Enlarging a hole in a wall.

[Roger Hayter]

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.

[John Rumm]

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)

[williamwright]

Did you used to write manuals?

Bill

[Roger Hayter]

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?

[GB]

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

[nothanks]

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.

[John Rumm]

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.

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.

[John Rumm]

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 :-)

[Roger Hayter]

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??

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]

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]

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

[John Rumm]

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 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.

[John Rumm]

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.