The back slopes of the 'conical' drill bit should in fact fall away from the cutting edge (this is the 'clearance angle'); if this angle is not enough (or worse, is zero!), instead of just the cutting edge biting into the metal, the whole conical tip of the drill comes into contact with the work piece; so instead of cuttting, it rubs, the friction causes heat, and the drill will burn (really more like melting!)

If you need any more explanation, please feel free to e-mail me privately, and I can send you a sketch...

We say "bottoms" because the drill is rubbing against the bottom of the hole it is making

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Note added at 2 hrs 24 mins (2004-12-17 11:16:08 GMT)
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Regarding your added, note, I think you\'ve NEARLY understood :-)

The clearance angle is NOT the \'point angle\' --- i.e. the angle that makes it conical; that is not what is involved here.

The clearance angle is the slope backwards behind the cutting edge (on ordinary twist drills, there are usually 2 cutting edges) --- although the tip LOOKS as if it is conical, if you examine it more closely, you will see that the metal behind the cutting edge actually slopes down away from it; it\'s like a razor: after the cutting edge has cut into the metal, there needs to be a small space [= clearance] for the cut-off metal shavings to move away (and then up the flutes...)

It\'s this angle that makes it tricky if you have to re-sharpen drills by hand!