Acetal CNC Tolerances: What's Achievable (2026)

Q: What tolerance is achievable when machining acetal POM?

Turned OD tolerances of ±0.001 in (±0.025 mm) are routine on properly set up CNC lathes. Bored IDs achieve ±0.001–0.002 in. Flatness and parallelism hold to 0.001–0.002 in over 4 in. These are repeatable process capabilities, not one-off heroics — acetal's dimensional stability makes tight tolerances achievable without special humidity conditioning or temperature-controlled machining rooms.

Q: How does acetal tolerance compare to nylon tolerance?

Acetal holds significantly tighter tolerances than PA6 or PA6/6 nylon. Nylon absorbs 1.5–8% moisture, which changes dimensions after machining when the part equilibrates to ambient humidity. Acetal absorbs less than 0.9% at equilibrium — dimensions are stable immediately after machining. In practice: acetal achieves ±0.001 in routinely; nylon is typically designed to ±0.003–0.005 in for any application that will see humidity variation.

Q: Do I need temperature-controlled machining for tight acetal tolerances?

No. Acetal's coefficient of thermal expansion (CTE) is 110 µm/m·°C. For a 50 mm (2 in) part, a 10°C temperature change causes approximately 0.055 mm (0.002 in) dimensional change. Standard shop temperature control (20–25°C) is sufficient for ±0.001 in work. For tolerances tighter than ±0.0005 in, temperature stabilization becomes relevant — but this level of tolerance is rarely needed in acetal applications.

Q: What is the tightest tolerance achievable in acetal?

On a high-quality CNC lathe with fresh carbide tooling, ±0.0005 in (±0.013 mm) on a turned OD is achievable but requires careful temperature control, fresh tooling on every part, and in-process gauging. Most precision acetal applications target ±0.001 in as the practical tight-tolerance threshold. Tolerances tighter than ±0.0005 in in acetal are rarely justified by application requirements.

Q: How should I callout tolerances on acetal drawings?

Use standard ASME Y14.5 / ISO 1101 GD&T callouts. Running fits for shafts through acetal bushings: H7 bore (e.g., 25.000/+0.021/+0.000) with f6 shaft for sliding fit, or g6 for close sliding fit. For individual features, explicit bilateral tolerances (e.g., Ø25.000 ±0.025 mm) are preferred over implied general tolerances for any precision feature. State the inspection method for bores: go/no-go gauges, CMM, or bore gauge as appropriate.

Acetal Holds Tight Tolerances Immediately — No Conditioning Required.

With nylon, you must condition stock to equilibrium moisture before machining — otherwise the part swells after delivery and the tight bore you machined becomes a loose bore. With acetal, you machine to final dimension and it stays there. This property alone makes acetal the preferred plastic for precision bearing fits, gear hubs, and locating features.

Section 1 of 5

Why Acetal Holds Tight Tolerances

If your part requires a precision bore fit or a datum surface, acetal's dimensional stability gives you tolerances that most plastics can't hold. Two material properties make acetal outperform most other engineering plastics in dimensional stability.

Low Moisture Absorption

Acetal absorbs less than 0.25% moisture in 24 hours and reaches equilibrium at less than 0.9%. Compare to PA6 nylon at up to 8.5% saturation — acetal is 9× more dimensionally stable in humid environments.

Material	Equil. moisture	Dim. change (50 mm part)
Acetal POM-H	0.9%	~0.045 mm
Nylon PA12	1.6%	~0.080 mm
Nylon PA6/6	2.5%	~0.125 mm
Nylon PA6	8.5%	~0.425 mm

Thermal Expansion (CTE)

Acetal's CTE is 110 µm/m·°C — higher than steel (12 µm/m·°C) or aluminum (23 µm/m·°C) but consistent and predictable. For room-temperature machining, the thermal contribution to dimensional variation is small. Plan for CTE in assemblies where acetal runs against metal at elevated temperature.

10°C temp change, 50 mm part0.055 mm (0.002 in)

20°C temp change, 50 mm part0.110 mm (0.004 in)

10°C temp change, 100 mm part0.110 mm (0.004 in)

5°C variation (standard shop)<0.030 mm for a 50 mm part — exceeds ±0.001 in tolerance; temperature-stabilize inspection for precision work on parts over 25 mm

Section 2 of 5

Achievable Tolerance Reference Tables

Before you callout a tolerance on your acetal drawing, check what's actually repeatable — over-tolerancing costs money and under-tolerancing costs function. These are repeatable process capabilities on quality CNC lathes and machining centers with fresh tooling. "Standard" = achievable on any properly maintained CNC. "Precision" = achievable with specific setup attention and in-process gauging.

Turned Outer Diameter (OD) Tolerances

Tolerance Class	OD Range	Tolerance Band	Notes
Standard	0.125–2.000 in	±0.002 in (±0.05 mm)	Default process capability — no special setup required
Precision	0.125–2.000 in	±0.001 in (±0.025 mm)	Fresh tooling, in-process gauging, controlled temperature
Standard	2.000–6.000 in	±0.003 in (±0.075 mm)	Workpiece compliance increases tolerance at larger diameters
Precision	2.000–6.000 in	±0.002 in (±0.05 mm)	Steady rest support required for long-diameter parts
Standard (metric)	3–50 mm	±0.05 mm	Approximate ISO IT10–IT11 across 3–50 mm range.
Precision (metric)	3–50 mm	±0.025 mm	Approximate ISO IT8–IT9 across 3–50 mm range.

Bored / Reamed Inner Diameter (ID) Tolerances

Method	ID Range	Tolerance Band	Notes
Boring (standard)	0.250–2.000 in	±0.002 in	Single-point boring bar on lathe or machining center
Boring (precision)	0.250–2.000 in	±0.001 in	Final spring pass with fresh insert; in-process bore gauge
Reaming (H7)	0.250–1.500 in	+0.001/0.000 in	Achieves H7 fit for running fits with shaft; most common precision bore method
Drilling only	0.125–0.500 in	±0.003–0.005 in	Drill wanders — use for clearance holes only, not precision fits
Metric boring (standard)	6–50 mm	±0.05 mm	H8 class or better achievable with single-point boring
Metric reaming (H7)	6–50 mm	+0.025/0.000 mm	H7 bore tolerance — standard for shaft running fit applications

Form and Positional Tolerances

GD&T Control	Achievable (standard)	Achievable (precision)	Notes
Flatness (over 4 in / 100 mm)	0.002 in	0.001 in	Face milling in climb direction; support entire workpiece surface
Parallelism (4 in span)	0.003 in	0.001 in	Precision parallel ground tooling plate required for 0.001 in
Perpendicularity (4 in height)	0.003 in	0.002 in	Achieved with rigid fixturing and square setup
Runout (turned OD)	0.002 in total	0.001 in total	Driven by chuck TIR and workpiece setup
Concentricity (OD to bore)	0.003 in	0.002 in	Turn OD in same setup as bore for tightest concentricity
Position (hole to hole, 4 in span)	±0.003 in	±0.001 in	Precision: digital scales on machining center, no tool deflection

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Section 3 of 5

Shaft and Bore Fit Selection

If you apply metal-to-metal fit tables to your acetal bushing bore, the fit will be too tight — acetal's CTE is 9× higher than steel. Acetal bushing bore fits with metal shafts require more clearance than metal-to-metal fits to account for CTE mismatch. Use these guidelines for initial design.

Fit Type	ISO Fit	Clearance Range	Application
Running fit — acetal bushing on steel shaft	H7/f6	0.020–0.065 mm	Default for precision rotating shafts through acetal bushings
Close sliding fit — acetal on steel	H7/g6	0.010–0.040 mm	Precision sliding or slow rotation; shaft and bore must be concentric
Free running fit — acetal on steel	H8/f7	0.040–0.110 mm	Higher-clearance applications; allows for POM CTE expansion at elevated temp
Loose running fit — acetal on steel	H9/d9	0.065–0.250 mm	Axial sliding applications; tolerance band allows for thermal expansion
Press fit — steel insert into acetal bore	P7/h6 or H7/r6	Interference 0.010–0.030 mm	For retaining pins or bearing cups; use low assembly force to avoid cracking
Clearance for thermal expansion (elevated temp)	Add 0.001–0.003 in	Per 10°C above ambient	Acetal CTE 110 µm/m·°C vs steel 12 µm/m·°C — gap increases with temp

Section 4 of 5

Fine Feature Limits

Know the physical limits before specifying — some features hit the tool geometry limit before the material tolerance limit.

Feature	Minimum Practical Size	Limit Reason	Notes
Drilled hole diameter	0.040 in (1 mm)	Small drill fragility — acetal is stiff and can snap small drills	Below 1 mm: laser or EDM required; not standard CNC
End milled slot width	0.031 in (0.8 mm)	End mill rigidity — 1/32 in end mills deflect in acetal	Deep slots require width ≥ 2× depth for adequate tool stiffness
Minimum wall thickness (milled)	0.060 in (1.5 mm)	Workpiece deflection under cutting force	Thinner possible with vacuum fixture and very light cuts
Minimum wall thickness (turned)	0.040 in (1 mm)	Workpiece compliance in chuck	Thin tubes require mandrel or split collet fixturing
Minimum internal radius (corner)	0.015 in (0.4 mm)	End mill corner radius	Square corners impossible — specify radius equal to tool radius
Thread size (tapped in POM)	#0-80 min (M1.6 metric)	Tap fragility in stiff material	Small threads risk tap breakage; micro-tapping fixture recommended
Engraved text (V-groove)	0.030 in (0.75 mm) deep	Engraving cutter access	Shallower text may close up from surface finish and be unreadable

Section 5 of 5

How to Callout Tolerances on Drawings

How you callout tolerances on your acetal drawing determines whether the shop machines to your intent or their default interpretation. Correct callouts prevent ambiguity and avoid tolerance interpretation disputes.

Correct Callout Format

Explicit bilateral: Ø25.000 +0.025/-0.025 mm (not relying on title block general tolerance)
ISO fit callout: Ø25 H7 for bore; Ø25 f6 for shaft mating to H7 bore
GD&T: Flatness 0.025 mm symbol with datum reference for precise form control
Inspection method note: "Verify bore with go/no-go gauge per drawing" for production
Temperature note for precision work: "All dimensions at 20°C (68°F)"

Callouts That Create Problems

Relying solely on title block "general tolerance ±0.005 in" for precision features
No surface finish callout on bearing or mating surfaces
Thread callout without specifying class of fit (2A/2B, 6H/6g)
Implied sharp corners on all internal radii — always specify minimum corner radius
No flatness callout on reference surfaces that will be used as datum planes

Frequently Asked Questions

What tolerance is achievable when machining acetal POM?

Turned OD tolerances of ±0.001 in (±0.025 mm) are routine on properly set up CNC lathes. Bored IDs achieve ±0.001–0.002 in. Flatness and parallelism hold to 0.001–0.002 in over 4 in. These are repeatable process capabilities, not one-off heroics — acetal's dimensional stability makes tight tolerances achievable without special humidity conditioning or temperature-controlled machining rooms.

How does acetal tolerance compare to nylon tolerance?

Acetal holds significantly tighter tolerances than PA6 or PA6/6 nylon. Nylon absorbs 1.5–8% moisture, which changes dimensions after machining when the part equilibrates to ambient humidity. Acetal absorbs less than 0.9% at equilibrium — dimensions are stable immediately after machining. In practice: acetal achieves ±0.001 in routinely; nylon is typically designed to ±0.003–0.005 in for any application that will see humidity variation.

Do I need temperature-controlled machining for tight acetal tolerances?

No. Acetal's coefficient of thermal expansion (CTE) is 110 µm/m·°C. For a 50 mm (2 in) part, a 10°C temperature change causes approximately 0.055 mm (0.002 in) dimensional change. Standard shop temperature control (20–25°C) is sufficient for ±0.001 in work. For tolerances tighter than ±0.0005 in, temperature stabilization becomes relevant — but this level of tolerance is rarely needed in acetal applications.

What is the tightest tolerance achievable in acetal?

On a high-quality CNC lathe with fresh carbide tooling, ±0.0005 in (±0.013 mm) on a turned OD is achievable but requires careful temperature control, fresh tooling on every part, and in-process gauging. Most precision acetal applications target ±0.001 in as the practical tight-tolerance threshold. Tolerances tighter than ±0.0005 in in acetal are rarely justified by application requirements.

How should I callout tolerances on acetal drawings?

Use standard ASME Y14.5 / ISO 1101 GD&T callouts. Running fits for shafts through acetal bushings: H7 bore (e.g., 25.000/+0.021/+0.000) with f6 shaft for sliding fit, or g6 for close sliding fit. For individual features, explicit bilateral tolerances (e.g., Ø25.000 ±0.025 mm) are preferred over implied general tolerances for any precision feature. State the inspection method for bores: go/no-go gauges, CMM, or bore gauge as appropriate.

Related Resources

CNC Machining Acetal

Speeds, feeds, tool selection, and DFM rules

Acetal / POM Guide

Complete material guide with all properties

CNC Tolerances Guide

General tolerance limits across all materials

Tight-Tolerance Acetal Parts — Ships in 5–7 Days

CNC machining to ±0.001 in in acetal POM and Delrin. Free DFM review on every order.

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Acetal CNC Tolerances: What's Achievable for Machined POM Parts