PTFE machines easily and holds tolerances poorly unless you control temperature, clamping and stress.
Why dimensions move
- Thermal expansion about 100 to 125 x 10-6 per K.
- Room-temperature transition near 19 degrees C changes volume about 1 percent.
- Stress relief. Moulded stock holds internal stress. Machining releases it and parts may distort over days.
- Elastic recovery after clamping or cutting force.
- Creep under clamp load.
Practical shop rules
- Use sharp, positive-rake tools with high clearance. Dull tools smear PTFE.
- Take light, final cuts. Leave allowance after roughing.
- Anneal heavy parts before final machining to relieve stress (slow heat and cool cycle matched to the stock thickness; follow the resin supplier's guidance).
- Hold lightly with soft jaws, mandrels or vacuum. Over-clamping deforms the part, which then springs back out of tolerance.
- Use coolant that is clean and compatible. Air blast or mist avoids swelling or contamination in clean parts.
- Measure at a controlled temperature, about 23 degrees C. Let parts rest before final inspection.
- Keep fillers in mind: glass wears tools quickly (carbide or diamond), bronze is heavy and abrasive.
Tolerance guidance
For plain PTFE, a typical machined tolerance is around plus or minus 0.05 mm on small parts, increasing with diameter and wall thinness. Filled grades hold tighter. Seal grooves and critical bores should be specified in the same temperature and with a stated reference method.
Design to reduce problems
- Keep walls uniform.
- Avoid very thin, long features.
- Add generous radii.
- Leave expansion room in housings.
- State which dimensions are critical and which are reference.
Inspection
Use non-contact measurement or low-force gauges. A pin gauge forced into PTFE reads small. Define the measuring force in the drawing note for critical bores.
Welding and bonding
PTFE does not weld by normal plastics methods. Bonding needs a chemically etched surface (sodium naphthalenide) and a suitable adhesive. Mechanical locking, bolting or sintering to metal are common alternatives.
About this article. It is original writing prepared from public sources, standards and general industry practice. It is not a copy of any book or standard and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application. Only analyse material you own or are entitled to analyse.
