Is the GD&T Blog Giving Away Secrets?

A funny question arrived in the inbox yesterday, and it reminded me to get on here and post another entry. The questioner wondered if I’m undercutting some of my training business by giving out GD&T tips and explanations via this blog.

Not at all. GD&T is not a secret society! It’s a very useful language and the more people that know it, the better.  By explaining some of the symbology here on the blog, we hope to do two things: educate people, especially those that need assistance with a specific topic; and at the same time, generate interest in our complete GD&T seminars (wink).  So it is possible to be helpful and advertise at the same time!  Here’s today’s GD&T “secret”:  the tangent plane modifier:

First, a review of the parallelism symbol in general. The top of this block is to be parallel to datum A within .006 inches (notice that the distance between the top and bottom is a separate, more generous tolerance).  Regular parallelism, without the T symbol, would require all points across the top to be within our zone of .006.

But the T symbol changes things — instead of controlling every point across the top surface, now it says that the “tangent plane” formed across the top surface must be parallel within .006. This imaginary tangent plane is formed by the highest points only:


Thus, the tangent plane modifier is loosening the parallelism idea because we ignore the valleys, or low points! (Of course, the low points cannot dip below 4.405 because of the height requirement.) Tangent plane makes sense for a mating surface whose counterpart only feels the high points anyway. The T symbol can be used with parallelism, angularity, or perpendicularity when those are applied to surfaces.

So there’s another GD&T secret out of the bag.

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