Tapping Drill Bit Size

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.
 
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
 
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.
 
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.
 
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.
 
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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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.
Click to expand...
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.

.Thread Geometry.JPG
 
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