Here’s another common question that comes up in a GD&T class: Suppose we are applying a position tolerance to a hole. It would typically have a diameter symbol in front of the number, as in the first example below. But what if we omit the diameter symbol in front of the number, as in the second example? Would the shape of the tolerance zone still be assumed as cylindrical?
Answer: No, it’s not cylindrical. Without a diameter symbol, a tolerance zone defaults to two parallel planes (unless the BOUNDARY concept is invoked). So the second drawing above is ambiguous; the zone will be two parallel planes, but we don’t know the direction of those two planes.
Two solutions: First, if the intent is to control the position in all directions, you must add a diameter symbol as in the first example above. Second, if we really intended two parallel planes, we must graphically indicate the direction of those planes:
Now, it is clear that we are controlling position in the left/right direction. Of course, that means that there is no position control in the up/down direction. So let’s take it one more step:
This example creates two sets of parallel planes — one in the vertical direction and one in the horizontal direction. The result is a square tolerance zone; thus, it’s very similar to using the traditional coordinate or plus/minus tolerancing method. But here we still have the advantage of clearly identifying the datum references, and we also have the MMC modifier to gain bonus tolerance, something that the coordinate method cannot do.