Sorry that it’s been a while since my last blog post! That simply means that our training schedule has been busy. Though a good portion of the U.S. economy is still sluggish, I have definitely seen an uptick in the number of requests for GD&T training. So remember, if you have a group of 4 or more people that need the basics or a refresher in GD&T, don’t hesitate to drop us a line or call for a customized price quote for a group class.
Today I should begin by answering the question posed in the title of this blog entry: No — there isn’t always one right way to tolerance something. Recall that GD&T is a language, and like any language there may be more than one way of accomplishing something. People often criticize prints for “bad” or “misapplied” GD&T. And there are certainly many examples of that. But many times what people are calling “bad GD&T” is simply a different way of doing something! That being said, however, I’d like to present the most common GD&T mistakes which are definite no-nos.
Here are my “Top 5” of the most common GD&T errors:
—Failure to show a diameter symbol in the feature control frame (if a cylindrical zone is desired
—Having parallelism (or perpendicularity) on a surface, and then adding a flatness tolerance of the same amount or greater
—Using the position symbol on a single feature related to a perpendicular datum (use perpendicularity!)
—Using concentricity when position RFS would be adequate (assuming ASME standard is invoked)
—Showing the datum feature symbol on a center line (yes, a center line can be a datum, but the symbol MUST be tagged with a physical feature)
Of course there are others, but if any of these items shows up on your drawings, you should at least be comfortable in raising a question to the designer/engineer. If you aren’t sure what I’m referring to in the items given above, or think that they are OK, then that would make for some good homework for you! Think about why those things don’t make sense; use your GD&T reference books, or if you need further explanation, feel free to add a comment here and I’ll try to address it in a future blog entry.
If those are your top 5, you’re leading a charmed life.
How about datum selected arbitrarily, and in such a way as to make your life miserable? I have a part right now, a hatch of sorts, that is some 8 feet long, it has two hinge pin holes, one in each end of the 8 foot long piece. The primary datum is the flat surface the hatch closes against, not a bad primary datum…. The secondary datum, however, is a flat surface on the end of the part surrounding the hinge pin hole that is about 3″ long. It is not a separate requirement for each end, so this surface (3% or so of the overall length of the part) is the sole rotational constrain for a .002″ positional tolerance on the hinge pin hole on the opposite end of this long part. Logically, the pin holes would be a composite datum to constrain the part’s rotation. Sadly this is not the case, the customer of course is not interested in complaints about the GD&T of this part as it has been in production for a number of years, the incumbent supplier never addressed this issue.
Well, “top 5” means just the 5 most common. I could list dozens and dozens of goofy GD&T things that people do 🙂
Regarding your scenario, yes I agree that the secondary datum is kind of strange. Too bad that the customer isn’t open to having a better part design; in the long run it’s only affecting their bottom line.