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Thread: Inline Keyways

  1. #1

    Inline Keyways

    I feel like I'm on the edge of overcomplicating a drawing and am curious if someone has a better method for relating these features.

    My drawing is of a base plate with two inline keyways on the bottom surface and another two keyways on the top surface (Note that the axial alignment of these keyways is unimportant). The intent is to align various features to the center plane created by the bottom keyways and bottom surface. For reference: the two keys assembled with this plate will locate to a mating slot but allow axial movement.

    The bottom surface is Datum A and one of the bottom keyway widths is Datum B. The other bottom keyway is located with a basic dimension on the side of the slot and position tol on the width to datums A & B; The width of this is then Datum C.

    For the top side, each keyway is located with a basic dimension to the center of the slot (this could be changed to the side for consistency). Also, each keyway width has a position tolerance to datum's A & B-C. The compound datum B-C is because I want the plane created by these two features together.

    An additional feature is that there is a threaded hole in the center of each of the key slots. There is enough clearance where I wouldn't normally care about using a position tolerance on these threads, but since they are in-line with the keyways it becomes ambiguous if I give them a linear tolerance. i.e. I don't want someone assuming the dimension is for the slot centerline. Thus, I instead dimensioned from the previously defined slot edge to the thread centerline and made it basic. Then I put a position tolerance on the thread callout itself.

    Mind you, on the actual drawing I am running out of room and greatly dislike placing dimensions on the part itself.

    BottomView.jpgTopView.gif

  2. #2
    Administrator Kelly Bramble's Avatar
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    Well, your dimensional specifications look good to me.. The datum's B & C define a center plane which are being used to locate the threaded hole feature symmetrically about the related slot features.

    You have related the slot feature on the opposite side to a center plane defined about datum B-C. I would have gone a step further and related the datum slot features to one-another though I suspect the default tolerance block and the center line does this for you.

    Additionally, there is not a horizontal relationship for the threaded features defined or shown on the drawing so, I don't know how well these features will be centered relative to the related slot features (left and right).

  3. #3
    Thanks for the reply Kelly. I really felt like I was starting to get a little to crazy (even if technically correct) with this drawing. This is what I am going to default to for reference though from now on.


    <<I would have gone a step further and related the datum slot features to one-another though I suspect the default tolerance block and the center line does this for you.>>

    Yes, I debated doing that here but determined standard TB tolerance is fine. For more critical parts I would certainly relate them together.

    <<there is not a horizontal relationship for the threaded features defined or shown>>

    What I did not show was a typical coordinate tolerance relating those threads horizontally. By design, there is a fair amount of wiggle room in the key slot to allow for a greater tolerance horizontally while I keep the threads centered in the slots a little tighter with a position tolerance. Even the position of the key slots can be relatively sloppy in the horizontal alignment.

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