Every machined part has tolerances, but not all tolerances are created equal. GD&T for machined parts—Geometric Dimensioning and Tolerancing—is the engineering language that tells a machine shop exactly what matters and what does not. Used well, it yields parts that assemble first try and cost less to make; used poorly, it either strangles the shop with impossible requirements or hides the features that actually need control. This guide covers the basics that buyers and designers should know.

What GD&T Is For

Coordinate tolerancing (± on every dimension) treats each size independently and assumes a perfect coordinate system. Real parts are imperfect and how they sit in an assembly matters more than any single number. GD&T defines a part by its function: which surfaces locate it, how much a hole can deviate from true position, and how flat a sealing face must be. That functional focus lets shops use the loosest tolerances that still guarantee fit—saving time and money. It also makes drawings language-independent, so a shop on another continent reads the same intent you designed, reducing the misinterpretation that causes scrap.

When you upload a drawing, our DFM support reviews the GD&T for manufacturability and flags callouts that conflict or over-constrain the part.

Datums and Position

Datums are the reference surfaces (A, B, C) from which everything else is measured—usually the surfaces that locate the part in its assembly or fixture. Position tolerance then controls where a feature (a hole, a boss) sits relative to those datums, using a diameter zone rather than a plus/minus box. This is far more meaningful than separate X/Y tolerances.

For round work, CNC turning pairs naturally with GD&T because datums are often the OD or a face. On milled parts, CNC milling holds position relative to machined datums you define.

Together, the three datums form a datum reference frame—the coordinate system the whole part is measured in. Choosing them to match how the part actually locates in assembly, rather than arbitrary faces, is the single most important GD&T decision, because every position and profile callout is interpreted relative to that frame.

Bonus Tolerance

One of GD&T’s most useful ideas is bonus tolerance. When a feature’s size is within its size tolerance, the position tolerance “grows” by the amount of that deviation—because a larger hole, for example, has more clearance. This gives shops real breathing room without harming assembly. Designers who understand bonus tolerance can specify tighter position callouts safely, knowing the shop has latitude on size.

Common Mistakes

• Over-tolerancing everything – tightening every dimension raises cost with no benefit to function.
• Conflicting datums – referencing a feature from two incompatible frames forces impossibility.
• Missing datum features – you cannot locate a part from a surface you did not call out.
• Ignoring the inspection method – a tolerance is only as good as how it is measured; specify it.

Learning to read a feature control frame pays off quickly: the symbol tells you the type of control, the tolerance value sets the zone, the datum letters show the reference frame, and any modifier (like maximum material condition) changes how the zone is applied. Once a buyer can read these, drawings become unambiguous and quotes become comparable—two shops quoting the same GD&T are truly quoting the same part.

For flat and parallel-critical faces, surface grinding delivers the flatness GD&T often demands, and 5-axis work benefits from the tighter relational control described in our 5-axis CNC machining capabilities.

How to Call Out Tolerances

Start from function: identify locating surfaces, then assign the loosest tolerances that still assemble and perform. Use position for features of size, flatness/parallelism for sealing and mating faces, and concentricity/runout for rotating parts. Reserve the tightest numbers for the few features that truly matter. Our deeper reads on precision machined parts and CNC machining services show how tolerance strategy affects cost, and our 3-axis CNC machining guide explains datum setup on simpler parts.

A practical habit is to start with the loosest reasonable tolerances on a first article, then tighten only the features that prove troublesome in assembly. This loosen-then-tighten approach avoids paying for precision you do not need and keeps the first batch manufacturable. The inspection report then tells you exactly where to invest in tighter control.

Frequently Asked Questions

Do I need GD&T on simple parts?

Not always. A basic bracket with loose fits may only need a few plus/minus dimensions. Use GD&T when assembly, function, or interchangeability depend on feature relationships.

Why is GD&T cheaper than tight plus/minus?

Because it targets control where it matters and allows slack elsewhere, including bonus tolerance. Shops machine to functional limits instead of fighting every dimension.

What does a CMM report check?

A coordinate measuring machine verifies features against the datum frame and GD&T callouts, producing a report that proves the part meets the drawing—critical for qualified production.

Can GD&T be applied to cast parts?

Yes, though casting has larger natural variation, so GD&T for castings uses broader zones and often references machined datum surfaces added after casting.

Specify with Confidence

Good GD&T for machined parts turns a drawing into a clear, manufacturable contract between you and your shop—reducing cost and rejects while guaranteeing fit. Send us your drawing for a DFM and tolerance review. Contact Huaxiao Precision today and let our engineers help you call out what truly matters.