PTFE Machining Tolerances: Why Tight Numbers Fail

PTFE machining tolerances cause more drawing arguments than almost any other plastic, because PTFE (Teflon) doesn't stay the same size.

Can a Machine Shop Really Hold an 8-Micron Tolerance on a PTFE Part?

Rarely, and it's usually not useful. PTFE moves with temperature, relaxes after machining and changes volume close to room temperature, so a very tight band is hard to hold and even harder to measure.

An r/machining post shared a drawing that called for 8 microns on a PTFE part. Replies were blunt. One machinist had seen a similar tolerance on a PTFE ring that press-fit into a stainless bore with a reasonable tolerance, and suspected the engineer didn't know which part needed the tight number. Another said a tight PTFE callout turned out to be on a handle that could have been held to half a millimeter, a CAD default left in place.

Why PTFE won't hold a tight number

The PTFE fabrication guidelines from Plastics International explain the main causes:

• A transition near room temperature. PTFE goes through a transition around 19 °C (66 °F), with a 1–1.5% increase in volume, which matters when measuring critical parts.
• Stress in the stock. For extremely close tolerances, this supplier's procedure is to heat above the expected service temperature (below 327 °C), hold about 1 hour per 2.5 cm of thickness, cool slowly, rough-cut to within 0.38–0.76 mm of final size, and re-anneal before the finish cut.
• Measurement. The guide warns that PTFE compresses, so a micrometer in inexperienced hands can read 0.13–0.25 mm under the true size.
• Typical practice. The same guide says PTFE shapes are normally machined to about ±0.13 mm (±0.005 in), and that closer tolerances usually aren't necessary.

Setting PTFE machining tolerances

• Ask what the feature does. PTFE's resilience lets it conform. The guide notes a press-fit part with interference can perform as well as one machined to an exact size, at lower cost.
• State the measurement temperature for any critical dimension. The guide says to check dimensions at the expected service temperature.
• Put tight numbers on the mating metal part where you can, not on the PTFE.

Related: what to do about overly tight tolerances on a drawing, what drives the cost of a CNC quote and first article inspection for low-quantity orders.

At DigiForge

Our materials page lists PTFE for CNC machining. The CNC service lists "ISO 2768-m standard, down to ±0.01 mm", but that's a general capability, not a promise for every material, and tighter tolerances carry a price uplift shown in the quote. If a PTFE dimension really has to be tight, mark it on the drawing and ask us when you quote.