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)


--
Cheers,

John.

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