js.b1 wibbled:

Before you go mad on holesaws, consider a step drill?

They are a round drill thus avoid the 3-sided hole of normal drills
and self-centre so if you centre-drill a hole to begin with they do a
very good job of ensuring the resulting hole is where you want it. The
downside is their depth is limited to about 4.5mm (it varies, just
look at the engineering drawing of the tool if available). Wonderful
tools for enclosures, glass reinforced polyester, ABS, polycarbonate
etc.

Thanks to you and John. My holesaws are bi-metal and in addition to Dave's
suggestion of water as a coolant, the bloke in Screwfix who's keen on kit
cars (much fibre glass) recommended applying tape over the surface too to
avoid chipping.

On the step drill front - I've never used on but it also sounds
interesting - I'll have a look into this.

Cheers

Tim