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Non welding Q. (need help reading a drawing)

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  • Non welding Q. (need help reading a drawing)

    Can anyone help me read how to machine his port.


  • #2
    see: http://en.wikipedia.org/wiki/Geometr...nd_tolerancing for a summary.

    see also: http://www.engineersedge.com/gdt.htm
    http://www.brownandsharpe.com/geomet...nd-tolerancing
    http://www.efunda.com/designstandard...troduction.cfm

    Your drawing specifies, among other thins, a diameter, tolerance for the diameter, how perpendicular the bottom needs to be to the axis of the bore and, radius for the corner at the bottom, and position tolerance.

    Start by identifying the symbols meaning and referents, and ask further questions as you need.

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    • #3
      Having had to pay for machine shop time when the machinist couldn't figure out that the datum was in the BOTTOM LEFT CORNER (in case he reads this forum) and even though the machinist knew he screwed up, sent the part for painting!!!!!, I can say this, if there is a question, contact the drafts person who is listed in the bottom right corner and _don't_ contact the engineer (we are dummies, at this point we nod our heads and say, "sounds good").

      Disclaimer: I haven't done mechanical engineering in 15 years.... I hesitate even answering but I love a puzzle. So take everything I say as an educated guess.

      So.. it's marked as a counter sunk hole - but it not, its threaded...

      Through hole is 9.5 (I'm guessing MM) maximum. Looks like a good opportunity for a pilot hole.
      The note talks about counter sink is 21mm +- 1 but the sides need to be straight (90 deg +- 5) and the bottom of the counter sink needs to be flat to .15mm. They are allowing a small radius on the corners of the counter sink.
      Warning, it looks like they really want a 13/16-20 thread which is 20.5mm!
      The 19.2 +- .1 is a tight bolt to screw clearance for 13/16 (I think that is about 90% thread engagement). The typically inner thread diameter of a 13/16 is .7512". They are calling for a minimum inside diameter of .7520" So they are giving you less than .0008 to play with. Its about a 90% thread engagement. I assume this is aluminum? If its steel, go ask them if its a mistake.

      And then is looks like they want it tapped for 13/16 and the threads need to be 10.2 mm deep.

      So... pilot hole + mill + tap + bottoming tap. 5 minutes max (actually I have no idea, complete amateur machinist!!!)

      I would talk to the drafts person anyway. There is some critical dimension that they are worried about. Will the screw bottom out in this hole? (is that why they marked the bottom as being flat to .2 degrees?) Not sure. I can say engineers loose sleep over how the shoulders of bolts lay on the material. If its critical, you need to know that.
      Last edited by con_fuse9; 03-15-2010, 10:08 AM.
      Con Fuse!
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      • #4
        Your drawing is a mix of inch and metric.
        the threads being the "INCH" part. The rest is metric.
        The arrows pointing down are the depth from surface.
        There is a countersink that is 21mm dia. x 90deg.
        the 13/16-16 thread is 10.2mm deep.
        the truepostion is .25mm with a maximum material condition taken into consideration. That means that if you hold the hole dias. to the high end you can add some tolorence to the location.

        All in all, you need to learn the symbols and the formulas as was suggested from "enlpck"
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        • #5
          Originally posted by 1havnfun View Post
          the truepostion is .25mm with a maximum material condition taken into consideration. That means that if you hold the hole dias. to the high end you can add some tolorence to the location.
          Could you explain that. Does it mean that whatever will connect with this has a slop essentially of .25mm - so if I hold my tolerances down, I would need to be more accurate, but if I hold my tolerances to the high side (sloppier fit) I don't need to be as accurate with placement?


          Thanks
          Con Fuse!
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          • #6
            True position is tricky when you don't use it all the time.
            example; ( I don't have all the symbols on this computer)
            true position of |.010|A|B|C
            means you can be off position by X.0033 and Y.0033 and "C" would be the base.
            When the Max material condition is call (the "m" in the circle) the tolorance of the hole in this case is considered in the true position. If you hold the dia to the low you must subtract tolorance if you hold to the high you can add tolorance. so if you hold to .001 over nominal the you .010 bcomes .011.
            the formula is 2*(sqrrt of ("x(sqr)"+"y(sqr)")

            That helps insure the assembly will go together.
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            • #7
              I would contact the designer/draftsmen if there is questions. Sounds like they have over toleranced...but maybe not.

              It's been a while for me but I took course in geometric tolerancing and found it is very useful and also very misunderstood and abused. There used to be a tendency for designers to add geo tol since they made drawing look "pretty" but they had no clue what the true tolerance meant. They would try to relate it to common limit tolerances.

              You could interpret all the tolerances and hold them and charge accordingly. As long as you stay within the parameters it's their bill. But you might be holding Nasa tolerances for a part that will prop up an engineers bookcase. We used to get wild tolerances for perpendiculars, flatness, surface finish etc. But since most was designed and drawn in plant we usually knew what had to be nitty gritty and what had to be shiny to mesmerize the engineers.

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              • #8
                How many do you want??? ...Bob
                Bob Wright

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                • #9
                  Thank you all for your suggestions. I have been trying to contact the drafter with little luck.

                  Ok back to googling some of your answers and trying to put my thoughts together!

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