Projected Tolerance Zone: Equivalent to Tightening the Zone?

If you’re familiar with the different GD&T modifiers, then you probably know that the circled P creates a “projected tolerance zone.”  This is often used on threaded holes to keep any fastener that protrudes beyond the threaded hole from causing interference with a mating part:

Without the “P” modifier, the tolerance zone exists only within the depth of the threaded hole itself.  The result is that the threaded hole could tilt, and be passed for position tolerance, yet cause interference:

So “P” is a good thing.  However, when this concept is presented in our GD&T classes, someone will occasionally ask if we could — as an alternative to “P” — simply tighten the position tolerance number instead.  The dialog might go like this:

“Couldn’t we just change the 0.3 to 0.2 (or 0.1) and achieve the same effect of preventing too much tilt?”

“Yes, that would be legal,” I answer. “But using the P allows us to keep a larger tolerance, while preventing interference.”

“But it has the same effect of tightening the position tolerance anyway,” the student might reply.

This is where we have to be careful.  It’s true that projecting the tolerance zone has the effect of tightening the perpendicularity aspect of a position tolerance (because it’s extended higher), but it still permits the threaded hole to use the 0.3 for lateral position.  In other words, if the threaded hole doesn’t tilt, then the 0.3 is still allowing the axis to drift left or right within that zone.  By eliminating the “P” and dialing the tolerance down to 0.2, we’d be robbing ourselves of some tolerance which might still be helpful in terms of x-y location.

So a projected tolerance zone is not the same as merely tightening the tolerance number. That said, there are times when a projected tolerance zone doesn’t make sense, even on a tapped hole.  An example might be for a hole whose fastener doesn’t protrude very far up — such as a bolt that engages the threads for 16 mm but then clears through an adjacent plate of only 4 mm thickness. 

But in general, a designer should at least consider the “P” option when imposing GD&T on threaded or press-fit holes.

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