Cast iron, brass and plastics dry for all operations. For general machining I use Biokool14, a soluble oil, from Hallett Oils. Most of my machines have flood coolant systems built in. Specifics as follows:
General turning: dry in all materials unless I'm using HSS tooling, or parting off where I always use coolant. For drilling in the lathe with HSS drills I run coolant.
Repetition lathe: coolant, to get rid of the swarf.
Vertical mill: Mostly dry as I use carbide tooling.
Horizontal mill: Mostly with coolant, as I use HSS cutters.
CNC mill: I always use coolant, except CI, B and plastic, for swarf removal rather than cooling
Cylindrical grinding: always with coolant
Surface grinding: Dry, no coolant system
Hand tapping: in stainless steel and/or taps over 1/2" I use Rocol RTD
Machine tapping: with coolant, except for aluminium where I use WD40
Regards,
Andrew