Understanding CNC Machining Tolerances

When specifying CNC machining tolerances, it’s easy to default to tight numbers across an entire drawing. ±0.1 mm feels safe. ±0.05 mm feels even safer.

But what do those numbers actually mean in practice? And when do they add unnecessary cost?

Understanding machining tolerances properly can reduce lead time, improve manufacturability and lower production cost without compromising performance.

What is a machining tolerance?

A tolerance defines how much a dimension is allowed to vary from its nominal value. If a feature is specified as 20 mm ±0.1 mm, the acceptable range is 19.9 mm to 20.1 mm.

In CNC machining, achievable tolerances depend on:

Material type
Part geometry
Machine capability
Tool condition
Fixturing stability
Environmental factors

For general CNC machining, ±0.1 mm is widely achievable. Tighter tolerances are possible, but they often require additional setups, slower machining speeds or secondary finishing operations.

Why tighter tolerances increase cost

Every time a tolerance tightens, risk increases. To manage that risk, machinists must:

Reduce feed rates
Increase inspection frequency
Use specialised tooling
Allow for additional finishing passes
Potentially scrap more parts

This increases machining time and quality control effort.

Specifying ±0.02 mm on non-critical features does not improve performance. It simply increases cost and lead time.

The key question should always be: Does this feature genuinely need that level of control?

General tolerances vs critical tolerances

A practical approach is to apply:

A general tolerance for most features
Specific tighter tolerances only where function demands it

Critical features often include:

Bearing fits
Sealing faces
Mating components
Alignment features
Press-fit diameters

By isolating these areas, you allow the rest of the part to be machined more efficiently.

Tolerance stacking and assemblies

Tolerance stacking becomes important when multiple components interact.

Individually acceptable variations can accumulate in assemblies, leading to misalignment or performance issues. This is where careful tolerance strategy and, where appropriate, GD&T become valuable.

However, over-complicating drawings can also slow production. Clear communication between design and machining teams is often more effective than layering complexity into every feature.

Inspection and verification

Achieving a tolerance is one thing. Verifying it is another.

Inspection methods may include:

Digital callipers or micrometers
Height gauges
CMM inspection
Surface measurement tools

The tighter the tolerance, the more rigorous the inspection process. That inspection time is part of the overall cost.

A balanced approach

The most effective CNC designs:

Apply tight tolerances only where function demands
Use standard hole sizes and threads where possible
Avoid unnecessarily tight flatness or parallelism requirements
Consider how the part will be fixtured

If you are unsure whether a tolerance is appropriate, a short DFM discussion before machining can prevent delays later.

Good tolerance strategy is not about pushing limits. It is about aligning design intent with manufacturing reality.

CNC Tolerances in Practice

At Prototype Projects, our general quoted tolerance is ±0.1 mm – already considerably better than what most 3D printed parts can achieve. In reality, CNC machining is usually accurate to much less than ±0.1 mm, and critical dimensions can be held to tighter tolerances on request.

Understanding this helps engineers make smarter decisions about when CNC machining is the right solution for prototypes, low-volume runs, or precision components.