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.

2 Comments

  1. This gets tricky when trying to inspect/verify that the threaded hole axis is indeed within the specified tolerance. With most threads, there is the play between the mating threads which can make measurement difficult.

    Does one use a threaded gage pin that will bottom out on the reference surface to eliminate that play, or is there a better way? In my application, the threaded hole is shallow and a relatively large diameter (1.0 x 32 TPI).

    • Yes, I would suggest a threaded gage pin that goes into the full depth of the hole, but then protrudes upward (in this case, 14 mm). Then gage it as if it were a pin being positioned. There’s not really a bottoming out; as long as the two faces (datum features A) are in full contact.
      Your question brings up a side note; on the threaded hole there is an MMC modifier — which is the “ticket” to using a fixed-size gage.
      In the GD&T community there is often a debate about using the MMC modifier on screw threads because MMC typically implies that there is a bonus tolerance. I think your question kind of touches on that: even threads are subject to a little looseness (until the part reaches final torque).
      If we didn’t show the MMC modifier, then it would be RFS and you’d be obligated to use an expanding-thread gage, but… good luck with that. So the play that you speak of can be used to your advantage to verify the position tolerance.

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