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Thread: Profile tolerance

  1. #1
    Associate Engineer
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    Profile tolerance

    Hello Forum,

    Newbie here. I'd like to get some input on the following:

    The company I currently work for has instituted some "across the board" tolerances on certain parts. First of all
    I'll just say that most of this is standard sheet metal panels, brackets etc... I'm new here and have a fairly good understanding of GD&T practices. I'm working on a series of panels with "standard" D-hole cutouts for circular military connectors. As most know the flat surface on the connector and subsequent cutout is simply to prevent the connector from clocking while the jamnut of the connector is being torqued. Their "standard" d-hole cutout detail shows a surface profile tolerance of .002 on the flat. (see attach) They do this on all D cutouts regardless of connector size/hole diameter. I contend that this is way overkill. I would say that not only is the tolerance way too tight for the intended application, but a "surface profile" tolerance is not needed at all. True position is fine, but I would think a standard +/- tolerance from lower quadrant to flat would suffice. Thoughts? (They insist on keeping this, no explanation)
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  2. #2
    Administrator Kelly Bramble's Avatar
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    The Profile of surface tolerance is more explicit than a limit tolerance and position tolerance would not really be applicable exclusively to the "flat".

    I assume that the datums A|D|B are appropriate to orient the flat clocking/holding feature.

    As far as the profile of surface tolerance .002 which roughly translates to +/- .001 the manufacture-ability depends on the process employed to create this feature. I tend to agree that a profile of .002 seems a little tight for this particular clocking feature.

    The position tolerance for the hole is dia. .125 at MMC and .135 at LMC which is quite generous and large with respect to the Profile of surface tolerance on the flat .002. - seems an odd combination.
    Last edited by Kelly Bramble; 10-30-2015 at 08:22 AM.
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  3. #3
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    Thank you Kelly

  4. #4
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    Maybe they want to hold that profile so tight because if the diameter of the actual hole is toward the largest, the expanding circumference of that circle "eats" into the area of the flat. If the profile tolerance were generous, then the flat could drift up, and suddenly you wouldn't have much of a flat left at all.
    A solution to that could be to reduce the basic dim of .830 to a smaller number, thus giving more surface area to the flat.
    Just a random thought...

  5. #5
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    I think my first sentence was backwards... I should've said: "Maybe they want to hold that profile so tight because if the diameter of the actual hole is toward the smallest, the reducing circumference of that circle "eats" into the area of the flat.

  6. #6
    The bilateral tolerance +.010/-0 means that the hole is shown at the smallest diameter (LMC).

    I agree with tebaugh61 on a couple of points: A .002 tolerance does seem a bit tight but we cannot know for sure without having the mating part tolerance. I also see no difference in this example between using GD&T profile or +/- coordinate tolerance. If the profile was intended for the entire hole - d-cut and circle - then it makes more sense. However there is a nifty little circle that should go around the jog point of the leader if that was the actual intent, and I don't believe it is.

    Last, the position tolerance of .125 is much larger than I am used to using, but if it is just a dummy light position or plug bulkhead on a dash, then it's purely visual and sounds appropriate. Conversely, the profile of the d-cut needs to be vertical... especially if there is some writing or other visually aligned feature on the mating part.

    Still .002!?

  7. #7
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by JFleck View Post
    The bilateral tolerance +.010/-0 means that the hole is shown at the smallest diameter (LMC).
    I've never heard of that.. Which standard (ASME, ISO, ANSI( states this?
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  8. #8
    For clarity I should rephrase my statement: The hole is currently dimensioned with smallest diameter shown (I only say this because it has a tolerance of minus nothing), thus LMC. To paraphrase Belanger's point: The length of the flat does get smaller as the hole gets smaller and at some point the hole will be too small to even have an effective flat.

    I am not quoting a standard, I'm just reading the drawing. The length of the flat won't be any smaller than what is shown in the detail because it is dimensioned accordingly. In fact, the hole would have to approach 1.660 before the flat is completely gone... The diameter where it is effectively useless would be dependent on the clearance of the mating part (which we do not currently know); presumably somewhere between 1.660 and 1.760.

    Untitled.png

    Side note: Technically the drafter may have drawn the circle and the flat not to scale and thus it may not visually represent the actual form. However, that does not appear to be the case and it is (at least in my opinion) bad practice to do so without stating it as such (NTS).

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