Tapping Drill Bit Size

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
 
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
 
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"
 
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"
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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.
 
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.
Click to expand...

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.
 
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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