GD&T & Tolerancing — Engineering Decision Guide

Use GD&T for features whose function is fit, orientation, or location — hole patterns that must mate, sealing faces, bearing bores, assemblies. Use simple plus/minus (coordinate) tolerances for non-critical sizes and one-off features where function is obvious and a square tolerance zone is fine. Don't mix the two ambiguously on the same feature.

Coordinate tolerancing puts a square zone around a location; GD&T position gives a round zone that is ~57% larger in area for the same tolerance — matching how a round pin actually fits a round hole, so it accepts more good parts without loosening the fit. GD&T also carries the datum reference (where the tolerance is measured from) and material-condition bonuses, which plus/minus can't express. That's why functional, mating features belong in GD&T and simple sizes don't need it.

Plus/minus on a mating hole pattern rejects good parts (square zone) and leaves the datum ambiguous, causing inspection disputes. GD&T on every trivial feature bloats the drawing and inspection cost. Mixing conventions on one feature makes the requirement unclear.

Choose datums by function and how the part is located in assembly — the primary datum is the feature that seats first (usually the largest/most stable face, constraining the most degrees of freedom), then secondary and tertiary in the order the part registers. Call them out A, B, C in that functional order, and prefer real, contactable features that mirror the mating interface and the fixture.

The datum reference frame is the coordinate system every geometric tolerance is measured from, so it must match how the part actually locates in use. A primary datum plane constrains three degrees of freedom (one translation, two rotations); the secondary and tertiary lock the rest in sequence. If the drawing's datums don't mirror the assembly interface, a part can measure perfectly against the print and still not fit — the measurement was made from the wrong reference.

Datums chosen for convenience rather than function let parts pass inspection and fail at assembly. The wrong datum precedence (order) shifts the tolerance zones. Non-repeatable datum features (a rough or interrupted surface) make measurements irreproducible between supplier and customer.

Use position to control the location of features of size — holes, pins, slots, bosses — relative to datums. Use profile (of a surface or line) to control the form, orientation, and location of surfaces and contours at once — molded shapes, blends, sealing faces, and complex geometry. Profile is the versatile "one control does several jobs" tool for surfaces; position is the tool for holes and pins.

Position is defined around an axis or center plane of a feature of size, so it's the natural control for round holes and pins that mate — and it accepts the round tolerance zone and material-condition bonuses. Profile controls a boundary around the true surface, simultaneously bounding form, orientation, and location, which is why it's used for contoured and non-size surfaces where there's no axis to locate. Picking the control that matches the feature type keeps the drawing clean and inspectable.

Trying to position a contoured surface (no feature of size) doesn't work; trying to profile a hole pattern loses the bonus tolerance and is awkward to inspect. Using multiple redundant controls where one profile would do clutters the drawing and over-constrains the part.

Apply MMC (maximum material condition) to position on mating features (clearance holes, pins) to gain bonus tolerance as the feature departs from MMC — it matches assembly fit and is the most common. Use LMC where minimum wall or minimum material matters (keeping a boss from getting too thin). Use RFS (regardless of feature size) when the location must hold regardless of size — e.g. a bearing or a dynamically balanced part — accepting no bonus.

Under MMC, when a hole is made larger than its worst-case (tightest) size, the extra clearance is available as additional position tolerance — "bonus" — because the part will still assemble. That directly models how clearance fits behave, so it accepts more good parts and lowers cost. RFS forbids that trade because some functions (press fits, running bearings, balance) care about location at every size, not just worst-case fit. LMC protects minimum material where thin walls or edge distance govern.

RFS on a simple clearance hole throws away free bonus tolerance and raises cost. MMC on a bearing bore or balanced feature wrongly loosens location where it mattered at every size. LMC misapplied can over-constrain. The modifier must match whether the function is clearance fit, minimum material, or size-independent location.

Tolerances far tighter than function needs (driving up cost), or drawings so ambiguous that supplier and inspector disagree on whether a part passes. Rejected good parts, accepted bad parts, and recurring quote-time and receiving-inspection disputes.

Copying tight default tolerances onto non-critical features, missing or unclear datum references, redundant/conflicting controls on one feature, and no clear statement of which features are functionally critical. Tighter-than-needed tolerances are one of the biggest silent cost drivers in machining.

1) Tolerance to function — loosen anything that doesn't affect fit, performance, or safety. 2) Give every geometric tolerance a clear datum reference frame. 3) Remove redundant/conflicting controls; one clear control per requirement. 4) Flag the few truly critical characteristics so the shop and inspector focus there. 5) Have the drawing checked against the standard before release to kill ambiguity.

Parts measure in-spec against the print but won't assemble, or measurements don't repeat between the supplier's CMM and the customer's. The numbers look fine; the parts don't fit.

Datums that don't match the assembly interface, wrong datum precedence, datum features that aren't repeatable (rough, interrupted, or too small to stabilize the part), and inspection that establishes the datums differently than the fixture or mating part does.

1) Set datums to mirror how the part actually locates in the assembly and fixture. 2) Verify datum precedence (A/B/C) reflects the seating order. 3) Choose repeatable datum features large and stable enough to constrain the part the same way every time. 4) Align inspection setup with the functional datum scheme (datum targets where needed). 5) Do a fit/assembly check on first articles, not just a dimensional report.