Machining Tolerances & Standards Guide

Guide to machining tolerances, fits, and dimensional standards used in Chinese manufacturing. ISO 2768, GB/T 1804, and practical tolerance selection for sourcing.

In this guide:

Tolerance Basics
ISO 2768 General Tolerances
Fits & Clearances
Surface Finish
Practical Tolerance Selection
Chinese Tolerance Practice

Tolerance Basics

Tolerance is the permissible limit of variation in a dimension. Tighter tolerances cost more. A general rule: every halving of tolerance (e.g., ±0.1mm → ±0.05mm) roughly doubles machining time and cost.

Common tolerance classes for machined parts:

Fine (f): ±0.05mm ISO 2768-f. Precision fits, mating parts, critical features
Medium (m): ±0.1mm ISO 2768-m. Standard machining, most mechanical parts
Coarse (c): ±0.5mm ISO 2768-c. Non-critical dimensions, clearance holes, external profiles
Very Coarse (v): ±1.0mm. Weldments, castings, rough blanks

ISO 2768 General Tolerances

ISO 2768 is the most widely referenced machining tolerance standard for linear and angular dimensions. The Chinese equivalent is GB/T 1804.

Class	Linear (0.5-3mm)	Linear (3-6mm)	Linear (6-30mm)	Linear (30-120mm)	Linear (120-400mm)	Linear (400-1000mm)
Fine (f)	±0.05	±0.05	±0.1	±0.15	±0.2	±0.3
Medium (m)	±0.1	±0.1	±0.2	±0.3	±0.5	±0.8
Coarse (c)	±0.2	±0.3	±0.5	±0.8	±1.2	±2.0

On drawings: Specify ISO 2768-m or ISO 2768-f in the title block. Mark tighter tolerances individually with (±0.02) near the dimension.

Fits & Clearances

Hole-basis system (ISO 286) is standard. Reference fit classes:

Fit Type	ISO Symbol	Typical Use
Clearance (loose)	H11/c11	Shafts running in bearings, thermal expansion
Clearance (medium)	H8/f7	Sliding fits with lubrication, precision shafts
Clearance (close)	H7/g6	Precision sliding, spool valves, guide pins
Transition	H7/k6	Pulleys on shafts, gears (can be assembled by hand)
Interference (light)	H7/p6	Press fits, bushings in housings
Interference (heavy)	H7/s6	Permanent assembly, bearing outer race in bore

Surface Finish

Ra (μm)	Appearance	Process	Typical Use
0.1-0.2	Mirror finish	Superfinishing, lapping	Seal faces, precision bearings
0.4-0.8	Smooth	Grinding, fine turning	Bearing journals, mold cavities
1.6-3.2	Fine machine	General turning, milling	Standard machined surfaces
6.3-12.5	Rough machine	Rough turning, sawing	Non-functional surfaces

Practical Tolerance Selection

Rules for specifying tolerances on parts sourced from China:

Default to ISO 2768-m unless function demands tighter — saves 15-30% cost vs. always specifying ±0.1mm
Only mark critical tolerances on drawings — excessive tight tolerances confuse factories and inflate price
For mating parts, use reference dimension system (A-B-C datums) per GD&T
Tolerances below ±0.02mm require grinding or EDM, increasing cost significantly
Allow for tolerance accumulation in multi-part assemblies
For injection molded parts: typical tolerance is ±0.3mm for general dimensions, ±0.1mm for critical features
For die cast parts: typical ±0.2-0.3mm for general, ±0.1mm for critical

Chinese Tolerance Practice

What to expect when working with Chinese machine shops:

Most CNC shops can hold ±0.05mm without issue on 3-axis machines
±0.01-0.02mm requires grinding or EDM, available from precision shops
Chinese factories typically default to GB/T 1804 (identical to ISO 2768)
5-axis CNC is increasingly common but still commands a premium
Request a CMM (Coordinate Measuring Machine) report for parts with critical tolerances
CMM inspection is standard for automotive and aerospace work; affordable for any precision project