When a CNC-machined part does not fit during assembly, the problem is often not the CNC machine itself. In many cases, the real issue starts with how the part was designed and how tolerances were specified on the drawing.

A common mistake is to assume that tighter tolerances always mean better quality.

They do not.

A tighter tolerance can improve the function of a critical feature, but it can also increase machining time, inspection requirements, tooling needs, production difficulty, and overall cost.

For engineers and product designers, the goal should not be to make every dimension as precise as possible. The goal is to specify the right tolerance for the function of the part.

In this guide, we explain how CNC machining tolerances work, when tight tolerances are necessary, which features usually require more attention, and how good DFM practices can help control manufacturing cost.

What Are CNC Machining Tolerances?

A tolerance defines the acceptable variation from a nominal dimension.

For example, if a dimension on a drawing is:

20.00 ± 0.05 mm

the acceptable size is between:

19.95 mm and 20.05 mm

The nominal dimension is 20.00 mm, while the total tolerance range is 0.10 mm.

In CNC manufacturing, tolerances are used to control the size, position, form, orientation, and relationship between features.

Depending on the part, the drawing may include:

• Dimensional tolerances
• Geometric tolerances
• Positional tolerances
• Flatness
• Parallelism
• Perpendicularity
• Concentricity or related location controls
• Surface roughness requirements

A complete drawing should make the functional requirements clear rather than leaving critical requirements open to interpretation.

Why Do CNC Tolerances Matter?

Tolerances are important because real-world manufacturing cannot produce every part at one mathematically perfect dimension.

Material properties, machine capability, tool wear, temperature, workholding, machining strategy, inspection methods, and part geometry can all affect the final result.

The purpose of tolerancing is therefore to define what variation is acceptable for the application.

Consider a simple mounting bracket.

The overall length of the bracket may not need extremely tight control.

However, the diameter and location of a mounting hole may be critical because the hole must align with another component.

This means the two features should not necessarily receive the same tolerance.

The function of the feature should determine the tolerance.

That principle is one of the most important ideas in cost-effective CNC part design.

Tight Tolerances Increase Manufacturing Cost

One of the most common CNC manufacturing mistakes is specifying unnecessarily tight tolerances.

Tight tolerances generally require greater process control and may require additional machining and measurement operations. Depending on the feature, they can also limit manufacturing options and increase inspection effort.

For example, a part with several dimensions specified to extremely tight limits may require:

• More careful tool selection
• Additional machining passes
• More controlled setups
• Additional measurements
• Specialized inspection equipment
• Secondary finishing operations
• More frequent process checks

This does not mean that tight tolerances should be avoided.

It means they should be used only where they provide a functional benefit.

A good DFM approach is to separate critical dimensions from non-critical dimensions instead of applying the same tight tolerance everywhere.

Standard vs. Tight Tolerances

A practical drawing usually contains a mixture of general tolerances and specific tolerances.

General Tolerances

General tolerances can be used for dimensions where a tighter tolerance is not functionally required.

For example, the overall size of a non-critical cover may not need the same precision as a bearing seat.

Standards such as ISO 2768 provide frameworks for general tolerances for certain linear and angular dimensions without individually specified tolerances. ISO has also been progressing a revision of ISO 2768 in 2026, so engineers should check the standard and revision required by the project rather than assuming an older drawing note is automatically appropriate.

Specific Tolerances

Specific tolerances should be applied to features that directly affect:

• Assembly
• Movement
• Alignment
• Sealing
• Load transfer
• Bearing fit
• Interchangeability
• Product performance

This approach keeps the drawing clear while avoiding unnecessary manufacturing cost.

Example: Not Every Dimension Needs the Same Tolerance

Imagine a CNC-machined aluminum housing with:

• A bearing bore
• Several mounting holes
• An external length
• Cosmetic chamfers
• Internal pockets

The bearing bore may require a close dimensional or geometric control because it affects fit and rotation.

The mounting hole locations may require positional control because they must align with mating components.

The external length may have a more relaxed tolerance if it does not affect assembly.

The cosmetic chamfers may not need tight dimensional control at all.

Trying to control all of these features to the same ultra-tight tolerance would add cost without necessarily improving the product.