Specifying Surface Finish on Machined POM: What the Shop Can Hold

Polyoxymethylene (POM) machined components are specified according to surface texture parameters not as a single roughness callout but as a process capability envelope defined by grade, tool edge preparation, stock removal, and post-machining thermal stabilization. Unfilled copolymer POM with a melt flow index of 9 g/10 min under ISO 1133-1:2022 or ASTM D1238-23 at 190 °C / 2.16 kg exhibits a tensile modulus of 2.6–3.2 GPa under ISO 527-1 and a coefficient of linear thermal expansion of 110–130 × 10⁻⁶ K⁻¹ under ISO 11359-2. These values establish the metrology baseline: a 50 mm feature changes by 110–130 µm for a 20 °C temperature swing. Surface texture specifications below 0.4 µm Ra therefore require not only tool control but also thermal equalization and, in many shops, a documented post-machining conditioning window.

What Ra, Rz, and Rmax Values Are Considered Holdable on Unfilled POM?

For a given profile parameter, the holdable roughness on unfilled POM depends on whether the operation is turning, milling, reaming, or lapping. Under ISO 21920-2:2021 profile evaluation, general CNC turning with polished carbide inserts can hold 0.8–1.6 µm Ra on POM copolymer when feed is not more than 0.15 mm/rev. Finish turning with polycrystalline diamond (PCD) inserts and an edge radius below 10 µm can hold 0.4–0.8 µm Ra. A quoted limit of 0.2 µm Ra is occasionally reported for light finishing passes with feed below 0.05 mm/rev and depth of cut below 0.25 mm, but published supplier data for this specific configuration is limited. Rz values typically fall between 1.5 µm and 6 µm when Ra is 0.4–1.6 µm, with Rmax roughly 1.5–2 times Rz. The relevant cutoff length selected according to ISO 4288:1996 is 0.8 mm for an expected Ra in the 0.1–2 µm range, with an evaluation length of 4 mm. A specification reading 0.8 µm Ra without a cutoff and filter designation leaves the measurement open to misinterpretation because ISO 1302:2002 and ASME B46.1-2019 require the roughness parameter, cutoff, and evaluation length to be stated. Centerless grinding and abrasive finishing are less common on POM because the material can load the wheel surface; lapping or film polishing may produce 0.1 µm Ra or lower, but the process window is narrow and operator-dependent.

Cutter Geometry, Feed Rate, and the Burr-Dominated Surface Limit

On production machining lines, surface finish on POM is governed less by material hardness than by side flow, elastic recovery, and burr generation at the cut exit. For turning, a sharp edge with a positive rake of 10°–15° and clearance of 8°–12° prevents rubbing; when clearance falls below 6°, the flank rubs and creates a smeared surface with an Ra increase of 0.2–0.4 µm. Feed rate is the controlling threshold. Above 0.15 mm/rev the cut zone compresses POM enough to cause elastic recovery that reduces effective clearance and produces a burr at the shoulder. Surface roughness can then shift from 0.8 µm Ra to 1.6–2.0 µm Ra even on stable machines. Cutting speed for unfilled POM with PCD is usually set between 200 m/min and 500 m/min on small-diameter parts. High spindle speeds on a 12-station Swiss-type lathe assist chip separation, but continuous chips can wrap around the part and scratch the finished surface if the through-tool coolant pressure is below 20 bar. Because POM yields continuous ductile chips, edge burrs cannot be fully eliminated by tool geometry alone. Programming a spring pass of 0.00–0.02 mm radial engagement removes some formed burrs but may smear material. Parts with sharp-edge callouts require a secondary deburring operation such as vibratory finishing or manual scraping under magnification.

Edge condition is part of the surface finish envelope even when the drawing does not call out a burr specification. On a production run of turned POM copolymer bosses, burr height at the shoulder is typically held below 0.05 mm only when the insert nose radius is 0.2–0.4 mm and the feed is below 0.08 mm/rev. If the shoulder geometry permits a 45° edge break, the burr tendency decreases because the exit angle is altered. Deburring by tumbling in ceramic media can produce a uniform matte surface but may locally increase Ra from 0.8 µm to 1.2 µm on flat sealing faces. In such cases, the roughness specification should be interpreted after the final deburring operation, not after turning alone.

Measurement of machined POM surface texture requires a stylus force low enough to avoid plastic micro-indentation. In instruments conforming to ISO 3274, a diamond stylus tip radius of 2 µm and a measuring force of 0.75 mN are common. Because POM has a tensile modulus of 2.6–3.2 GPa, the contact stress is below the yield surface, but shallow elastic recovery can occur. The part should be thermally equalized for a minimum of 4 h at the metrology room temperature after machining, particularly if high-pressure coolant was used. The coefficient of linear thermal expansion of 110–130 × 10⁻⁶ K⁻¹ can change the effective roughness profile height through substrate distortion. Equilibrium moisture uptake of 0.2% at 23 °C / 50% RH per ISO 62 produces dimensional swelling that may affect micrometer-scale topography; published data quantifying the exact Ra shift under that condition is limited. A part measured immediately after machining and again after 24 h can show variation greater than the instrument repeatability because of stress relaxation and thermal drift. For this reason, surface finish capability studies on POM should include a stabilization interval and a defined measurement location on the feature.

Values in the table are typical ranges from industrial tooling and material supplier data; they are not specification limits unless validated on the production lot.

When Pre-Machining Annealing Shifts the Metrology Baseline

Annealing of POM copolymer before final finishing is performed at 150–160 °C in circulating air or oil, with a soak time of 1–2 h per 25 mm of section thickness according to material supplier bulletins. This step relaxes molded-in or prior machining stress and changes the crystalline order sufficiently to alter the way the tool shears the surface. Production lots that skip annealing often show dimensional drift after machining, which can invalidate both tolerance and surface roughness measurements. The annealed workpiece should be allowed to cool slowly, ideally at 20–30 °C/h, and a roughing allowance of 0.25–0.5 mm should be removed after annealing to eliminate the oxidized skin. Published comparative data on annealed versus unannealed POM surface roughness is limited, but stress-relaxed POM reduces the risk of post-machining distortion that appears as profile slope error rather than roughness height. Post-machining distortion from residual stress is a recognized failure mode in extruded POM plate; dimensional changes can occur after 24 h, altering profile waviness. A surface finish of 0.8 µm Ra may be achieved immediately but can degrade to 1.2 µm Ra after 24 h on heavily stressed workpieces; published data for specific roughness change is limited.

Coolant selection is an operational boundary because POM is susceptible to environmental stress cracking in contact with some chemical agents. Water-miscible emulsions at 5–10% concentration are typically used for both cooling and chip flushing. High-pressure coolant delivery at 20–70 bar through the tool reduces built-up edge and the resultant smearing of the machined surface. Without through-tool coolant, localized temperatures can approach the heat deflection temperature under 1.8 MPa of 95–110 °C per ISO 75-2, producing a glazed surface layer and burr adhesion. POM should not be machined with certain chlorinated solvent-based cutting fluids or strong acid additives because environmental stress cracking can occur. Published data for the specific surface finish effect of individual coolant improvers is limited; the material supplier’s chemical resistance data should be consulted before switching coolant chemistry on a production line.

Interaction Between Surface Finish, Tolerance, and Batch-to-Batch Melt Flow

The relationship between surface finish and tight dimensional tolerance is not independent in POM. A shop can hold 0.8 µm Ra more repeatably on turned POM if the tolerance band is no tighter than ±0.025 mm. When the same part carries a ±0.005 mm diameter tolerance, the required thermal compensation, tool offset updates, and in-process measurement make the surface finish less repeatable because the finishing pass may need to be interrupted for dimensional corrections. Batch-to-batch variation in POM melt flow index—from 7 g/10 min to 12 g/10 min under ISO 1133-1—is an experience-based source of slight roughness drift, typically less than 0.2 µm Ra on production turning lines. Shops compensate by adjusting cutting speed rather than feed because feed changes have a stronger effect on profile peak spacing and burr size. These holdable ranges apply to unfilled POM homopolymer and copolymer; glass-filled or mineral-filled grades produce different surface finish as a result of fiber pull-out and abrasive tool wear. Published machined surface ranges for reinforced POM are typically 1.6–3.2 µm Ra, and drawing requirements below 0.8 µm Ra on filled grades require a prior validation study.