Learn with this article why positioning tolerances are the better way of dimensioning hole patterns.

Why should I use GD&T to detail simple parts?
Do you want to make sure that your part always fits the counterpart? Do you want to reduce production costs? Do you want to map complex relationships in one part? This article looks into the subject of GD&T and why utilising this capability enables greater control of dimension hole patterns.

No tolerance vs. Position tolerance
Let’s have a look at this simple plate with six holes in Figure 1 and see how they are dimensioned. This is a typical example of what you may find in a number of companies.

Figure 1

Let in Figure 2 assume that the general tolerances according to ISO 2768-m apply to the dimensions. What does this mean for the allowed deviation of the holes? For each dimension we have a different tolerance.

Figure 2

The tolerance zone resulting from the deviation in Figure 2 (Detail Z) is the cyan rectangle in Figure 3. If we assume that a round screw will be placed in the round hole, does a rectangular tolerance zone really make sense? Or would a round tolerance zone be better?
When we circle the rectangle, we get a round tolerance zone with a diameter of 1.2 mm (blue circle). Using this method, we can increase the permissible deviation under realistic conditions without changing the function and produce the component more cost-effectively, because we are allowing a wider deviation.

Figure 3: Detail Z of Figure 2

The dash double dot lines show the maximum permitted position deviation of the hole with diameter 17,5 in one direction. The grey circle shows the diameter 16 of the bolt mounted in the hole. Without looking at the tolerances of the holes and bolts we always can guarantee a clearance of 0,3, if the counterpart is toleranced in the same way.

Figure 4

If you compare the dimensioning in Figure 4 with Figure 1, it might look a bit more complicated but it is much closer to reality. Morever, you have increased the allowed deviation realistically and reduced manufacturing costs.

There is not always just one solution
In Figure 5 we see the dimensioning of a hole pattern as it is often used.

Figure 5

What do these annotations say?

The axes of the holes must be within the cylindrical tolerance zone.
The position of the tolerance zones is defined by basic dimensions related to the datum B and C.
The tolerance zones are perpendicular to datum A.

Hole patterns in a part often form functional groups. What if the large hole is the centering and the small holes are for the screws? Do the small holes really need the same tolerance as the large hole? Wouldn't it then be more important that the small holes are correct in their position in relation to the large hole?

Figure 6

An example of how functional groups can be displayed is shown in Figure 6. The position of the tolerance zone of the large hole is defined by basic dimensions related to the datum B and C. The tolerance zone is perpendicular to datum A.
At the same time the axis of the centering forms datum D. The is axes of the small holes must lie within the cylindrical tolerance zones relative to the centering.
Position tolerances make it possible to display even complex relationships in a part easy to understand.

Functional dimensioning and production cost savings
Here are three reasons why you should use more position tolerances from now on:

Position tolerances are much more functional, since round components are usually joined into round bores and the tolerance zone should therefore also be circular.
Position tolerances allow even complex relationships in a part to be shown in a practical manner.
Position tolerances often make it possible to select a larger permissible deviation and thus reduce production costs.