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| GD&T Surface Profile Question |  | ||
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| Posted by: waynorth ® 05/22/2009, 13:52:14 Author Profile eMail author Edit |
Hi All, GD&T surface profile question: If I have a drawing with only basic dimensions, no datums, and I point to the part with a single call-out [profile of a surface|.12] with a circle (all around) does that somehow imply that all of my part surfaces are now evaluated together as one big 'best fit'? I'm on the receiving end of this and I can't make any headway with our GD&T guru. It's a casting and I want datums so I can measure and adjust accurately but first I need to break through the issue of whether this is actually saying what they think it is. I'm of the opinion that without datums each surface should be evaluated independently to itself with no magic 'best fit' of the entire part. Any thoughts? |
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| Posted by: traingdt ® 05/22/2009, 19:26:20 Author Profile eMail author Edit |
The interpretation you gave in your first paragraph is essentially what the designer meant, although technically the little circle is "all around," which means a continuous tolerance zone around the perimeter in the given view. He probably should have ditched the circle and added a note under the callout saying "all over" to communicate that it's not just in that one view. Yes, this means that the perfect CAD model (basic dimensions) now has a tolerance "shell" hovering inside and outside of it. You don't really need datums called out; just start measuring anywhere, then move over and sample another point, and move over again, etc. It's like connect-the-dots (of course, the more points sampled, the better). If your gaging or inspection machine requires datums, just choose one surface and go from there. I'm not saying this is the best way to design parts, but a profile tolerance all over the entire part with no datums mentioned is indeed legal. |
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| Posted by: waynorth ® 05/25/2009, 17:26:02 Author Profile eMail author Edit |
Ok, thanks a bunch for the response. Whether I'm right or wrong it's always good to learn something new. Gives me something to move forward with. Appreciate it! |
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