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NW Washington | Just off the top of my head, I think a No. 7 bit is recommended for 1/4 - 20 threads (NC). Better check that before tapping a hole.
When tapping threads using a good cutting fluid like Tap Magic will save a lot of heartburn. The tap guide that Ed posted is a help too, as is using the right taps, as there are different types. Good new taps are much easier to use than those found in twenty year box. And back up about every 1/2 turn and break the thread cuttings.
If you have a through hole you can burn out a tap with a cutting torch, if you are careful and have a steady hand. Get the tap red hot and just touch the oxygen lever and the tap will disappear. You may need a little clean up and if you flinched you may need to go to the next larger thread.
Edited by JohnW 1/16/2010 13:28
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Broken drill tap
