In article ,
"Paul S" writes:
I will shortly be changing the position of my electrical sockets in one of
my bedrooms. Currently they are surface mounted sockets, but for neatness I
would like to take this opportunity to change them to flush fitting sockets.
When I have previously attempted to fix back boxes into walls on previous
jobs, I have always had problems getting the box square, and sitting flush
in the wall. I have previously only used a drill, hammer and bolster to
create the hole for the box. Recently I have seen these Box Sinker Sets that
fit to an SDS drill to create a perfect hole and they also are used to
channel the wall out for the cable, has anyone any experience of using
these? If so, are they worth the investment? (not just for one job), or has
anyone got any other tips for me to make the job easier and neater.

I bought one. There are two types -- I bought the type which drills
a circular hole of the right diameter, and then you use a square
box attachment to make it square. The other type has lots of hardened
fingers in a share shape, which I've not tried.

I was drilling into house bricks and I worked out that the unit cost
about £5 per hole before it was knackered (too blunt to work anymore).
It did a speedy job and you get a nice flat back to the box (unless the
wall crumbles to pieces). It makes one hell of a mess -- basically the
volume of removed material gets turned into dust which is thrown into
the air and goes everywhere. The square frame part was completely
useless -- it would just jam in the wall. An SDS chisel was much better
for finishing off the box corners.

I wouldn't buy another one, unless I had a number of holes to do and
time saved was worth £5/box sunk. I doubt it would work on soft
material like thermal blocks. You have to drill a pilot hole first,
and a shaft on the hole borer has to be located in that hole to keep
the borer in the right place. If the pilot hole ends up near a mortar
line, it usually breaks through and you get a rather uncontrolled and
oversized box hole, and nothing at the back suitable to screw the box
into. I would imagine this would be much worse with thermal blocks.

I generally sink boxes by drilling postage stamp type perferation
pattern around the hole. I then run the drill in the holes diagonally
so it joins them up, and if the wall is soft enough, twist it back
straight and then diagonally in the other direction until all the
holes join up. Then take the centre out with a hammer and bolster
or SDS chisel bit. ~8 times out of 10 I can do this so no filler is
needed around the hole and the socket/switch plate completely covers
the cut edge. My score for perfect holes (need no filler) with a box
sinker was probably lowerer than 3 out of 10.

--
Andrew Gabriel