T i m wrote:
On Mon, 13 Feb 2006 19:46:29 -0000, "Mungo \"Two Sheds\" Toadfoot"
wrote:

Pete wrote:
Hi.

Does anyone know of a good way to cut a 7.25 inch (185 mm) circular
hole through a wall?


Based on those plastic drill jigs you get for chain drilling
electrical socket metal back boxes howabout this.

As an exansion of what Dave Baker suggested ..

Drill right through at the centre of the proposed hole with (say) a
10mm dia long bit. Maybe use a square on the hole as you start to
ensure it's a square to the wall as you can get.

Get a lump of 8 x 2 sawn (or similar) and scribe yer 7.25" (or maybe
scribe it at 7" or whatever depending what sort of final fit you need)
and using a pillar drill, drill through yer scribed line a series of
holes the diameter of your long drill bit tip at regular intervals
around the circle. I would leave half the diameter of the drill bit
between the hole to try to stop the holes breaking into each other
till you were ready (especially on the inner layer).

Counter bore the middle and use a long 10mm Fischer plug / screw to
pull it tight to the wall then drill through your gide. You could
possibly slacken the fixing screw and rotate your guide to get inter
spaced holes?

If you didn't want to go right through in one go go half way (mark the
drill shank with red tape) and repeat on the inside.

Revove yer drilling jig and redrill at a slight andgle (from both
directions) to join the holes together. When sufficiently broken away
you could probably use the drill bit like a router and cut the
remaning 'bridges' away till the plug falls out ;-)

Trim up with suitable half round 'universal' rasp ;-)

Look at the two plugs, the pretty neat hole through yer wall and go
down the pub with the 100 quid! (or use it to buy a half decent SDS
drill and a couple of long 10mm bits that you can use on the next job)
;-)

All the best and have fun ..

T i m

one has to wonder why the diamond tipped core drill was invented and
why drilling rigs were invented and why hundreds of enterprising people
have bought drilling rigs and franchises for drilling concrete )

about the only inexpensive solution for the OP is to scribe a circle on
the outside wall, drill a hole in the middle right through the outer and
inner walls to give a reference, repeat the circle scribe on the inside
and then drill series of holes around the scribed circles until the desired
~185mm hole is achieved.

as for finish, it's only the outermost 2 or 3 mm which has to be perfectly
circular as the render / plaster will do what it's desigend to do: hide the
underlying imperfections from a critical eye which most people don't have.

personally, hire the rig or I'd get someone in. someone who does a job
day in, day out usually produces a better result and there are times when
even the most avid DiYer needs to box a job off by paying someone else
to get the whole project over a time consuming obstacle.