On 02/11/2020 00:53, Roger Hayter wrote:
On 1 Nov 2020 at 17:07:55 GMT, "GB" 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.




You could just fill the hole with some (old) plaster? Or even sand and
cement.

That's good idea. I'll have to block the cavity though. Wooden disks against
the insulation, perhaps?

Another trick that can work with some hole saws, is you can stick two of
them on the arbour at once. The smaller diameter inside of the larger.
Then the smaller one acts as a pilot in the existing hole. It will
depend on the type of core bit and arbour as to whether enough thread
will poke through into the larger core to mount the second one.


--
Cheers,

John.

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