CNC Machining Tolerances and GD&T: A Buyer's Guide (2026)

A part comes back late and over budget, and the culprit is not the geometry, it is the print: every dimension toleranced to plus or minus 0.001 in because that felt safe. The shop had to treat an ordinary bracket like an inspection fixture. Tolerances are the single biggest lever a buyer controls on machining cost, and most parts are toleranced far tighter than they need to be. This guide covers sensible defaults, what the standards actually say, when you need GD&T, and how each tolerance decision shows up in the quote.

TL;DR

Tolerance only what has a functional reason and leave the rest to a general-tolerance note. A sensible default for machined features is about plus or minus 0.005 in (0.13 mm), or a published class like ISO 2768-m, and that is the cheapest tier. Step to precision (around plus or minus 0.001 in) for fits and mating features, and to tight (plus or minus 0.0005 in or below) only where function truly demands it, because cost climbs at every step. Use plus/minus tolerances for sizes and GD&T (ASME Y14.5) for feature relationships like hole position and flatness. The rule is simple: the loosest tolerance that still works is the right one. Tell us what you need and we route the part to a shop that holds the print.

What a Tolerance Actually Is

A tolerance is the allowed range on a dimension. A shaft called out at 20.00 mm plus or minus 0.05 mm is in spec anywhere from 19.95 to 20.05 mm. Nothing is ever made exactly to nominal, so every real dimension carries a tolerance, whether you state it or not.

That last point is the one buyers miss. If you leave a dimension untoleranced, it is not "make it perfect," it defaults to the drawing's general-tolerance note, which is why that note matters so much.

General Tolerances: The Default That Saves You Money

You do not tolerance every dimension by hand. You set a general tolerance once, and it applies to everything you did not call out individually. The common standard for this is ISO 2768-1, which assigns default tolerances by feature size and class. The four classes are f (fine), m (medium), c (coarse), and v (very coarse); medium is the usual default on mechanical drawings.

The medium-class linear tolerances look like this:

Nominal size (mm)	ISO 2768 class m (medium)
0.5 to 3	±0.1 mm
3 to 6	±0.1 mm
6 to 30	±0.2 mm
30 to 120	±0.3 mm
120 to 400	±0.5 mm

Put ISO 2768-m in the title block and every unremarkable dimension is handled, at a tolerance any shop holds without effort. Then you spend your tight tolerances only where they earn it.

The Three Practical Tolerance Tiers

Beyond the general note, individual features fall into three cost tiers. Rough figures, because exact capability varies by feature and machine:

Tier	Typical tolerance	When to use it	Cost
Standard	±0.005 in / ±0.13 mm	Most features; the default	Lowest
Precision	±0.001 in / ±0.025 mm	Fits, bearing seats, mating faces	Higher
Tight	±0.0005 in / ±0.013 mm or below	Critical interfaces only	Premium

Standard covers the large majority of features on a typical part. Precision is for the handful that locate or mate with something else. Tight is a deliberate, budgeted decision, not a default.

When Plus/Minus Is Not Enough: GD&T

Plus/minus tolerances control sizes. They cannot, on their own, say "these four holes must sit true to each other" or "this face must be flat regardless of its thickness." That is what geometric dimensioning and tolerancing (GD&T) does, using symbols and datums to control form, orientation, location, profile, and runout.

In North America, GD&T is defined by ASME Y14.5 (the current edition is 2018); the international equivalent is the ISO GPS system (standards such as ISO 1101). You do not need to master it as a buyer, but you should recognize where it helps:

• True position on a bolt-hole pattern that has to mate with another part.
• Flatness on a sealing or gasket face.
• Concentricity or runout on a rotating shaft.
• Profile on a curved surface that has to match a mating contour.

If plus/minus fully captures the intent, skip GD&T; it only adds value where feature relationships drive function.

How Tolerance Drives Cost

Here is the part worth internalizing: tolerance is a price dial. Tightening it does several expensive things at once.

• Slower cutting. Tight tolerances mean lighter finishing passes and lower feeds.
• Better tooling and more setups. Holding a tenth is a different setup discipline than holding five thou.
• More inspection. Tight features get measured, sometimes on a CMM, sometimes every part.
• Higher scrap. A part that drifts out of a narrow band is scrap, and that risk is priced in.

None of this is the shop padding a number. It is real time and risk. Which is why loosening an unnecessary tolerance is close to free money: same function, lower price.

What to Put on the Drawing

A clean tolerance package quotes fast and makes right the first time:

1. A general-tolerance note (for example ISO 2768-m) so undimensioned features are covered.
2. Explicit tolerances only on functional features: fits, seals, mating interfaces, bearing seats.
3. Datums and GD&T where feature relationships matter, per ASME Y14.5.
4. Surface finish callouts where they matter (a tolerance is not a finish).
5. Material and grade, since achievable tolerance interacts with the material.
6. A clean STEP file plus the drawing; the model carries geometry, the drawing carries intent. See also how to prepare a CAD drawing for CNC.

Common Mistakes Buyers Make

Mistake 1: Blanket-tightening everything. Toleranceing every dimension to plus or minus 0.001 in is the number-one way to overpay. Most features do not need it.

Mistake 2: No general-tolerance note. Without one, untoleranced dimensions are ambiguous, and the shop either guesses or comes back with questions, both of which cost time.

Mistake 3: Confusing tolerance with finish. A tight size tolerance does not mean a smooth surface, and vice versa. They are separate callouts.

Mistake 4: GD&T for its own sake. Adding position and profile controls where plain plus/minus would do just makes the part harder to quote and inspect.

Material Changes What Is Achievable

Tolerances do not live in a vacuum. A free-machining brass or aluminum part holds a tight tolerance more easily and cheaply than a work-hardening stainless steel one, where slow, careful cutting is the price of precision. Long, thin, or turned features deflect and are harder to hold than short, stiff ones. If a tolerance is critical, the material and geometry are part of the conversation, not an afterthought.

Sourcing in Canada

Tolerance capability is standard across Canada's $931.2 billion manufacturing sector, from general machining to CMM-inspected precision work. Sourcing domestically keeps freight and lead times short and puts you a phone call, not a time zone, away when a tolerance question comes up mid-job. Compare local capacity in Toronto, Vancouver, and Montreal, and if you are still selecting a supplier, start with how to choose a CNC machine shop.

How FrankWorks Handles Tolerances

You send the part and the print; we match it to a shop equipped for the tolerance you actually need, whether that is a general-tolerance bracket or a CMM-inspected precision component, and return pricing and a lead time. Send a STEP file and drawing with your tolerances and finishes called out, or describe the part, and the quote reflects the real requirement rather than a worst-case guess.

Frequently Asked Questions

What is a standard CNC machining tolerance? When a drawing states no tolerance, shops typically work to about plus or minus 0.005 in (0.13 mm), or to a general standard like ISO 2768 medium class. That is the cheapest, comfortably achievable tier. Specify tighter numbers only where function needs them.

What is ISO 2768? A general-tolerance standard that sets default tolerances by feature size for dimensions you have not individually toleranced. Part 1 has four classes (f, m, c, v); medium (m) is the common default. Calling out ISO 2768-m covers every unremarkable dimension.

What is the difference between plus/minus tolerances and GD&T? Plus/minus controls the size of one dimension. GD&T (ASME Y14.5 in North America) controls feature geometry such as flatness, position, and profile relative to datums. Use plus/minus for sizes and GD&T when feature relationships drive function.

How do tolerances affect CNC machining cost? Strongly. Tighter tolerances mean slower cutting, careful setups, better tooling, more inspection, and higher scrap risk. Loosening a tolerance that need not be tight is one of the cheapest ways to lower a part's price.

What tolerance should I put on my drawing? Tolerance only functional features (fits, seals, mating interfaces) and leave the rest to a general-tolerance note like ISO 2768-m. Over-tolerancing every dimension is the most common way to overpay.

Do tighter tolerances always cost more? As a rule, yes, and the curve steepens fast below about plus or minus 0.001 in. Aim for the loosest tolerance that still works.

What is the tightest tolerance CNC can hold? On the right setup, roughly plus or minus 0.0005 in (0.013 mm) and sometimes tighter on specific features, but that is a premium operation, not an everyday spec.

Do I need GD&T on my part? Not always. Many parts are fully defined with plus/minus plus a general note. Use GD&T when relationships between features (position, flatness, concentricity) drive function.

How do I communicate tolerances to a machine shop? Send a STEP file for geometry plus a drawing (or annotated model) carrying tolerances, datums, GD&T, finishes, and the general-tolerance note. The 3D model alone does not carry tolerance intent.