Geometric tolerancing earns its keep on four or five features per drawing. Everywhere else it is decoration that slows down inspection.
GD&T exists to say something that plus-minus dimensioning cannot: how a feature relates to a datum under real conditions. Used deliberately it removes ambiguity and cost. Sprayed across a drawing it does the opposite.
Start with the datum structure
Datums should reflect how the part is located in the assembly, and ideally how it is held in the fixture. If your primary datum is a surface the machinist cannot clamp on, the callout is theoretical and inspection becomes an argument.
The controls that pay for themselves
- .01Position on hole patterns — replaces a stack of coordinate tolerances with one honest number
- .02Perpendicularity on bores that carry bearings
- .03Flatness on mating faces that seal or bolt down
- .04Profile on cast or moulded surfaces where a linear dimension means nothing
- .05Runout on rotating features, in place of three separate callouts
Maximum material condition, used properly
MMC on a clearance hole pattern gives the machine shop bonus tolerance for free, and costs assembly nothing. It is the single highest-value modifier on most fabricated parts, and the one most often left off.
IF YOU CANNOT DESCRIBE HOW A CALLOUT WOULD BE INSPECTED, IT SHOULD NOT BE ON THE SHEET.
Know which standard you are drawing to
ASME Y14.5 and ISO GPS are not interchangeable, and mixing them within one drawing pack causes real inspection disputes. Pick the standard your client and their supply chain work to, state it in the title block, and stay inside it.