Machining Delrin components: tolerance and stress-relief notes begin with the stock form rather than the final drawing. Polyoxymethylene (POM) homopolymer bar and plate are produced by continuous extrusion or press sintering; the as-supplied skin typically contains compressive residual stress introduced during cooling from the melt. A facing cut across a 300 mm length of as-extruded 40 mm plate can release bowing of 0.2 mm to 0.5 mm before any further dimensional inspection is attempted. For this reason, ISO 2768-1 general tolerances applied directly to raw stock are not adequate for POM parts where the machining sequence has not stabilised the workpiece.

Homopolymer grades such as Delrin 150 and 570 exhibit higher yield strength and hardness than copolymer acetal grades, but the homopolymer is more sensitive to internal stress concentration at sharp shoulders. Copolymer grades such as Celcon M90 and Hostaform C13021 absorb slightly less moisture at elevated humidity and are consequently specified for parts machined to total tolerance bands below 0.05 mm when relative humidity can exceed 60 %. The difference is small in absolute terms, but it is significant when the available tolerance band is only 0.03 mm to 0.05 mm.

What Tolerance Band Can Be Held in CNC Turning of POM Homopolymer?

With sharp positive-rake carbide inserts, polished chip grooves, and flood coolant, turned diameters between 6 mm and 50 mm can be held to ±0.03 mm in a single setup on a rigid lathe. Bored holes from 10 mm to 50 mm are more difficult because POM recovers elastically after tool withdrawal; a boring bar is preferred to a reamer, and the bore may close by 0.01 mm to 0.03 mm after the tool passes. ISO 286-1 IT7 is attainable on as-machined POM-H for bores in this range only when the blank is stress-relieved between rough and finish passes. On larger features above 100 mm, the achievable band widens to ±0.10 mm because the linear thermal expansion coefficient of 11×10⁻⁵ K⁻¹ under ISO 11359-2 means a 5 °C shop-floor shift alters a 100 mm dimension by 0.055 mm. Inspection at 23 °C ± 1 °C is therefore required before accepting tighter than ISO 2768-1 medium class.

For milled pockets and slot walls in 30 mm thickness plate, climb milling with a two-flute carbide end mill at cutting speed near 250 m/min and feed per tooth 0.10 mm produces wall parallelism of 0.03 mm over 50 mm depth only when the blank is not locally heated above 45 °C. If the part is machined dry and surface temperature rises above 70 °C, elastic recovery after unclamping can produce a negative dimensional shift of 0.04 mm to 0.08 mm in the machined slot width. This is a process conflict: removing material at high rates lowers cycle time but creates thermal input that consumes most of the available tolerance band before the part reaches inspection. The thermal effect is larger than the moisture effect over a short machining cycle.

The values in the accompanying table are integration ranges from ISO 286-1 and ISO 2768-1 general tolerance classes; they are not material-specific guarantees. Published data for this specific configuration is limited because achievable POM machining tolerance depends on part stiffness, clamping force, and tool condition. A thin-walled ring machined from 100 mm diameter bar with wall thickness below 3 mm will not hold the same band as a solid boss of equal outside diameter.

Residual Stress Redistribution and Annealing Temperature Limits

POM stress-relief annealing is performed between roughing and final finishing, not after final inspection. A heated oil bath at 160 °C for POM-H parts with maximum cross-section under 25 mm, with dwell time of 1 h/25 mm and slow cooling at ≤30 °C/h to 50 °C, reduces subsequent post-machining distortion. For POM-C the upper setpoint is normally reduced to 150 °C to limit hydrolytic degradation and surface oxidation. The oven or bath must maintain temperature uniformity within ±3 °C because polyoxymethylene homopolymer has a melting region near 175 °C; overshoot above 165 °C risks local softening, surface blisters, and nonrecoverable deformation under the part’s own weight.

Parts must be jigged flat during annealing; unsupported plates sag because the heat deflection temperature under 1.8 MPa by ISO 75-1/2 is approximately 95 °C for POM-H and 105 °C for POM-C. Creep modulus at 160 °C is low, and a dead load of even 0.1 MPa can produce nonrecoverable warpage. In forced-air ovens, air circulation should exceed 1 m/s and parts should be spaced at least 25 mm from heating elements. Oil immersion minimises oxidation but introduces a cleaning step; surface water on parts entering hot oil can cause spatter, so a pre-drying step at 80 °C for 2 h is required when ambient relative humidity is above 60 %.

When stress relief is omitted prior to final dimensional inspection, POM parts can pass incoming inspection and then exhibit out-of-roundness of 0.05 mm to 0.15 mm within 24 h after the first elevated-temperature cycle. This delayed movement is not moisture absorption alone; it is residual stress relaxation assisted by the part being above the low-temperature beta relaxation. For POM the beta transition occurs near −60 °C to −40 °C, while the alpha relaxation associated with longer-range chain mobility lies above 100 °C. Post-machining annealing therefore shifts the relevant relaxation processes close enough to operational conditions that further movement becomes less probable.

If Interrupted Cuts and Sharp Internal Corners Are Required, Notch Sensitivity Controls Tolerance Strategy

POM homopolymer has notched Charpy impact values under ISO 179-1/1eA of approximately 7 kJ/m², which is lower than many unfilled engineering thermoplastics. Sharp internal radii below 0.5 mm create stress concentrators that can fail in service even when machining tolerances are acceptable. For load-bearing features, root radius should not be less than 0.75 mm; this value is derived from notch geometry rather than final drawing dimension. The machine tool path should be generated with trochoidal engagement for slot widths less than 8 mm to limit radial cutting force below 15 N/mm of flute length, reducing corner breakout at the pocket exit.

Tool edge geometry exerts direct control over edge chipping. A carbide insert with rake angle between 8° and 12°, clearance angle between 5° and 10°, and a polished cutting edge is required for POM homopolymer. Negative-rake tools generate compressive cutting forces that can indent the soft workpiece and create local stress peaks. When machining continuous chips, a chip breaker with feed per revolution between 0.05 mm and 0.15 mm prevents chip wrap around the workpiece; chip wrap can scratch finished bores and alter surface geometry beyond the specified roughness. For close-tolerance bores, the spindle should not dwell at the shoulder because friction heating can raise wall temperature by more than 15 °C within seconds and change the final diameter.

Coolant selection is governed by chemical compatibility rather than cutting performance alone. A water-miscible emulsion with pH between 8.5 and 9.5 is typical, but chlorinated cutting fluids should be avoided at high cutting temperatures because hydrogen chloride formation accelerates acid-catalysed degradation of POM. Continuous alkaline coolant at pH above 10 can also attack the surface over hours of recirculation. This limitation means that machining centres used for POM must be cleaned of residues from fluids used on other materials before close-tolerance work begins.

The property differences in the accompanying table influence tolerance retention rather than initial machinability. Higher modulus reduces clamping deflection in homopolymer, but higher thermal expansion sensitivity increases dimensional error when shop-floor temperature varies. These effects are not linear and must be evaluated with the part at thermal equilibrium; measuring immediately after machining creates bias because the part surface can be 5 °C to 15 °C above room temperature while the core remains at ambient temperature.

Thermal Expansion and Moisture Shift Are Not Linear During First Conditioning

Conditioning a finished POM part at 23 °C and 50 % RH for 24 h before final metrology removes much of the short-term thermal and moisture hysteresis, but it does not eliminate all post-conditioning movement. Moisture absorption under ISO 62 for POM at 50 % RH is approximately 0.2 %, which is much lower than polyamide values. However, the through-thickness moisture gradient in a 30 mm thick slab creates asymmetric expansion of 0.01 mm to 0.03 mm if one face is sealed by a mating component. This is a tolerance problem only when the part is constrained on one side and free to bow on the other.

For food-contact machined components, natural POM homopolymer falls under FDA 21 CFR 177.2470 and EU regulation 10/2011 when migration testing is performed on the final part, not on raw stock. The machining operation must remove the outer stock skin because the skin may contain processing aids that are not representative of the bulk material. In practice this means that rough blanks should be oversized by at least 5 mm per side before facing; finishing passes then remove the stabilised layer produced by annealing.

When a POM component is to be mounted with a press-fit bushing, the machined bore should be re-checked 24 h after stress relief and conditioning at 23 °C and 50 % RH. This delayed verification captures the remaining short-term viscoelastic recovery that otherwise appears only in first assembly, making it a mandatory gate for acceptance of parts with bore tolerance below 0.04 mm.