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Thread: Slot True Position

  1. #1
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    Slot True Position

    Hi all,

    To preface, I understand tolerancing between holes and fasteners ( floating: MMC_hole - MMC_screw / fixed: (MMC_hole - MMC_screw)/2 ). However, I am having a difficult time finding formulas applying them to slotted holes.

    I speculate that you can apply float/fixed fastener formulas to the width of the slot, but I'm not sure what to do for its length. Every example I found seems like its length tolerance is some arbitrary multiple of its width. ASME or any guide doesn't really explain this.

    I appreciate the help.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    Fixed and floating fasteners determine positional tolerance by distributing the nominal clearance between the hole and bolt/shaft to the perspective parts.

    For an oblong (slot) hole there would be two calculations for either floating and fixed fastener condition. The narrow width and the wide width.

    Simply substitute the mmc size of each width into the formulas to determine the distribution of tolerances.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Thank you for your time.

    I understand that it would work for the slot's narrow width, but how would that work for its wide width?

    Take for example a slot whose MMC dimensions are .257 x .500. For a fixed fastener situation where you'd have a 1/4 size screw, I can interpret the narrow width TP tolerance as .0035 [(.257-.250)/2]. How does this apply to its wide width? It doesn't seem right to have a TP of .1215.
    Unless I'm misunderstanding your post.

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    For the math one would use the MMC from tangent edge to tangent edge. The Position tolerance specification would be a separate specification ASME refers to as "Bidirectional Positional Tolerancing, Rectangular Coordinate Method"

    See ASME Y14.5-2018, Figure 10-29 paragraphs 10.4.6 and 10.4.4.1

    or

    ASME Y14.5-2009, Figure 7-28, paragraphs 7.4.6 and 7.4.4.1
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