What Are Tolerance Standards Commonly Used in Metal Fabrication?

When you send a part out for manufacturing, one of the first questions a fabricator will ask is: “What tolerances do you need?” If your drawing doesn’t specify tolerances clearly — or worse, specifies the wrong ones — you’re looking at one of two outcomes: parts that don’t fit, or parts that cost 3× more than they should.

Tolerance standards exist to prevent both problems. They give engineers and manufacturers a shared language for how much deviation from the nominal dimension is acceptable. But there isn’t just one standard — there are two major families: ISO 2768 (used globally) and ASME Y14.5 (dominant in North America).

This guide covers both in detail: what each standard means, how to read their tolerance tables, how to choose between them, what it costs to get it wrong, and how to specify tolerances on your drawings so your manufacturer gets it right the first time.

What Is ISO 2768?

ISO 2768 is an international standard published by the International Organization for Standardization. It defines general tolerances — default permissible deviations that apply to any dimension on a technical drawing that doesn’t have an individually specified tolerance.

The standard is split into two parts:

• ISO 2768-1: Tolerances for linear and angular dimensions
• ISO 2768-2: Geometrical tolerances for features (flatness, straightness, perpendicularity, symmetry, circular run-out)

Instead of cluttering a drawing with tolerance callouts on every single dimension, an engineer simply writes a designation like “ISO 2768-mK” in the title block. The “m” refers to the tolerance class from Part 1, and the “K” refers to the geometrical tolerance class from Part 2.

ISO 2768-1: Linear and Angular Dimension Tolerances

Part 1 defines four tolerance classes, each corresponding to a level of precision:

Here are the actual permissible deviation values for linear dimensions (in millimeters):

How to read this table: If your drawing says “ISO 2768-m” and has a 50 mm dimension without an individual tolerance, the acceptable range is 49.7 mm to 50.3 mm (±0.3 mm for the 30–120 mm range, class m).

For angular dimensions, the standard uses degrees and minutes:

ISO 2768-2: Geometrical Tolerances

Part 2 covers form and position tolerances. It defines three classes — H, K, and L — for characteristics like straightness, flatness, perpendicularity, symmetry, and circular run-out:

“ISO 2768-mK” — what it means on a drawing: The part follows medium tolerances for linear dimensions (class m) and class K for geometrical tolerances (straightness, flatness, perpendicularity). This is the most common specification for general CNC machined parts and is what many shops — including BravoFabs — use as their default unless the customer specifies otherwise.

What Is ASME Y14.5 (GD&T)?

ASME Y14.5 is the American standard for Geometric Dimensioning and Tolerancing (GD&T). Unlike ISO 2768, which provides default “blanket” tolerances, GD&T is a symbolic language that lets you precisely control how a feature relates to other features — not just its size.

GD&T defines 14 geometric characteristic symbols across five categories:

The Feature Control Frame

In GD&T, tolerances are communicated through a Feature Control Frame (FCF) — a rectangular box attached to the dimension it controls:

┌──────────────┬──────────┬──────────┐
│  ⌖  │  Ø0.2 Ⓐ │  A │ B │ C │
└──────────────┴──────────┴──────────┘
   Symbol    Tolerance    Datums

• First compartment: The geometric symbol (⌖ = position)
• Second compartment: The tolerance value (Ø0.2 mm) and any material condition modifier (Ⓐ = regardless of feature size)
• Third compartment(s): The datum references (A, B, C) — the reference surfaces that define the coordinate system

Datums are the foundation of GD&T. A datum is a theoretically perfect point, line, or plane from which measurements are taken. On a typical machined part, Datum A might be the bottom face, Datum B a side edge, and Datum C a front edge — together forming a 3-2-1 alignment system that fully constrains the part in space.

ISO 2768 vs. ASME Y14.5: Key Differences

How to Choose the Right Tolerance Standard

Your choice depends on three main factors:

1. By Industry

2. By Region

If your customer is in Germany, Italy, or Japan, they will expect ISO 2768 on the drawing. If they’re in the US or Canada, ASME Y14.5 is the default. When manufacturing in China for export, the best practice is to specify both — for example, “ISO 2768-mK unless individual tolerances per ASME Y14.5 are indicated.”

3. By Part Criticality

A simple bracket holding a sensor doesn’t need GD&T. A turbine blade housing that mates with 12 other components does. A practical rule of thumb:

• Non-critical fit: ISO 2768-mK is sufficient (saves cost)
• One or two critical fits: ISO 2768-mK as default, add individual GD&T callouts on the critical features
• Complex multi-part assembly: Full ASME Y14.5 with datums and feature control frames

The Cost of Getting Tolerances Wrong

The relationship between tolerance and cost is not linear — it’s exponential:

Real example: A 100 mm × 50 mm aluminum bracket quoted at ISO 2768-m (±0.3 mm on the length) might cost $15 per part. The same bracket with a ±0.01 mm tolerance on every dimension could cost $80–120 — and in many cases, the tighter tolerance adds zero functional value.

Common Tolerance Mistakes (And How to Avoid Them)

Mistake #1: Over-Tolerancing — “If Some Is Good, More Is Better”

Putting ±0.01 mm on every dimension of a sheet metal enclosure is the most common (and expensive) error we see. Sheet metal parts, by the nature of the bending process, have inherent variation from springback and material thickness variation. A ±0.5 mm tolerance on an enclosure is perfectly functional; insisting on ±0.05 mm just means the shop will quote you 3× the price and you’ll still get ±0.3 mm in practice.

Mistake #2: No Tolerance at All

The opposite problem: a drawing with dimensions but zero tolerance information. The shop has to guess. Most will default to their internal shop standard — but if your part needs something specific, that information needs to be on the drawing. Write “ISO 2768-mK” in the title block. It takes five seconds and prevents a week of back-and-forth emails.

Mistake #3: Missing Datums in GD&T

A position tolerance without a datum reference is ambiguous. Which surface is the reference? If you call out ⌖ Ø0.2 but don’t specify Datum A, B, C, the manufacturer doesn’t know where to measure from. Always define your datums first, then reference them in your feature control frames.

Mistake #4: Ignoring the Process Capability

Different manufacturing processes have different natural tolerance ranges. Don’t put a ±0.02 mm tolerance on a welded assembly — welding introduces distortion that makes that tolerance impossible to hold without post-weld machining. Match the tolerance to what the process can actually deliver:

BravoFabs Default Tolerance Standards

At BravoFabs, our default manufacturing tolerance is ISO 2768-mK for CNC machined parts and ISO 2768-cK for welded fabrications, unless the customer’s drawing specifies otherwise. Here’s what that means in practice for our three core processes:

If your project requires tighter tolerances — such as ±0.01 mm for bearing fits or GD&T per ASME Y14.5 — we can accommodate. Just specify it on your drawing, and our engineering team will review it before quoting so you know exactly what’s achievable and at what cost.

Key Takeaways

• Always put a tolerance standard on your drawing. “ISO 2768-mK” in the title block is the minimum. No tolerance = ambiguity = delays.
• ISO 2768 is for general tolerances; ASME Y14.5 is for precision GD&T. They work together — use ISO for default coverage, ASME symbols for critical features.
• Match the tolerance to the process. Don’t put CNC-grade tolerances on a weldment. Don’t put sheet metal tolerances on a press-fit bore.
• Tighter tolerances cost exponentially more. Ask yourself: does this feature need ±0.01 mm, or would ±0.1 mm work just as well? The difference can be 5× the part cost.
• When in doubt, ask your manufacturer. A good fabricator will review your drawing and flag tolerances that are unnecessarily tight — saving you money before production starts.