GD&T for Sheet Metal: What Buyers Should Know
Engineering Team | Anjue Metal | Updated Nov 5, 2026 | 8 min read
Plus/minus dimensions are fine until two holes must line up with a mate, or a panel must sit flat in a sealed enclosure. That is where Geometric Dimensioning and Tolerancing (GD&T) earns its place. You do not need to be a metrologist to use it well - you need to know when it helps and the few symbols that matter for sheet metal.
1. GD&T vs plus/minus dimensions
A ±0.5 mm on every dimension tells the shop each feature is independent. But features are rarely independent - a hole pattern must relate to an edge, and a flange must relate to a base. GD&T expresses those relationships, so the part is tolerated the way it is actually used and assembled.
2. Datums on a formed part
A datum is a referenced feature (face, edge or hole) that everything else is measured from. On a formed enclosure, pick primary, secondary and tertiary datums that match how the part seats in assembly - usually a major mounting face first, then two locating edges. Vague or missing datums are the #1 reason "in tolerance" means different things to you and the inspector.
3. Position tolerances on holes
For hole patterns that must align with a mating part, a position tolerance (the circled crosshair symbol) with a datum reference controls where the pattern sits as a group - far more useful than tolerancing each hole independently. For sheet metal, also state whether the tolerance is measured before or after bending, because bending moves holes.
4. Flatness and profile
Flatness controls a single surface (a sealing face that must not leak). Profile controls the shape of a 2D or 3D feature - useful for a bent profile that must match a gasket or a cover. Use these when the function depends on shape, not just a linear size.
5. Mistakes that cost money
Three recur: (1) over-tolerancing everything to a tight number "just in case" - you pay for precision you don't need; (2) datums that don't match assembly, so the part passes inspection but won't fit; (3) GD&T on the drawing with no corresponding note on the RFQ, so the shop quotes to the ± dims and ignores the symbols. Put the symbols on the drawing and mention them in the RFQ.
Frequently Asked Questions
Frequently Asked QuestionsUsually not. For a part with independent features and loose fit, plus/minus dimensions are simpler and cheaper. Use GD&T when features must relate to each other - hole patterns that mate, or surfaces that must seal or sit flat.
Pick the primary datum as the major mounting or seating face, then two locating edges as secondary and tertiary - the scheme should mirror how the part is positioned in your assembly, not an arbitrary corner.
Yes. Bending shifts and rotates hole patterns. State clearly whether position tolerances are measured on the flat blank before bending or on the formed part after bending, or the shop will measure the wrong state.
Almost always a datum problem: the drawing's datums don't match how the part seats, so inspection passes against the wrong reference. Align datums to assembly and the fit problem usually disappears.
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