DIYbanter - View Single Post - Using Carbide Concrete Drill Bit on Metal

Smitty Two

In article ,
(Chris Lewis) wrote:

According to Toller :

Has anyone else used these bits on steel?

I have heard it doesn't work, but your experience obviously shows it does.

From your description you were drilling the steel dry? That isn't going to
work.

Carbide drills once they're used, have fairly rounded tips - they "cut"
largely by bashing their way thru masonry. Which is why hammer
drills make masonry drilling so much easier, but does almost nothing
for wood or metal.

Mild steel drilling is with sharp-edged tools - ordinary twist
drills with proper edges and relief angles. In a drill press with
a good bit, you see these nice continuous swirls of metal swarf
coming off the drill bit. A carbide tripped drill ain't sharp.

_However_, particularly with hardened steel (which can't really be "cut"
at all with edged tools), you can cut with a fairly dull bit - by
melting your way through. Problem being that the drill bit tip has to
stay hard while the stuff you're drilling gets soft. With an ordinary
drill bit, the tip just deforms and you don't get anywhere, and you
feel like you're trying to push cooked spaghetti thru a cinder block.

Carbide has a considerably higher melting/softening point than steel.
So, in some cases a carbide drill bit would work fine. If you use
a lubricant or coolant, this won't work at all, because the work
simply won't get hot enough.

There are special drill bits specifically designed to get hot and melt
their way through hardened steel. Had a shop instructor demonstrate one
once - piece of hardened steel could not be scratched by a file or a
normal cutting tool. With the special bit in a hand drill, he got
through it in seconds. Threw droplets of molten steel, sparks, and the
hole glowed for a few minutes... Eye protection _definately_ advised.

I'm going to disagree with you here, to some extent. "A carbide tipped
drill ain't sharp?" Um, no correlation there. A drill is either sharp or
it's not, has nothing to do with whether it's carbide. And carbide
cutters are certainly appropriate for cutting steel, particularly
hardened steel, where "high speed steel" drills certainly won't make a
dent. Masonry bits? No comment.

Drilling is like all machining -- "Speeds and Feeds" are crucial. It's
easy to try to drill steel at too high of a speed, and too low of a
feed. Then the tool gets dull, and hot, and what's worse is the steel
gets hot and "work hardened." Turns mild steel into hardened steel, and
then for damn sure you're going to need carbide to cut it.

I've never heard of a "special drill bit designed to get hot and melt
through steel." Citation, other than your shop teacher? Link?