A drawing with a tolerance block that just says "±0.005 unless noted" tells a machine shop almost nothing about which features actually matter. Geometric Dimensioning and Tolerancing (GD&T) is the language that fixes that gap — it tells us not just how much variation is allowed, but what kind of variation, and relative to what. If you're sourcing CNC machined parts and want quotes that come back accurate on the first pass, understanding what's on your own drawing is the fastest way to get there.
This isn't a full GD&T course. It's the subset that shows up on most of the drawings we quote, explained the way we actually read them on the shop floor.
Why Tolerance Blocks Alone Aren't Enough
A general tolerance block (e.g., ±0.1 mm on two-place decimals, ±0.05 mm on three-place) controls size. It says nothing about:
- Whether a face has to be flat, or just within a size range
- Whether two holes have to line up with each other, or just with the outside edge
- Whether a bore has to run true to a shaft, or can wobble within the size tolerance
GD&T callouts — the feature control frames you see near a dimension — answer those questions explicitly. When they're missing, a machinist has to guess intent, and guessing intent is exactly where inspection disputes and RFQ re-quotes come from.
Reading a GD&T Feature Control Frame
A feature control frame reads left to right, and every symbol has a fixed meaning: geometric characteristic, tolerance zone, then datum references in priority order.
- Geometric characteristic symbol — what's being controlled (position, flatness, runout, etc.)
- Tolerance zone — the allowed variation, often with a diameter symbol (⌀) if it's a cylindrical zone
- Datum references — the reference surfaces or features (A, B, C) the tolerance is measured from, in priority order
The datum order matters. Datum A gets set up first on the inspection fixture, then B, then C. If a part fails inspection, the datum sequence is usually the first thing to check.
The Symbols You'll Actually See
CNC machined part drawings lean heavily on a handful of the 14 GD&T characteristics.
Flatness
Controls how much a single surface can deviate from a perfect plane, independent of size. Common on mounting faces and sealing surfaces where a size-only tolerance wouldn't catch a bowed part.
Position (true position)
Controls the location of a feature — usually a hole or boss — relative to datums, defining a cylindrical tolerance zone rather than a simple linear ± box. This is the callout that shows up most often on bolt patterns and dowel holes, because it directly controls whether mating parts will actually assemble.
Runout
Controls how much a rotating feature wobbles relative to a datum axis, critical on turned parts like shafts, pins, and anything with a bearing fit.
Perpendicularity, parallelism, and angularity
Control the orientation of a surface or axis relative to a datum, tighter than what a basic angle dimension implies.
Profile of a surface
Controls a surface's shape against a theoretical, usually CAD-defined, profile — common on cams, contoured faces, and organic shapes a simple dimension can't fully describe.
What "True Position" Actually Buys You
True position is worth calling out on its own because it's the most misread symbol on a drawing. A linear tolerance of ±0.1 mm on both X and Y creates a square tolerance zone — a hole can be off-position diagonally by up to 0.14 mm and still pass a naive linear check. A true position callout of ⌀0.1 mm defines a circular zone of that same 0.1 mm diameter, centered on the theoretical exact location — a tighter, more honest way to control where a feature actually sits.
When a Tolerance Block Is Enough — and When It Isn't
Not every feature on a part needs a GD&T callout. As a rule of thumb:
- Non-critical faces, chamfers, and clearance holes — a general tolerance block is fine.
- Mating features, bolt patterns, sealing surfaces, anything with a fit or assembly requirement — GD&T callouts remove ambiguity a ± dimension can't resolve.
- Rotating or symmetric parts — runout and concentricity callouts are usually worth adding wherever a bearing, seal, or press fit is involved.
If your drawing is missing callouts on features that clearly need them, that's exactly the kind of thing an engineering-led RFQ process should catch and flag back to you before quoting, rather than silently machining to a "reasonable" interpretation.
What This Means for Your Next RFQ
The single biggest lever you have for quote accuracy is a drawing that says exactly what you mean. GD&T does that. If your drawing already has feature control frames, make sure the datum scheme is unambiguous — datums should be called out on the drawing itself, not implied. If it doesn't have them yet, flag the features you're most concerned about and let the quoting engineer confirm inspection requirements with you before the part goes to the machine. See our CNC machining, CNC milling, and CNC turning services for the processes behind these callouts.
