Hi Randy


Your safest bet is to core drill the hole. Do not try to scratch it.

Thanks for the advice.....
.....unfortunately I don't have a core drill. I'm going to need to be
able to do this to produce a range of specific sizes - might involve
quite a few core drills.

Time is the one commodity that I have plenty of - I'll just have to
try the other techniques that people have suggested - but thanks for
the suggestion anyway....

Regards
Adrian
Suffolk UK