I've been building more nozzles and drilling some holes in metal. I tried drilling out a stainless steel bolt to make the extruder barrel, but it was hard work and didn't feel right. Usually when that happens, I'm doing something wrong...
So, I googled and read up on drilling into stainless steel.
Apparently, when the drill rubs over the metal in the hole, it starts to heat and work harden a thin layer of steel - making it harder and stronger - and more difficult to drill.
The advice is to drill at a slow to medium speed, apply lots of pressure, and drill few mm then pull right back to clear the swarf. This is pretty much the opposite of my original technique.
This works *really* well - you get lovely long spirals of swarf (razor sharp splinters) and the hole is much easier to drill. I found it much easier to drill out welding tips and have drilled a couple of stainless bolts using the afghan lathe technique.
Top Tip: Read instructions before using.