On Nov 12, 8:43*am, Peter Purdie wrote:
The suggestion to use 2-1/4" or 3-1/8" hole saws is absolutely correct.
Those were the original standard instrument sizes (and you will find that
the P.C.D. of the fixing holes is also a neat measurement in inches.

Over here in metricland where they don't understand inches, it was
converted to the nearest integer millimetre.

And a hole saw (used on a panel clamped down to a good rigid drill press
bed for accuracy) is a damn sight less costly than a hole punch.

Although Bob's idea of a piece of det cord looks interesting, I must try
it. *On someone else's panel...............

Surely you can order metric hole saws by the internet? The hard work
way of doing it is to scribe the correct size hole onto the panel,
drill out a series of adjacent holes just inside the scribe mark,
knock out the centre, and them finish to size with a half round file.

Derek C