Polyurethane CNC Machining Services: A Buyer's Checklist

• 1
  Five things to settle before you send a drawing
• 2
  Which process fits your polyurethane part
• 3
  Durometer grade decides your tolerance callout
• 4
  Why sharp tooling and light passes matter more than spindle speed
• 5
  Fixturing soft elastomer without crushing it
• 6
  Five checks on any polyurethane cnc machining services quote
• 7
  What drives price, and when to skip the finish
• 8
  How to prepare a polyurethane RFQ that gets a usable quote

Polyurethane is a family, not a grade. Suppliers cast or cure it across a wide hardness band, from roughly Shore 30A (soft, rubber-band territory) to Shore 85D (hard, closer to a rigid plastic). For machining, thermoset PU in sheet, rod or tube is the usual stock form. The durometer number is the single most useful piece of information on a drawing, because it predicts how the material behaves under a tool.

Here is the practical rule. Above roughly 70D, polyurethane machines much like a tough engineering plastic. You can hold ±0.02 mm on a bored hole without heroics, and ±0.005 mm is achievable on a rigid grade with good fixturing and a finishing pass. Between 80A and 60D, expect ±0.05 mm on external features and looser on thin walls. Below 70A, stop thinking in thousandths and start thinking in functional fit.

Soft grades compress under the cutting tool and under the clamp. A 50A part measured on the machine can read 0.1 mm oversize once the pressure is released. That is not a machining error; it is elastic recovery. If the design needs a hard tolerance on a soft grade, the honest answer is to change the design, not to demand a tighter number from the shop.

Ask your supplier which specific grade they stock. A shop that stocks 60D and 95A sheet can start in 24 hours. A shop that has to order a custom cast grade will add days to your lead time. Grade availability is a lead-time question, not just a material question.

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  Above 70DTreat like a tough plastic; ±0.02 mm is routine, ±0.005 mm with a finish pass.
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  60D to 80APlan ±0.05 mm on external features, looser on walls under 3 mm.
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  Below 70ASpecify functional fit, not decimals. Elastic recovery dominates.

Polyurethane cuts best when the tool slices rather than pushes. Experienced machinists use razor-sharp, highly polished carbide with a positive rake angle. A dull edge rubs the surface, generates heat, and leaves a torn finish that no amount of polishing will fix cleanly. For soft grades, some shops run tooling ground for aluminum, with a high helix to clear chips fast.

Heat is the second enemy. Polyurethane has low thermal conductivity, so friction heat stays at the cut edge. Above roughly 80 °C to 100 °C, depending on the grade, the surface can smear or gum. The fix is moderate surface speed, generous feed per tooth, and air or mist cooling rather than flooding coolant. Flood coolant on an open-cell or soft PU can also swell the material if it absorbs moisture.

Chip clearance matters more than on metal. PU chips are light, stringy and static-prone. They pack into pockets and recut, which ruins the finish. A high-helix tool plus air blast keeps the pocket clean. On deep pockets, peck or trochoidal paths reduce radial engagement and keep the tool from loading up.

For finishing, take a light spring pass. On grades between 80A and 60D, a 0.1 mm to 0.2 mm finish pass at reduced feed removes the elastic springback left by the roughing cut. Skip it and your final dimension will drift high on external profiles and low on bores.

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  Tool geometryPolished carbide, positive rake, high helix for chip evacuation.
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  CoolingAir or mist. Avoid flooding soft grades that can absorb moisture.
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  Finish pass0.1–0.2 mm spring pass at reduced feed to remove springback.

Clamping is where most polyurethane jobs go wrong. A standard vise on a 60A block will compress the material and distort every feature cut while it is held. The part springs back after unclamping, and the dimensions are wrong in a way that is hard to trace. The fix is to spread the clamping force over a large area and support the part from below.

Common approaches: soft jaws machined to the part profile, a vacuum plate for flat sheet, or a sacrificial sub-plate that the part is bonded to with a low-tack adhesive. For thin walls, the shop may leave a web of stock that is cut away in a second operation. Each of these adds setup time, and setup time is a real cost you should expect to see in the quote.

Ask how the shop plans to hold the part before you approve the quote. If the answer is vague, the first article will likely fail inspection. A competent shop will describe the fixture in the DFM report, often with a sketch of the soft jaw profile or the vacuum nest.

For parts with both a rigid insert and a soft PU body, sequence matters. Machine the metal insert first to its final tolerance, then machine or bond the PU around it. Trying to machine the assembled part in one pass usually deflects the PU where the tool crosses the bond line.

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  Soft jawsMachined to the part profile; spreads clamp load over a wide area.
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  Vacuum plateGood for flat sheet and large panels over 500 mm.
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  Sacrificial platePart bonded down, then cut free in a second operation.

A quote for polyurethane cnc machining services should read differently from a quote for aluminum. If the two look identical, the shop has not thought about the material. The first check is grade: does the quote name the durometer and the stock form? The second is tolerance: does the shop flag which features are hard to hold, or does it accept every callout without comment?

The third check is fixturing. A good DFM report will mention soft jaws, vacuum workholding or a bonded plate. The fourth is inspection. Polyurethane moves after machining, so a measurement taken on the machine is not the final number. Ask whether the part is measured after a rest period, and whether the report includes the measurement method.

The fifth check is lead time. Polyurethane stock in common grades is widely available, so a shop that quotes two weeks for a 20-piece run is either busy or ordering material. With stock on the shelf, a 3–5 day ship window is normal. Ask what grade they have on hand before you commit to a date.

One more thing worth checking: certifications. For medical or automotive programs, ISO 13485 or IATF 16949 may be a hard requirement. For general industrial work, ISO 9001 is usually enough. Confirm the certificate is current and covers the site that will actually run your parts.

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  Grade namedQuote states durometer and stock form, not just 'polyurethane'.
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  Tolerance flaggedDFM notes which features are hard to hold on this grade.
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  Fixture describedSoft jaws, vacuum nest or bonded plate mentioned in the plan.
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  Inspection methodMeasured after rest, with method and report stated.
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  Lead time realistic3–5 days with stock on hand; longer if material is ordered.

Polyurethane parts are usually more expensive per cubic centimeter to machine than aluminum, mainly because feeds and speeds are lower and setups take longer. The cost drivers are durometer (softer is slower), feature count, tolerance class, and fixturing complexity. A simple 80D bushing may cost less than a 60A gasket with a complex profile, even though the gasket is smaller.

Finishing is often unnecessary. Polyurethane is chosen for its surface properties, and a clean machined finish is usually the goal. Bead blasting can hide tool marks but also rounds edges and changes the fit. Polishing works on harder grades and does little on soft ones. If the part is a seal or a wear pad, the as-machined surface is often the correct one.

Surface finish targets on PU are looser than on metal. Ra 1.6–3.2 μm is a normal as-machined result on 80D. On 60A, Ra 3.2 μm or rougher is realistic. Asking for Ra 0.8 μm on a soft grade is possible with a spring pass and a polished tool, but expect the shop to push back, and expect a higher price.

Deburring deserves a mention. A sharp edge on a soft PU part is a stress riser. Most shops will break edges with a hand tool or a light tumbling pass. Specify an edge break if the part will be handled or press-fitted, and say whether a chamfer or a radius is acceptable.

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  Softer is slowerLower feeds and larger springback add cycle time.
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  Skip bead blastingIt rounds edges and can change a press fit.
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  Realistic Ra1.6–3.2 μm on 80D; 3.2 μm or rougher on 60A.