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  • unnessecary callouts?

    Discussion in 'The main mechanical design forum' started by topher5150, Jan 30, 2017.

    1. topher5150

      topher5150 Active Member

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      I've been going through a lot of these old prints, and a lot of them have depths called out for drilling and tapping through sheet metal. For example, I just found this one today, it calls for .15 dia drilled .59" down, and tapped 10-24 .38" down. Wouldn't a callout for "drill and tap 10-24 be much more useful?
      Keep in mind that these parts are 10-14 ga steel, no where near the drilling, and tapping depths called out?
       
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    3. Dana

      Dana Well-Known Member

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      Sounds like somebody mistakenly used a callout appropriate for a thicker piece, or perhaps an earlier revision of the part used thicker material. Actually a "drill and tap" callout isn't correct either; you should simply specify the desired tap size and depth (or "thru"). The machinist should know what size to drill for any standard tap size. The only time the drill should be specified is when you want a non standard drill size, or need to control the drill depth.
       
    4. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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      I'm guessing that it's a case of an inexperienced CAD user using a hole generator in the software, without understanding all the parameters. They specified a 'deep enough' hole, it looks good in the CAD, and they just leave the call-out to generate automatically.

      As far as Dana's point, I've had this argument (at length) in the past, but I believe that the tap drill should be on the drawing. It is functionally important, because the drill you use determines the thread percentage of your hole. Yes, a competent machinist will choose the 'right' drill that will give about 75% thread engagement, which works in 95% of all cases, but that is relying on the machinist to make a design decision, not just a 'how is it made' decision, which is poor practice.
      I consider the drill depth to be less important, in most cases, but once you're specifying your drill diameter, you may as well put it on.
       
    5. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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      I put in some time to review the standards, and I stand partially corrected.

      ASME Y14.6 specifies that you should specify the depth of a blind threaded hole.

      Further, you have a choice. You can either specify a tap drill diameter, and so long as your tolerance range is within the overall tolerance range specified in B1.1, it considered a standard hole. OR you can explicitly state in your general notes that your holes conform to B1.1 in all aspects, in which case your drill hole tolerance is set roughly between 65% and 85% thread engagement, as per the tolerances given in the table there.
       
      Last edited: Jan 31, 2017
    6. sasham

      sasham New Member

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      Hi, I think, 10-24 it is tapped thread 0.19"-24 with depth of 0.38". You can find the deffinition in I-net.
       

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