By Roger W. Thomas
Oversized taps are available in the marketplace and the common size is +.005". They are typically used when sending a part to be heat-treated. Since you normally get shrinkage when heat treating, it is proper to use an oversized tap.
What do you do if you have an odd tap size or just need one hole? There is a simple solution.
What causes a tap to tap oversized? Chip build-up. In some applications you may be tapping a deep hole or you may have some gummy material that will not eject chips properly and you get an oversized tapped hole. So, create the situation intentionally to get the same results.
On a four-flute hand tap, get the tap started and then pack the flutes with strips of wood. Just keep a small piece of wood handy and shave off strips as needed. Pack the flutes, the chips will have nowhere to go and you will get an oversized hole.
Using metric taps and need the tap drill size? People are always calling my business looking for that information. It’s pretty simple: Subtract the pitch from the diameter, and that is the drill size! So, an M6 x 1.0 tap would require a 5 mm drill. An M10 x 1.5 tap, would require an 8.5 mm drill. Check it out, works every time!