2026-05-14
Traditional dimensional tolerancing (±0.005") tells you how big a feature is, but says nothing about where it sits or how straight it is. A hole can be perfectly sized at Ø0.500" yet still be tilted, off-center, or oval — and your mating part won't go on. Geometric Dimensioning and Tolerancing (GD&T), codified in ASME Y14.5, fixes this by controlling form, orientation, location, and runout independently of size.
The 14 symbols fall into five categories:
Datums are the foundation. Every orientation, location, and runout callout references one or more datums (A, B, C) — physical surfaces or features the inspector uses to set up the part. The order matters: A is the primary (3 points of contact), B is secondary (2 points), C is tertiary (1 point). This 3-2-1 rule fully constrains the part in space.
Real-world example: A motor mount has four bolt holes that must align with a gearbox flange. If you only specify hole-to-hole distance as 4.000" ±0.005", any individual hole can drift within a square tolerance zone — and stack-up can prevent assembly. Replace it with Position Ø0.010 M | A | B | C, and each hole must fall within a Ø0.010" cylindrical zone relative to datums. The bonus: the M (Maximum Material Condition) modifier means if a hole is drilled larger than minimum, you get extra positional tolerance — a "bonus" that reflects real assembly clearance.
Quick rule of thumb for position tolerance:
Tolerance Zone Diameter = (Min Hole Ø − Max Bolt Ø) / 2 × (1 or 2)
For floating fasteners (clearance holes both sides), multiply by 2. For Ø0.260" holes and Ø0.250" bolts: (0.260 − 0.250)/2 × 2 = Ø0.010" position tolerance at MMC for each part.
GD&T also shrinks drawings: one position callout replaces a grid of ±X/±Y dimensions, and it communicates function rather than just geometry. Machinists know what matters most because the tightest tolerances tell the story.