"Alex" wrote in message ...

Sorry if I'm stating the obvious, but you do withdraw the bit regularly to
let it cool don't you? I find when drilling old concrete posts etc (70+ yrs)
that the bit will get red hot on the tip and melt if it's not withdrawn
often. It's almost like the concrete melts in there too, no idea what's
actually going on though!

I can remember a job I was on where that was happening. The foreman
was very angry with us, suspecting theft but even he was only getting
one or (occasionally) two holes drilled. We were drilling through
concrete just deep enough for brown plugs. It was a mystery that never
got solved.

I suspect there was steel in it and we were hitting that each time.