CNC Machining Tolerances — What’s Achievable, and When You Need Tight

What a Tolerance Is

A tolerance is the allowed deviation from a dimension — for example ±0.05 mm means the feature may be 0.05 mm larger or smaller than drawn. Calling out tolerances tells the shop which features must be exact and which can be left standard.

What CNC Holds as Standard

For general features, ±0.1 mm is routine and needs no special process. Most non-critical dimensions are fine here, and leaving them standard keeps cost down.

What CNC Holds When Tight

With careful setup, single-point boring and measurement, features reach ±0.005 mm to ±0.01 mm, and some can go tighter. Tight tolerance costs more — slower cycles, more inspection, higher scrap risk — so reserve it for features that actually fit against something.

When to Specify Tight

Specify tight tolerance on mating faces, bearing seats, threaded holes, locating pins and sealing surfaces — anywhere a gap or interference breaks the function. Everything else can stay at standard.

When Standard Is Enough

Outer envelopes, non-mating surfaces, cosmetic faces and internal features that never touch another part rarely need tight control. Leaving them at ±0.1 mm saves cost with zero functional loss. Our tolerance chart maps common classes.

Why Over-Tolerancing Costs More

Every tighter callout adds cycle time, inspection and scrap risk the shop prices in. Tolerancing everything tightly — “just to be safe” — is the most common way buyers pay more than needed. Call out tight only where function demands it.

GD&T Basics for Buyers

GD&T (Geometric Dimensioning and Tolerancing) describes shape and position — flatness, perpendicularity, true position — beyond simple plus/minus. If your drawing uses GD&T, we read it directly; if not, clear dimensions and a few key tolerances are enough to quote. See what to send for a quote.