When cutting new threads in a blank hole, the hole size must be appropriate for the tap. This is size is called the Tap Drill Size. The tap cuts and removes a certain amount of metal, leaving the internal thread. If the hole is too small, the tap will have difficulty removing material, and a great deal of force in turning the tap will be required. The tap will bind in the hole and begin to gall and pull up material, leaving a poor internal thread. If the hole is too large, the tap will cut with little problem, but the internal thread will not be the correct size for the bolt/screw, and failure during tightening or use is very likely.
Manuals on machining contain tables and charts to determine the correct tap drill size. If no chart is available, a simple formula for both SAE (fractional) and metric threads can be used.
Tap Drill Size = Nominal size of tap - pitch
As an example, a hole is required for a 1/4″ x 20 threads per inch. There is a simple conversion for thread per inch into pitch. The pitch (distance from crest to crest) is the inverse of the threads per inch. For 20 threads per inch, 1/20 is equal to 0.05-inch pitch. The correct size hole in this example would be
0.25 -.05 = 0.20-inch, or a 13/64-inch drill bit.
Notice that the coarser the thread, greater the difference between tap drill size and tap size. Finer threads would have less of a difference between the tap and hole size. The thread of 1/4-inch x 28 tpi would require 0.214-inch tap drill size. In another example, the tap drill size for a 9/16-inch x 20 tpi thread would be: 0.5625 - 0.05 = 0.5125 inch. In a fractional size, this is nominally 33/64-inch.
Using a metric example, a hole is required for a thread of 5mm x 0.8mm. The correct size hole would be 5mm - 0.8mm = 4.2mm, which is approximately 5/32-inch.