Projected Tolerance Zones in GD&T: A Hard Lesson Learned

by Kyle Zachary, MSOL, Continuing Education Instructor, Goodwin University

When I was 19, I owned a ’73 VW Beetle. It was orange and primer grey, and I relied on it to get me around. It almost never let me down, but when I did have to fix it, it was simple to work on. I loved that car, both for its service and for the lessons it taught me about mechanics, engineering, and design, both good and bad.

One lesson I learned working on the Bug was the importance of the alignment of parts BEFORE they are assembled. As it turns out, 40 years later it ended up providing an example of the application of a very useful GD&T concept: the projected tolerance zone, though I didn’t know it at the time. But let me start from the beginning.

My exhaust system was gradually getting louder because the muffler was rusting through, and I knew that eventually I would have to replace it. On a VW Bug of that vintage, the entire exhaust system was a single unit, pipes, muffler, and manifold, all one piece. It was held on by four nuts that screwed onto studs extending out from the engine block. The nuts and studs were very rusty, making them difficult to remove. So difficult, in fact, that I snapped one of the studs off while trying to spin the rusted nut. No problem, I thought, I’ll just drill out the remaining stud from the block, re-bore it and tap the hole.

Being young and optimistic, I thought I could get the job done with a hand drill while underneath the car. Sure enough I was able to drill and tap the new bore, and install a new stud. At that point I noticed that the replacement stud was not as straight as the other three that had been installed in the factory. In other words, it wasn’t perpendicular to the surface of the engine block from which it emerged. Again my optimistic young mind thought, no problem! I’ll get it on one way or another.

The hole in the exhaust flange and stud was misaligned by nearly half the diameter of the stud, about 3/16 of an inch. No amount jiggling, twisting, or forcing would get things to line up correctly. As it turned out, I ended up using the universal tool of 19 year olds everywhere: the trusty 20-ounce framing hammer. After a few bashes, I noticed the threads at the end of the stud were beginning to collapse, buying me a little bit better alignment. Excitedly I kept bashing, each time driving the flange a little further down over the mashed threads of the stud.

Gradually it got easier to move the flange down the stud because the alignment was better as it got closer to surface of the engine block. In other words, the location of the bore on the plane of block surface was good, but the end of the stud out in space was off because the axis of my bore wasn’t positioned correctly while I was drilling. If I had understood the concept of an extended tolerance zone in GD&T, I might have avoided making that mistake.

Unless otherwise specified, the tolerance zone of a geometric tolerance is limited to the feature of a part. A true position tolerance applied to a bore is only as long as the length of the bore; it does not pass outside of the edge of the bored feature. But what if we need to control the position of a bore axis that is going to receive a pin or stud during assembly, so that they are properly aligned before they’re assembled? Having a way to do this would be handy in avoiding the “bash it with a hammer” maneuver.

Projecting a tolerance zone outside the tolerance feature is the way to do this. The (circle P) symbol is added to the feature control frame after the tolerance value, and the projection distance value is specified after the (circle P).

The projected value is usually the maximum thickness of the mating part or the maximum height of the pin or stud.

Now we have good control of the position of the feature at the mating surface AND out at the end of the stud/pin where the features come in contact when the parts are being assembled. No hammer needed!

Why is this specification important? Considering all of the trouble I got into with an old VW and just four studs, imagine the problems that could arise with something more complex. We certainly don’t want to be bashing a helicopter gearbox housing together. Helicopter builders are touchy about that. A projected tolerance zone can help keep us out of trouble with these types of assemblies. Having a basic understanding of this concept is helpful in interpreting drawings, and a good understanding of GD&T in general helps keep everyone in the production process on the same page.

And my Bug? Well, I was able to get the flange all the way down to the face of the block where the alignment was actually pretty good. I managed to get the nut over the damaged threads, and tightened everything down nicely. Drove the car two more years before I sold it, nice and quiet (for a Bug), no problems.


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Kyle Zachary is an instructor and member of the Business Management and Manufacturing faculty at Goodwin University. He has worked in the wood technology industry and financial markets. Kyle has been a full-time teacher for 10 years, teaching a diverse range of subjects from ESL to Manufacturing and Six Sigma, including teaching positions in the U.S. and Brazil.