GD&T Tolerances Guide for Machined Metal Parts

Tolerances are how you tell a shop exactly what “good” means. Slapping ±0.05 mm on every dimension invites scrap and argument; GD&T (Geometric Dimensioning and Tolerancing) states function instead. This guide gives a practical GD&T tolerances framework for machined metal parts so your drawings inspect clean, your supplier quotes with confidence, and you stop paying for tight numbers you never needed.

Why GD&T Beats Coordinate Tolerances

Coordinate (±) tolerances stack up and do not say which feature matters. GD&T ties controls to function—this hole pattern must locate a shaft, this face must seal—so the shop holds what counts and relaxes what does not. It also removes ambiguity about which surface is the reference, which is the single biggest source of scrap on coordinate-only drawings. Our CNC machining tolerances guide shows typical achievable numbers so you can set callouts the machine can actually hit without special effort.

Datums and Reference Frames

Datums are the surfaces or axes everything references from—usually the mating face and two locating features. Pick stable, accessible datums and label them A/B/C in a sensible order (primary, secondary, tertiary). From a single frame, position and form controls become unambiguous, which is why even 5-axis machining setups rely on clean datum chains to keep multi-side features aligned. A poorly chosen datum—say a non-machined casting surface—imports its variation straight into your tolerances.

Form and Orientation Controls

Flatness, straightness, and circularity control a single feature. Perpendicularity and parallelism tie one feature to a datum. Use these instead of tightening a size callout—you get the function you need without over-machining the whole part. After stock removal, surface grinding and CNC grinding hit the tight flatness and finish GD&T demands on sealing and bearing faces, while the rest of the part stays at a looser, cheaper tolerance.

Position and Runout

Position (⌈ with an LMC/MMC bonus) is the workhorse for hole patterns and bosses; runout governs rotating parts such as shafts and disks. Specifying MMC lets the shop use bonus tolerance when features are at max material, cutting cost while protecting assembly. These are the controls most precision machined parts live by, because they describe how parts fit rather than just how big they are.

Model-based definition (MBD) takes GD&T further by embedding the controls in the 3D model itself, so there is one source of truth instead of a drawing that can disagree with the CAD. Shops program inspection directly from the model, cutting translation errors and speeding first article. Even without full MBD, attaching your GD&T balloon to the native model and exporting a clean STEP keeps everyone aligned. Our design-for-machining notes show how to set this up without overcomplicating the file.

Tolerance Stack-Up Basics

Every tolerance in a chain adds. Keep critical stacks short, put the tight callouts only where function demands, and let non-critical features float. A turned shaft with a loose diameter but tight runout assembles better than one with uniformly tight sizes, because runout is what the bearing cares about, not the absolute diameter. A simple stack-up sketch during design surfaces the one dimension that actually needs ±0.01 mm.

Communicating with Your Shop

Send a CAD model plus a GD&T drawing, not a sketch. State the inspection method (CMM, gauge) and acceptables, and resolve any conflict between model and drawing in writing. Early design-for-machining review catches impossible callouts—like a position tolerance tighter than the feature can be produced—before metal moves, which is far cheaper than rejecting a finished lot.

Inspection and CMM

A CMM program built from your datums verifies true position and form, not just coordinates. First-article reports should quote the GD&T actually measured, with ballooned drawings, so both sides agree the part passed. For production, a simple functional gauge often checks the few critical controls faster than a full CMM scan, lowering inspection cost on every piece.

Common GD&T Mistakes

The classic error is zeroing every tolerance and calling it “precise”—this inflates cost and still fails, because the part was never tied to function. Another is using a machined surface as a datum when a casting or forged face would be the real locator; the variation you import then eats your budget. Position callouts without an MMC/LMC modifier forgo free bonus tolerance, and a runout symbol on a non-rotating part adds inspection for nothing. Our GD&T machined parts reference and the precision machining overview show correctly ballooned examples worth copying.

Finally, keep the model and the drawing in agreement. A CAD file at nominal with a drawing at true position creates a fight the inspector resolves conservatively—against you. State which controls are inspected on a CMM versus a functional gauge, and accept the report format before the run. Clean GD&T is not bureaucracy; it is the shortest path from drawing to a part that assembles first try.

Frequently Asked Questions

When should I use GD&T? Anytime features must assemble or seal; skip it only for non-critical plates.

What tolerance can CNC hold? Roughly ±0.01–0.025 mm routinely, tighter with grinding.

What does MMC mean? Maximum material condition—it grants bonus tolerance when features are at max size.

Do I need a CMM report? For critical parts, yes; it proves the GD&T was met.

Can GD&T lower cost? Yes—by relaxing tolerances where function allows.