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Thread: Which dim to use?

  1. #1
    Associate Engineer
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    Which dim to use?

    In the picture you'll find 5 dim labeled A,B,C,D1,&D2. I originally defined the drawing with dim. A, B, C, and D2 and the redline I received was to change D2 to D1.

    Questions:
    1. Why is D2 wrong and D1correct?
    2. Is there a standard way to dimension the circular shape with the end cut?
    3. is the grader just nitpicking?

    Thank you
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  2. #2
    Administrator Kelly Bramble's Avatar
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    D2 is not wrong however it would be easier to measure and open machine for the D1 dimension. Moreover, dimension D2 is to the intersection of two surfaces top and bottom. From a manufacturing point of view the dimensioning scheme amplifies the variability of the dimension tolerance and that variability would need to be understood in the tolerance assignment. Most likely the D1 dimension reflects functional requirements. BTW, this feature will require either broaching or EDM to create.

    Again, D2 is not wrong just not perfered...

    For a drafting, and mostly dimensioning and tolerance reference or standard see, ASME Y14.5-2009, Paragraph 1.8.17 and Figure 1-46.

    Quote Originally Posted by michael_buller828 View Post
    In the picture you'll find 5 dim labeled A,B,C,D1,&D2. I originally defined the drawing with dim. A, B, C, and D2 and the redline I received was to change D2 to D1.

    Questions:
    1. Why is D2 wrong and D1correct?
    2. Is there a standard way to dimension the circular shape with the end cut?
    3. is the grader just nitpicking?

    Thank you
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    are you referring to the picture I included in the attachment? The Key way?
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  4. #4
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    QUESTION:
    In post #3, referring to the picture on the right, why is the dim 17.4 given? How does it relate to the functional requirement of the part?
    Last edited by michael_buller828; 09-25-2017 at 07:34 PM.

  5. #5
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by michael_buller828 View Post
    QUESTION:
    In post #3, referring to the picture on the right, why is the dim 17.4 given? How does it relate to the functional requirement of the part?
    Mating diameter and keyway... Most correctly, dimensions and tolerances should define mating features..
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  6. #6
    Technical Fellow jboggs's Avatar
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    On your first post, D1 is preferred over D2 because it is easier to measure accurately. In dimensioning you need to remember that the more accurately the shop can measure the part they're making the more accurate the end result will be. The geometry is completely defined either way, D2 or D1. Its just a matter of real world issues.

    On your second post, the same theory applies. Without a vertical dimension to the horizontal keyway surface the geometry is not fully defined. That vertical dimension could have been to the hole center or to the opposite edge. Dimensioning to the opposite edge is preferred, again because it is easier to measure accurately. Measuring to a non-existent feature like a centerline can be pretty difficult, resulting in inaccuracies.

    No matter how technically perfect your drawing is, if interpreting it can lead to imperfect parts it's not good enough.

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