Incresing Hole Size

[Brian1960]

Hi all, currently have a 100mm hole in thermalite block, what's the best way to increase this to 150mm (for a cooker hood)?
1. I've read to stich drill and chip away
2. Cut away with Pad Saw
I thought of cutting square, insert round pipe and make good in the 4 corners.
Any advice appreciated please.
Thanks in advance.

 

[mikeey84]

When I had to cut a hole in thermalites I used a jigsaw with a metal cutting blade. Ruins the blade, but I used an old one.

They are so soft you could use a bread knife

 

[robinbanks]

Personally I'd get a 152mm core drill out, use it to drill a 152mm hole in something else (that is portable, like a scrap of thin ply) then hold/screw that thing to the wall so it acts as a guide for the core drill to start without wandering all over

It's also possible to start them with the drill at an angle so the bit is only dragging on the wall in one place, then gradually turn the drill as you go so it's straight but it's a bit of a fight with a large bit

..but then again you might not have such a core drill to hand. As noted , the blocks are so soft they can be cut with many things; an old blunt wood saw would work- stick it into the hole and use it to cut several lines around the edge of the hole (so it ends up like a kid would draw a shining sun) out to the radius of the material you want to remove, then use a chisel to break the bits off

 

[RandomGrinch]

currently have a 100mm hole in thermalite block, what's the best way to increase this to 150mm (for a cooker hood)?

Depending on the duct run length, 100mm may still be adequate for a cooker hood - a suitably located reducer could be suitable?

Manrose PVC White Diameter Reducer - 150mm to 125mm to 100mm | Wickes.co.uk

Manrose PVC White Diameter Reducer - 150mm to 125mm to 100mm

www.wickes.co.uk

 

[Dereekoo]

As you have only 25mm to take off either side I would use the smallest drill you can get away with, more holes but less chipping and neater edge

 

[jeds]

For a one-off, a jab saw would do it in thermalite. Mark the hole and work your way around.

 

[johnny2007]

Personally I'd get a 152mm core drill out, use it to drill a 152mm hole in something else (that is portable, like a scrap of thin ply) then hold/screw that thing to the wall so it acts as a guide for the core drill to start without wandering all over

It's also possible to start them with the drill at an angle so the bit is only dragging on the wall in one place, then gradually turn the drill as you go so it's straight but it's a bit of a fight with a large bit

..but then again you might not have such a core drill to hand. As noted , the blocks are so soft they can be cut with many things; an old blunt wood saw would work- stick it into the hole and use it to cut several lines around the edge of the hole (so it ends up like a kid would draw a shining sun) out to the radius of the material you want to remove, then use a chisel to break the bits off

This is what I would do and have done many times.
But if you don't have the core kit, use a drill bit.
Thermalite is very easy to drill and cut, so if you stitch it all the way around, you can then use the same masonry bit to cut between the holes you've drilled.
Once I saw a guy using a round file in a drill to do the same job after having stitch drilled the hole.
To my amazement it worked perfectly

 

[Brian1960]

For a one-off, a jab saw would do it in thermalite. Mark the hole and work your way around.

Thanks for that, that was the option I took, worked a treat.
Thanks to all replies I received, much appreciated.