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Thread: Question about tap and drill size

  1. #1
    Boolit Master
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    Question about tap and drill size

    Is there a chart somewhere that will tell you if you are using "x" tap use this "y" size drill bit to drill your initial hole with?
    My mother always said I was the Flower of the Family, The Blooming Idiot

  2. #2
    Boolit Master hickstick_10's Avatar
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    hmmm if only there was a drill and tap chart.............

    google it. And notice the charts.
    http://www.google.ca/images?hl=en&q=...w=1241&bih=754

  3. #3
    Boolit Master
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    Quote Originally Posted by hickstick_10 View Post
    hmmm if only there was a drill and tap chart.............

    google it. And notice the charts.
    http://www.google.ca/images?hl=en&q=...w=1241&bih=754
    Have Googled, only prob is the 100 or so that I have looked at do not show 6x48 or 8x40 or some of the more common screws used on guns. What I am doing is installing a scope base on Browning Gold shotgun, semi auto, for a holographic type sight. Since it is an aluminum receiver, I wondered about going with 8 x 40 screws or 8x36 screws in the base for a stronger grip. Gun is only used for turkey hunting and 31/2" shells are the only shells shot in it. I know I can get diameter of screw and go somewhat smaller for the drill, but was just wondering if there was a formula to go by to figure how much smaller the drill bit needed to be.
    My mother always said I was the Flower of the Family, The Blooming Idiot

  4. #4
    Boolit Master

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    6-48 NS #31 drill (.120")

    8-40 NS #28 drill (.1405")

    Jack

  5. #5
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    For tsp drill sizes you can use this equation:

    Tap drill diameter = nominal size - pitch

    For single lead threads the pitch is 1/Threads per inch.

    Example for a 1/4-20 tapped hole:

    Tap drill diameter = .250 - 1/20 = .250 - .050 = .200

    Most charts will tell you to use a drill of about .201 to .203 for this.

    This equation will always work for any 60* thread form.
    I am now in full production of top punches for Lyman/RCBS and Saeco lubers ($8 each including shipping, two business day turnaround), and blank sizing dies for Lyman/RCBS, Saeco, Herters, and Star machines. Other products will be added as time and health permit. PM me for details.

  6. #6
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    Quote Originally Posted by giz189 View Post
    Is there a chart somewhere that will tell you if you are using "x" tap use this "y" size drill bit to drill your initial hole with?
    Yes, it's called a Starrett card.
    Hell, I was there!

  7. #7
    Boolit Master

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    Most of the standard drill charts will not cover special thread pitches such as 6-48 and 8-40. If you get a set of number drills the box will have the drill size to use for standard taps. Same with fraction size drill bits, most metal boxes will have the sizes needed for standard taps
    John Taylor, Taylor Machine, gunsmith

  8. #8
    Boolit Master
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    The perfessor nailed it. I have used this formula for decades. There isn't one in ten machinists that know this information, especially the know-it-alls. Just look at a metric chart..... OD - pitch= tap drill size. Example 14mm x 2mm, drill a 12mm hole. In the inch system just use the reciprocal of the TPI. 3/8-16.... 3/8 minus 1/16 = 5/16 tap drill size, 1"-8 thread..... 1" minus 1/8 = 7/8 tap drill size. Most can be figured in your head, (13 is a little difficult). To look at it another way, any 32 pitch thread, subtract 1/32 from the major diameter of the screw, if a 40 pitch thread just subtract .025 from the major diameter of the screw and so on. Remember this and it will serve you a lifetime.

  9. #9
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    Yep, should be a rifling height standard formula as well. Every gun with a 0.004 height for the lands makes it pure luck that it works in the majority of land configuration styles. Maybe we can come up with a standard formula which includes all these factors, one of which would be projectile hardness. ... felix
    felix

  10. #10
    Skook

    Some men you just cain't reach...

  11. #11
    Boolit Master
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    Nice chart, but common gun screw sizes like 6-48 and 8-40 are not included.

  12. #12
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    jmh54738;
    My father was a tool and die man (Jig and Fixture man) at Wright Aeronautical during WW II after a life time as a machinist. He taught me the reciprocal protocol over sixty years ago. It has stood me in good stead all of these years.

    Dale53

  13. #13
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    Quote Originally Posted by jmh54738 View Post
    The perfessor nailed it. I have used this formula for decades. There isn't one in ten machinists that know this information, especially the know-it-alls. Just look at a metric chart..... OD - pitch= tap drill size. Example 14mm x 2mm, drill a 12mm hole. In the inch system just use the reciprocal of the TPI. 3/8-16.... 3/8 minus 1/16 = 5/16 tap drill size, 1"-8 thread..... 1" minus 1/8 = 7/8 tap drill size. Most can be figured in your head, (13 is a little difficult). To look at it another way, any 32 pitch thread, subtract 1/32 from the major diameter of the screw, if a 40 pitch thread just subtract .025 from the major diameter of the screw and so on. Remember this and it will serve you a lifetime.
    I dont know if one in ten machinists can figure out and do bolt hole circles on a manual machine any more???

  14. #14
    Boolit Master
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    yes there is
    central wire has one
    Hit em'hard
    hit em'often

  15. #15
    Boolit Master
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    Quote Originally Posted by arjacobson View Post
    I dont know if one in ten machinists can figure out and do bolt hole circles on a manual machine any more???
    I agree, for we old farhdts, there are a few magic numbers that just stick in one's head, like .86603 and .70711. The 3 -4- 5 rule is also one that everyone should know. I have to admit that is nice to just push a few buttons on the DRO for a BC and just crank the x & y to zero. Would you believe that until 2005, the machine tool students at Dakota Cty Tech in MN were required to purchase a set of toolmakers buttons. Now that is going back to the stone age. A couple of the grads that I hired had them in their tool boxes. They had no idea as to what they were for. Somewhere on my computer is an old MS-DOS program called "Trig Man". It will solve all gear, thread, and triangle computations. One is old to just remember MS-DOS.

  16. #16
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    ahh .70711 wasn't that the sine of 45deg?? .866 was 30 if I remember right

  17. #17
    Boolit Master
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    Quote Originally Posted by theperfessor View Post
    For tsp drill sizes you can use this equation:

    Tap drill diameter = nominal size - pitch

    For single lead threads the pitch is 1/Threads per inch.

    Example for a 1/4-20 tapped hole:

    Tap drill diameter = .250 - 1/20 = .250 - .050 = .200

    Most charts will tell you to use a drill of about .201 to .203 for this.

    This equation will always work for any 60* thread form.
    That is really what I was looking for, since most all tap drill charts do not list the sizes commonly used in sight mounting, I needed a way to determine what size other than by comparing sizes. Thanks Perfesser and jmh54738
    My mother always said I was the Flower of the Family, The Blooming Idiot

  18. #18
    Boolit Master

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    Laying out holes on a circle is why I bought a rotary table. If I need to know something about angles I get out my 1935 machinist hand book. I didn't go to machinist school, learned on the job and still learning.
    John Taylor, Taylor Machine, gunsmith

  19. #19
    Boolit Master

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    Quote Originally Posted by theperfessor View Post
    For tsp drill sizes you can use this equation:

    Tap drill diameter = nominal size - pitch

    For single lead threads the pitch is 1/Threads per inch.

    Example for a 1/4-20 tapped hole:

    Tap drill diameter = .250 - 1/20 = .250 - .050 = .200

    Most charts will tell you to use a drill of about .201 to .203 for this.

    This equation will always work for any 60* thread form.
    Funny, I have been using almost the same thing for years and never knew there was actually a formula for it. Just figure out how many thousandth inch there is in the thread pitch and subtract it from the diameter of the screw. Like 1/16" for 16 TPI is .062".
    John Taylor, Taylor Machine, gunsmith

  20. #20
    Boolit Master

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    Quote Originally Posted by John Taylor View Post
    Laying out holes on a circle is why I bought a rotary table. If I need to know something about angles I get out my 1935 machinist hand book. I didn't go to machinist school, learned on the job and still learning.
    Depends on how close you need the holes. If they need to be dead nuts a rotary table will not cut it..They will get close..........but..

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