I need to make a 2' 6" x 1' hole in a lightly reinforced 8" deep concrete
floor. There is already a 12" diameter hole in the middle but it ain't big
enough. I was thinking of using a 4 1/2" diamond core drill that I already
have to cut out the corners, a 9" angle grinder to join the corners up and
then hammer in steel wedges to try to crack the concrete out cleanly.

If I can get beneath any rebars with the angle grinder I reckon that I've got
a chance to do it, however I've got one concern...

The diamond core drills that I have are general purpose, light weight dry
cutting ones. To cut this sort of depth in concrete and through any rebars
that I find, I suspect that I would do better with vacuum or water cooling and
dust removal for the drill. Vacuum would be difficult and the core drill and
arbour aren't really designed for water injection.

I can make a shroud to go over the top of the drill, drill some holes in its
top face and introduce water that way, hoping that the zero-pressure water
will find its way down the inside face of the drill and up the outside face.

I could drill a small hole across the path of the big drill with a 1 1/2" core
drill pecking to remove the dust and cool the drill. I could then get a
sufficient supply of water to the big drill to stop it from overheating or
clogging.

In either case I plan to use an old (late 40's) bench drill anchored to the
floor, with its head back-to-front to avoid the base for driving the drill.
This should keep the drill straight and help to avoid the muscle spasms that
would result from trying to do 24" of hole with a hand held drill motor

Anyone got better ideas, other than hiring it out?


Mark Rand
RTFM