[TR] drill bits

[Michael Marr]

Dave Massey wrote:

> Good point.  Let me just add that the second most common mistake is too 
> much
> speed.  Speed makes for friction, friction makes heat and heat will 
> de-temper
> the drill bit and it will quickly become dull.
>
An average cutting speed for a typical cold drawn mild steel is around 70 to 
80 fpm.  For a 1/4 drill bit, this equates to1070 rpm.  For 3/8, it is 713 
and for 1/2, it is 535 rpm.  The harder the steel, the slower you drill - 
thus, for a quenched and tempered mild steel you might reduce these speeds 
by half.

Also, don't forget that when drilling non-ferrous metals it is best to have 
zero or negative rake on the cutting edge of the drill, to avoid the 
possibility of the drill digging in, picking up the workpiece (which you 
didn't bother to secure to the table of the drill press because "it is just 
one hole"), and spinning it wildly while it raps the knuckles of the left 
hand.  I have the scars to prove it.

Mike