Tolerances are the acceptable ranges for the degree of dimensional error. It tells how much a dimension can deviate from the true value. Engineers and designers consider the permissible tolerances while creating the drawings so machining processes and quality control can ensure the desired accuracy and precision requirements.
Key Takeaways
- CNC machining tolerance refers to the acceptable range of dimensional error in manufactured parts, ensuring accuracy and precision.
- Tolerance includes both design tolerance (specified in CAD models) and machine tolerance (accuracy of the CNC machine).
- ISO 2768 is a standard that simplifies tolerances for mechanical parts, providing guidelines for permissible size deviations.
- There are various types of tolerances, including unilateral, bilateral, limit, profile, orientation, location, form, and runout.
- DIN ISO 2768 is a popular CNC machining tolerance standard, divided into T1 (for linear and angular dimensions) and T2 (for forms and positions).
- CNC tolerance charts help designers and manufacturers determine appropriate tolerances based on nominal lengths and precision requirements.
Whether milling, drilling, turning, or any other operation, CNC machining services use tolerances to ensure the accurate fit and functionality of designed parts. Till the end, we will discuss the concept, types, and standards of machining tolerances.
What is Meant by CNC Machining Tolerance?
Tolerance in machining involves both desing tolerance and the accuracy of the CNC machine l. The Desing tolerance refers to the allowable deviation range in the dimension specified in the CAD model. For example, 20±0.5 mm means if the manufactured dimensions fall under 19.5 to 20.5, it does not affect the intended fit in assembly or performances.
On the other hand, the tolerance of a CNC machine refers to how accurately the machine can convert the design. For example, the ±0.5mm accuracy of a CNC machine means it produces the part without deviating dimensions more than ±0.5mm from specified dimensions in G-codes.
If you want to machine any part, the accuracy of the CNC machine must fall under the desired tolerances in your design. For example, to machine a design with the highest tolerance of ±0.5mm, you need a CNC machine that has an accuracy of ±0.5mm or lower ( 0.4, 0.3, 0,2, etc).
ISO 2768 Tolerances
Adhering to precise tolerances is critical for ensuring the quality and interoperability of CNC machined parts. Different applications require different levels of precision, which are often guided by specific international standards. One such standard is ISO 2768, which simplifies the wide range of tolerances for mechanical parts. For manufacturers and engineers, referencing the ISO 2768 tolerance chart provides essential guidelines on permissible size deviations for general tolerances, helping to streamline the design process and ensure compliance with global quality norms.
Types of CNC Machining Tolerances
There are different types of tolerances describing the allowable directions, limits, profile curvatures, locations, forms, etc. The following table outlines the concept of tolerances useful in CNC precision machining;
| Tolerance Type | Description | Example |
| Unilateral | Variation in one direction only. | 20 mm ±1, only in + or – direction |
| Bilateral | Variation in both directions. | 20 mm ±1 (e.g., 9 mm to 11 mm) |
| Limit | Max and min limits without ± symbols. | 19 mm – 21 mm |
| Profile | Curvature along a line. | Surface profile to ±0.5 mm |
| Orientation | Variation relative to a datum for perpendicularity or angularity. | Perpendicularity to 0.02 mm |
| Location | Checks feature position relative to a datum. | True position tolerance |
| Form | Maintains flatness, and roundness without references to datum. | Flatness within 0.1 mm |
| Runout | Rotation around an axis. | Circular runout of 0.05 mm |
| Unequally Disposed | Unequal tolerances in the machining profile. | 20 mm +0.1/-0.2 mm |
| GD&T | Standardized symbols for communication of tolerances and geometry | GD&T symbols |
CNC Tolerance Standards
The main purpose of CNC tolerance standards is to bring uniformity between designers and manufacturers. The use of standard tolerances is key for the design for manufacturing (DFM) and it provides consistency in machined parts between different manufacturers. For example, a local supplier can send the design labeling with standard tolerances to an on-demand manufacturer like Proleantech, and then the manufacturer converts that design interpreting those standards, and sends back the machined parts.
DIN ISO 2768 is the most popular CNC machining tolerance standard that guides general dimensional tolerances for machined parts. It has two sub-division DIN ISO 2768-T1 and DIN ISO 2768-T2.
DIN ISO 2768-T1
DIN ISO 2768-T1 provides guidelines for tolerancing the linear and angular dimensions in engineering design. The machining tolerance chart for this standard is categorized into three types; fine, medium, and coarse. The fine class indicates tight precision, and coarse means rougher or maximum deviation. Moreover, their value differs according to the nominal size ( length, radius, etc. ). For example, ±0.05 is fine for up to 3mm length, but ±0.5 tolerance is regarded as a fine class for 1000 mm length.
DIN ISO 2768-T2
DIN ISO 2768-T2 deals with forms and positions; perpendicularity, flatness, and runout. It also has three classes High(H), medium(M), and Low(L). Like the T1 standard, all of their values depend on the nominal length.
How to Read a CNC Tolerance Chart?
A CNC tolerance chart includes different columns for allowable tolerance corresponding to their nominal length. First, you need to identify your dimensions fall under which nominal length, then find out standard fine, medium, and coarse tolerances in consecutive columns of the same row you find the nominal length.
For example, the format of the DIN ISO 2768-T1 linear tolerance chart looks like this ;
| Nominal Length (mm) | Fine (f) | Medium (m) | Coarse (c) | Very Coarse (v) |
| 0.5 to 3 | ±0.05 | ±0.1 | ±0.2 | ±0.3 |
| 6 to 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
It contains numerous rows each with a specific nominal length range in increasing order downward. You need to choose the tolerances according to your nominal length. Next, reading the DIN ISO 2768-T2 CNC machining tolerance chart is not much different. The only difference is that the fine, medium, and coarse columns are replaced with H, M, and L.
Here is an example table for the perpendicularity chart;
| Nominal Length (mm) | H | K | L |
| Up to 100 | 0.2 | 0.4 | 0.6 |
| 100 to 300 | 0.3 | 0.6 | 1 |
