On Thursday, 17 November 2016 20:08:32 UTC, jim wrote:
"Andrew Mawson" Wrote in
message:
OK chaps - suggestions please.

I need to cut a 12" diameter hole in a workshop wall construction of which
is a single skin of 9" x 18" x 4.5" 7 newton concrete blocks. Now I've all
sorts of drills, angle grinders, hammers and even demolition breakers, so
the problem isn't removing the material, the problem is doing it without the
wall collapsing !!!

Current plan is drilling a circle of (say) 1/2" holes with a masonry drill

sounds good, though I hate doing that myself. It makes a simple job take ages and look a mess.

and wiggling it from both sides to hopefully join them up without disturbing

wiggling a drill bit in a hole is asking for trouble.

the rest of the structure. (I'll probably select a spot where the hole is
fully bridged by an 18" block - position within a few inches doesn't matter.

Any other constructive suggestions very welcome.

I'd be tempted to see if I could cut the perimeter with an angle grinder. Held at the right angle the curves would roughly match. AGs are faster than drills and remove a wide slot, they can easily clean up unevenness, and the result would be far neater. If you cut as far as it'd go from both sides, then sliced the remaining centre what's left would be weak.


(this is for a huge 3900 cuM/Hour extractor for a CNC plasma table down
draught exhaust)

Andrew


Can you position the hole so the uppermost cut block is left
enough "meat" to work as an arched lintel?

not much of an issue with a 12" hole in 18" blocks.


NT