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Need some help with GD&T

  1. May 11, 2007 #1
    Now that I've completed my first CAE course I'm attempting to design a part (to actually have built) using solidworks. I'm want to build this part to mil spec, problem is mil specs are given in GD&T and I only know BD&T.

    I've been trying to convert the GD&T to BD&T so I can dimension and tolerance the part, but I haven't been having much sucess. I've even considered simply giving the machist the actual mil spec document (and letting him decifer it), but I would at least like to have the dimensions correct on my model.

    So I'm looking for any help or suggestions about how to deal with this situation, or how to convert GD&T into BD&T. I would even be willing to pay if someone would take the time to do to conversions for me. It's a fairly simple design, and I'm sure it wouldn't take someone who knows GD&T very long to do.

    To give you a better idea of what I'm working with I've attached a picture of my model so far, it is a handgaurd for a rifle. Also, here is a link to the mil specs (specifically for the rails): http://www.biggerhammer.net/picatinny/" [Broken]


    Attached Files:

    • rail.jpg
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    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. May 12, 2007 #2


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    Well, with any tolerancing scheme, we have to understand the role and function of the part. What are the important characteristics? I could GDT this thing until I am blue in the face and make the cost per part about a million and a half bucks.

    I guess it would help if you gave us some ideas about the current tolerancing.

    Do you have access to ASME Y14.5?
    Last edited: May 12, 2007
  4. May 12, 2007 #3
    No, I don't have access to ASME Y14.5. I thought my school library would have it but a quick search revelaed nothing.

    I'm trying to tolerance the whole part as BD&T, not GD&T, so for most of the part I already know the tolerances I want and how to apply them. My issue is specifically about the "rails". Looking at the picture the rails are the four longitudional sections that at located at the top, bottom, and sides of the part.

    To dimension and tolerance these rails I have found the military specifications for them, as linked to in my previous post. I think I have been able to convert the dimensions from GD&T to BD&T, but I am uncertain how to convert the tolerances.

    So I know the tolerancing I want (mil spec) I just am uncertain how to convert the tolerancing from GD&T and BD&T.
  5. May 12, 2007 #4


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    Do you understand the concepts behind the true position tolerance? Do you know the implications of having the max material condition symbol on the tolerance?
  6. May 12, 2007 #5
    No, I do not.
  7. May 12, 2007 #6
    To clairify from my 2nd post, I know that I want mil spec dimensions and tolerancing for the rail sections, but I do not know what those dimensions and tolerances are because the mil specs are given in GD&T and I don't understand GD&T.
  8. May 14, 2007 #7


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    True position is probably the toughest of GD&Ts because it can have a lot of others implied or wrapped up in a single tolerance. Essentially, the tolerance creates a circular zone around the theoretical perfect point where the feature is allowed to lie. There's a fair amount to it. I would highly suggest finding a GD&T book. Your library should have something...anything. There are some background materials you need to be familiar with in order to help you understand what is being toleranced.

    Start here:
  9. May 15, 2007 #8
    Thanks for the replies and the link Fred. I'll be working on it and see what I can come up with.
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