Guides / Design

Tolerance Every Feature and You Will Pay for Every Feature

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

The most expensive line on a lot of drawings is a general tolerance that somebody tightened because it felt more professional. Tolerance is not a measure of quality. It is a budget, and spending it where nothing needs it is money that buys you nothing.

The bands, and what they mean at the machine

Band	Value	What it takes	Where it belongs
Standard	±0.13 mm	Normal machining, spot checked	Profiles, clearance holes, non-critical
Precision	±0.05 mm	Careful setup, more frequent checks	Locating features, counterbores
Tight	±0.025 mm	Bored or reamed, most parts measured	Bearing bores, dowels, shaft fits
Very tight	±0.013 mm	Temperature control, every part measured	Rare, needs justification

The jump that costs money is not in the machining. It is in the inspection. A standard dimension might be checked on one part in ten. A tight one gets measured on every piece, and that inspection time lands in your unit price whether you see it broken out or not.

Three habits that cost nothing and save a lot

Name the critical features. Two or three per part is normal. Everything else goes in the general note. A shop reading a drawing with three called out features knows exactly where to concentrate, and it quotes accordingly.

Tolerance the function, not the dimension. A bore that takes a bearing needs a bore tolerance. The distance from that bore to the outside edge of the part usually does not. Ask what the feature has to do before you decide how tightly to hold it.

Use fits where they apply. O20 H7 tells a shop far more than 20.00 ±0.01. It says this is a hole in a standard fit system, the shop knows what goes in it, and there is an accepted way to machine and check it.

Datums matter more than numbers

A tolerance without a reference is ambiguous. If three holes have to line up with each other, what matters is their position relative to one another, not their position from an arbitrary edge.

Even basic geometric callouts help enormously:

• Flatness on a face that seals or mounts.
• Position on a hole pattern, referenced to a datum.
• Perpendicularity where a bore has to be square to a face.
• Concentricity between two bores that carry the same shaft.

A position tolerance on a bolt pattern is usually cheaper to hold than tight ± dimensions on every hole, because it gives the shop the bonus tolerance the fit actually allows.

Aluminum specific traps

Thermal expansion. Aluminum moves roughly 23 um per metre per degree C, about double steel. A 300 mm part that comes off the machine at 30 C and gets measured at 20 C will have shrunk about 0.07 mm. On tight tolerances, agree the measurement temperature.

Stress movement. A part can be in tolerance on the machine and out of tolerance an hour later as internal stress releases. Tight tolerances on large plates need the right material and often a rough, rest and finish sequence.

Coating growth. Anodizing changes dimensions. A tolerance that does not say whether it applies before or after coating is an unfinished specification.

Thin walls. A wall that deflects under the cutter cannot hold a tight tolerance at any price, because it springs back after the tool passes.

A drawing note that works

GENERAL TOLERANCE ISO 2768-m UNLESS NOTED.
CRITICAL: BORES 'A' AND 'B' O25 H7, CONCENTRIC WITHIN 0.02 A-B.
FACE 'C' FLAT WITHIN 0.05.
ALL TOLERANCES APPLY AFTER ANODIZING.

Four lines. The shop now knows what matters, what does not, how the features relate and when to measure. That drawing will be quoted faster and more cheaply than one with ±0.02 written beside forty dimensions.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• ISO 2768, general tolerances
• ISO 286, limits and fits
• Geometric dimensioning and tolerancing
• Thermal expansion coefficients

Frequently asked questions

Keep reading

• The Part Was Flat Until the Clamps Came Off
• The Wall Is Not Too Thin Until It Starts Moving