What are CNC Machining Tolerances?
Tolerances refer to the permissible limit of variation in a physical dimension. In CNC machining, tolerances specify how much a given measurement can deviate from its nominal value without affecting part performance.
Ø10.00 mm ±0.05 mm
The actual diameter can range between 9.95 mm and 10.05 mm — still within spec.
Why Are Tolerances Important?
Tight tolerances ensure parts fit together correctly in assemblies, especially in high-speed or high-load applications.
In mechanical systems, small deviations can affect performance, efficiency, or safety.
Precision tolerances allow parts to be replaced without modification.
Overly tight tolerances increase machining time and inspection requirements. Realistic tolerances help reduce cost.
Classification
Types of Tolerances in CNC Machining
Tolerances are not one-size-fits-all. Different aspects of a part require different tolerance types, each controlling a specific characteristic of the finished component.
LINEAR DIMENSIONS
Defines how much a machined edge or hole can vary in length, width, or height. Applied to external and internal features such as widths, diameters, and hole spacing.
GEOMETRIC TOLERANCES
Specifies allowable deviations in form, orientation, position, and runout, including flatness, parallelism, and concentricity. Expressed using GD&T symbols.
SURFACE FINISH
Describes the texture or smoothness of a machined surface, typically measured in Ra (roughness average) in µm or µin. Critical for sealing surfaces and mating faces.
Standard CNC Machining Tolerances
The tables below give you reference values for standard and ISO-compliant tolerances. Use these as a baseline when defining requirements — tighter values are achievable but may increase cost.
ISO 2768-m General Tolerances (Medium Class)
| Nominal Size Range (mm) | Tolerance ± (mm) |
|---|---|
| 0.5 to 3 | ± 0.1 |
| >3 to 6 | ± 0.1 |
| >6 to 30 | ± 0.2 |
| >30 to 120 | ± 0.3 |
| >120 to 400 | ± 0.5 |
| >400 to 1000 | ± 0.8 |
| >1000 to 2000 | ± 1.2 |
| >2000 to 4000 | ± 2.0 |
General Standard Tolerances by Process
| Process | Tolerance (± mm) | Tolerance (± inch) |
|---|---|---|
| CNC Milling (3-axis) | ± 0.05 – 0.1 | ± 0.002 – 0.004 |
| CNC Turning (lathe) | ± 0.02 – 0.05 | ± 0.0008 – 0.002 |
| 5-Axis CNC Milling | ± 0.02 or better | ± 0.0008 |
| EDM (Wire or Sinker) | ± 0.005 – 0.01 | ± 0.0002 – 0.0004 |
| Grinding | ± 0.002 – 0.005 | ± 0.00008 – 0.0002 |
| Laser Cutting | ± 0.1 – 0.2 | ± 0.004 – 0.008 |
| Waterjet Cutting | ± 0.1 – 0.2 | ± 0.004 – 0.008 |
| Sheet Metal Bending | ± 0.2 – 0.5 | ± 0.008 – 0.020 |
High Precision CNC Machining At Davantech
±0.005 mm
Tight tolerances achievable on precision machining runs
CMM Inspection
Full dimensional reports available on request
Material Expertise
Aluminium, stainless steel, brass, titanium, plastics
ISO Compliance
ISO 9001:2015 quality management system
DFM Support
Engineering guidance for tolerance optimisation
Common ISO Tolerance References
These ISO standards provide the foundation for specifying tolerances in a universally understood format, reducing ambiguity and simplifying supplier communication.
| ISO Standard | Description |
|---|---|
| ISO 2768-m | Medium tolerance for general machining |
| ISO 2768-f | Fine tolerance, tighter than standard |
| ISO 286 IT6 | Fine grade — high-precision fits |
| ISO 286 IT9 | Coarse grade — general engineering |
| ISO 1101 | Geometric tolerancing (GD&T) |
| ISO 1302 | Surface texture / roughness indication |
GD&T — A Practical Guide
Linear tolerances control size, how long, wide, or deep a feature is. Geometric tolerances control shape, whether a surface is truly flat, whether two holes are truly parallel, whether a shaft is truly round. Both are needed to fully define a precision part. GD&T (Geometric Dimensioning and Tolerancing) is the international language for specifying these requirements, using standardised symbols defined in ISO 1101 and ASME Y14.5 so that every manufacturer reads the drawing the same way.
Controls how much a surface can deviate from a perfect plane, not its height, only its waviness. A callout of 0.05 mm means the entire surface must fit between two parallel planes 0.05 mm apart. Critical for sealing faces, base plates, and mating surfaces.
Controls how much a line or axis can deviate from a perfect straight line. Used on shafts, bores, and profiles where bowing must be limited regardless of the diameter tolerance. Prevents bowed shafts from passing a size check but failing in a bearing housing.
Controls whether two surfaces or axes are truly parallel to a reference datum. A callout of 0.02 mm means the controlled surface must lie within a zone 0.02 mm wide, parallel to the datum. Used on mating faces, guide rails, and bearing housings.
Controls whether a surface, axis, or slot is exactly 90° to a reference datum. Used on bolt hole patterns, stepped features, and any feature that must stand truly square to a base face. Without it, a hole can be within size tolerance but angled.
Controls whether the centre axis of a cylindrical feature is aligned with a reference axis. A callout of 0.03 mm means the controlled axis must lie within a cylinder 0.03 mm in diameter centred on the datum. Used on shafts with multiple diameters and rotating components where eccentricity causes vibration.
Controls the overall form of a cylindrical surface, combining roundness, straightness, and taper into a single callout. The surface must fall within two coaxial cylinders separated by the tolerance value. Used on precision bearing bores and hydraulic cylinder surfaces.
Controls the location of a feature, typically a hole, relative to datum references. Unlike a ± coordinate tolerance, true position uses a circular zone centred on the exact nominal location. More generous for assemblies, more precise where it matters. Widely used on bolt hole patterns and pin locations.
Measures total variation of a surface as a part rotates about its datum axis. Total runout combines concentricity and cylindricity. The indicator must not move more than the tolerance across the full surface during one full revolution. Used on shafts, flanges, and any rotating component.
Choosing When to Use GD&T
Specify a geometric tolerance when a linear ± tolerance alone does not fully control the functional requirement. A shaft diameter of Ø20.00 ±0.01 mm controls size, but without a straightness or runout callout, the shaft could still be bowed or eccentric and pass inspection. If the shaft drives a bearing, that matters. Add geometric callouts only where they are functionally necessary — unnecessary GD&T increases inspection cost and supplier setup time without improving part performance.
ISO 1101 vs ASME Y14.5
Both standards use the same symbols and largely the same principles. The differences are minor but worth noting. Specify which standard applies in the title block of your drawing to avoid ambiguity.
Not sure which tolerances to specify? Share your drawing with Davantech's engineering team as part of our free DFM review. We will flag any features that are under- or over-toleranced before production begins, at no cost and no obligation.
Best Practice
How to Specify Tolerances On your drawings
To ensure clear communication and avoid delays, include the following in your technical drawings. Our engineering team can help if you’re unsure.