A dimension with a plus/minus tolerance tells you how big a feature is allowed to be. It doesn’t tell you where that feature is allowed to sit relative to everything else on the part, or how far it’s allowed to tilt, or how round a supposedly round bore actually has to be. That’s the gap GD&T (Geometric Dimensioning and Tolerancing) closes — and it’s what a drawing reviewer is actually checking for on a manufacturing drawing, beyond just “are the numbers right.”
Why plain tolerances aren’t enough on their own
A bolt-hole pattern dimensioned with only linear plus/minus tolerances can technically pass every individual measurement and still not bolt up to its mating flange — because linear tolerances on each hole don’t constrain how the pattern as a whole is positioned relative to the part’s true center, only how far any one hole can be from its nominal location in isolation. GD&T solves this by tolerancing the relationship between features, not just each feature’s size, using a defined set of symbols with precise mathematical meanings rather than descriptive words open to interpretation.
What a reviewer is actually checking
On an oil and gas equipment drawing, a handful of GD&T controls come up constantly, and each answers a specific functional question:
- Flatness and perpendicularity on a flange face — is the sealing surface true enough for a gasket or metal seal to seat correctly across its whole area, not just close to nominal at a few measured points.
- Position tolerance on a bolt-hole pattern — will the holes actually line up with the mating flange’s holes, expressed as a single tolerance zone rather than a pile of individual linear dimensions that don’t add up to a guarantee of fit.
- Concentricity or runout on a bore or a sealing diameter — is it actually centered on the part’s rotational axis, which matters directly for anything that has to seal or rotate around that axis.
- Datums — the reference features every other tolerance is measured from. A drawing with a well-chosen datum scheme tells you unambiguously how the part will actually be measured and inspected; a drawing without one leaves that open to interpretation, which is exactly where inspection disputes come from.
Why this matters more for pressure equipment than most
A cosmetic part with a slightly out-of-position hole is a nuisance. A wellhead flange with a slightly out-of-position bolt pattern, or a sealing bore that’s not truly concentric with the part’s centerline, can mean a joint that won’t make up correctly or won’t seal reliably under pressure — the same seating and alignment concerns covered in our bolted-connection analysis post, but caught (or missed) at the drawing stage rather than in an FEA or, worse, in the field. GD&T is what lets a drawing specify exactly how much positional and form error is tolerable before that happens, instead of leaving it to whatever “reasonably accurate” means to whoever’s machining the part.
The practical takeaway
When reviewing a drawing, the questions worth asking are simple: are the critical sealing and mating features controlled with GD&T rather than just linear tolerances, is there a clear, sensible datum scheme those controls are measured from, and do the tolerance values actually match what the joint or feature functionally needs — not tighter than necessary, which drives up manufacturing cost for no functional benefit, and not looser than the design can actually tolerate.
We produce manufacturing drawings with GD&T, weld symbols, and full bills of material, built to your templates and drawing standards. See our CAD Services, or talk to an engineer about a drawing package.