Tolerances in CNC Machining

Types of Tolerances

There are primarily four types of tolerances used in CNC machine parts. Let’s take a closer look at each of them.

1. Limit tolerances

Example of limit tolerance

Limit tolerances are the range of two-dimensional values, where the measurement of CNC-machined parts must fall under that range. The higher number denotes the most allowable dimension, and the lower value indicates the lowest allowed dimension.

For Example, if a part has a limit tolerance (width) of 0.82-0.90 mm, the acceptable width is between 0.82 and 0.90 mm. That part is unacceptable if the width does not fall under the range during manufacturing.

2. Unilateral tolerances

Example of unilateral tolerances

Unilateral tolerances dictate the range of allowable dimensions in one direction only, which can be either negative or positive. For Example, if a part has diameter tolerance in a hole 3 mm+ 0.007/ – 0.00, it represents that the diameter can go from 3 to 3.007 mm only.

It is used in components that require fittings later. For Example, in the manufacturing of shafts, unilateral tolerances are applied. Here If the shaft diameter is slightly more than the hole size, it will not fit, so only using negative tolerancing will save time and material wastage.

3. Bilateral tolerances

Unlike unilateral tolerances, bilateral tolerances allow error in both positive and negative sides with respect to base dimension by the same value. The base value may be slightly higher or lower than the value of the permitted measurement.

For Example, the bilateral tolerance of ± 0.005 mm dictates the manufacturer to create the part that can only be 0.005 mm shorter or longer than the stated value.

4. Standard tolerances

Standard tolerances are the most prevalent tolerances used in CNC machining and other metal fabrication methods.

It enables the product to function broadly. The standard tolerances may vary depending on the kind of requirements. The most commonly used standard is ISO 286-1:2010. However, the tolerances vary based on the grade of CNC machines. It is applicable for creating holes, threads, pipes, pins, and other features.

Standard tolerances

The standard tolerances are defined for angular & linear dimensions, rounded portions, chamfers, and others. There are four classes of standard tolerances fine, medium, coarse, and very coarse.

All tolerance ranges for each dimension are based on the standards of EN 20286, JIS B 0401, ISO 286, ISO 1829, ISO 2768, ANSI B4.1, and ANSI B4.2.

5. Geometric dimensioning & tolerancing (GD&T)

More than any other manufacturing method, CNC machining extensively uses GD&T. GD&T ensures further characteristics of the dimensions, such as positioning, flatness, roundness, and concentricity. It defines all the physical dimensions of the part going to CNC-machined.

Example of GD&T

GD&T system contains various symbols that define the part’s geometric properties, such as datum line, modifier, and tolerances. It does not define the deviation from the base dimension but dictates the tolerances in specifications. Using GD&T enables manufacturers to produce a variety of features within a specific accuracy range.

• True position: The true position of any feature is determined by the location of the datum line (reference line). For Example, a coordinate (X, Y) might describe the true position of a hole. A datum line is fixed at the start of the tolerancing process.
• Flatness: Tolerancing of flatness is defined by the two parallel planes within the milling surface. It is crucial during milling because the stress of clamping force might cause the warpage and affect the flatness.
• Cylindricity: The two concentric cylinders that the machined hole must lie inside are what characterize cylinders.
• Concentricity: The circular boss or coaxial counter-bore is ensured by concentricity. For instance, a car’s wheels are always concentric with the axle.
• Perpendicularity: Perpendicularity is measured by the deviation of the horizontal surface to the adjacent vertical surface within a CNC-machined part. Perpendicularity helps regulate Squareness as well.

There are additional criteria in addition to the ones mentioned above, such as angularity, straightness, parallelism, and others.