"N. Thornton" wrote:
Various contributors said:

What can happen with a quality drill, is that the tip changes
colour and the cutting edge gets rounded. The result is a hot
drill. The cause is too fast rotation or lack of push to keep
the drill drilling. Or both.

It doesn't help when the bit is used on another job without
being re-sharpened (and possibly re-hardened). Once it's got
too hot it's useless without re-hardening/sharpening, both
of which it is *possible* to do at home.


The result is that one has to go slow to avoid blunting the
thing while drilling just ONE hole. Yes I've done it. Even
on wood I've had bits that have got smoking hot and lost
their cutting ability.

This *could* be due to the flutes becoming choked. If so,
lifting the bit partly out of the hole will fix the problem.
This also goes for drilling holes in masonry - keep pulling
the bit back every so often to prevent dust causing the bit
to bind in the hole (and possibly shear off there).


Wrong drill angles, that's all that was wrong. Drill angle
depends on the material to be drilled. Change the material
and you change the drill angles to those that work best.

I suspect thats the key to all this. Twist drills are generally
multipurpose, which means their angles are wrong for almost
everything. For wood theyre terrible.

They're fine for drilling steel, timber, plastic - *if they're
sharp*. It takes very little to blunt a drill, and then problems
multiply fast.


Along comes my a with a steeper angle, its better matched and
sails through. But presumably it would not do well on harder
materials like stone, concrete and iron.

The problem with HSS and masonry is that the bit will blunt
very quickly indeed if you touch (say) the bit on concrete,
render, brick or whatever. Just one hole drilled with a HSS
bit into masonry will shag it. It won't be just blunt, the
flutes will wear too, and the whole thing tend towards a
taper shape. That's why for TCT bits the diameter of the
hole is distated by the TC piece brazed into the end of
the drill. The flutes haven't got any guiding job to do,
they just clear masonry fragments from the hole. N.B. you
should still sharpen TCT bits every so often, it makes a
great difference to speedy results. Do *NOT* quench hot
TCT bits after grinding, let them cool naturally or even
wrapped in cloth (N.B. rags!).


J.B.