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5 Common Tolerance Mistakes Engineers Make Before CNC Production

Created on 09.15

By Selina Sun | Marigold Rapid

Tolerance is one of the most important parts of a CNC drawing.

But tighter does not always mean better.

In CNC machining, an overly tight tolerance can increase machining time, inspection requirements and production cost without improving the function of the part. On the other hand, a tolerance that is too loose can cause poor fit, alignment problems or assembly failure.

For engineers preparing parts for CNC production, the goal is not to specify the tightest possible tolerance.

The goal is to specify the right tolerance for each feature.

Here are five common CNC tolerance mistakes to avoid before sending a part to production.

1. Using Tight Tolerances on Every Dimension

This is one of the most common tolerance mistakes in CNC machining.

A drawing may specify a very tight tolerance such as ±0.01 mm or ±0.02 mm across many dimensions, even when those dimensions are not functionally critical.

Tighter tolerances can require more careful machining, additional measurement and tighter process control. They can also increase inspection time and the risk of rework.

For example, a bearing seat, precision hole or mating feature may need tight dimensional control. A non-critical external dimension may not.

Better approach

Separate your dimensions into:

Critical featuresUse tighter tolerances where fit, motion, alignment or performance depends on them.

Non-critical featuresUse realistic general tolerances where extreme precision is not required.

This approach can reduce CNC machining cost and lead time without changing the functional performance of the part.

Marigold Rapid currently lists a standard CNC tolerance of ±0.05 mm, with high-precision machining available down to approximately ±0.01 mm depending on the part, material and production conditions.

2. Relying on General Tolerances for Functional Features

General tolerance standards are useful, but they should not replace specific tolerances where function requires tighter control.

For example, a drawing may state a general tolerance such as ISO 2768-m and assume that it will be sufficient for every feature.

That can create problems when a hole, shaft, mating surface or mounting location has a functional requirement that is tighter than the general tolerance.

A general tolerance tells the manufacturer how to handle dimensions that do not have individual tolerance values. It does not remove the need to define critical dimensions clearly.

Better approach

Use your drawing to identify important features individually.

For example:

Precision hole: Ø10.00 ±0.02 mm

Non-critical linear dimension: general tolerance

This gives the CNC machinist a much clearer manufacturing target.

Marigold Rapid currently uses DIN ISO 2768-m (Medium) as its default for unmarked dimensions, while custom tolerances can be specified on the 2D drawing.

3. Ignoring Datum Structure and Tolerance Stack-Up

A part can meet every individual dimensional tolerance and still fail during assembly.

Why?

Because tolerances can accumulate.

This is known as tolerance stack-up.

Imagine a component with several related holes and mounting surfaces. Each individual dimension may be within tolerance, but the combined variation can shift the final hole position enough to create an assembly problem.

This is especially important for:

· Multi-part assemblies

· Precision alignment

· Bearing locations

· Gear and shaft systems

· Mating components

· Medical device components

· Automated equipment

Better approach

Define clear datums and think about how important features relate to them.

For functional parts, engineers should consider not only:

"Is this dimension within tolerance?"

but also:

"How does the variation of this dimension affect the final assembly?"

For more complex designs, GD&T can provide a better way to control feature relationships such as position, flatness, perpendicularity and concentricity.

The goal is to control the function of the part, not simply increase the number of decimal places on the drawing.

4. Specifying Tolerance Without Considering Material and Part Geometry

The same tolerance can be easy to achieve on one part and difficult on another.

Material and geometry both affect dimensional stability during CNC machining.

For example, a thin-wall aluminum component can behave differently from a rigid stainless steel block during machining. Deep pockets, long unsupported walls and small features can also increase the risk of vibration, deflection or deformation.

This means the question:

"Can this CNC machine achieve ±0.01 mm?"

does not have a useful answer without considering the actual feature.

The real question is:

Can this specific feature, in this material, on this part, be held to the required tolerance consistently?

Better approach

Before production, review:

· Material

· Part size

· Wall thickness

· Feature size

· Tool access

· Machining direction

· Number of setups

· Required inspection method

For complex components, 4-axis or 5-axis CNC machining may sometimes reduce setups and improve access to multiple surfaces. Marigold Rapid provides 3-, 4- and 5-axis CNC milling as well as turning and turn-mill machining.

5. Treating the 3D Model and 2D Drawing as the Same Source of Truth

Another common problem appears when the 3D CAD model and 2D drawing do not match.

An engineer may update a dimension in the CAD model but forget to update the drawing.

Or the 3D model may show one hole diameter while the drawing specifies another.

For the manufacturer, this creates uncertainty.

Before CNC production begins, the supplier may need to stop and ask:

Which version should we manufacture?

This can delay quoting, programming and production.

Better approach

Before sending your CNC RFQ, make sure:

· 3D CAD and 2D drawing match

· Material is clearly specified

· Critical dimensions are identified

· Tolerances are clearly defined

· Threads are specified

· Surface finish requirements are clear

· Any special inspection requirements are stated

For CNC prototypes, this can make a major difference because the goal is usually to move quickly from engineering design to physical validation.

Marigold Rapid accepts 3D CAD files such as STEP and IGES together with 2D drawings for quotation and DFM review.

How Tight Should CNC Machining Tolerances Be?

There is no single tolerance that is right for every CNC part.

The correct tolerance depends on:

· Part function

· Material

· Geometry

· Feature size

· Machining process

· Production volume

· Inspection method

A good engineering drawing should therefore avoid using extremely tight tolerances simply to make a part "more precise."

Instead, ask:

What does this feature need to do?

Then assign the tolerance required for that function.

This approach can help reduce unnecessary machining cost while maintaining part quality.

What Tolerance Information Should You Put on a CNC Drawing?

For a practical CNC drawing, make sure you clearly define:

Critical dimensionsIdentify dimensions that affect fit, function or assembly.

General tolerancesState the general tolerance standard for dimensions without individual tolerances.

Geometric tolerancesUse GD&T where the relationship between features matters.

Threads and holesDefine thread size, class or fit where required.

Surface finishSpecify the required surface roughness when it is functionally important.

DatumsUse appropriate datums for features that must be accurately located or aligned.

A clear drawing reduces questions during quoting and helps the manufacturer choose the appropriate manufacturing and inspection process.

How Proper Tolerances Can Reduce CNC Cost

Tolerance optimization does not mean accepting poor quality.

It means controlling the right features to the right level.

For example, reducing unnecessary tight tolerances can help:

· Shorten machining time

· Reduce inspection time

· Lower tooling and process requirements

· Reduce rework risk

· Improve production efficiency

At the same time, truly critical features can remain tightly controlled.

This is particularly valuable for CNC prototypes and low-volume production, where engineers often need to balance development speed, cost and functional performance.

CNC Tolerance Checklist Before Production

Before sending your next CNC prototype or production drawing, ask:

Are all tight tolerances functionally necessary?

Have critical dimensions been clearly identified?

Are your 3D CAD model and 2D drawing consistent?

Have you considered tolerance stack-up in the assembly?

Are the material and part geometry suitable for the required tolerance?

Are datums and GD&T clearly defined where needed?

Are inspection requirements clear to the manufacturer?

Five minutes of tolerance review before production can prevent much more expensive changes after machining has started.

Final Takeaway

The most common CNC tolerance mistakes are not caused by engineers asking for too little precision.

They are often caused by asking for the wrong precision in the wrong places.

The best CNC drawings clearly distinguish between critical and non-critical features, use appropriate general tolerances, define functional relationships and take material and machining conditions into account.

In CNC machining, good tolerance design is not about making every dimension tighter. It is about making every tolerance meaningful.

At Marigold Rapid, we support CNC prototypes and low-volume production with 3-, 4- and 5-axis machining, CNC turning and DFM review. Our quality system is certified to ISO 9001:2015 and ISO 13485:2016, supporting precision manufacturing for industrial, medical and other demanding applications.

Have a CNC part that requires tight tolerances?

Send us your 3D CAD file and 2D drawing. Our engineering team can review the design, identify potential manufacturing issues and recommend a practical machining approach.