Turning PEEK Engineering Plastics: Heat, Tools and Tolerance Control

• 1
  Why Turning PEEK Engineering Plastics Is a Heat Problem
• 2
  Carbide Grades and Geometry That Survive the Cut
• 3
  Speeds, Feeds and Depth of Cut in Practice
• 4
  Chip Control, Burrs and Surface Finish
• 5
  Part Features That Turn Well and Features That Do Not
• 6
  Grade and Process Selection for Turned PEEK Parts

PEEK turns cleanly on a lathe. The trouble is where the heat goes. In aluminum, most cutting heat leaves with the chip and the rest spreads into the part and tool. PEEK keeps it at the cutting edge. The chip comes off hot, the insert stays hot, and the machined surface stays soft long enough to smear.

The number to remember is the glass transition temperature, near 143 °C. Below it the material shears cleanly and the chip breaks. Above it the same cut starts to push material instead of cutting it. You get torn surfaces, burrs that fold rather than snap, and dimensions that move after the part cools.

Thermal conductivity explains the rest. Unfilled PEEK sits around 0.25 W/m·K, roughly a thousandth of aluminum. Heat has nowhere to go except into the insert and a thin skin of the part. That skin is the surface you are trying to hold a tolerance on, so the surface is the first thing to suffer.

One consequence surprises people who come from metal work: a light finishing pass can be hotter than a heavy roughing pass. Rubbing generates more heat than shearing. If the insert is below the nose radius engagement, the tool rubs, and the finish gets worse as you take less material.

Use uncoated or lightly coated micro-grain carbide. Aluminum oxide and titanium nitride coatings are hard, but they hold heat at the edge and they do not slide on PEEK the way they slide on steel. A sharp polished uncoated insert usually outlasts a coated one on this material.

Geometry matters more than grade. A positive rake, usually 10° to 15°, lowers cutting force. A small nose radius reduces the contact patch and the heat it generates. Inserts sold for aluminum finishing, with a polished top face and a sharp edge, are close to ideal for unfilled PEEK.

Do not reuse an insert that has already run steel or aluminum. A worn edge rubs, and rubbing is what melts PEEK. Change tools on schedule, not on failure. On long runs we log tool changes by part count and check the surface with a comparator before the finish drifts.

For carbon fiber filled grades the rules shift. Abrasive fiber wears the edge fast, so expect shorter tool life and plan for it. Polycrystalline diamond inserts cost more per edge but hold size far longer on CF30 and similar compounds. On a 500-part run the PCD edge usually wins on total cost.

Start conservative and read the chip. For unfilled PEEK, a surface speed of 150 to 250 m/min with a feed of 0.05 to 0.15 mm/rev is a workable window on a rigid lathe. Take depth of cut of 0.5 to 2.0 mm on roughing so the edge shears instead of rubs.

If the chip comes off as a continuous string that whips, you are cutting too hot or too slow on feed. Increase feed per revolution. If the chip is powder, you are rubbing. Increase depth of cut or change the insert. Stringy chips on a finishing pass usually mean the feed is below the nose radius.

Carbon filled grades want less speed and more feed. Drop surface speed to roughly 100 to 150 m/min and keep the feed up so the fiber is cut, not pulled. Pulled fiber leaves a fuzzy surface and a dimension that creeps as the fiber springs back.

Coolant is a choice, not a default. Flood coolant helps carry heat away on deep bores and long parts. Air blast alone works well on short, stiff parts and avoids the moisture uptake that PEEK can show over time. Whichever you pick, be consistent, because the thermal state sets the final size.

Rough, then let the part rest. A PEEK turning that measures on size while warm will measure small after it cools. On tight work we rough, wait, then take a light finishing pass. That rest period is often the difference between holding ±0.005 mm and chasing it all afternoon.

PEEK does not break chips the way brass does. You manage the chip with feed, depth and geometry rather than with a chipbreaker. A string that clears the tool and falls away is fine. A string that wraps the workpiece is a sign to change feed or insert.

Burrs on PEEK form as thin, tough fringes at the part edge. They do not snap off. A sharp positive insert and a light chamfer pass remove most of them. For sealing faces, plan a finishing pass that cuts toward the supported end so the edge does not lift.

Surface finish is mostly a heat story. A cool, sharp cut on unfilled PEEK reaches Ra 0.8–1.6 μm without extra work, and Ra 0.2–0.8 μm is achievable on facing and outside diameters with a fine feed and a fresh edge. Chasing the same finish on a dull insert only raises the temperature.

Carbon filled PEEK sits coarser, typically Ra 1.6–3.2 μm as machined, because the fiber ends stand proud of the resin. That is normal and usually acceptable. If a sealing surface needs better, say so at quoting so the process can include a dedicated finishing pass.

Turning suits round, axially symmetric parts: bushings, seal rings, valve seats, insulators, wear pads, support and locating pins. PEEK's wear resistance and low friction make those parts last where a metal would gall or corrode.

Thin walls are the common failure. PEEK has a high thermal expansion for a plastic and low stiffness, so a 1 mm wall on a Ø60 mm tube will move during and after the cut. Keep wall thickness above roughly 2 mm where you can, and support the part with a mandrel or tailstock on long bores.

Long slender parts deflect. If the length to diameter ratio passes about 4:1, plan for a travelling steady or a support. Otherwise the tool pushes the workpiece away, the cut goes quiet, and the diameter drifts along the length.

Cross holes, slots and flats need a second operation. A mill-turn center handles them in one setup, which matters when a cross hole has to line up with a turned bore. Trying to hold that relationship across two setups adds error for no gain.