# Tapping Drill Bit Size



## planeflyer21 (Mar 14, 2016)

A couple of quick hacks on selecting the correct drill bit for tapping:


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## T Bredehoft (Mar 14, 2016)

Diameter minus pitch (4mm -.7) may equal tap drill size in Blighty, but 3/8 -16 won't work in Imperial threads. Glad to see discussion of drills and taps.


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## Steve Shannon (Mar 14, 2016)

3/8 - 16 would not work, but 16 is the number of teeth per inch, not the pitch.  The pitch is actually 1/16 of an inch, or 0.0625"


 Steve Shannon


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## RJSakowski (Mar 14, 2016)

T Bredehoft said:


> Diameter minus pitch (4mm -.7) may equal tap drill size in Blighty, but 3/8 -16 won't work in Imperial threads. Glad to see discussion of drills and taps.


It will if you convert tpi to pitch.  16 tpi =1/16",  3/8" -1/16" = 5/16".  This is the recommended tap drill size.


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## T Bredehoft (Mar 14, 2016)

Shucks.... rubs toe in dust.


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## ch2co (Mar 14, 2016)

Am I missing something here, or just dense.  I always look up at my Starrett drill/tap chart and use that or about that size drill.
Never failed me yet.
CHuck the grumpy old guy.


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## ch2co (Mar 14, 2016)

I guess I should have looked at the video first.  I just saw it as a picture and blabbed on.
CHuck the same old grumpy guy.


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## Wreck™Wreck (Mar 14, 2016)

A 60° V-thread is an equilateral triangle, the relationship between the height and the base is .866 therefore the depth of the sharp V of a 16 TPI thread would be .0625 X .866 =.054, in the case of a 3/8-16 thread this will be a minor diameter of .321,Threads in the ANSI standards are measured by the Pitch Diameter for all practical purposes.


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## Bob Korves (Mar 14, 2016)

You are not required to use the tap drill listed in the chart.  I often use a (edit)larger drill, particularly with hard materials or with  joints that will be low stress.  It makes it much easier to tap, and stresses the taps less.  If I need the strength, I use the listed drill, and occasionally even one slightly (edit)smaller.  There are tap drill calculators on the web that give you a drill size after you enter the percentage of full thread engagement that you want.  I also have multiple tap drill charts which give different drill choices for certain tap sizes.  It's a moving target...


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## RJSakowski (Mar 14, 2016)

Wreck™Wreck said:


> A 60° V-thread is an equilateral triangle, the relationship between the height and the base is .866 therefore the depth of the sharp V of a 16 TPI thread would be .0625 X .866 =.054, in the case of a 3/8-16 thread this will be a minor diameter of .321,Threads in the ANSI standards are measured by the Pitch Diameter for all practical purposes.


Wreck, You are forgetting that the major diameter of the thread is not at the apex of the equilateral triangle. The triangle is truncated with a flat  equal to P/8 in width, P being the thread pitch.  This defines the major diameter.  The base of the triangle is truncated with a flat P/4.   Which defines the minor diameter.   

From the diagram below, the minor diameter is less the major diameter by 2 *(5/8)*H where H is the altitude of the equilateral triangle with sides equal to the pitch, P = sqrt(3)/2 =.866*P.   So the major diameter -minor diameter = 2  * 5/8 * .866P =1.082P.  Since internal threads are usually cut with 75% thread engagement, the approximation of the tap drill diameter  equal to the major diameter less the pitch is good.  

For a 3/8 -16 thread, the minor diameter will be .307".  The tap drill would be .3125. 

.


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## BGHansen (Mar 15, 2016)

I was taught a quick and dirty for tap drill size was the major diameter - 1/tpi.  For a 3/8 - 16, it's 3/8"- 1/16 or 5/16".  1/4" - 20 would be .250" - 1/20  ->  .250" - .05"= 0.200".  A #7 drill bit is 0.201", #8 is 0.199".  Or as Chuck mentions above, use a readily available tap/drill  chart and use Bob's tip and go up or down a size depending on class and/or material being threaded.

Bruce


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## Dan_S (Mar 15, 2016)

people might find the calculators on this page helpful.

http://theoreticalmachinist.com/TapDrillSizeCalculators

thread engagement is usually between 50% & 75%, but I've seen as high as 90%


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## Dan_S (Mar 15, 2016)

I found my old link, Guhring has what i think might be the best calculator.

http://www.guhring.com/tech/TapDrill/


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## Woodemp (Mar 20, 2016)

TDS = D - 1/N.  For 70% engagement.

D = Tap Dia. in inch's
N = Number of threads per inch.

For  example for a 1" dia. tap NC. The number of threads per inch is 8. So 1" minus 1/8 equals 7/8" which is the TDS.
Works every time for Dia's  from 1/4" to 2". for 60 deg threads.

Good Luck


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## Bill Gruby (Mar 20, 2016)

Did everyone miss what was wrong in the video? It may be OK for the poster but storing cutting tools, taps are cutting tools, together where cutting edges can touch is not a good idea. Sharp edges hitting each other can break each other. I know a lot do it but that does not make it the correct way. All of my taps and dies are separated. This  is my personal opinion which has been formed over many years of practice.

 "Billy G"


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## fretsman (Mar 20, 2016)

Bill Gruby said:


> Did everyone miss what was wrong in the video? It may be OK for the poster but storing cutting tools, taps are cutting tools, together where cutting edges can touch is not a good idea. Sharp edges hitting each other can break each other. I know a lot do it but that does not make it the correct way. All of my taps and dies are separated. This  is my personal opinion which has been formed over many years of practice.
> 
> "Billy G"



agreed Billy, I learned this early on from the more experienced folks in the shop as I was growing up. 

Same as putting drills in an index cutting edge down.....yes, they cut metal, but do they have to be constantly rubbing against it? Not for me.


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## Wreck™Wreck (Mar 20, 2016)

RJSakowski said:


> Wreck, You are forgetting that the major diameter of the thread is not at the apex of the equilateral triangle. The triangle is truncated with a flat  equal to P/8 in width, P being the thread pitch.  This defines the major diameter.  The base of the triangle is truncated with a flat P/4.   Which defines the minor diameter.



No, I haven't forgotten the thread truncation at root and crest, was merely pointing out the relationship between the base and the height of an equalateral triangle, you will notice that I wrote "sharp V", no threads are made this way. The Minor Diameter is described within a range of minimum to maximum.
In this case Min. Minor Diameter .307, Max. Minor Diameter .321. ANSI Unified Thread Standard.


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## benmychree (Mar 21, 2016)

The whole matter is crazy, just look at the wall chart and do it, never had a problem doing that in my 50 some odd years of service; sure do like that big black dog, 'tho!


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