On Wed, 28 Jun 2006 10:25:50 -0700, wrote:

Never used such a large core drill, but with a 45mm I always go
striaght in with guide bit and core as one. I can't see why you wanted
to pilot the guide bit right through first.

Because it's often useful to be able to check where the hole will (or
should) come out.

It can be a bit of a pain to get the core drill started if the surface
layer (plaster or whatever) is too flakey to guide the centre drill in. I
suppose a longer centre drill or some sort of rod might be nice, but I
usually find I can get the main core drill cutting to the point it
self-stabilises by carefully offering up the rotating core drill to the
wall slightly off-axis so that just one part of the rim starts to cut at
first, and then straightening it out as it cuts deeper.

Industrial grade language may also help :-)