On 1 Nov 2020 at 18:03:10 GMT, "John Rumm"
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.

Start the hole dead square - once you are in quarter of an inch (or
through most of the tile) you can rotate the angle a little so you start
cutting the required angle, but still have the rim of the hole cut so
far engaged in the already cut grove.

That's a great suggestion. I've sort of done this in the past, but worried
that I'd break the saw when changing direction. I shouldn't worry??




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?

I might be tempted to take an angle grinder to the porcelain to at least
notch the surface since porcelain tiles can be ridiculously hard.

Yes. I think I can afford to cut a square in the tiles outside the circle, but
experience laying the tiles suggests I'll need an assistant with a water
hose.

--
Roger Hayter