Machining PEEK, Delrin, and UHMW: A Precision Shop Guide
Machining PEEK, Delrin, and UHMW looks simple on paper. All three cut cleanly on a CNC mill or lathe, and all three produce parts that look finished straight off the machine. The trouble starts when a part has to hold a tolerance of a few thousandths of an inch after it leaves the fixture, or when it has to survive months of sliding contact without drifting out of spec. At Summit City Precision Machining, we see the same mistakes repeatedly: the wrong material chosen for the job, the right material cut with the wrong tooling, and parts measured before they have finished settling. This guide covers how each plastic behaves on the machine and what we do about it.
Table of Contents
Quick Takeaways
Key Insight | Explanation |
|---|---|
Choose Delrin when a precision fit must hold under load | Delrin's low creep keeps bores and bushings close to nominal. UHMW drifts under sustained load, and drift of 0.1 mm or more is enough to misalign a fit. |
Use UHMW for sliding and impact, not for dimensions | Its dry friction of roughly 0.10 to 0.22 and its impact resistance make it a strong choice for wear strips, liners, and chain guides. |
Treat PEEK as a heat problem first | PEEK quality depends on sharp carbide edges and controlled heat. A dull tool rubs and smears the surface, and the friction leaves stress in the part. |
Rough, rest, then finish critical Delrin and PEEK | Stock releases internal stress as material is removed. Leave a finish allowance and let the part settle before the final pass. |
Measure after the part reaches room temperature | UHMW expands far more than metal with temperature change, and all three plastics can move after machining. Inspect a stable part, not one still warm from the cut. |
Do not specify PEEK by default | PEEK commonly costs roughly 7 to 20 times as much as Delrin or nylon, and Delrin covers many jobs that do not need PEEK's heat or chemical resistance. |
How PEEK, Delrin, and UHMW Differ
The three plastics share one trait that causes trouble: they all cut well enough to make a shop think the job is easy. Underneath, they are built from very different polymers, and those differences decide what each one does on the machine and in service. PEEK is a high-performance aromatic polymer. Delrin is an acetal, also called POM. UHMW is polyethylene with an extremely long molecular chain.
The published numbers show the spread. Delrin has tensile strength around 60 to 70 MPa and a flexural modulus of roughly 3.0 GPa, which is why it holds its shape under load. One CNC plastics guide we reviewed puts PEEK tensile strength near 100 MPa, which places it above Delrin on strength and in a different class on heat. UHMW sits at 20 to 40 MPa with a much lower modulus, so it bends and absorbs impact rather than resisting deflection.
Strength and stiffness, in practical terms
Stiffness matters most for thin walls and long unsupported features. A Delrin bushing that drifts out of its bore in service usually traces back to material movement, not to the cut itself. UHMW flexes under clamping force that a Delrin part handles without complaint, so a UHMW part that looks perfect at the mill can distort the moment it leaves the vise.
Temperature limits that change the material choice
Heat is where the three materials separate most sharply. The CNC plastics guide lists continuous service for PEEK up to 480 °F (250 °C), while Delrin's maximum service temperature is 180 °F (82 °C). If a part sees a hot washdown, an oven cycle, or a warm engine bay, Delrin may be the wrong call before dimensional accuracy even enters the picture.
Material cost as part of the decision
Material cost is the third variable. The same guide cites rough per-foot prices for a 2 inch diameter rod, putting nylon 6/6 around $10 to $20, Delrin around $15 to $25, and PEEK around $180 to $300. Those figures move with supplier and market conditions, so treat them as a snapshot. The ratio is the useful part: PEEK commonly costs roughly 7 to 20 times as much as the other two, and that premium should be earned by the application.
Machining PEEK Without Heat Damage
PEEK is the material most likely to be machined badly by a shop that already runs Delrin well. It looks forgiving because it is rigid and holds its shape, but heat from a dull or rubbing tool softens the surface and leaves stress behind. The part may pass a quick check and then shift once it cools and sits in its fixture.
Tooling for PEEK
Machinists disagree about tooling. Some report running PEEK with the same new aluminum tooling they use for Delrin and getting stable results. The comparison guide we reviewed recommends carbide, and that matches what we see when tolerances tighten. Our position is simple: use sharp carbide with a positive rake, and replace the insert before it starts to rub. Worn tooling is the real problem, regardless of the insert material.
Pro tip: Log the edge condition of every PEEK tool at part changeover. PEEK tolerates a slightly worn edge for a while, but the surface smears gradually, and the smear is hard to spot until the part reaches inspection.
Heat and chip control
Run cutting parameters that keep the chip thin and clean, and avoid long dwell in one spot. Heat that stays in the cut zone softens the material, so use coolant or air blast consistently rather than intermittently. Chips should leave the cut as separate pieces, not as a stringy ribbon wrapping the tool. If the surface looks glossy or smeared, the first fix is usually a higher feed that thins the chip, because slower cutting keeps the tool rubbing longer.
Relieving stress after roughing
PEEK stock carries internal stress from extrusion or molding. Remove the bulk of the material, leave a finish allowance, and let the part rest before the final pass. For parts with heavy material removal or thin walls, a stress relief step between roughing and finishing is often the difference between a part that measures correctly on the day it ships and one that is out of tolerance a week later. It is a shop habit as much as a material rule, and it pays for itself on tight work.
Machining Delrin to Tight Tolerances
Delrin is the material we reach for when a precision part needs low friction and clean, predictable chips. The comparison guide lists tolerances as tight as ±0.001 in (0.025 mm) for Delrin on CNC equipment, and its low water absorption, around 0.20 percent, helps the part hold size through humidity changes. Those numbers only hold when the process respects how Delrin moves during machining.
Why Delrin moves after it is cut
Delrin rod and sheet carry internal stress from how they were produced. When you remove a lot of material from one side, the part can bow or twist as that stress releases. The fix is a roughing pass that leaves generous finish allowance, a rest period, and then the finish cut. Clamp with light, even force, because a hard squeeze can distort thin walls, and the spring-back after release is what ruins a bore size.
Moisture and dimensional stability
Nylon absorbs enough moisture to swell by a percent or two, which is why nylon parts often need a different tolerance strategy. Delrin's water uptake is far lower, so humidity has much less effect on size. That stability is a strong reason to choose Delrin over nylon for bushings and gears. It does not make Delrin immune to chemistry, though. The same guide notes that Delrin is flammable (HB rated) and is attacked by strong acids and oxidizers, so check the process fluids before you commit to it.
Fixturing and chip handling
Delrin produces good chip formation, which makes it easier to run unattended. The main risk is over-clamping. Use soft jaws or pads, support long features near the cut, and confirm the part is not flexing when the tool enters. A common mistake is specifying Delrin to a tight tolerance on a setup that clamps the part harder than the tolerance allows. Fix the holding first, then the program.
UHMW CNC Machining: Where It Works
UHMW is one of the most useful plastics in our part mix, but only for the jobs it was made for. Its dry friction of roughly 0.10 to 0.22 and its impact and abrasion resistance make it a strong choice for wear strips, liners, chain guides, and impact pads. An engineering forum discussion we reviewed reached a similar conclusion: for a sliding surface, UHMW is plenty good, while Delrin is the better choice when dimensional stability matters.
Creep and why it ruins fits
The biggest UHMW failure we see in the field is creep. Under sustained load, UHMW slowly deforms, and the CNC guide we reviewed notes that precision fits can drift by 0.1 mm or more. That is enough to change the clearance on a guide rail or push a shaft out of alignment. If the design needs a bore to stay at size under load, specify Delrin. If the part slides and absorbs shock, UHMW is the better material.
Thermal growth during machining and service
UHMW expands considerably more than metal with temperature change. A part that measures correctly at the machine can read differently once it reaches the inspection room, because it is still settling to ambient temperature. Machine UHMW in a stable room, let it come to temperature, and measure after it has stabilized. Do not assume a warm part is at final size.
Pro tip: Use sharp tools with generous clearance on UHMW and take light finishing passes. The material is soft and gummy, so a dull edge tends to tear and smear rather than cut, and the surface can look fuzzy even when the dimension is correct.
Cutting UHMW cleanly
UHMW has very low surface energy, which helps it slide but makes it hard to hold on a tool and easy to smear. Keep chips clear of the cut, because they can be pulled back into the surface. Use light holding force, since UHMW deflects under pressure and springs back when released. Sawing rough stock before milling can cut machine time, but the finish passes still need the same care.
Side-by-Side Comparison
Most material mistakes come from choosing by price or by familiarity instead of by the failure mode the part must avoid. The table below lists the primary use case and the machining watch-out for each material, based on how these plastics behave in our shop.
Material | Best Use | Machining Watch-Out |
|---|---|---|
PEEK | High-temperature parts, sterilization cycles, and harsh chemical exposure where the cost premium is justified | Heat buildup from dull or rubbing tools. Use sharp carbide, manage heat, and plan stress relief for tight parts. |
Delrin (acetal) | Precision gears, bushings, and housings that must hold size under sustained load | Internal stress release after material removal. Rough, rest, then finish, and avoid over-clamping thin walls. |
UHMW | Wear strips, liners, chain guides, and impact pads on sliding or shock-loaded surfaces | Creep and thermal growth make it unsuitable for tight fits. Measure after the part stabilizes and keep tools sharp. |
PEEK and Delrin overlap in some applications, such as small gears and bushings, but the decision usually turns on temperature. If the part never sees more than 180 °F (82 °C) and does not face aggressive chemistry, Delrin is the more economical choice. Once the environment demands PEEK's heat or chemical resistance, the machining plan has to change with it.
Most plastic scrap traces back to a material chosen for the wrong reason, not to a machine that cut it badly.
UHMW is the odd one out because its failure is rarely visible at the machine. A UHMW part can measure perfectly and still fail in service after weeks of load. That is why we pair material selection with a clear statement of the function the part must perform, not just its drawing dimensions.
Inspection and Dimensional Verification
Measurement is where plastic parts reveal whether the machining process was right. A dimension that is correct at the machine can drift once the part relaxes and reaches room temperature, so the inspection plan must account for time as well as geometry. At Summit City Precision Machining, we treat plastic inspection as its own step with its own timing rules, not a quick check at the end of the cut.
Wait before you measure
For Delrin and PEEK, we let the part stabilize at shop temperature before final measurement, particularly for bores and thin walls. For UHMW, the wait matters more because thermal change is larger. Record the time between machining and inspection on the traveler, so a result can be compared with the same conditions later.
Programming a CMM for soft features
CMM probes can deform soft plastic if touch force is too high or the probe path crosses a fresh edge. Use low-force probing where the system allows it, and keep probe paths away from burrs that may tear. For critical bores, pair the CMM result with a gage pin or go/no-go plug check at the same temperature state, and compare the two. CMM programming for plastics is a different discipline from programming for metal, and the difference shows up in the data.
Documentation for first article and PPAP
Customers buying plastic components for automotive, aerospace, or production tooling often need first article inspection and PPAP documentation. Those reports should carry the plastic-specific details: material grade, machining sequence, stress relief step, and inspection temperature. A report that lists only final dimensions gives the next machinist nothing to repeat, and it leaves the customer with no way to trace a drift back to its cause.
Frequently Asked Questions
Can I run PEEK on the same tooling and setup I use for Delrin?
You can share a machine and many fixtures, but you should not share the tooling plan. Delrin is forgiving of a slightly worn edge, while PEEK is not, because heat builds quickly when the tool starts to rub. Use sharp carbide for PEEK and track tool life separately.
Why does my UHMW part measure differently the next day?
UHMW changes size with temperature and under sustained load. A part measured while still warm from machining can read differently once it reaches room temperature, and a part under continuous compression can creep over time. Measure after the part stabilizes and keep the time between machining and inspection consistent.
Is Delrin suitable for food contact or medical parts?
Some Delrin grades are sold as FDA-compliant, according to the CNC plastics guide we reviewed. Confirm the specific grade, its certification documents, and the requirements of your application before you release the design. Do not assume that every acetal grade carries the same approval.
What tolerance can I realistically hold on machined plastic?
Delrin can be machined to tolerances as tight as ±0.001 in (0.025 mm) on CNC equipment, according to the comparison guide we reviewed. Achieving that depends on fixturing, stress relief, and measurement timing as much as on the machine. UHMW and PEEK need their own tolerance plans rather than the numbers used for Delrin.
Is PEEK worth the cost for most applications?
For most applications, no. PEEK commonly costs roughly 7 to 20 times as much as Delrin or nylon, so it should be specified only when the part sees temperatures above Delrin's limit, sterilization cycles, or chemicals that Delrin cannot survive. If none of those apply, Delrin is usually the better engineering and budget decision.
Does Summit City Precision Machining provide inspection reports for plastic parts?
Yes. Our shop provides first article inspection and PPAP documentation for machined components, including plastic parts, and our MetroLab division supports calibration needs for the gages used in that work. Share your inspection requirements at the quoting stage so the plan can account for the material's timing and temperature needs.
Tell us which plastic has given your shop the most trouble, and what the part was doing when it failed, in the comments or by reaching out directly.
Comments