CNC Machining Tolerances Guide: Standards, Charts, and How to Choose the Right Tolerance

When ordering CNC-machined parts, one of the most important questions is how accurately each dimension must be controlled.

A part may look correct but still fail to fit its mating component if a hole is too small, a shaft is too large, or a mounting surface is not flat enough.

A CNC machining tolerances guide helps engineers and purchasing teams understand which dimensions need tight control, which can use general tolerances, and how tolerance requirements affect manufacturing cost.

A machining tolerance is the permitted variation around a specified dimension.

For example, a drawing specifies a part length of:

100.00 ±0.10 mm

This means the acceptable length is between 99.90 mm and 100.10 mm.

The part does not have to measure exactly 100.00 mm. Any measured length within the specified limits meets that dimensional requirement.

The total tolerance band is 0.20 mm, because the upper and lower limits differ by 0.20 mm.

Tolerances help manufacturers produce parts that fit and function reliably despite the small variations inherent in machining.

There is no single tolerance that applies to every CNC part or supplier.

As a general reference, many machine shops use approximately ±0.125 mm (±0.005 in) for ordinary linear dimensions when no tighter requirement is specified. Your drawing, quotation, and agreed manufacturing standard should determine the actual default.

Our Design Tips for Low-Volume CNC Machining Production Runs explains why specifying practical tolerances and simplifying features can help control production cost.

These ranges are practical design references, not universal standards. The appropriate tolerance depends on part size, geometry, material, machining process, and inspection method. A supplier must confirm whether a particular feature can be held consistently at the requested tolerance.

For Shiye's published capabilities and inspection equipment, see our updated Inside Shiye Hardware's 60-Machine CNC Facility.

This controls the size of a feature, such as length, width, thickness, or diameter.

For example, a shaft specified as 10.00 ±0.02 mm must measure between 9.98 and 10.02 mm.

Geometric tolerancing controls features beyond their basic size. It can define requirements for form, orientation, position, or surface profile.

Common examples include:

1:Flatness: How much a surface can deviate from a flat plane.

2:Parallelism: How parallel two surfaces or features must remain.

3:Perpendicularity: How close a surface or axis must be to 90 degrees relative to a reference.

4:Position tolerance: How accurately a hole or other feature must be located relative to datums.

5:Runout: How much a rotating surface can vary as the part rotates.

For example, four mounting holes might each have the correct diameter but still fail assembly if their positions are wrong.

Our newer guide, Introduction to GD&T for CNC Machining, explains how datum references and geometric tolerancing communicate these requirements more clearly.

Tighter tolerances reduce the range of acceptable finished dimensions. Maintaining them consistently may require more control during machining and inspection.

The main cost drivers include:

1:Slower finishing operations

2:More frequent dimensional checks

3:Additional machining or grinding

4:Closer control of tool wear and workholding

5:Greater risk of rejection or rework

For example, a non-critical mounting plate may work perfectly well with a general dimensional tolerance, while a bearing seat may need a much tighter diameter tolerance to achieve the required fit.

Applying ±0.01 mm to every dimension can therefore increase cost without improving the part's function.

Start by asking what each feature does.

For surfaces that do not control assembly, movement, or sealing, a general tolerance may be sufficient.

Examples include non-contact external dimensions and some cosmetic features.

Hole location may be more important than hole diameter when multiple parts must align.

Specify positional tolerances when the drawing needs to control the relationship between holes or between a hole pattern and functional datums.

Bearing seats, shafts, locating pins, and sliding components may need tighter dimensional or geometric control.

The exact requirement should come from the intended fit and function rather than the assumption that every precision part needs the tightest available tolerance.

Achievable tolerance depends on more than the CNC machine's nominal capability.

Aluminum is relatively easy to machine, but large thin-wall parts can move when material is removed. Clamping forces, tool deflection, heat, and residual stress can also affect the final dimensions.

For example, a thick aluminum block with an accessible bore may be easier to control than a large plate with several deep pockets and thin walls.

That is why tolerance review should consider:

1:Material and stock condition

2:Part dimensions and geometry

3:Workholding and machining sequence

4:Cutting tool access

5:Surface treatment

6:Inspection requirements

If anodizing or another finish affects a functional dimension, specify whether the final tolerance applies before or after finishing.

Tolerance stack-up occurs when dimensional variations from multiple features add together in an assembly.

Consider two dimensions, each specified as 20.00 ±0.10 mm.

Their combined nominal length is 40.00 mm.

If both dimensions are at their upper limits, the total becomes 40.20 mm. If both are at their lower limits, the total becomes 39.80 mm.

The total assembly dimension may therefore vary between 39.80 and 40.20 mm under this simplified worst-case calculation.

This is why engineers should consider the full assembly rather than evaluating each part's tolerance independently.

Where assembly performance is critical, a tolerance-stack analysis can help identify which individual dimensions need tighter control.

Consider Mike, an automation engineer in the United States, who is ordering a CNC-machined aluminum mounting plate.

His initial drawing specifies ±0.01 mm for the overall length, width, thickness, mounting holes, and every pocket.

During the design review, the team separates the functional requirements:

1:Overall dimensions: general tolerance where suitable

2:Mounting-hole positions: controlled to support assembly alignment

3:Critical locating bore: tighter dimensional tolerance based on the mating component

4:Non-functional pocket edges: practical general tolerance

This approach allows the manufacturer to focus inspection and process control on the features that matter most.

It does not mean tighter tolerances should be removed wherever possible. They should be retained whenever the engineering function requires them.

For related guidance, see our Design Tips for Low-Volume CNC Machining Production Runs, which covers practical ways to reduce unnecessary machining work and improve manufacturability.

Before requesting a quotation, prepare:

1:2D engineering drawing

2:3D CAD file

3:Material and grade

4:Critical dimensional tolerances

5:GD&T and datum requirements, where applicable

6:Surface finish

7:Quantity

8:Inspection requirements

9:Delivery requirements

If a feature is critical to assembly, identify it clearly. This helps the supplier determine the appropriate machining process and inspection method before production.

Our Custom CNC Machining Services support drawing-based custom components, with tolerance and inspection requirements evaluated according to the project.

A good CNC machining tolerance guide is not about making every dimension as precise as possible. It is about choosing tolerances that allow the part to function reliably without making manufacturing unnecessarily difficult.

Use general tolerances for non-critical dimensions, tighter tolerances for functional interfaces, and GD&T when the form, orientation, or position of a feature matters.

Have a CNC-machined part that requires specific dimensional or geometric tolerances?

Send your 2D drawing, 3D CAD file, material, quantity, and critical requirements.