Starting a core drill hole

Quoted text begins.how does one start one of these really big hole saws?
End of quote.

Hi Norman,

You can either start the hole manually, or use a template.

To start a hole manually, allow the core drill to spin wet in your spare hand
(the one not on the drill trigger) and hold the drill at an acute angle to the
wall. Let the rim of the drill contact the wall just at one place on the outside
of the cutting edge. As it cuts a small groove, gradually ease the drill over to
a right angle or whatever you need, and continue drilling. Knock out the centre
core with a hammer & chisel when the drill bit reaches full depth, and continue
drilling until the hole is deep enough.

This method would be difficult if the drill bit is too large to hold, or if you
are drilling on a finished surface. If this is the case, use the core drill to
first cut a template through a scrap of plywood or similar, and clamp it against
the surface being drilled. The template can be located accurately, and will
guide the drill bit until it cuts a groove, and then it can be removed.

Vertical (downwards) drilling is easiest, you can use your boots to hold the
drill steady while it is starting the hole. The weight of the drill provides
enough pressure for abrasion at the tip. These machines are not meant for uphill
drilling, because the water could run back into the motor. Dry core drills are
used for overhead drilling.

Norman, I have to ask: if you're drilling holes in your walls by guesswork, how
do you avoid contact with power cables and water & gas pipes?

Simon