CNC Machining Tolerances: What's Achievable and How to Spec Them

                                                                                              

Tolerances are the language of precision manufacturing. They define the acceptable deviation between a machined part's actual dimensions and its nominal design values. Getting tolerances right is one of the most impactful decisions in CNC machining — too loose and parts won't assemble; too tight and costs go up quickly.

This guide provides a practical, engineer-to-engineer overview of CNC machining tolerances: what's achievable with different materials and processes, how to specify them correctly on drawings, and how to balance precision requirements against cost and lead time. Whether you're designing your first prototype or sourcing high-volume production parts, this article will help you make better tolerance decisions.

Understanding CNC Machining Tolerances

A CNC machining tolerance defines the permissible range of variation for a given dimension. If a shaft is specified as 25.00 ± 0.05 mm, the acceptable range is 24.95 to 25.05 mm — any part outside this range is rejected.

Types of tolerances in CNC machining:

•  Dimensional (linear) tolerance: ±0.05 mm on a diameter or length

•  Geometric tolerance: Flatness, roundness, cylindricity, concentricity, perpendicularity, parallelism, position (GD&T)

•  Angular tolerance: ±0.5° or tighter for angles

•  Surface finish: Ra value that constrains surface texture

Understanding which type of tolerance applies to each feature is essential for communicating design intent to your CNC machining supplier.

Standard CNC Machining Tolerances: What You Can Expect

Most CNC machining shops follow ISO 2768 (general tolerances for linear and angular dimensions) when no specific tolerance is called out on the drawing. The standard defines four tolerance classes:

ISO 2768 Class	Name	Typical Range (1-10mm)	Application
f	Fine	±0.05 to ±0.10 mm	Precision gauges, optical mounts, medical devices
m	Medium	±0.10 to ±0.20 mm	General engineering, automotive, industrial parts
c	Coarse	±0.20 to ±0.50 mm	Structural frames, non-critical housings
v	Very Coarse	±0.50 to ±1.00 mm	Rough prototypes, non-functional parts

For most CNC machined parts, ISO 2768-m (medium) is the default and provides a good balance of precision and cost. Tighter tolerances should only be specified where functionally required.

Tightest Achievable CNC Machining Tolerances

Modern CNC machining centers can achieve remarkably tight tolerances under the right conditions. However, the achievable tolerance depends on multiple factors:

By Material（H3）

Material	Standard Tolerance	Tightest Achievable	Notes
Aluminum (6061/7075)	±0.05 mm	±0.01 mm	Excellent machinability
Stainless Steel (304/316)	±0.05 mm	±0.015 mm	Work hardening must be managed
Carbon Steel	±0.05 mm	±0.02 mm	Good rigidity, predictable behavior
Titanium (Ti-6Al-4V)	±0.075 mm	±0.025 mm	Low thermal conductivity challenges
PEEK (unfilled)	±0.05 mm	±0.01 mm	Requires annealing for best results
PEEK (30% CF)	±0.075 mm	±0.025 mm	Fiber orientation affects anisotropy
Brass / Copper	±0.05 mm	±0.015 mm	Soft, excellent surface finish
Delrin (POM)	±0.05 mm	±0.01 mm	Excellent dimensional stability

By Process

•  CNC milling (3-axis): ±0.025 mm typical, ±0.01 mm with precision setup

•  CNC milling (5-axis): ±0.03 mm typical due to compounded axis errors, ±0.015 mm achievable

•  CNC turning: ±0.015 mm typical, ±0.005 mm achievable with diamond tooling on non-ferrous materials

•  CNC grinding (post-process): ±0.005 mm or tighter — but adds cost and lead time

Note: Tolerances tighter than ±0.01 mm typically require temperature-controlled environments (20°C ± 1°C), premium tooling, and extended machining time — all of which increase cost significantly.

How to Specify Tolerances on Your Drawings

Proper tolerance specification is a balance between function and cost. Over-specifying is one of the most common and costly mistakes in CNC machining procurement.

General Rules

•  Use ISO 2768-m as the default general tolerance for all dimensions unless otherwise specified.

•  Apply tighter tolerances only to critical features: mating surfaces, bearing bores, sealing faces, and alignment datums.

•  Use geometric tolerancing (GD&T) for features where form, orientation, or location matters more than size alone.

•  Specify tolerances that your chosen process can reliably hold — don't specify ±0.01 mm on a 3-axis milled part when ±0.05 mm will work.

•  Consider the measurement method: If you can't measure it to the specified tolerance, don't specify it.

GD&T for CNC Machined Parts

Geometric Dimensioning and Tolerancing (GD&T) provides a more complete and unambiguous way to specify part requirements than simple ± tolerances. Key GD&T controls commonly used in CNC machining include:

•  Flatness: Controls surface planarity — critical for sealing faces and mating surfaces

•  True Position (Position): Controls hole and feature location relative to datums — essential for assembly

•  Concentricity / Coaxiality: Controls alignment of cylindrical features — important for shafts and bores

•  Perpendicularity: Controls 90° relationships between features — critical for squareness in fixtures

•  Profile of a Surface: Controls complex 3D contours — common in aerospace and medical device parts

For a comprehensive introduction to GD&T, see our detailed guide on GD&T Explained: Geometric Dimensioning and Tolerancing for Machined Parts.

How Tolerances Affect CNC Machining Cost

Tolerance is one of the biggest cost drivers in CNC machining. Here's why tighter tolerances cost more:

Factors That Increase Cost with Tighter Tolerances

•  Slower machining: Tighter tolerances require slower feed rates, finer stepovers, and multiple passes (rough → semi-finish → finish)

•  Better tooling: Premium carbide or diamond tooling costs 3-10× more than standard tools

•  More inspection: Tighter tolerances require CMM or optical inspection instead of simple calipers

•  Higher scrap rate: Statistical variation means more parts fall outside tolerance bands

•  Environmental control: Temperature-stable environments add overhead

•  Secondary operations: Grinding or lapping may be needed to achieve the final tolerance

Cost Multiplier Guidelines

Tolerance Range	Relative Cost	When to Use
±0.1 to ±0.05 mm	1.0× (baseline)	Standard machining — most features
±0.05 to ±0.025 mm	1.3-1.8×	Mating surfaces, precision bores
±0.025 to ±0.01 mm	2.0-3.5×	Gauge components, optical mounts
±0.01 to ±0.005 mm	4.0-8.0×	Only for critical aerospace/medical features
< ±0.005 mm	10×+ (requires grinding)	Extreme precision — avoid CNC if possible

Rule of thumb: Every time you halve the tolerance, the cost approximately doubles.

General Tolerances Without Drawings: ISO 2768 and ISO 20457

Many CNC machining orders are placed based on 3D models without detailed 2D drawings. In these cases, general tolerance standards provide the quality framework:

•  ISO 2768: General tolerances for linear and angular dimensions (metals and plastics). Defines classes f/m/c/v.

•  ISO 20457: General tolerances specifically for plastic parts made by machining, accounting for material-specific behavior like thermal expansion and moisture absorption.

For engineering plastics like PEEK, POM, and PTFE, ISO 20457 provides more realistic tolerance expectations than ISO 2768, because it accounts for the inherent dimensional instability of polymers.

Tolerance Stack-Up: Thinking Beyond Individual Dimensions

When parts assemble together, individual tolerances accumulate — this is called tolerance stack-up. A stack-up analysis ensures that even at worst-case conditions, assembled parts will fit and function correctly.

Example: Shaft and bore assembly

•  Shaft: 10.00 ±0.02 mm → range: 9.98 to 10.02 mm

•  Bore: 10.10 ±0.03 mm → range: 10.07 to 10.13 mm

•  Worst-case clearance: 10.13 - 9.98 = 0.15 mm (maximum)

•  Best-case clearance: 10.07 - 10.02 = 0.05 mm (minimum)

This simple analysis confirms adequate clearance. For more complex assemblies, statistical (RSS) stack-up methods provide a more realistic prediction than worst-case analysis.

CNC Machining Tolerance Specification Checklist

•  General tolerance standard specified (ISO 2768 class or ISO 20457 for plastics)

•  Critical dimensions have explicit ± tolerances

•  GD&T used for form, orientation, and location requirements

•  Datum reference frame clearly defined

•  Material selected — tolerances appropriate for material capabilities

•  Surface finish requirements specified where needed

•  Tolerance stack-up analysis performed for mating assemblies

•  Measurement method feasible for specified tolerances

•  Cost vs. precision trade-off reviewed for each tight tolerance

•  Post-machining requirements (annealing, grinding) identified for tight tolerances

Why Partner with Eastmaster for CNC Machined Parts

At Eastmaster Manufacturing Limited, we hold tolerances to the standards your parts demand — no more, no less. Our capabilities include:

•  Standard tolerances: ±0.05 mm (ISO 2768-m default)

•  Precision tolerances: ±0.01-0.02 mm with CMM inspection

•  Full GD&T capability: True position, flatness, concentricity, profile, and all geometric controls

•  Material expertise: Aluminum, stainless steel, titanium, engineering plastics (PEEK, PTFE, POM), and more

•  ISO 9001 and ISO 13485 certified quality management

•  D-U-N-S verified: 66-849-8033

Contact Eastmaster to discuss your CNC machining requirements. Our engineering team will review your design, recommend appropriate tolerances, and provide a detailed quote — typically within 24 hours.

• Email: info@eastmaster.com 
• Phone: +86 755 22676100
• WhatsApp / WeChat available on the right side.