Modern CNC machining allows for very high precision in the manufacture of parts. However, in production reality, the most important question is not: how precisely can we manufacture a part, but what level of precision its function actually requires.
In practice, many designs include tolerances that are far stricter than necessary. Every tightening of the accuracy class affects the production technology, cycle time and unit cost of the part.
In many industrial applications, CNC machining uses IT7–IT8 classes, which ensure adequate accuracy whilst maintaining process stability. Moving into the IT6 range or higher usually involves increased technological requirements and can lead to higher production costs, particularly for more complex parts.
IT tolerance classes in CNC machining – the basis for communication between designer and manufacturer
IT (International Tolerance) tolerance classes define the permissible range of deviation from the nominal dimension. The lower the IT class number, the narrower the tolerance range and the greater the demands on the manufacturing process.
What is the IT accuracy class in CNC machining practice
In accordance with ISO 286, classes IT01 to IT5 are mainly used in measuring instruments and reference elements. In machine design, the IT6–IT11 range is most commonly used.
For example:
for a shaft with a diameter of 50 mm, IT7 denotes a tolerance of approximately 0.025 mm. By way of comparison, the diameter of a human hair is approximately 0.05–0.08 mm.
Such minute dimensional differences demonstrate the high level of stability required in the CNC machining process at high accuracy grades.
Thermal expansion of the material
One of the factors affecting the maintenance of tolerances is the temperature of the material.
Maintaining IT6 class for larger components can be challenging, particularly with materials that have high thermal expansion, such as aluminium. Temperature variations between the production and measurement environments can affect the measurement result and the maintenance of the required tolerance.
For this reason, temperature stabilisation is often employed in precision CNC machining prior to final quality control.
How tolerances affect production costs in CNC machining
The relationship between tolerance class and production cost is not linear. Each successive level of accuracy results in a significant increase in process complexity.
Cycle time and number of operations
Moving from IT9 to IT6 very often means a change in production technology.
Instead of a single finishing operation, the following may be required:
- Rough machining,
- workpiece tempering (stress relief),
- precision finishing,
- additional processes, e.g. grinding.
Each of these stages extends production time and increases the unit cost of the component.
Approximate impact of the IT class on the cost of the workpiece
| IT class | Process complexity | Estimated production cost |
| IT11 – IT13 | Rough machining | Base level |
| IT8 – IT9 | Standard CNC machining | +20–40% |
| IT7 | Precision finishing | +60–100% |
| IT6 and above | Grinding / controlled conditions | +200% and above |
These figures are indicative, as the actual cost depends on the material, the part’s geometry and the size of the production run.
Tools and tooling
Higher accuracy also requires more precise tooling.
In CNC machining, the following are used, among others:
- Heat-shrink collets with minimal runout,
- carbide tools with very tight tolerances,
- stable workpiece clamping systems.
Importantly, the tools may still cut the material correctly, but they no longer maintain the required accuracy class. In practice, this means they need to be replaced sooner.
The higher the accuracy class, the more complex and costly the production process becomes. Therefore, optimising tolerances is one of the key elements in the design of parts intended for CNC machining.
Fits in mechanical assembly
Tolerance classes are directly linked to the selection of fits between components.
Three basic types of fits are used in mechanical assembly:
Loose fit
There is always clearance between components. This is used in moving components, e.g. in plain bearings.
Mixed fit
Depending on the actual dimensions of the components, there may be clearance or interference.
Press fit
Joining the components requires the application of considerable force or a temperature difference, for example by heating one component or cooling the other.
With large industrial components, incorrect selection of tolerances can cause serious problems as early as the assembly stage.
The selection of the appropriate fit has a direct impact on the functioning of mechanical joints. Incorrectly selected tolerances can lead to assembly problems or excessive wear of components.
Quality control and measurement of parts in CNC machining
Precision CNC machining requires equally precise quality control methods. The measuring system must be more accurate than the tolerance we wish to verify.
Depending on the geometry of the workpiece, the following are used:
- Micrometers and diameter gauges,
- fixed gauges,
- coordinate measuring machines (CMMs).
In the case of complex geometries, it is precisely coordinate measurements that allow both linear dimensions and geometric tolerances to be controlled.
We discuss this topic in more detail in our article on part measurement methods and the role of the measurement room in CNC machining.
Summary
Optimising tolerance classes is one of the most effective ways to reduce production costs without compromising the functionality of the part.
In many cases, applying a more stringent IT class does not improve the product’s performance, but merely increases its manufacturing cost. Therefore, it is worth analysing what level of accuracy is actually required as early as the design stage.
At SIM Gdynia, when carrying out projects, we not only manufacture parts in accordance with the documentation, but also support our clients in selecting tolerances and CNC machining technologies to maintain a balance between precision and production economy.Does your project require specific fits or non-standard tolerance classes? Consult our technical department – we will help you select the parameters to strike a balance between precision and production economy.