WidenersInline FabricationRotoMetals2Reloading Everything
MidSouth Shooters SupplyTitan ReloadingRepackboxLee Precision
Load Data
Page 1 of 2 12 LastLast
Results 1 to 20 of 32

Thread: Question about tap and drill size

  1. #1
    Boolit Master
    Join Date
    Apr 2005
    Location
    Magnolia, Arkansas
    Posts
    553

    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 Buddy hickstick_10's Avatar
    Join Date
    Feb 2010
    Location
    Kelowna British Columbia
    Posts
    363
    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
    Join Date
    Apr 2005
    Location
    Magnolia, Arkansas
    Posts
    553
    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

    Join Date
    Dec 2005
    Location
    Central Massachusetts, U.S.A.
    Posts
    1,283
    6-48 NS #31 drill (.120")

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

    Jack

  5. #5
    Boolit Master

    theperfessor's Avatar
    Join Date
    Dec 2007
    Location
    Evansville Indiana
    Posts
    2,746
    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.

  6. #6
    Boolit Grand Master


    swheeler's Avatar
    Join Date
    Mar 2005
    Location
    Montana
    Posts
    5,471
    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.
    Charter Member #148

  7. #7
    Boolit Master

    Join Date
    Feb 2007
    Location
    Lewiston, Idaho
    Posts
    2,732
    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

  8. #8
    Boolit Buddy
    Join Date
    Nov 2005
    Location
    140 acre farm, west central WI
    Posts
    237
    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
    Boolit Master on Heavens Range
    felix's Avatar
    Join Date
    Mar 2005
    Location
    fort smith ar
    Posts
    9,679
    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
    Boolit Buddy SkookumJeff's Avatar
    Join Date
    Jan 2011
    Location
    Western Washington
    Posts
    141

  11. #11
    Boolit Buddy
    Join Date
    Nov 2005
    Location
    140 acre farm, west central WI
    Posts
    237
    Nice chart, but common gun screw sizes like 6-48 and 8-40 are not included.

  12. #12
    Boolit Grand Master



    Join Date
    Dec 2005
    Location
    Southwestern Ohio
    Posts
    8,456
    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
    Boolit Master
    arjacobson's Avatar
    Join Date
    Oct 2010
    Location
    Mason City Iowa
    Posts
    768
    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 Grand Master
    white eagle's Avatar
    Join Date
    Jul 2010
    Location
    718 miles E. of Wall Drug
    Posts
    6,158
    yes there is
    central wire has one
    Hit em'hard
    hit em'often

  15. #15
    Boolit Buddy
    Join Date
    Nov 2005
    Location
    140 acre farm, west central WI
    Posts
    237
    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
    Boolit Master
    arjacobson's Avatar
    Join Date
    Oct 2010
    Location
    Mason City Iowa
    Posts
    768
    ahh .70711 wasn't that the sine of 45deg?? .866 was 30 if I remember right

  17. #17
    Boolit Master
    Join Date
    Apr 2005
    Location
    Magnolia, Arkansas
    Posts
    553
    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

    Join Date
    Feb 2007
    Location
    Lewiston, Idaho
    Posts
    2,732
    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.

  19. #19
    Boolit Master

    Join Date
    Feb 2007
    Location
    Lewiston, Idaho
    Posts
    2,732
    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".

  20. #20
    Boolit Master
    arjacobson's Avatar
    Join Date
    Oct 2010
    Location
    Mason City Iowa
    Posts
    768
    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..

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check