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Thread: Which drill bit for a 3/8"x16 tap?

  1. #1
    Join Date
    Oct 2009
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    Louisville, KY
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    Default Which drill bit for a 3/8"x16 tap?

    I'm drilling cast iron. This web page says I need a .3320" (size Q) drill. However, my Machinerys Handbook says a 5/16" (.3125) hole is called for (assuming my threads are American Standard Form...) My tap is an H3 tap if it matters.

    What's a newb to do?

    (curiously, MH says a Q is appropriate for 3x8"x24 TPI threads.)

  2. #2
    Join Date
    Nov 2002
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    Maine
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    Default

    I'd use the 5/16" drill and not worry about it.

  3. #3
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    Dec 2007
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    Kent Bridge, Ontario
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    Default

    akk the charts I have seen say 5/16 thats what I use.

  4. #4
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    Jul 2007
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    West Michigan
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    Default

    Yup, 5/16

    Brian
    OPEN EYES, OPEN EARS, OPEN MIND

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  5. #5
    Join Date
    May 2002
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    Beaumont, TX
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    Default

    My tap drill calculator gives a value of 0.3141" diameter for a 3/8-16, 75% thread. This is close to the 5/16" (0.3125") drill size and 5/16" is probably as close as you are going to get.

    The Q (0.332") drill would give you a thread that is only about 52% of the full thread form. This is a bit too small for my liking. For one thing, your actual hole would probably be several thousanths larger than the drill's size so the 52% would be reduced even further.

    CI is relatively easy to tap. I would go with the 5/16" size.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  6. #6
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    Sep 2003
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    Louisville, KY
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    Default

    Going from memory, I use a letter O, .316", for 3/8-16 threads.
    Harry

  7. #7
    Join Date
    Jan 2003
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    2,879

    Default

    My favorite tap drill chart lists drill sizes from .307" to .325" for a 3/8--16 thread. The length of thread engagement is the determining factor in drill size.

    The longer the thread engagement the larger the drill.

  8. #8
    Join Date
    Nov 1999
    Location
    SE OZ
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    2,068

    Default 5/16

    Use 8mm x 1/25.4 = 0.315" ~ 0.3125" = 5/16"

    I keep both inch drills in 1/64" (0.0156") increments and metric drills in 0.1mm (0.004") increments for just this reason.


  9. #9
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    Aug 2009
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    Canada, Bc
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    Default

    I use an awsome thread chart that lists 50% and 75% drill size, in both fractional/number/letter/whatever is closest form, AND in decimal form, So I can actualy use my calipers to pick out a 'close enough' bit, And see just how close a perticular bit is.

    Usally, In aluminum you want 75%, And steel/etc 50%, though I usally aim for somewhere around 60%~70% in steel.. because It makes me feel like im cheating and getting a better thread, but not stressing the tap too much with a 75% depth.

    http://www.shender4.com/thread_chart.htm

  10. #10
    Join Date
    May 2002
    Location
    NL
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    Default

    Write This on the wall in ur shop for quick reference

    TD = D - 1/N

    and get a chart off the net...just a tip with stainless go 60 to as low as 50%

    e

    PS The book that Oldtriffie has a pic of is excellent for the price!!
    Last edited by motorworks; 12-31-2010 at 07:57 PM.
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