// RESOURCE — TOLERANCES & GD&T
CNC Machining Tolerances Explained
Standard vs precision tolerances, the basics of GD&T, surface finish, what is actually achievable, and why tightening a tolerance you do not need quietly inflates your cost.
A tolerance is the allowable variation on a dimension — the difference between the largest and smallest acceptable value for a feature. No part is ever made perfectly to a nominal number; tolerances tell the shop how much deviation the function will accept. Specifying them well is one of the highest-leverage things a designer can do: too loose and the part fails its function, too tight and you pay for precision you never needed. This guide explains the levels of tolerance, the basics of GD&T, and exactly what drives cost.
What a tolerance is
Every dimension on a drawing carries an implied or explicit tolerance. A callout of 1.000" ±0.005" means any part measuring between 0.995" and 1.005" is acceptable. Tolerances exist because real-world cutting, tool wear, thermal expansion, and material spring-back all introduce small variations. The designer's job is to assign each feature the loosest tolerance that still guarantees the part functions and assembles correctly.
Standard vs precision tolerances
Tolerance capability is a spectrum. The following are typical, useful planning figures for machined metal parts; the achievable value for any given feature depends on its size, geometry, material, and the equipment. Treat these as guidance, hold tight tolerances to print, and send your drawing so we can confirm what is realistic for your critical features.
| Standard / general | ~±0.005" — economical, suits most non-critical features. |
|---|---|
| Precision | ~±0.001" — tighter fits, mating features, bearings. |
| High precision | down to ~±0.0002" on appropriate features & materials. |
| Reality check | Tighter than function requires only adds cost and scrap risk. |
Rigid Concepts routinely holds tight tolerances to print and can achieve down to ±0.0002" on appropriate features and materials — the same capability described on our precision machining page. The discipline that makes those numbers repeatable is documented inspection; see quality inspection.
GD&T basics
Geometric Dimensioning and Tolerancing (GD&T), governed by ASME Y14.5, is a symbolic language for controlling not just size but the form, orientation, and location of features relative to datums. It communicates design intent far more precisely than ± dimensions alone, and it can actually loosen manufacturing while tightening function through bonus tolerance at maximum material condition.
- Datums: the reference surfaces everything else is measured from.
- Form: flatness, straightness, circularity, cylindricity.
- Orientation: parallelism, perpendicularity, angularity.
- Location: true position, concentricity, symmetry.
- Profile: profile of a line or surface for complex contours.
Calling out a clear datum scheme and using true position for hole patterns instead of stacked ± dimensions usually produces a more functional part at lower cost, because it tells the shop exactly what matters.
Surface finish
Surface finish (roughness), typically specified as Ra in microinches, is distinct from dimensional tolerance and carries its own cost. An as-machined finish is adequate for most parts; a fine, polished, or lapped finish requires extra operations. Specify a tight finish only on sealing faces, sliding surfaces, or cosmetic areas that need it.
What drives tolerance cost
Tightening a tolerance increases cost in several compounding ways. Understanding them helps you spend your tolerance budget where it matters.
- Slower cutting: finishing passes and lighter cuts to hit tight dimensions.
- More inspection: tighter features need more measurement, sometimes a CMM.
- Higher scrap risk: less margin means more parts rejected.
- Tooling & setup: special tooling, temperature control, and refixturing.
How to specify tolerances well
- Default the drawing to a sensible general tolerance, then tighten only critical features.
- Use GD&T and clear datums for anything that mates or locates.
- Reserve fine surface finishes for faces that actually seal, slide, or show.
- Send the model and drawing for a DFM review before release.
A free DFM review will flag tolerances that are tighter than they need to be and features that fight the toolpath — saving cost before metal is cut. When your print is ready, send it for a quote.
// FAQ
Frequently asked questions
A common general machining tolerance is about ±0.005". It is economical and suits most non-critical features. Precision work holds about ±0.001", and high-precision features can reach roughly ±0.0002" on suitable materials.
On appropriate features and materials we can achieve down to ±0.0002". The achievable value depends on the feature size, geometry, and material, so send your drawing and we will confirm what is realistic for your specific part.
Geometric Dimensioning and Tolerancing (ASME Y14.5) is a symbolic language that controls the form, orientation, and location of features relative to datums — not just their size. It communicates design intent precisely and can lower manufacturing cost while tightening function.
Tighter tolerances require slower finishing cuts, more inspection, sometimes special tooling or temperature control, and they raise scrap risk because there is less margin for variation. Specify tight tolerances only on features whose function requires them.
Yes. Surface finish (Ra) is separate from dimensional tolerance. As-machined finishes are inexpensive; fine, polished, or lapped finishes need extra operations, so reserve them for sealing, sliding, or cosmetic surfaces.
Send us the hard one.
Upload your drawing or STEP file and we'll come back with pricing and lead time — from a single high-mix part to full production runs, held to exacting tolerances.