J-G wrote, QUOTE: WD40 is a water repellent not a lubricant. UNQUOTE:
and Music Man wrote, QUOTE: I would not use WD40 for lubricating anything but aluminium, but it is good for that, in my experience. UNQUOTE:
The first statement about WD 40 comes up so many times, but sorry, it's just plain wrong. Q: What is a "lubricant"? A: ANY liquid (or semi liquid/paste/slimy stuff) which will keep 2 pieces of metal apart while they're rotating or sliding or rubbing. In other words, water (OFTEN used as a lubricant, AND fuel of various types) and even raspberry jam COULD be used as a lubricant - though perhaps there are better uses for raspberry jam (my wife's is lovely)!
And although WD 40 IS primarily a water dispersant, it definitely IS also a lubricant - and maybe even better than raspberry jam for that purpose! Seriously, in certain special applications, WD 40 is a very GOOD lubricant and for some special purposes (in aviation for example) it's sometimes specified for exactly that purpose.
Coming back to the practical hobby workshop world. Music Man (amongst others) is perfectly correct when he states that although you'll find paraffin (kerosene) specified as lubricant for ali in all the text books, WD 40 works just as well as paraffin for cutting ali - that includes threading and lathe turning when you're after a fine finish. In addition, when turning brass, (which always "squeals" at you) though it's impossible to stop that squeal completely, a judicious squirt of WD 40 does a lot to at least reduce the teeth-jarring frequencies!
I also use WD 40 as a cleaner/lubricant for all my garden tools, with and without cutting edges. Some of these tools live in the relative warm/dry cellar workshop, the bigger ones such as the lawn mower and tree lopper live outside year round in a small timber tool store (unheated but with a small high level ventilator). In all cases cutting edges remain clean, sharp, and untarnished, and there is no rust at all on any tools, stored inside or out. And WD 40 is ALWAYS used (a quick squirt) as a lubricant on the pivot points on my wife's secateurs before and after use.
The "trick" of course is that after the "carrier fluid" has evaporated off, WD 40 leaves a thin film of silicon, which I think is why its use is frowned on in many wood working applications (where I understand silicon can adversely affect many traditional wood finishes).
But to say WD 40 is not a lubricant is just plain wrong, sorry - and to repeat, in some (limited) special applications it is actually specified as the only lubricant to use.