MaterialsOctober 15, 2026

Machining PEEK and Other Plastics: How They Differ from Metals, Cutting Conditions and Tools, Annealing, Dimensions and Plastic Gears

Plastics grow and shrink with heat and moisture. Measure right after cutting, and the size might be different tomorrow!

Machining PEEK and Other Plastics: How They Differ from Metals, Cutting Conditions and Tools, Annealing, Dimensions and Plastic Gears

Plastics such as PEEK, POM and nylon (PA) can be cut on the same lathes and machining centers as metals. Yet people still run into trouble: the size changes after machining, holes come out small, parts crack, burrs won't come off. The reason is that plastics are materials that trap heat, spring back elastically, and expand and shrink with temperature and moisture. This article covers how plastics differ from metals, types of plastics and where to use them, cutting conditions and tools, cooling, annealing and dimensional stability, a calculator for dimensional change with temperature, workholding and burrs, tool wear with glass- and carbon-fiber-filled grades, and plastic gears and how they are used in humanoid and collaborative robot joints, based on technical documents from plastic manufacturers, JIS standards and research papers (as of October 2026).

What happens when you cut plastics. Heat builds up at the cutting edge and surface, and the material springs back a little after the edge passes. As a result, holes tend to shrink and outer diameters tend to grow^[1][2].

How plastics differ from metals

Even though it is all "cutting," plastics behave differently from metals in the following five ways. Each one directly affects dimensions and surface finish.

Difference	What happens	What to do on the shop floor
Heat doesn't escape	Low thermal conductivity and a low melting point mean heat builds up at the cutting edge and surface. The material melts, gets gummy, or keeps residual stress^[1][3]	Clear chips quickly. Cool with air. Use a sharp edge that cuts rather than rubs
Soft and springs back	The elastic modulus is lower than metal's, so gripping hard distorts the part. The material springs back after the edge passes, so holes tend to shrink and ODs tend to grow^[2][1]	Grip lightly and remove stock symmetrically. Aim ODs at the low side and holes at the high side
Expands and shrinks a lot	The coefficient of linear expansion is about 4–17 times that of steel (chart below). Hand heat or cutting heat changes the measured value	Measure after the part has cooled, close to 20 °C. Use the Calculators to estimate the temperature effect
Absorbs water	Nylon can absorb up to about 7% of its weight in water when immersed, changing dimensions by up to about 2%^[5]. PEEK and POM absorb far less	For materials that absorb moisture, keep storage and measuring conditions the same. Use low-absorption plastics for precision parts
Cracks from chemicals	Amorphous plastics such as PC and PEI can stress-crack from mineral-oil-based cutting oils^[1][4]	Use air or water-soluble coolant. If coolant is used, wash with IPA and deionized water

Watch out for slow deformation, tooUnder sustained load, plastics keep deforming over time (creep). Seating faces under tightly torqued bolts, press fits, and clamp marks from long fixturing may not recover, so take measures such as adding metal collars at fastening points or widening the gripping surfaces.

types of plastics and where to use them

Here are plastics commonly machined from stock, compared by service temperature, expansion, water absorption and machining notes. The values are typical figures from plastic manufacturers and vary by manufacturer and grade. Detailed values and sources for each material are also in the Materials database.

Plastic	Continuous service temperature (guide)	Coefficient of linear expansion (×10⁻⁵/K)	Water absorption (23 °C, immersed)	Machining note
POM-C (acetal)	100 ℃	14	24 h 0.05%, 96 h 0.1%	The standard choice: no voids and easy to machine. Gears and sliding parts^[16]
PA6 (cast nylon)	100 ℃	12	Saturated 6.5%	Dimensions move with water absorption. Large parts, rollers^[17][18]
UHMW-PE	80 ℃	18	24 h 0.01%	Soft and deflects, prone to welding onto the tool. Raise feed and speed to break chips away^[22][1]
PTFE	260 ℃	10–13	0.01%	Low strength and large expansion, so discuss tight tolerances at the design stage^[20][21]
PC (polycarbonate)	120 ℃	8	24 h 0.03–0.06%	Amorphous. Can crack from mineral-oil-based cutting oils^[19][1]
PPS	230 ℃	About 6 (about 11 at 100–150 °C)	0.01% or less	Expansion nearly doubles above the 97 °C glass transition. PCD for reinforced grades^[11][12]
PEI	170 ℃	5	24 h 0.05%, 96 h 0.1%	Amorphous (glass transition 216 °C). Watch for stress cracking from cutting oil^[10][1]
PEEK	260 ℃	5 (12 above Tg 143 °C)	24 h 0.02–0.03%, saturated 0.45%	Expensive, so plan roughing setups carefully. Can be annealed at up to 250 °C^[6][7][1]
PEEK GF30, CF30	260 ℃	4 (extrusion direction)	24 h 0.02–0.03%	The fibers wear the tool. PCD or CVD diamond tools^[8][9][1]
PAI (polyamide-imide)	210 ℃	4.3	Dimensions change with moisture absorption	Hard and strong. PCD for reinforced grades^[13][14][1]
PI (polyimide)	290 ℃	4.3–4.4 (50–200 °C)	24 h 0.47%	Low cutting speeds of 25–100 m/min. PCD is best^[15][1]

Continuous service temperature, coefficient of linear expansion and water absorption are typical values from Ensinger product pages (POM-C: TECAFORM AH, PA6: TECAST T, PI: TECASINT 2011, PAI: TECAPAI CM XP403, etc.). Saturated water absorption of PA6 from MCAM; saturated water absorption and Tg of PEEK from Victrex.

Comparison of coefficients of linear expansion. For the same 100 mm, a 10 °C temperature difference changes S45C by about 0.012 mm, PEEK by about 0.05 mm, and POM by about 0.14 mm^{[6][16][23][24][25]}.

How to choosePOM is cheap and easy to machine. Avoid PA where dimensions must hold around water. For areas above 100 °C or exposed to chemicals and steam, use PPS or PEEK; for even hotter areas or where strength is needed, PAI or PI. PEEK is often chosen for precision parts that need both dimensional stability and heat resistance because its linear expansion is small for a plastic, it absorbs little water, and it can be annealed at 250 °C^[6][1].

Starting cutting conditions and tools

Below are values from plastic manufacturers' machining guides. They are all starting points; adjust them for the machine, workholding and part shape. The table from Mitsubishi Chemical Group (MCAM, formerly Quadrant) divides plastics into several groups^[1].

Plastic group (MCAM table)	Turning: speed, feed	Milling: speed, feed	Drilling: speed, feed	Tools
POM, PA, UHMW-PE, etc.	200–500 m/min, 0.05–0.5 mm/rev	200–500 m/min (POM up to 400), 0.5 mm/tooth or less	50–100 m/min, 0.1–0.3 mm/rev	HSS or carbide
PC, PEI, PSU, PPSU	200–400, 0.05–0.4	200–400, 0.4 or less	50–100, 0.1–0.3	Carbide
PEEK (unfilled), PAI (unfilled), PET	200–400, 0.05–0.5	150–300, 0.4 or less	50–80, 0.1–0.3	Carbide
GF/CF-filled PEEK, PAI, PPS; PA66 GF30	100–200, 0.05–0.3	75–150, 0.3 or less	50–80, 0.1–0.3	Carbide or PCD/CVD
PI, PBI	25–100, 0.05–0.2	25–100, 0.15 or less	25–50, 0.1–0.3	Carbide or PCD/CVD

Tool angles for both turning and milling: relief angle 5–15°, rake angle 0–10° (0–15° for milling, 3–5° for turning PI and PBI). For drills: relief angle 5–10°, rake angle 3–5°, point angle 90–120°^[1].

For PEEK, the plastic manufacturer Victrex also publishes conditions. For unfilled PEEK: turning at a cutting speed of 100–300 m/min, feed 0.1–0.7 mm/rev, relief angle 5–15°, top rake angle 5–15° and depth of cut 1–6.5 mm; milling with carbide at 180–230 m/min; drilling at 120 m/min, feed 0.05–0.20 mm/rev and a 118° point angle^[2]. Victrex's lower speed limit is lower and its upper feed limit is higher than MCAM's, so a safe start is where the two sources overlap (turning at around 200–300 m/min and 0.1–0.5 mm/rev).

Sharp edges, positive or 0° rake: When plastic is rubbed, the heat makes it melt or tear. HSS works, but carbide lasts longer for large quantities^[1]
Generous relief angle: The material springs back after the edge passes, so with too little relief the flank rubs and generates heat
End mills: 2–3 flutes, high helix: Give the chips plenty of room and prevent them from wrapping around the tool^[1]
Don't drop the feed too low: Too slow, and the tool rubs the same spot and heat builds up. Find conditions where chips come off continuously^[1]
Even diamond tools wear: According to a report from a tool manufacturer's research lab, cutting PEEK with single-crystal diamond tools wears and damages the cutting edge, and the type of damage changes with edge preparation^[26]. Even for mirror-like finishes, tool life must be managed

Cooling: air, water-soluble coolant and cutting oil

Except for drilling and parting off, the basic rule for plastics is to machine them without coolant. When surface finish and tolerances matter, water-soluble coolants free of aromatics, mist, or compressed air are effective^[1].

Don't use mineral-oil-based cutting oils on amorphous plasticsAmorphous plastics such as PC, PPSU, PEI and PSU can stress-crack (chemical cracking) from mineral-oil-based cutting oils. If you use coolant, wash the parts with isopropyl alcohol (IPA) and deionized water right after machining^[1][4]. Even if no cracks appear right after cutting, you can't relax while oil remains on the part.

On the other hand, for PEEK, which is semi-crystalline and chemically resistant, Victrex says both water-based and oil-based coolants can be used^[2]. Deep holes (deeper than 2× the diameter) tend to heat up, so use coolant and peck drill to clear the chips^[1].

Cooling also reduces burrs. In a study of micro-milling (fine slot cutting) of a biomedical polymer, burrs 185 µm wide blocked the slot with 16 °C air, but blowing air chilled to −2.2 °C with a vortex tube reduced them to 70 µm. The cold plastic becomes stiffer, which changes how it tears^[28].

annealing and dimensional stability

Plastic stock (plate and rod) contains residual stress from extrusion or molding. Machining off the surface upsets the stress balance and causes warping, and cutting heat also introduces new stress. Relieving this stress is the job of annealing (stress-relief heat treatment)^[2][1].

Plastic	Holding temperature (MCAM)	Atmosphere
PE-UHMW, PC	130 ℃	Air, nitrogen (oil also for PE)
POM-C, POM-H, PET	155 ℃	Air, nitrogen, oil
PA	160 °C (170 °C for PA4.6 and PA66 GF30)	Air, nitrogen, oil
PSU	170 ℃	Air, nitrogen
PPS, PEI, PPSU	200 ℃	Air, nitrogen
PEEK	250 ℃	Air, nitrogen

Common procedure: heat at 10–20 °C/h, hold for 10 minutes per mm of wall thickness, and cool at 5–10 °C/h. Allow a further 3 minutes per mm of wall thickness to return to room temperature. Keep the oven temperature within ±3 °C, and keep the part held in a fixture until it has cooled^[1].

Annealing temperature profile. A 10 mm thick PEEK part takes nearly two days. Cooling too fast brings stress back, so cooling is the longest stage^[1].

Victrex publishes a different procedure for PEEK: heat evenly to 200 °C, hold for 1 hour per mm of wall thickness, then cool at 10 °C/h to below 140 °C before switching off the oven. To stabilize the dimensions of parts used at high temperature, hold at a temperature at least 20 °C above the maximum service temperature instead^[2]. Both the temperature and the time differ from MCAM's (250 °C, 10 min/mm), so record in the process sheet which source you followed.

Roughing: Leave the finished size plus the amount annealing will move it plus finishing stock^[1]
Annealing: At the temperatures in the table above. For parts with heavy stock removal, anneal once more before final finishing^[2]
Light finishing cut: Small depth of cut and a fresh edge. Even just letting parts rest for 24 hours after roughing helps the dimensions settle^[1]

If you want to skip annealingPlate and rod from stock manufacturers are normally annealed before shipping. For parts with little stock to remove, a symmetrical shape and generous tolerances, roughing and finishing in one go often causes no problems. If warping occurs, first check whether you removed a lot of material from one side only (machine both sides evenly and symmetrically about the extrusion centerline)^[1].

Dimensional targets and measurement (with calculator)

Three things matter when measuring plastic parts: Temperature and moisture and whether the part was just machined. JIS dimensions are values at 20 °C, so for materials with large expansion like plastics, pass/fail judgments go wrong unless you allow for the measuring temperature (see Measurement error basics).

The coefficient of linear expansion is an average over a temperature range and varies with grade, direction (reinforced grades differ between the extrusion direction and perpendicular to it) and degree of crystallinity. Plastic manufacturers' documents give no formula for moisture growth, so this field takes a value you already know (for nylon, up to about 2% when saturated in water^[5]).

Don't measure right after cutting: The part is swollen from cutting heat. Measure after it cools, ideally after it has settled to the measuring room temperature
Don't hold parts in your hand for long: If hand heat raises the temperature a few °C, a 100 mm POM part moves by more than 0.01 mm (you can check with the calculator)
Aim ODs at the low side of the tolerance and holes at the high side: In turning, ODs tend to grow and holes tend to shrink. Also check how tap drill holes actually cut^[1]
How tight can you go?: MCAM's "best tolerances" table gives, for sizes of 10, 50, 100 and 200 mm: 0.02, 0.04, 0.06 and 0.10 mm for PEEK, PEI, PPS and PC; 0.03, 0.05, 0.09 and 0.15 mm for POM and PA; 0.05, 0.08, 0.15 and 0.26 mm for HDPE; and 0.02, 0.03, 0.05 and 0.08 mm for filled PEEK. It notes that tighter tolerances need special tools and processes and may increase cost^[1]

Workholding, burrs and surface finish

Plastics have a low elastic modulus, so if you grip them with the same force as metal, they are machined in their clamped shape, then spring back and lose their form after unclamping. Victrex notes that strong clamping easily causes warping and distortion, so stock should be removed evenly and symmetrically^[2].

Chucks: soft jaws, wide contact: For thin-walled rings, form the jaws to support the full circumference, or insert a plug in the bore before gripping
Plates: vacuum, double-sided tape or fixtures rather than a vise: Spread out the clamping force. Removing a lot from one side only causes warping, so machine both sides evenly^[1]
Burrs: prevent them first: Burrs depend on the material, tool condition, feed, speed and cooling^[28]. Keep the edge sharp, and use a backing plate or down milling on the exit side
Deburr without heat: Trim lightly with a blade, or cool first. Rubbing hard with a file melts the plastic again and raises a lip

Glass- and carbon-fiber-filled grades and tool wear

With PEEK GF30 (30% glass fiber), CF30 (30% carbon fiber) and reinforced grades of PAI and PPS, the plastic itself may be soft, but the fibers wear the tool like abrasive grains. MCAM states that for glass- or carbon-fiber-filled and graphite-filled plastics, polycrystalline diamond (PCD) or CVD diamond tools are essential. Cutting speeds are also lowered to about half those for unfilled grades (100–200 m/min for turning)^[1]. Victrex also says abrasive reinforced grades require carbide, coated carbide or diamond tools^[2].

Cut stock with a band saw with 4–6 mm tooth pitch: An ordinary circular saw can cause cracking^[1]
Expansion depends on direction: The coefficient of linear expansion of GF30 and CF30 is 4×10⁻⁵/K in the extrusion direction. Since the value varies with fiber orientation, match the measuring direction to the drawing^[8][9]
Don't breathe the dust: Chips from filled grades turn into fine dust. Use dust extraction and protective equipment

When fibers are cut while the plastic is still soft, heat can also weld the plastic onto the tool. In a study of trimming a thermoplastic CFRP with a PA66 matrix (45% carbon fiber), the chips were continuous, and when grinding with an electroplated diamond tool, the higher the speed and feed, the more molten plastic stuck to the tool, leaving fuzz, and the machining force rose from 49 N to over 100 N. On the other hand, cold air about 15 °C below ambient showed no clear effect under these conditions^[27]. With reinforced grades, before "cooling," what works is to avoid generating heat through tool grade and speed.

Plastic gears: molding vs. cutting, strength and wear

Plastic gears are injection-molded for mass production, and machined from stock (hobbing or milling) for prototypes, small lots, large gears, and expensive materials like PEEK. Their advantages are light weight, quiet running, operation without lubrication, and no rust^[35]. For gear cutting tools and gear basics, see Gear calculations and Gear study.

Standard	What it specifies	How it applies to machined gears
JIS B 1702-3:2020	Accuracy grades for injection-molded plastic cylindrical gears (flank and double-flank composite deviations). Module 0.1–2.0 mm, reference diameter 1–280 mm, 3 or more teeth, helix angle 45° or less, face width 0.2–40 mm^[29]	A standard for injection-molded gears. For machined gears, confirm on the drawing and purchase order which standard and grade apply
JIS B 1759:2019	Method for evaluating the tooth root bending strength of plastic cylindrical gears. The material's allowable bending stress is found by running tests, and safety against the nominal tangential force is evaluated. There is no corresponding international standard^[30]	Intended for injection-molded gears, but also applicable to machined gears. However, only where the failure mode is tooth root breakage; it cannot be used when wear is extremely advanced^[30]

Strength standards for plastic gears were also developed by the Technical Committee on Molded Plastic Gears of the Japan Society for Precision Engineering. Because there were many cases of "designing plastic gears without understanding them and causing accidents" and "plastic gears not even being considered, even though they could cut costs if used well," the committee created a strength calculation framework modeled on ISO 6336 for metal gears, and set values starting with the tooth root bending strength of POM, based mainly on endurance tests running against a steel master gear without lubrication^[31][32].

Strength and wear: temperature affects everything

Life is often set by wear rather than bending: In tests where gears machined from extruded POM rod were run without lubrication against steel and cast iron gears, wear rather than breakage set the usable limit under low-speed, long-life conditions, and wear was lower against steel than against cast iron^[33]
Heat reduces backlash: A follow-up report from the same study found that when expansion from friction heat exceeded wear, backlash during running became smaller than just after start-up. The wear rate was determined by load rather than speed^[34]. Set the center distance and backlash of plastic gears allowing for expansion when they warm up
Higher speed doesn't mean longer life: The tooth root breakage life of POM gears (oil-lubricated at 50 °C, constant torque) peaked at about 750 min⁻¹; at higher speeds, thermal failure took over and life became shorter. Shaping the root fillet to reduce stress concentration extended life at every speed^[35]. With machined gears, the root shape can be built into the cutting tool
Raise the load and they melt: For PVDF gears (module 3, 17 teeth, steel pinion, unlubricated at 1000 rpm), B10 life was 6.3×10⁶ cycles at 4 Nm; at 6 Nm the tooth root temperature rose to 123 °C, and at 8 Nm the gears melted and failed^[36]
Adding oil doesn't always help: In PEEK-on-steel sliding, depending on the conditions, lubricating oil can make wear worse than running dry^[37]. Choose lubricants by testing

Victrex, the plastic manufacturer, makes PEEK gears by injection molding as "VICTREX HPG," claiming 70% less weight, 80% less moment of inertia, and up to 50% (3 dB) less powertrain noise and vibration compared with metal gears. It also cites one-step injection molding that eliminates secondary machining, hardening and finishing as a cost advantage^[38]. The division of labor is molding for mass production and machining for prototypes and small lots.

Humanoid and collaborative robot joints and plastic gears

In robots that work alongside people (collaborative robots, service robots and humanoid robots), there is strong demand for lighter, quieter joint reducers, and there is a move toward plastic gears. Here we list only cases confirmed by company announcements and technical documents (as of October 2026).

Structure of the strain wave gearing (Harmonic Drive) used in collaborative robot joints (illustration). An elliptical cam flexes a thin external gear so that it meshes with an internal gear that has slightly more teeth, giving a large reduction. The parts targeted for plastic are mainly the thin external gear and the internal gear^[40][39].

Company	What they are doing	How plastic gears are used
Riken NPR × Harmonic Drive Systems	On September 18, 2026, announced an agreement to jointly develop a "plastic strain wave gear." For applications that share space with people, such as service robots, wearable devices and small mobility vehicles, the aim is safety on contact with people and lower power consumption through weight reduction. The development has future commercialization in view^[39]	Riken NPR has already developed a strain wave reducer with a flexspline (thin external gear) and ring gear (internal gear) made of super engineering plastics, and lists applications such as the tips of collaborative and SCARA robots, precision equipment, and each joint and the hands of humanoid robots as targets^[40]
igus	At Hannover Messe in June 2022, unveiled a prototype humanoid service robot. Its arms are the company's ReBeL robot arm, each able to carry 2 kg^[41]	At the heart of each ReBeL joint is a lubrication-free plastic (tribo-polymer) strain wave gear, integrated with the motor, encoder, force control and controller^[41]
Victrex	Handles PEEK gears (VICTREX HPG) from design to manufacturing for automotive, robotics and e-mobility^[38]	Says they can replace metal planetary gear sets in collaborative robots. Has not disclosed specific robot customers^[38]

What we left outNews reports and analyst views that a particular humanoid robot has adopted PEEK gears or parts could not be confirmed by company announcements, so they are not included. How far plastic gears will be used in robot joints depends on the results of the developments above.

From a machining standpoint, plastic gears for joints are expected to be mostly molded in mass production, but prototype and evaluation gears, molds, and the metal parts that work with plastic gears (housings, shafts, cams) are machining jobs. If you cut thin plastic gears, the "workholding," "annealing" and "temperature and dimensions" sections of this article apply directly.

8 common mistakes

Passed when measured right after cutting, failed the next day: Cutting heat and stress recovery moved the dimensions. Measure after cooling; for precision parts, rough → anneal → finish^[1][2]
The hole won't take the gauge: Holes in plastic shrink after machining. Set the pilot hole and reamer sizes based on how the material actually cuts^[1]
PC or PEI cracked later: Stress cracking from mineral-oil-based cutting oil. Switch to air or water-soluble coolant, and wash with IPA and deionized water after use^[1]
The plate warped: A lot was removed from one side only. Machine both sides evenly and symmetrically, and anneal if needed^[1]
The ring went oval: The chuck gripped too hard. Support the full circumference with soft jaws or a plug^[2]
Tools wear out fast on GF30: The fibers wear the tool. Switch to PCD or CVD diamond tools and lower the speed^[1]
UHMW-PE or PA wraps around the tool: Feed too slow or edge dull. Raise feed and speed, and use 2–3 flute, high-helix end mills^[1]
A plastic gear jammed during operation: It warmed up, expanded and lost its backlash. Set center distance and tooth thickness allowing for expansion^[34]

FAQ

Do I need special tools to machine plastics?: Unfilled POM, PA and PE can be cut with sharp HSS or carbide tools. For large quantities, carbide lasts longer. Glass- or carbon-fiber-filled grades and PI need PCD or CVD diamond tools^[1]. In every case, the common requirements are "a sharp edge, positive or 0° rake, and a generous relief angle."
Can I use the same tools I used on metal?: A tool whose edge has been rounded on metal will rub plastic and generate heat, causing tearing, burrs and dimensional errors. It is safer to use a fresh edge or tools set aside for plastics only.
Does PEEK always need annealing?: Stock is often already annealed by the manufacturer, and if there is little stock to remove and the tolerances are generous, annealing can sometimes be skipped. For parts with heavy stock removal, thin walls, tight tolerances or high-temperature service, anneal after roughing and then finish. The conditions differ between MCAM (250 °C, 10 min/mm) and Victrex (200 °C, 1 hour/mm), so decide which one you will follow^[1][2].
At what temperature should plastic parts be measured?: Drawing dimensions are referenced to 20 °C. If the measuring room is not at 20 °C, estimate the temperature effect with the calculator above and check whether it is negligible relative to the tolerance. For 100 mm of PEEK, an 8 °C difference moves it by about 0.04 mm, which is not negligible for a ±0.05 mm tolerance.
My nylon parts changed size during the rainy season.: Nylon absorbs water and grows. When saturated in water it can absorb up to about 7% of its weight, and its dimensions can change by up to about 2%^[5]. Where dimensional stability is needed, switch to a low-absorption plastic such as POM or PEEK, or condition the parts to the service environment before measuring.
Are machined plastic gears as strong as molded ones?: No. Molded and machined parts differ in material state, such as the surface skin, fiber orientation and residual stress, so their strength can differ too. JIS B 1759 says it can be applied to machined gears, but the material's allowable stress is to be found by running tests^[30].

Values for each material are in the Materials database, measurement principles in Measurement error basics, gears in Gear study and Gear calculations, and cutting condition calculations in Calculators.