolerance band. The practical consequence: any PEEK or PTFE part destined for dimensional inspection must be stabilised at room temperature (20 ± 1 °C — the same logic applied to precision machining medical parts) before measurement — never inspected while still warm off the machine. For asymmetric PEEK geometries or parts with large section variation, an intermediate thermal stabilisation step between roughing and finishing is recommended to release internal stress before the finishing pass that sets the final dimension.

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3. Cutting Parameters and Tool Geometry for a Burr-Free Surface Finish

3.1 Comparative Turning Parameters Table

Cutting data published by Michaud-Chailly (Plastiques Techniques catalogue, turning tables) and Experta (technical sheet "Machining Technical Plastics") converge on the following turning ranges:

Parameter	PEEK	Virgin PTFE
Cutting speed Vc	200–400 m/min (up to 500 m/min in light finishing)	100–300 m/min (up to 500 m/min in light finishing)
Feed f	0.05–0.5 mm/rev	0.05–0.3 mm/rev
Clearance angle α	5–15°	10–15°
Rake angle γ	0–10° (near-neutral)	15–20° (strongly positive)
Nose radius	0.5–1 mm	0.5–1 mm

Sources: Michaud-Chailly, Plastiques Techniques technical catalogue (turning tables); Experta, technical sheet "Machining Technical Plastics".

3.2 Why PTFE Requires a Two to Three Times More Positive Rake Angle than PEEK

The gap in rake angle (γ 0–10° for PEEK versus 15–20° for PTFE) is not an arbitrary setting: it is the direct translation, in tool geometry, of the stiffness gap measured in Section 1 (tensile modulus 4.34 GPa versus 0.4–1.8 GPa). On a material as compliant as PTFE, too neutral a rake angle pushes the material instead of shearing it — the edge locally "crushes" the polymer, which deforms elastically and springs back slightly after the tool passes, producing local oversize and an irregular surface finish. A strongly positive rake angle (15–20°) reduces the compressive component of the cutting force and favours shearing, letting the tool slice through the material instead of pushing it aside.

In both cases, inserts must be uncoated carbide, with a honed, polished edge (no PVD/CVD coating, whose surface roughness — even minimal — promotes local sticking of molten polymer to the tool, the "build-up" phenomenon specific to plastics). A dulled edge no longer cuts PTFE: it abrades it, generating local heat and cold flow that degrades dimensional accuracy long after machining.

3.3 Deburring: Why Cutting Direction Makes All the Difference on PTFE

PTFE does not snap cleanly like a brittle plastic — its high ductility (elongation at break up to 650%) produces long, stringy burrs that wrap around the part or the tool instead of breaking off as a short chip. The fix starts in the program itself:

• Always machine from the outside of the material toward the inside: sequence facing and chamfering passes so the last pass removes material from the already-cleared side toward the material still held (jaws, bar stock) — never the reverse. The ductile burr is pushed toward the cleared chip zone instead of being "pulled" onto the finished edge of the part.
• End every diameter transition with a reduced-feed chamfer pass, which shears the burr instead of tearing it.
• For batches requiring supplementary deburring, cryogenic deburring (immersion or blasting at −70/−100 °C, liquid nitrogen or CO₂) is the most reliable method on PTFE: at that temperature the polymer loses its ductility and becomes brittle, allowing stringy burrs to snap off cleanly instead of being torn off at ambient temperature — where they simply re-deform under the slightest stress.

PEEK, being more rigid, produces noticeably shorter, more brittle burrs, manageable with conventional manual deburring or vibratory tumbling — the difficulty on this material lies upstream, in internal stress management (Section 1.1), not in the burr itself.

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Conclusion — Two Materials, Two Machining Logics

Treating PEEK and PTFE with the same settings is the most common mistake in a plastics CNC turning shop: PEEK demands vigilance on internal stress and thermal stabilisation, PTFE demands a strongly positive tool geometry and a dedicated deburring strategy. On the application side, PEEK is increasingly replacing titanium in certain implants — spinal interbody cages in particular — for its radiolucency and its elastic modulus closer to cortical bone; for readers weighing the two solutions, our article on titanium TA6V CNC turning covers the logic on the metal side.

Our shop holds qualified parameters for PEEK and PTFE in series production, including under ISO 13485 medical machining requirements. See also our PEEK CNC turning machining page for the full detail of our capabilities.