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Thread: Complicated Parallelism Question

  1. #1
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    Complicated Parallelism Question

    I'll try to keep this as simple as possible. Attached is a basic 2D shape I drew real quick for this specific question.

    The left and right side of a machined casting needs to be parallel to each other. It gets complicated for us when we have to specify a minimum distance to keep the part parallel. In the attached drawing I set up the GD&T Tolerances how we have them set up on our drawing right now. We know it's not right, so I'm hoping someone here could help us out.

    Any advice would be greatly appreciated.
    Attached Files Attached Files

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by MikeE View Post
    The left and right side of a machined casting needs to be parallel to each other. It gets complicated for us when we have to specify a minimum distance to keep the part parallel.
    If I understand your question right, Rule #1 (Envelope Principle) is what you need to know and apply.

    Simply state on your drawing that the drawing is to be interpreted per ASME Y14.5-2009 and then apply a limit tolerance from the left to the right side surface you wish to control min. distance and parallelism.

    Rule number #1 Envelope Principle states that a feature of size tolerance at maximum material condition (Boundary) size defienes a 3D boundary that may not be violated due to the combined effects of size and form. Additionally, the limits of size dimension and tolerance will control the maximum parallelism as well.

    If the limits is size total variability is +/- .010 then the feature will not vary in parallelism greater than .020. You can refine the parallelism to a greater degree by applying a parallelism less in variability than the limits of size.

    What is the minimum distance required on your design as well as the parallelism?

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    Administrator Kelly Bramble's Avatar
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    The minimum distances required are exactly as shown on the pdf. I kept the dimensions exactly the same as the part I'm dealing with, just made a very simplistic shape to the design for confidentiality purposes.

    Your explanation kind of made sense, in a way. Since our company is still in the process of trying to get us training on GD&T we only know the extreme basics. If I apply the limit tolerance from the left to right how do I specify the up/down dimension tolerance for the vertical of the boundary? Would I use the "B" datum as I show in the pdf?

    The video you attached was a little useful. ​But like a lot of other examples I find online, it only deals with holes. Does this also apply to faces?

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    Administrator Kelly Bramble's Avatar
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    If I apply the limit tolerance from the left to right how do I specify the up/down dimension tolerance for the vertical of the boundary?


    The upper tolerance would be plus some acceptable variable (.020 just for fun) minus zero. Or, you could increase the nominal size/distance (surface to surface) by perhaps .010 and give a bilateral equal tolerance of plus and minus .010.

    So, 13.625 +.020/-.000

    or

    13.625 +/- .010
    Last edited by Kelly Bramble; 05-05-2014 at 07:11 AM.

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    Administrator Kelly Bramble's Avatar
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    Why the single datum target line? What is the purpose/goal?
    Last edited by Kelly Bramble; 05-05-2014 at 07:11 AM.

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    I believe he is trying to ensure one defined area on the side of the box is parallel to the opposite side only in the defined area. Not sure if he needs 2 datums for this callout. I'm not too experienced using GD&T either but I would assume there may bolted on plates that need to be parallel to one another.

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    Administrator Kelly Bramble's Avatar
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    Ok, here is a video that shows explicitly how to define a minimum distance as well as parallelism between two planes.

    Click Link -> Parallelism of Surface to Datum Center Plane Application

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    Awesome video Kelly.

    Still addressing his question and questions of my own, how do we define an "area" on one side to an "area" on the opposite side? I also have castings that have partially machined side areas that we inspect per notes currently without GD&T callouts. We don't inspect the parallelism of the entire side since nothing else bolts to it. Our datum A would likely be the bottom of the casting supported at three defined datum points, then inspecting the perpendicularity of the side "area" to the bottom, and finally the parallelism of the side to the opposite side "area".

  11. #11
    Administrator Kelly Bramble's Avatar
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    how do we define an "area" on one side to an "area" on the opposite side?
    Chain or phantom line an area
    and/or crosshatch then dimension the size with limit or basic dimensions and indicate with a delta (flag) note that applicable tolerance (or datum reference) only applies within the area indicated.

    Parallel_GDT (1).pdf


  12. #12
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    Quote Originally Posted by Kelly Bramble View Post


    Chain or phantom line an area
    and/or crosshatch then dimension

    I think this would be the best way to do it as well. I just wasn't sure if there was a different way to go about it or not. Thanks for the help!

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