Datums first. Tolerance zones. Function-driven.

Function over numbers.

Traditional ± tolerancing fails when feature relationships matter. A hole with ±0.05 diameter + ±0.05 location tolerance: the part can fail assembly even when every dimension is "in tolerance."

GD&T characteristic symbols.

Datum reference frames.

Datums define the reference system for GD&T measurements. Datum selection affects part functionality and inspection method.

The most common GD&T control.

What position tolerance does: Defines allowable location variation of feature center from theoretical exact location. Tolerance zone is a circle (or cylinder for holes) with diameter equal to position tolerance value. Feature axis must fall within zone.

Equivalent to ± tolerance: Position 0.2 tolerance is roughly equivalent to ±0.1 in X and ±0.1 in Y — but with a circular tolerance zone instead of square. This gives 27% more tolerance area for same drawing intent, so position is less restrictive (cheaper to produce) than equivalent ± tolerances.

At MMC modifier: "Position 0.2 at MMC" means 0.2 tolerance when feature is at maximum material. If hole is larger than MMC, additional tolerance allowed equal to the difference. For typical hole MMC 9.9-10.0 mm: at exactly 9.9, tolerance is 0.2; at 10.0, tolerance is 0.3; at 10.1, tolerance is 0.4. Very useful for hole patterns where fasteners need to assemble.

Datum references: Position tolerance must reference datums. "Position 0.2 |A|B|C|" means tolerance zone is positioned by datum A, then B, then C. Without datums, the tolerance has no reference — meaningless.

Practical example: Hole pattern for mounting a part to mating assembly: specify position 0.2 at MMC on the hole pattern, referenced to datum A (mounting face) and B (orientation reference). This guarantees assembly as long as individual holes are within position tolerance, while allowing machining flexibility.

FAQ

If drawings specify only ± tolerances and parts assemble fine, you don't need GD&T. Add GD&T when: (1) You have assembly problems with ± tolerances. (2) Critical features need defined relationships (concentricity, parallelism). (3) Cost is issue — GD&T often gives bigger tolerance zones. (4) Customer specifies GD&T (aerospace often requires it). For simple parts with obvious function, ± tolerances are adequate. For complex assemblies, GD&T is essential.

(1) Missing datums on controls that need them (position, parallelism, perpendicularity). (2) Over-constraining datums (redundant datum references). (3) Unclear datum features (using centerlines or theoretical axes as primary datums). (4) Position tolerance without MMC when MMC would be appropriate. (5) Specifying tighter control than function requires. (6) Using concentricity when runout would serve. (7) Surface profile applied to inappropriate features.

Concentricity is difficult and expensive to inspect — requires axis detection. Runout measures actual surface wobble at specific cross-sections or along entire length. For most applications, runout achieves the functional goal at lower inspection cost. Rule: specify runout unless you specifically need concentricity (very rare — concentricity has stricter theoretical meaning).

Surface profile controls the full shape of a surface — contour, location, form all in one tolerance. Position controls feature location (e.g., hole center). For complex 3D surfaces, use surface profile. For feature locations, use position. For faces that must be parallel to another face, use parallelism (simpler than profile). Match control to what you're actually trying to constrain.

Most GD&T features require CMM for efficient measurement. Key capabilities needed: (1) Datum establishment — probing datum features per DRF definition. (2) Feature axis and surface evaluation — for position, runout, surface profile. (3) Analysis software — reports actual vs tolerance zone. Manual inspection possible for simple features (flatness with surface plate, runout with dial indicator + V-block) but CMM is standard for modern GD&T.

Both current, both used. Y14.5-2018 added: continuous feature modifier, new symbol for "dynamic profile" (for form only without size), some datum clarifications. Most drawings still use 2009 conventions. For new drawings, specify which standard applies as drawing note. We work with both — most customer drawings reference Y14.5-2009.

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