10 Common CNC Machining Tolerances

Due to different part geometries and different types of machining processes, tolerances in CNC machining are expressed differently and are identified based on machining blueprint symbols.

Let’s take a look at these different tolerances one by one:

1.Single-Sided Tolerance

Single-Sided Tolerance in CNC machining implies that the allowed deviation can only occur in one direction. The basic dimension of the part is the same as the upper or lower limit, and the tolerance can only be positive or negative, not both.

For example, if the pipe diameter is 10 mm and the single-sided tolerance is +1 mm, the basic dimension and the lower limit of the process are both 10 mm. The upper limit in this case is 11 mm. All acceptable parts should be within this range, and any part that is less than 10 mm from the basic value will be rejected.

Similarly, if the pipe diameter is 10 mm and the single-sided tolerance is -1 mm, the basic dimension and the upper limit of the process are both 10 mm. In this case, the lower limit is 9 mm. The manufactured parts should be within this range, and all parts that are slightly greater than 10 mm from the basic value will be rejected.

2.Bilateral Tolerance

As opposed to unilateral tolerance, bilateral tolerance allows for variation in two directions. The basic dimension of the part is between an upper and lower limit, and the tolerance value can be either positive or negative. If the variation in both directions is equal, the bilateral tolerance is ±0.x mm. If the variation is unequal, the bilateral tolerance can be written as +0.x mm and -0.y mm.

For example, if a tube has a diameter of 10 mm and the bilateral tolerance is ±1 mm, the basic dimension is 10 mm, the upper limit is 11 mm, and the lower limit is 9 mm. All parts between 9 mm and 11 mm are acceptable. Therefore, the actual part can be smaller or larger than the basic expected part.

3.Limit Tolerance

Limit tolerance is another common way of expressing tolerances in CNC machining and other manufacturing methods. Limit tolerance does not use any “+”, “-” or “±” symbol language. Instead, it mentions the upper and lower limits of the part. Instead of using a basic dimension and making the actual dimension conform to the allowed variance of the basic dimension, the only requirement is to make the part within the limits provided.

Limit tolerances are easy to use and do not require any calculations. If the limit tolerance is represented in a diagram, the upper limit is above a specific dimension and the lower limit is below the upper limit and above a specific dimension.
An example of using limit tolerances is machining a pipe with a diameter between 9 mm and 11 mm, instead of requiring a pipe with a diameter between 10 ±1 mm.

An important thing to remember is that although the limit tolerance uses a different expression than the bilateral tolerance, the part results will be the same. The difference is only in the ease with which the blueprint reader understands the design criteria.

4.Profile Tolerances

Profile tolerances are very different from the other types of tolerances mentioned above. So far, the other tolerances have been variations in dimensional accuracy, while profile tolerances are related to the curvature of the cross section of the part. Its symbol is a semicircle located on the diameter of its cross section.

In order to understand the concept of profile tolerances in CNC machining, it is important to understand what a profile line is. A profile line is a line that runs along the cross-sectional area of ​​the workpiece. Profile tolerance range means that the curve of the line should be within an acceptable range of variance. This value is measured in dimensional units (millimeters or inches).

5.Orientation tolerance

Orientation tolerance is the variation of the workpiece shape relative to a reference shape. The reference shape or plane used to check the relative variance is called a datum. Orientation tolerance is measured for perpendicularity or angle of the workpiece. Even when measuring angular offset, orientation tolerance is measured in millimeters or inches instead of degrees.

6.Position tolerance

Position tolerance range is similar to orientation tolerance. Position tolerance in CNC machining refers to the deviation of the position of a specific feature of the workpiece. To measure the deviation, a reference line called a datum is used. The expected position of a feature is called its true position.

7.Form tolerance

Form tolerance is related to the physical characteristics of the workpiece, such as its flatness, roundness or straightness. These tolerances are also measured in millimeters or inches, using measuring tools such as height gauges, calipers, micrometers, etc.

8.Runout tolerance

Runout tolerance refers to the fluctuation of a specific feature of the workpiece relative to the datum when the workpiece is rotated 360 degrees around the center axis. Runout tolerance is a tolerance that is important and measurable for any or all features of a workpiece. The symbol for this tolerance is a box with an arrow pointing to the upper right corner.

9.Unequal Tolerances

Unequal tolerance zones are sometimes also called U modifiers because its symbol is the letter “U” in a circle. These tolerances are used when a specific profile of a workpiece requires unequal single-sided tolerances.

10.Geometric Dimensioning and Tolerancing (GD&T)

Geometric Dimensioning and Tolerancing is a system for detailing and communicating standard machining tolerances. Since there are many types of tolerances for many different types and shapes of parts, a standardized system helps the parties involved in manufacturing communicate with each other. GD&T is the most widely used standard tolerancing system in the world. GD&T assigns symbols to different types of tolerances and establishes a detailed set of rules for how to measure specific tolerance zones.