Hi,

I bought a plastic bulkhead light fitting [1] from Screwfix and tried
to drill a 20mm hole to pass the cable through. I thought that a hole
saw would be the most appropriate way to do this but halfway through
the hole saw got stuck! What was I doing wrong? Is this particularly
tough plastic, or do I have a blunt/poor quality hole saw or should I
have used something completely different all together?

Talking of holesaws, is there a logic behind the sizes they come in?
Written in metric the sequence 35, 38, 40, 44, 51, 57, 64, 70, 76, 86,
102, 127 doesn't seem to follow any pattern. Does it make more sense
when in imperial?

They don't even seem to match common pipe sizes (I realise you would
want the hole a couple of mm larger than the pipe). I suppose 35 and
44 saws could be used with 32 and 40 mm waste pipe but 127mm seems too
big if you wanted to fit 110mm soil pipe through it.

I would like to cut a 100mm hole to fit some ducting; would a 102mm
hole be too fit a fit? I notice there is a 152 mm hole saw, presumably
for 150mm ducting but again it only leaves one millimetre either side,
so there's little room to manouvre.

Thanks,
Fred






[1]
http://www.screwfix.com/prods/17936/...Black-Bulkhead