- Measure the overall diameter of the tap with the micrometer. In this particular instance, we want a drill bit for a ¼ - 20 tap. The ¼ is the diameter and the 20 is the thread pitch or threads per inch. The overall measurement of the taps diameter from the micrometer is 0.252 inches.
Use the following formula for finding the correct drill bit size: Dh = Dbm -- 0.0130 (percentage of the full thread desired/ Ni). Dh is equal to the drilled hole size in inches. Dbm is the basic major diameter of the thread, in our case 0.252 inches. The "percent" of full thread desired is your choice, but a good number is 70 percent to 85 percent depending on the material being used. "Ni" is the number of threads per inch, again, in our case, this is 20.
Apply the numbers to the formula using an 84 percent thread depth. The formula would be:
Dh = 0.252 -- 0.0130 (84/20) = 0.0130 (4.2) = 0.252 -- 0.0546 = 0.1974 inches.
Find the nearest drill bit size in 1/16ths of an inch. Multiply 0.1974 times 16 and the answer is 3.15. This corresponds to a 3/16-inch drill bit; however, this is too small.
Determine the drill bit size in 32nds of an inch. The answer is 6.31, so a 7/32-inch drill bit can be used for this tap. In fact, if you look to a chart, a 7/32-drill bit is recommended.
tap and CRY
well this tap and die stuff is much more complicated than i imagined. i went to the hardware store yesterday to get another bit for a different sized hole to drill and they said you want to use a small drill bit for whatever size tap you are making. i realized that i had just guess with the last one. i just grabbed a bit and drilled and it was okay but in hindsight it was very hard to turn the tap bit to make the threads and i always felt like i was going to just snap it in half. so i googled it and this is what i found. DOES ANYONE HONESTLY THINK I UNDERSTAND ANY OF THIS? ps the hardware store people didnt know what size i needed. on the tap bit package it said use size F. they only had sizes in numbers not the alphabet.