On 1 Nov 2020 at 17:51:45 GMT, "
wrote:

On 01/11/2020 17:04, Roger Hayter wrote:
On 1 Nov 2020 at 15:04:53 GMT, "John Rumm"
wrote:

On 01/11/2020 14:37, Roger Hayter wrote:
I have a (tiled) block cavity wall with a 100mm dia. hole in it, for a
fan.
I
want a concentric 150mm hole (for a bigger fan duct). It seems to me a
well-fitting cylinder (?mandrel) with an axial pilot hole would help, to
guide
a diamond core dril, especially for cutting the porcelain tiles neatly.
Is
such a thing commercially available? The two problems about making
one are
that it would be hard work to make it fit tightly and it would be an
expensive
chunk of wood. Also my carpentry is usually crooked.

Well you could take a 12" square bit of 1/2" plywood, offer your 150 mm
core bit up to it, and draw round. Now cut out the circle with a jigsaw.

Stick some wide blue masking tape to the rear of the ply, all around the
outer square perimeter. Stick a matching square of tape to the wall
centred on your existing hole. Apply some superglue to the tape on the
ply. Spray some activator onto the tape on the wall. Now offer the two
together and press and hold for 15 secs. You should now have a ring of
ply stuck to the wall.

Use that you guide the core drill (no pilot bit required). once you are
5mm into the surface, you can remove the guide by inserting a wide flat
bladed scraper behind the ply and prising it off the wall. (the masking
tape bond is very strong in shear, but not that strong in tension)

That's a very useful suggestion. I have actually done that with smaller
hole
saws, and it works fairly well although the wood is rapidly eroded by the
diamonds on the periphery. My concern would be that the hole needs to
slope
down. While it doesn't have to parallel the old hole, it does mean there
are
lots of perturbing forces on the hole saw, especially when starting to cut
the
porcelain surface of the wall. Do you think it would be easy to hold a 6"
hole saw steady?

A female guide should work but you could help it by stitch drilling with
(say) a long 10mm SDS bit at the desired angle. It's common to fit a
cover plate where a duct enters a wall, so that will hide any
unpleasantness.
Don't forget to use an oversize core drill, I failed to do that recently
and found that the bumps on the spiral ducting wouldn't go through the
hole - much cursing and SDS chiselling eventually solved the problem.

It's fan with integral duct, but I'll take the hint and make sure the saw's
definitely a few mm bigger. Not least because it's a rigid duct and the hole
may not be 100% straight.

--
Roger Hayter