GD&T, geometric dimensioning and tolerancing, is the language a drawing uses to say exactly how a part must fit and function. If you buy CNC parts, you do not need to author GD&T, but understanding the common callouts tells you why one print costs three times another and where you can save money without hurting the part. This GD&T basics for buyers guide covers datums, true position, and profile, then shows how each callout drives cost.
Why GD&T exists
Plain plus-or-minus dimensioning controls size but not relationship. A hole can be the right diameter and still be in the wrong place. GD&T adds control over location, orientation, and form relative to defined reference surfaces, and it does so with a functional bonus: features can gain extra tolerance as they move away from worst-case size. That functional logic is why aerospace, defense, and medical prints lean on it, and why CMM inspection is the natural way to verify it.
Datums: the reference frame
A datum is a surface, axis, or plane the drawing uses as a reference. Datums are labeled A, B, C and usually establish a 3-2-1 constraint: the primary datum (A) removes the most motion, the secondary (B) constrains rotation, and the tertiary (C) locks the last direction. Everything else on the part is measured from this datum reference frame.
Why datums matter to a buyer
- They tell the shop how to fixture and inspect the part, which drives setup time.
- They define what the tolerances are measured relative to, so the same 0.010 in callout can be easy or hard depending on the datum chain.
- Poorly chosen datums force extra setups. A feature referenced to a datum on the opposite side of the part may require a second operation and a second CMM alignment.
True position: locating features
True position controls where a feature, most often a hole, sits relative to the datums. Instead of a square plus-or-minus box, position defines a circular (or cylindrical) tolerance zone around the exact location. A callout might read a position tolerance of 0.010 in to datums A, B, C.
Maximum material condition and the bonus tolerance
Position is frequently modified by MMC (maximum material condition). When a hole is larger than its smallest allowed size, the extra material yields bonus tolerance, so the true position zone grows. This is good news for cost: MMC callouts are generally cheaper to hold than tight position at regardless of feature size (RFS), because the part gains slack exactly where a bolt or pin would still fit. When you see position at MMC, the designer is telling you the feature is about assembly clearance, not a precision locating pin.
Profile: controlling a whole surface
Profile of a surface controls the form, and optionally the location, of a contour against the datums. A profile tolerance of 0.005 in means every point on that surface must fall within a 0.005 in wide band that follows the true shape. Profile is powerful because a single callout can control size, form, and location of a complex surface at once. It is also verified efficiently by CMM scanning, which captures hundreds of points along the contour rather than a handful.
How GD&T callouts drive CNC machining cost
Tolerance is the single biggest lever on machining price after material and quantity. Tighter zones mean more passes, slower feeds, better tooling, temperature control, and more inspection. The table below shows typical illustrative cost multipliers relative to a general-tolerance baseline part. These are planning estimates, not binding quotes.
| Callout | Typical tolerance | Relative cost impact |
|---|---|---|
| General profile or size (baseline) | +/-.005 in | 1.0x baseline |
| Precision size or position | +/-.001 in | 1.3x to 1.7x |
| Tight position at MMC | 0.005 in at MMC | 1.2x to 1.5x |
| Tight position at RFS | 0.005 in RFS | 1.6x to 2.2x |
| Precision profile | 0.002 in to 0.005 in | 1.5x to 2.5x |
| Critical fit or bearing bore | +/-.0005 in | 2x to 4x |
| Fine surface finish add-on | Ra 16 uin or better | +10% to +40% |
Where buyers save money
- Open up non-functional tolerances. If a feature does not mate or seal, a +/-.005 in general tolerance costs far less than a blanket +/-.001 in.
- Ask for MMC where clearance allows. Bonus tolerance on clearance holes lowers cost with no loss of fit.
- Keep datums on one side when possible. Fewer setups and fewer CMM alignments mean lower labor.
- Relax surface finish off critical faces. Ra 63 uin is a normal machined finish and costs less than a specified Ra 16 uin.
Verifying GD&T
Geometric callouts are not something calipers can confirm. Position, profile, orientation, and runout are measured against the datum reference frame on a coordinate measuring machine, then documented in a report or a first article package. Wexmar verifies GD&T in-house on a Brown & Sharpe CMM, so the same building that mills, turns, and Swiss-machines your part also proves it.
See how tolerance and finish play out across our CNC milling and Swiss machining capabilities, review our inspection and quality process, or upload a print through our instant quote and we will flag any callout that is driving cost harder than it needs to.