Have you ever had to drill and tap a hole and have no idea what size drill bit to use? Here is the answer: The tap is marked with the size and threads per inch (TPI). Place 1 over the TPI and subtract the resulting fraction from the diameter. The answer is your tap drill size. For example: 3/8 - 16 (3/8 diameter, 16 TPI). 3/8 minus 1/16 = 5/16 1/4 - 20 1/4=.250 minus 1/20 (.050) = .200 closest fractional drill is 13/64 = .2031 5/8 - 11 5/8=.625 minus 1/11 (.0909) = .5341 closest drill 17/32 =.5312 This only works for fractional sizes and not metric or number sizes. I hope this is helpful.
A tap drill chart is a very handy tool to have . I have one on the wall next to my drill press . Drills come in three differant sizes Fractions , numbers , letters , 3/8-16 is a 5/16 drill size (.312 ) 1/4-20 is a # 7 drill size (.201 ) 5/8 -11 is a 17/32 drill size ( .531 ) jay
Actually it does work with metric but also for metric is easier to figure out for a M4.0X0.35 you just subtract the pitch from the diameter and then you have true drill size so it would be 4.0-0.35 your tap drill size would be 3.65mm or it rounds to 3.5mm (remember you are looking at fractions and trying to carry it over and associate it with standard drills. Metric taps are supposed to be drilled with metric drills )
