[To...] Posted April 16, 2020 Share Posted April 16, 2020 I'm wondering if anyone has ever seen something like this. We all know that when a straightness is applied to the size dimension of a cylinder, that we create a derived median line from all of the circular segments, which must fall within a cylindrical tolerance zone. We also know when a flatness is applied to the size dimension of 2 parallel planes, we create a derived median plane from individual center points from 2 opposing points on each surface from all over the plane surface. Sorry if I did not accurately state the true description but you probably get my point. Has anyone ever seen a straightness on a size dimension of 2 parallel planes? Seems like something that wouldn't be very common, but I'm only trying to discuss theory here. Would it be correct to say that we would build a derived median line from center points from 2 opposing points on each surface taken in a straight line? And, that all of the points would need to fall within 2 parallel lines equal to the straightness tolerance? Link to comment Share on other sites More sharing options...
[Ri...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. I've seen Straightness on the surface of a Plane that normally someone would have a Flatness call-out, so I would assume the same principle would apply. Think of it like Roundness of a Cylinder. It applies to any cross-section, so it is up to you to measure as many cross-sections to adequately represent the surface. What you are proposing to do seems correct. Link to comment Share on other sites More sharing options...
[To...] Posted April 16, 2020 Author Share Posted April 16, 2020 Please sign in to view this quote. Actually, I am working on a basic level GD&T training course and am concentrating on my verbiage, looking at various examples from the standard, online and other sources and it popped into my head. I couldn't find any examples of it. I also couldn't find an example of an surface straightness interpretation on a tapered cylinder. Straight one? Yes. Bent ones? Yes, Barrel shaped? Yes. Waisted shaped? Yes. Tapered? No. All of the examples are shown with the 2 parallel tolerance zone lines parallel to the axis of the cylinder. On a tapered cylinder (not a cone), the 2 parallel line are not necessarily parallel to the axis and I can't find any examples of that. Link to comment Share on other sites More sharing options...
[To...] Posted April 16, 2020 Author Share Posted April 16, 2020 Please sign in to view this quote. Update. Found an example of straightness on a tapered cylinder. Courtesy AGI training manual. (Mark Foster)...IMG_3642.jpg Link to comment Share on other sites More sharing options...
[Ia...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. Mark Foster is the man. Totally recommend AGI's GD&T training to anyone that needs it. (Off topic) Does anyone know if Mark still teach classes? Link to comment Share on other sites More sharing options...
[Da...] Posted April 16, 2020 Share Posted April 16, 2020 You are talking about True Position of a Plane if I am understanding your question. Controlling the location with in a nominal dim of a plane.Capture.JPG Link to comment Share on other sites More sharing options...
[An...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. Well,the designer has to define this straight line by means of basic dimensions. Otherwise it makes no sense to me! Link to comment Share on other sites More sharing options...
[Ri...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. No, he's talking about controlling the derived median points of lines on opposing sides. It isn't that common, but it is legal to do. Typically I've seen this for Flatness, but I can see it being applied to Straightness. Link to comment Share on other sites More sharing options...
[To...] Posted April 16, 2020 Author Share Posted April 16, 2020 Please sign in to view this quote. Andreas. Yes, it makes no sense. It was only meant to create a discussion. Does a designer specify exactly where a surface straightness is taken? No. We know that any line that we measure at any location on the surface, needs to be within the 2 parallel lines. I was thinking you could apply the same logic to measuring 2 sets of points from both sides to create the DML. I think I'm correct in the theory though I don't believe it would ever be used. I've since found something from Tec-Ease stating that the 1994 edition used the straightness symbol was used to control the derived median plane. In the 2009 standard, the symbol was changed to a flatness symbol. Link to comment Share on other sites More sharing options...
[Da...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. Sorry Tom for hijacking. Richard, you reminded me of something and I just want to challenge you with this https://www.eng-tips.com/viewthread.cfm?qid=391408 And I guess such stuff would be a food for our thing between ears only, in the real world the designer would probably want to put it either way only. Depending on the milling direction or something. Link to comment Share on other sites More sharing options...
[An...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. Contribution_16_04_2020.pdf Link to comment Share on other sites More sharing options...
[An...] Posted April 16, 2020 Share Posted April 16, 2020 Please sign in to view this quote. Seems to be a little bit erratic? Link to comment Share on other sites More sharing options...
[Da...] Posted April 16, 2020 Share Posted April 16, 2020 And I guess such stuff would be a food for our thing between ears only, in the real world the designer would probably want to put it either way only. Depending on the milling direction or something. [/quote] One thing I have learned over the years and this goes for all industry is Design rarely know what they are doing. Most of the answers that I get when I ask why they did the callout that way? 80% of the time you get an answer like we copied it from a different part. Only about 5% of the time do you get a engineers that can tell you why he did what he did. That goes for all the big Aerospace company's, Automotive, and Medical. One of the big discussion that comes up every year at the committee meetings ASME Y14.5m is that all requirements can fit in to callouts True Position and Profile. You would need no other callout. In this case you would replace the straightness and Flatness with True Position with no controlling datums. Link to comment Share on other sites More sharing options...
[Da...] Posted April 17, 2020 Share Posted April 17, 2020 Please sign in to view this quote. Andreas, thank you for graphic presentation. I've done something in Calypso a while ago:straightness-test.zip Link to comment Share on other sites More sharing options...
[An...] Posted April 17, 2020 Share Posted April 17, 2020 Unfortunately there isn't any access to a CALYPSO computer. Link to comment Share on other sites More sharing options...
[Da...] Posted April 17, 2020 Share Posted April 17, 2020 Please sign in to view this quote. It's nothing special, 35 theoretical points each moved slightly in Z direction, so that it creates an exact values and evaluated. Basically a proof for the right answer to that question.straight1.PNGstraight2.PNGstraight3.PNGstraight4.PNGstraight5.PNGstraight6.PNGstraight7.PNG Link to comment Share on other sites More sharing options...
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