Tolerances tell the machine shop how much a dimension is allowed to vary. Set them too loose and parts may not fit; set them too tight and cost climbs quickly. This guide explains the basics in plain language.
What is a machining tolerance?
No machine makes every part exactly to size. A tolerance is the amount a dimension is allowed to vary and still be acceptable. For example, a diameter of Ø20 ±0.05 mm is acceptable anywhere between 19.95 and 20.05 mm.
Tolerances can be written in different ways:
- Bilateral: 20 ±0.05
- Unilateral: 20 +0.05 / −0.00
- Limit dimensions: 19.95 – 20.05
- Fit symbols: such as Ø20 H7 for a hole or Ø20 g6 for a shaft (ISO system of limits and fits)
General tolerances and ISO 2768
Putting a tolerance on every single dimension makes a drawing hard to read. Instead, most drawings carry a general tolerance note such as “General tolerances ISO 2768-m”. Any dimension without its own tolerance then follows that standard.
ISO 2768-1 has four classes for linear dimensions: f (fine), m (medium), c (coarse) and v (very coarse). The allowed variation grows with the size of the dimension. As an example, in the medium class a dimension between 6 and 30 mm is allowed ±0.2 mm, while the fine class allows ±0.1 mm for the same range.
ISO 2768-2 covers general geometric tolerances such as straightness, flatness and perpendicularity, with classes H, K and L.
Typical tolerance ranges for CNC and VMC work
Achievable tolerances depend on the machine, the material, the part size and the feature. As a broad guide:
| Level | Typical range | Used for |
|---|---|---|
| General | ISO 2768-m (about ±0.1–0.3 mm for common sizes) | Non-critical dimensions |
| Standard precision | ±0.05 mm | Most functional features |
| High precision | ±0.01–0.02 mm | Bearing seats, fits, locating features |
Very tight tolerances are possible on specific features, but they usually need extra operations, controlled temperatures and more inspection. Always confirm what your supplier can hold for your particular part.
Geometric tolerances (GD&T) in brief
Size tolerances alone do not control form and position. Geometric Dimensioning and Tolerancing (GD&T) adds controls such as:
- Flatness – how flat a face must be
- Perpendicularity and parallelism – relationship between faces
- Concentricity / runout – how true a turned diameter runs to its axis
- Position – where a hole must be relative to datums
Datums (A, B, C) tell the machinist and the inspector which faces to measure from. Clear datums are one of the best ways to get consistent parts.
How tolerances affect cost
Surface finish follows the same logic – see surface finish (Ra) explained.
Tighter tolerances cost more because they need slower finishing passes, more stable set-ups, sometimes extra operations such as grinding or reaming, and more inspection time. The cost curve is steep: going from ±0.1 mm to ±0.01 mm can increase machining and inspection effort many times over for that feature.
Rule of thumb: use the general tolerance for everything, then tighten only the features that mate with other parts or affect function.
Tips for specifying tolerances
- Add a general tolerance note (for example ISO 2768-m) to every drawing.
- Use fit symbols (H7/g6 etc.) for shafts and holes that assemble together.
- Mark critical dimensions clearly so they are checked on every part.
- Define datums that match how the part is located in the assembly.
- Ask your supplier for a dimensional or first-article report if the part is critical.
For more on preparing drawings, see how to prepare a drawing for an accurate CNC machining quote. To learn how we check parts, visit our quality page, or send us your drawing for a quote.
Frequently Asked Questions
What is a standard CNC machining tolerance?
Many machine shops work to about ±0.05 mm to ±0.1 mm as a normal tolerance for general features. Tighter limits such as ±0.01–0.02 mm are achievable on critical features but take more time and inspection. Always confirm with your supplier for your material and part size.
What does ISO 2768-m mean on a drawing?
It means that any dimension without its own tolerance follows the ISO 2768 “medium” class. The allowed variation depends on the size of the dimension – smaller dimensions get tighter limits.
Does a tighter tolerance always mean a better part?
No. A tolerance should match the function. Unnecessarily tight tolerances add cost and lead time without improving how the part works.
CNC and VMC machining specialists in Metoda GIDC, Rajkot. Questions about your part? Send us your drawing.
