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