CNC Machining Tolerances for Plastic Parts: A Practical Specification Guide

Tight tolerances on a drawing don't guarantee tight tolerances on the part. We've seen ±0.05mm specifications result in ±0.2mm actual variation because the material wasn't factored in. Another client paid 40% premium for ±0.02mm tolerances their application didn't need.

This guide covers what actually matters when specifying tolerances for CNC-machined plastic parts—based on 15 years of production data and the specification errors we see repeatedly.

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Understanding Tolerance Standards: ISO 2768 vs DIN 16901

ISO 2768: The General-Purpose Standard

ISO 2768 is the default reference for machined parts, but it was written for metals. Applied blindly to plastics, it causes problems.

ISO 2768-1 Linear Dimensions (simplified):

Tolerance Class	0.5-3mm	3-6mm	6-30mm	30-120mm
Fine (f)	±0.05	±0.05	±0.1	±0.15
Medium (m)	±0.1	±0.1	±0.2	±0.3
Coarse (c)	±0.2	±0.3	±0.5	±0.8

The problem: These assume material stability. Plastics move. A 50mm nylon part held to ±0.1mm at 20°C will measure ±0.15mm at 30°C just from thermal expansion.

DIN 16901: Plastic-Specific Tolerances

DIN 16901 accounts for material behavior, classifying plastics by shrinkage characteristics.

Tolerance Groups (simplified for machined parts):

Group	Materials	Typical Tolerance for 50mm
1	PTFE, PE, PP	±0.3 - ±0.5mm
2	PA (Nylon), POM, PET	±0.15 - ±0.25mm
3	PC, PMMA (Acrylic), PSU	±0.1 - ±0.2mm
4	GF-reinforced materials	±0.08 - ±0.15mm

Key difference: DIN 16901 factors in moisture absorption, thermal expansion, and material memory. For plastic parts, reference DIN 16901 first.

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Material Impact on Achievable Tolerances

Acrylic (PMMA)

Best achievable: ±0.025mm | Practical production: ±0.05-0.1mm

Factor	Impact
Internal stress	High—cell-cast has less than extruded
Moisture absorption	Low—dimensionally stable
Thermal expansion	7×10⁻⁵/°C—significant over 20°C range

Specification recommendation: ±0.05mm for dimensions <50mm, ±0.1mm for 50-200mm.

Real data: A 100mm acrylic bracket at ±0.02mm had 23% rejection. Relaxed to ±0.05mm: 4% rejection, 35% cost reduction.

Polycarbonate (PC)

Best achievable: ±0.03mm | Practical production: ±0.05-0.1mm

Factor	Impact
Internal stress	High—requires annealing for tight tolerances
Moisture absorption	Moderate—0.15% at saturation
Machinability	Gummy chips—requires sharp tools

Specification recommendation: ±0.05mm for precision features, ±0.1mm general. Always specify annealed material.

Nylon (PA6, PA66)

Best achievable: ±0.05mm (dry) | Practical production: ±0.1-0.2mm (stabilized)

Factor	Impact
Moisture absorption	High—2.5-3% at saturation
Dimensional change	0.5-1% from dry to saturated
Thermal expansion	8-10×10⁻⁵/°C

Critical: Nylon continues to move after machining. A part measuring ±0.03mm dry shifts ±0.15mm after moisture conditioning.

Specification recommendation: Specify moisture conditioning (50% RH equilibrium). Tolerances apply to conditioned state.

POM (Acetal/Delrin)

Best achievable: ±0.02mm | Practical production: ±0.05mm

Factor	Impact
Dimensional stability	Excellent—low moisture absorption
Machinability	Excellent—clean chip formation
Creep	Low—maintains dimensions under load

Specification recommendation: POM is your best bet for tight tolerances. ±0.03mm reliably achievable for dimensions <100mm.

PTFE (Teflon)

Best achievable: ±0.1mm | Practical production: ±0.2-0.3mm

Factor	Impact
Creep	Extreme—continues to flow under load
Thermal expansion	10-12×10⁻⁵/°C—highest of common plastics
Machinability	Poor—tears rather than cuts

Specification recommendation: Avoid tight tolerances on PTFE. Design for adjustment or use filled grades.

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Process-Specific Tolerance Ranges

Milling

Operation	Typical Tolerance	Best Achievable
Roughing	±0.2-0.5mm	±0.1mm
Semi-finish	±0.05-0.1mm	±0.03mm
Finish (high speed)	±0.02-0.05mm	±0.01mm

Turning

Operation	Typical Tolerance	Best Achievable
Rough turning	±0.1-0.2mm	±0.05mm
Finish turning	±0.02-0.05mm	±0.01mm
Precision turning	±0.01-0.02mm	±0.005mm

Drilling

Drill Size	Standard Tolerance	Precision Tolerance
<3mm	+0.05/-0mm	+0.02/-0mm
3-10mm	+0.1/-0mm	+0.03/-0mm
10-20mm	+0.15/-0mm	+0.05/-0mm
>20mm	+0.2/-0mm	+0.08/-0mm

Reaming: Tighten to ±0.01mm for precision holes.

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Design for Manufacturability

Wall Thickness and Tolerance

Wall Thickness	Tolerance Multiplier
<2mm	2× standard
2-5mm	1× standard
5-10mm	0.8× standard
>10mm	0.7× standard

Feature Size vs Tolerance

Feature Size	Practical Minimum
<5mm	±0.05mm
5-20mm	±0.03mm
20-100mm	±0.05mm
100-300mm	±0.1mm
>300mm	±0.2mm

Aspect Ratio

Length-to-Thickness	Tolerance Impact
<5:1	Standard
5:1 to 10:1	1.5×
10:1 to 20:1	2×
>20:1	3× or redesign

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Quality Control Methods

Inspection Environment

Material	Temperature	Stabilization Time
Acrylic	20±1°C	4 hours
Polycarbonate	20±1°C	4 hours
Nylon	23±2°C, 50% RH	24-48 hours
POM	20±1°C	2 hours

Critical: Measuring immediately after machining gives false readings. Nylon measured dry fails after conditioning.

Measurement Methods

Tolerance	Method	Accuracy
±0.1mm+	Calipers, micrometers	±0.02mm
±0.05mm	Digital micrometers	±0.01mm
±0.02mm	CMM	±0.003mm
±0.01mm	CMM + temp control	±0.002mm

Statistical Process Control

Application	Recommended Cpk
General industrial	≥1.33
Automotive, medical	≥1.67
Critical aerospace	≥2.0

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Cost Impact of Tight Tolerances

Tolerance vs Cost Multiplier (POM, 100mm part)

Tolerance	Cost Multiplier	Rejection Rate
±0.2mm	1.0×	<1%
±0.1mm	1.2×	2-3%
±0.05mm	1.5×	3-5%
±0.03mm	2.0×	5-8%
±0.02mm	2.5×	10-15%
±0.01mm	4.0×	20-30%

Rule of thumb: Each halving of tolerance roughly doubles the cost.

Real Project Example

Part: 150mm × 100mm × 25mm mounting plate (100 pcs)

Specification	Part Cost	Inspection	Total
±0.2mm general	$45	$300	$4,800
±0.05mm critical	$68	$800	$7,600
±0.02mm all	$180	$2,500	$20,500

Savings: $12,900 by tightening only where functionally required.

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Specification Checklist

General Requirements

• Reference standard (ISO 2768-m, DIN 16901 Group X)
• Material grade (PA6, not just "nylon")
• Reference temperature (20°C or 23°C)
• Moisture conditioning for hygroscopic materials

Tolerance Specification

• General tolerance class on drawing
• Critical dimensions individually toleranced
• Geometric tolerances where needed
• Surface finish (Ra value)

Quality Requirements

• Inspection sampling plan (AQL)
• Cpk requirement
• Measurement temperature and time
• Certificate of conformance

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Common Specification Errors

1. Applying metal tolerances to plastics: ISO 2768-f on nylon causes rejection
2. Ignoring moisture effects: Tight tolerances on unconditioned nylon
3. Over-tolerancing: ±0.02mm where ±0.1mm functions
4. Missing reference temperature: 15°C vs 25°C shows false failure
5. Tight tolerances on large parts: 500mm at ±0.05mm is expensive
6. No geometric tolerances: Linear alone doesn't control form
7. Wrong material: PTFE when POM would work

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Quick Reference: Tolerance by Material

Material	General	Precision	Notes
POM (Acetal)	±0.05mm	±0.02mm	Best for precision
PC	±0.1mm	±0.03mm	Requires annealing
PMMA (Acrylic)	±0.1mm	±0.03mm	Stress-relieve
PA6/66 (Nylon)	±0.15mm	±0.05mm	Condition to 50% RH
PET	±0.1mm	±0.05mm	Good stability
PEEK	±0.05mm	±0.02mm	High cost
PTFE	±0.3mm	±0.1mm	Avoid tight

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What We Need From You

To quote accurately:

1. Material: Grade, filler, color
2. Tolerances: General class + critical dimensions
3. Reference conditions: Temperature, humidity
4. Quantity: Affects process and inspection
5. Application: Identifies critical features
6. Fit: What does this mate with?

Lead times: Standard 2-3 weeks | Precision 3-4 weeks | Ultra-precision 4-6 weeks

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Questions about tolerance specification? Send your drawing—we'll review tolerances against material and application at no charge.

Submit drawing for review

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