Taps are used to cut threads for bolts or screws into wood, plastic and metal. A hole of a specific size is needed for the tap. An improper hole size, say too small, would not allow the tap to fit. On the other hand, too large of a hole would not allow the tapped threads to hold the bolt in place causing a bolt to strip out the threads when tightened. You can use an on-line resource, listed below, for calculating the drill bit needed for a particular sized tap. However, there may be times when access to a handy chart or the internet is not feasible. By using a basic formula, you can find the correct drill bit for any machine thread tap.
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 "per cent" of full thread desired is your choice, but a good number is 70 per cent to 85 per cent 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 per cent 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.
The metric formula is generally the same, it reads as: Dh = Dbm -- (percentage of full thread desired/ 76.98). Note the thread pitch is a constant and there is no deduction constant as in the inch formula.