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.


--
Roger Hayter

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.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 01/11/2020 15:04, John Rumm wrote:
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)

Did you used to write manuals?

Bill

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?

--
Roger Hayter

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.

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.

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.

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.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 01/11/2020 16:46, williamwright wrote:
On 01/11/2020 15:04, John Rumm wrote:
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)

Did you used to write manuals?

I have done so on occasion... and then a proper technical author would
usually come along, rip them to bits and put them back together properly :-)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

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

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

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?

--
Roger Hayter

On 02/11/2020 00:49, Roger Hayter wrote:
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??

Not if you take it carefully - you will be able to feel when you have
enough depth to wander off the perpendicular, and once you do it will
cut to the new angle. Note that this will move the hole very slightly
toward the direction you are now pointing.

One tends to need to do this when cutting cores for a boiler flue, where
there is normally a requirement to slope it back toward the boiler by a
few degrees (so that any condensate runs back into the boiler).

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.

I find you can follow a circular path with a diamond disc, so long as
you are not cutting that deep - it will just carve a slot a bit wider
than the kerf of the disc. If you hold a vacuum hose close to the back
of the contact point with the tiles, you should be able to catch the
bulk of the dust.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

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.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/