What is the best way to turn down a piece or hardened lead screw material? What cutter should I use?

Paul, if it's an Acme type screw, they're usually only hardened to Rc 28-32. Any carbide used for steel cutting will work. If it's a ball screw, they're a lot harder. If you're using brazed carbide tools, a C2 like you'd normally use on aluminum will hold up better on hard steel than a C6 will. Give it a little speed and let the chips come off a little red. If you're using insert tooling, a cermet or a ceramic insert will work. I prefer the cermet on single piece jobs because they will cut with less force than the ceramics. Their edge is a lot sharper. Makes size control easier. Most ball screws are cased, so once you're past the case you can go back to a normal steel cutting grade. A cermet can stand about 100 fpm on steel in the 60C range, with light feed and depth of cut. Of course the cermet insert can also be used to take the part to final size as well if you prefer, but once you're thru the case it should be speeded up. Regardless of what you use, this type cutting is best done dry, as the heat at the point of the cut is your friend in this instance.

I use a bench grinder to grind down to the root diameter on the end you need to turn down. I have done this many times on my ball screws that I turn the bearing diameters on. You still need to use a carbide bit but you don't need to turn much when you remove most with the grinder.

Thanks guy's , my friend has a grinder attachment for his lathe I guess I will use that to get it to it's final size.

I found the technique Jeff describes to be the most economical, I first tried using cemented carbides on RC60 ballscrews then found it was getting quite expensive I then switched to using a 4" angle grinder to get below the hard surface then turned them.

This resulted in getting my lathe filthy as the coolant needed between grinds turned the dust to cement, I also had to source some discs for stainless to get almost acceptable wear (stock disc 2 minutes stainless disc 5 minutes to unusable)

The very last Ballscrew I did I used the bench grinder with a fine wheel turning it by hand to get the size I wanted + 1 mm then turned it in the lathe with a small radius cemented tip and really REALLY wish I'd done them all this way! (Thanks Jeff ;) )