In our Video Question Line, our GD&T instructors answer questions that we receive from our students. The topic of the question line video below is regarding diametric tolerance zones. Our student asked the question, “Is the diameter symbol supposed to be used (in the feature control frame) when the feature itself is circular or when the tolerance zone is circular?” In this video, we discuss several examples to identify when to use the diameter symbol in a feature control frame.

The short answer to this question is that we use the diameter symbol in the feature control frame when the tolerance zone is diametric. The examples below illustrate the unique tolerance zones shapes and sizes for position, perpendicularity, straightness, runout, and cylindricity controls.

## Position Control Examples

In this first example, we see the diameter symbol used in the feature control frame for a cylindrical hole. This feature control frame is controlling the position of the hole by placing a diametric tolerance on the position of the hole’s axis. In this case, the axis of the hole must fall within a .005 diametric tolerance zone.

Figure 1: Position Control for Cylindrical Feature

Next, we have a part where the position of a width feature is being controlled. Notice that there is no diameter symbol in the feature control frame. When we control the position of a width feature, the midplane of the feature must fall within a tolerance zone made up of two parallel planes, as shown below. This is not a diametric tolerance, therefore there is no diameter symbol used in the feature control frame.

Figure 2: Position Control for a Feature of Width

## Perpendicularity Control Examples

Now let’s look at these same parts, but with perpendicularity controls instead of position. With perpendicularity, we are controlling the orientation of the feature instead of the feature’s location. For the cylindrical feature, we are once again controlling the axis of the cylindrical feature. This axis must fall completely within the diametric tolerance zone of .010, indicated in the feature control frame.

Figure 3: Perpendicularity Controls for Cylindrical Feature and Feature of Width

For the width feature, we are again controlling the midplane. The midplane of the feature must fall completely within the tolerance zone made of up two parallel planes spaced .010 apart, as indicated in the feature control frame. Because the tolerance zone is bounded by two parallel planes, it is not a diametric tolerance, and no diameter symbol is used in the feature control frame.

## Straightness Control Examples

So far, we have seen examples where the cylindrical feature has had a diametric tolerance in the feature control frame. However, this does not hold true for all types of form controls. In the example below, we see a cylindrical bar with straightness applied. The example on the left has straightness applied to the surface, and the example on the right has straightness applied to the feature of size.

Figure 4: Straightness Control Examples

### Surface Straightness

Notice that there is no diameter symbol in the feature control frame for straightness applied to the surface. When surface straightness is called out, we are controlling the straightness of one linear element of the surface. The tolerance zone for this linear element is two parallel lines. Therefore, there will be no diameter symbol in the feature control frame.

Figure 5: Surface Straightness

### Derived Median Line Straightness

When we apply straightness to a feature of size, we are controlling the derived median line of that feature. The derived median line of the feature is the line made from the center points of the cross-sections of that feature. Notice that the feature control frame includes the diameter symbol. This is because the derived median line must fall completely within a diametric tolerance zone for it to meet the straightness requirement.

Figure 6: Derived Median Line Straightness

## Runout Control Example

Runout and total runout will mostly be applied to round or cylindrical features. This is another case where a cylindrical feature will not have the diameter symbol in the feature control frame. In the example below, we see a cylindrical feature with total runout called out in the feature control frame. Total runout controls the amount of variation in the entire surface as the part is rotated around the datum axis. This surface must fall completely within the tolerance zone, which is made up of two concentric cylinders. In the feature control frame, we are not identifying the diameters of the two cylinders that make up the tolerance zone – we are identifying the distance between them.  Because we are controlling the linear distance, not the diameter, we do not use the diameter symbol in the feature control frame.

Figure 7: Total Runout of a Surface

## Cylindricity Control Example

The cylindricity form control is yet another example where the diameter symbol is not used in the feature control frame. Cylindricity is a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis. The tolerance zone for cylindricity, like in the total runout example above, is made up of two concentric cylinders that run the length of the feature. The entire surface of the cylindrical feature must fall within these two concentric cylinders.  Once again, because the feature control frame controls the linear distance between the two concentric cylinders making up the tolerance zone, not their diameters, the diameter symbol is not used.

Figure 8: Cylindricity of a Feature

## Summary

“Is the diameter symbol supposed to be used when the feature itself is circular or when the tolerance zone is circular?”

The diameter symbol is used when the size in a feature control frame is the diametric size of the tolerance zone. If the size of the tolerance zone is the linear or radial distance between two cylinders or two circles, the value is a linear value and thus cannot have a diameter symbol before it.

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