Putting a square hole through 30mm concrete

[tommcdonna]

I need to install a cat flap in my conservatory wall, the base of the conservatory is constructed from moulded concrete panels, 30mm thick. A normal hammer drill was largely ineffective drilling holes though it, it took about 10 minutes and 3 bits to get a single hole through it.

I am mounting the flap in a wooden tunnel which will in turn be fixed into place into the hole (approx 20cm square) I make through the concrete so the edges don't have to be perfect (though the neater the better).

The two ways I can think of to do this are:

1.) Use a 9" angle grinder with a diamond disc to cut the hole. There will have to be a bit of overshoot on each cut though due to the curvature of the cutting disc.

2.) Use an SDS drill to perforate the perimeter then attack with a chisel bit.

Any thoughts on which method will work best? Or maybe there's another option I've not considered?

 

[theoldun]

Mark out front and back. Feed blade in from outside until it touches top and bottom lines. Pull out, do same to other three lines. Also put three vertical cuts in, one third, centre and three thirds so that cut out is split into three. Go inside feed blade into all outside cuts and push in until you hit top and bottom lines. Go outside and rattle the lot in. Clean up corners with sharp chisel and lump hammer.
old un.

 

[tommcdonna]

I should probably clarify a few points too...

The concrete appears similar to a concrete paving slab, it's very dense and is NOT like engineering brick it seems much harder.

Also, I don't current have either of the tools I mentioned in my first post (9" grinder or SDS drill), whichever method I use I will need to borrow or buy the tools. The SDS tool I'd be happier buying as it's likely to get re-used unlike the 9" angle grinder.

I do have a 6" grinder (only capable of cutting to 22mm) and a normal 240v hammer drill.

Finally, I can only attack the exterior with the grinder as there's not room between the plasterboard and the concrete to get the angle grinder to the edges of the hole. I can drill from the inside as long as the bits are long enough.

 

[Deluks]

A small grinder should still get you some of the way in, enough to weaken it for chiselling.
If there's any rebar in there, this may have been what damaged your drill bits.

 

[theoldun]

Tom. Did not know you did not have the tools. Me crystal ball is playing me up, bit like me back.
You can hire decent SDS for about £30 a day, 230mm angle grinder, about £22 a day, plus blade and sharpening charge.
If you want to buy something, buy SDS and couple 6mm bits and stitch drill round perimeter, and up middle.
Take it out with sharp chisel and lump. You may hit some re bar that you will have to cut out.
If all else fails put cat on diet and cut 25mm of bottom of door.
Wish yer luck. old un.