GD&T Basics for Buyers — What Tolerances Mean and When to Call Them Out

What GD&T Is, Plainly

GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language for saying exactly how a part may vary. Instead of only ± numbers on every size, it controls shape and position — flatness, roundness, how parallel two faces must be, where a hole is allowed to sit. It tells the machine shop what matters and what does not.

Why a Datum Matters

A datum is a reference surface or axis the whole part is measured from. If two features must line up, they need a shared datum, or the shop will measure them against different starting points and the part may not assemble. Calling out a datum turns "make these agree" into something measurable.

When You Need Tight Position Tolerance

Use GD&T position control when a feature must locate precisely relative to others — a bolt hole pattern that has to mate, a bore that must sit on axis, a pin that must clear. A true-position callout with a diameter zone (e.g. ø 0.1) says the centre may sit anywhere inside that small circle, which is exactly what assembly needs.

When a Standard Note Is Enough

Most non-critical sizes are fine with a general tolerance note like "all dimensions ± 0.1 unless noted." Over-using GD&T on every feature makes the part harder and more expensive to inspect without improving fit. Our CNC tolerance guide covers the standard bands we hold and when to call tight.

How We Read Your Drawing

Send the drawing with whatever symbols you have. If a tolerance or datum is missing but the function is clear, our engineers flag it in the manufacturability review before quoting — we will not silently guess on a feature that has to fit. See the GD&T symbol quick reference for the common characteristics.