Guides / Materials

Machining PEEK, PTFE and POM on Swiss-Type Lathes

September 21, 2026

Engineering plastics run on Swiss-type lathes for the same reason metals do. The parts are small, slender, feature rich and made in repeat volumes. Insulators, medical components, sealing elements, bushings, valve parts and instrument components all suit the machine.

What changes is almost everything about the material behavior. Heat, stiffness, dimensional stability and chip formation all differ from metal in ways that reach your drawing. This guide covers what to expect and what to specify.

The materials

PEEK is the high performance choice. High temperature capability, excellent chemical resistance, good strength and stiffness for a polymer, and available in implant grades for medical use. It is expensive in bar and it is the reason most plastics conversations start.

PTFE is chemically inert and has very low friction, used for seals, insulators and low friction bearing surfaces. It is soft, it creeps under load and it is difficult to hold tolerance on. Filled grades, with glass or bronze, machine and perform better dimensionally than virgin PTFE.

POM, also called acetal, is the workhorse. Good stiffness, low friction, dimensional stability better than most plastics and it machines cleanly. Where the application allows POM, it is usually the sensible answer on both cost and manufacturability.

Nylon grades absorb moisture, which changes dimensions after machining. Useful mechanical properties, but the moisture behavior has to be designed around.

PPS, PEI and PSU fill specific gaps in temperature and chemical resistance between POM and PEEK.

	PEEK	PTFE	POM	Nylon
Machinability	Good	Difficult	Best	Good
Dimensional stability	Good	Poor	Good	Moisture sensitive
Temperature capability	High	High	Moderate	Moderate
Chemical resistance	Excellent	Excellent	Moderate	Moderate
Bar cost	Highest	High	Low	Low

Heat is the constraint, not tool wear

On metal, the limit is usually what the tool can survive. On plastic, the limit is usually what the workpiece can survive.

Plastics conduct heat poorly and soften at modest temperatures. Heat generated at the cutting edge stays in the part, and if it builds up the material softens, smears, melts locally or deforms. The result is a part that measured correctly on the machine and is wrong on the bench.

Shops manage this with very sharp tooling, generous coolant, feeds chosen to keep the cut efficient and geometry that clears chips away from the part. Tool sharpness matters more than on metal, because a dull edge rubs and rubbing generates heat.

A practical consequence: parameters that work on a short part may not work on a long one where heat accumulates through the cycle. This is part of why a plastics quote is not simply a metal quote with a cheaper material line.

Chips

Plastics produce long continuous chips even more readily than aluminum. A ribbon of PEEK wrapping a slender part on an unattended machine is a job stopper.

Filled grades often break chips better than unfilled ones, which is one of several reasons glass filled PTFE is easier to machine than virgin. Beyond material choice, it is a tooling and parameter question the shop solves.

What this means for you: ask whether the shop machines your specific material regularly. Plastics are not interchangeable from a process standpoint, and a shop with a proven process in PEEK is worth more than a lower quote from one that will develop it on your order.

Tolerance expectations

Plastics hold looser tolerances than metals, and the reasons compound.

Low stiffness. A section deflects under cutting force and under gauging pressure. On thin walls this is the dominant effect, and the reading depends on how the operator measures. Define the method.

Thermal expansion. Plastics expand several times more than steel per degree. At tight tolerances, a few degrees between the shop floor and your inspection room produces a real difference. If it matters, state the measurement temperature.

Stress relaxation. Extruded or molded bar carries internal stress. Machining removes material asymmetrically and the part relaxes toward a new shape over hours or days. Annealing the bar before machining, or the part between roughing and finishing, is the standard answer where stability matters.

Moisture. Nylon in particular absorbs moisture from the air and grows. A part measured dry and used humid is a different size.

Typical production tolerances on modest sections land around ±0.025 mm. Tighter is achievable on specific features with the right process, and it costs more. The tolerances guide covers the general principle of tolerancing only what matters, which applies with extra force here.

What belongs on a plastics drawing

Beyond the usual geometry and tolerances:

1. The exact material, including filler, grade and any relevant specification. PEEK is a family, not a material.
2. Whether annealing is required, and at what point in the sequence.
3. The measurement method and temperature for critical features.
4. Moisture conditioning, if the material is moisture sensitive and the dimension matters.
5. Surface finish requirements, since plastics finish differently from metal and a visual expectation is not a specification.
6. Burr and edge requirements. Plastics produce a different kind of burr, often a soft flash that does not snap off the way a metal burr does.
7. Handling and packaging, since soft materials mark easily and a scratch on a sealing surface is a defect.

Point 1 is the one most often underspecified. Unfilled PEEK, 30 percent glass filled PEEK and implant grade PEEK are different in price, in machining behavior and in what they can be used for.

Bar stock behaves differently from metal

Plastics bar is not simply metal bar in a different material, and several of the differences reach your part.

Straightness varies more. Extruded polymer bar is less consistently straight than drawn metal bar. For guide bushing work this matters, since the bushing runs on the bar surface. Shops often run plastics bushing-less for this reason, which suits shorter parts. The guide bushing article covers the trade off.

Diameter tolerance is looser. Plastic bar is typically held to a wider diameter band than ground metal bar, which affects how much material must come off and how consistent the starting condition is.

Internal stress is built in. Extrusion leaves stress through the section, and it is not uniform. Two bars from different lots can relax differently after machining. Where dimensional stability matters, annealed or stress relieved bar is worth specifying rather than hoping.

Shelf conditions matter. Moisture sensitive materials change dimension in storage. Nylon in particular arrives at a different size depending on how long it sat and in what humidity.

Availability is narrower. Common grades in common diameters are straightforward. An unusual grade in an unusual size can carry a long lead time, and on an expensive material a shop may not want to hold stock. Ask about availability early, because on plastics it is more often the schedule constraint than the machining is.

Medical grades

PEEK has implant grade variants used in spinal and orthopedic devices. These carry the same documentation burden as implant metals: controlled material specification, lot traceability from bar to finished part, validated cleaning and full records.

If your part is implant grade, treat the sourcing the way you would a titanium implant part. The medical guide covers the documentation chain, and it applies to polymers without modification.

Cost

Plastics quotes behave differently from metal quotes.

Material can dominate. PEEK bar is expensive enough that on a small part the material line can exceed the machining. This makes bar utilization, cut off width and remnant more important than usual. Ask how many parts the shop gets per bar.

Cycle time is not always shorter. Plastics cut easily but heat management and chip control can force conservative parameters. Do not assume a plastic part is faster than the metal equivalent.

Secondary work is usually less. No passivation, no plating, no heat treatment in most cases. Cleaning and careful packaging often replace them.

Scrap has little recovery value, unlike titanium or brass turnings.

Getting plastic parts quoted

Name the exact material and grade, state whether annealing is required, define the measurement method for critical features and give the yearly volume. Send it through the RFQ form. We match the part with up to three shops that run Swiss-type lathes and reply within 24 business hours.

Say in the notes that the part is plastic rather than leaving it to the material field alone. It changes which shops are a genuine match, and a shop that runs polymers daily is a different proposition from one that runs metal and will try. For how plastics sit against the metal options, see the materials guide.

Where a part could be made in either a polymer or a metal, it is worth quoting both. Engineers often reach for PEEK on weight or chemical grounds and find that a stainless part costs less, holds tighter tolerances and removes the stress relaxation question entirely. The comparison takes one extra line on an RFQ.

Frequently asked questions

Keep reading

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• Tantalum and Niobium on Swiss Lathes: Machining Gummy Refractory Metals