On Tue, 17 Apr 2007 18:20:03 +1000, Duracell Bunny
mused:

Lurch wrote:
On Mon, 16 Apr 2007 13:18:48 +1000, Duracell Bunny
mused:

Ben Blaukopf wrote:
Lots of questions tonight...

The old cooker hood in my kitchen has a 4" hole at the right height, and
about as far left as I can possibly have the new hole if I'm having a
chimney hood.

Modern cooker hoods appears to want 5", or more usually, 6" holes.

Is it possible, using a core drill, to drill over the the top of an
existing hole? Or is it going to wander so far that I end up with a
complete mess? Obviously I'd need to offset the centre at the same time,
so that the left edge of the hole didn't move any further left.

Never used a core drill, no idea if this is going to work or not. The
alternative is to find a hood that takes a 5" hole and use the 5"-4"
reducer I have hanging around in the garage.

Ben
Yes, easily done with a core drill & pilot. I usually do it by hot melt glueing
a sheet of plywood over the existing hole, then use the core drill on that. The
thickness of the ply is more than adequate to stabilise the core drill once the
core is out of the ply.

Use a thickness of ply to suit the job, access & your strength. In a door, you
can get away with 3 ply, in a ceiling you need much thicker.

Are you on about a normal holesaw for plasterboard and the like and
not a dry diamond core? I have never seen a dry diamond core drill
used in a door or ceiling before.

Doesn't matter, the principle is the same ... you use a new surface to guide the
core drill.

Yes, but you would really want a decent thinckness guide for a core
drill though. Depends on what your wall is made from and how well the
core drill will stay in it with minimal guidance.
--
Regards,
Stuart.