Lathe Turning Copper Bar- HSS blank shape?

[Odin]

I've got a problem. A friend of mine, a watchmaking student, is trying to turn what I believe to be electrical connector grade standard
copper bar on a nice Levin watchmaker's instrument lathe, and he's getting terrible finish.

I'm also a watchmaking student, but I was a beginner manual machinist and CNC programmer before I got into this. Hand grinding custom
HSS turning and form tools for the lathe is no problem for me, learned a long time back.

I am teaching my friend how to hand grind HSS blanks to custom cutters on simple bench grinders like I learned, without the rotary
diamond hones we use in school- but the problem is, I've never done copper before. It's gumming like crazy on him. Machinery's says the
grade I think he has has a machinability index of 10- ouch.

Oddly enough, I can't find anything on cutter forms for copper in the 28th or my old 7th edition of Machinerys. I know this stuff can be cut
to a mirror, I've seen it. I just didn't see what shape their tool was. I'm thinking negative rake, as it's soft, but can't find anything!

Can someone give some advice on HSS (yes, must be HSS!) cutter form for copper?

We do have access to rotary diamond hones, with marble wheels and diamantine for sharpening cutter blanks if a mirror finished cutter is part of this.

Thanks!

[txcncman]

I would be more concerned about speed, feed and depth of cut first. As far as tool geometry, minimum nose radius with plenty of relief. I have not cut copper with HSS, only carbide inserted tools. Might want to hone the edge of the HSS to get a radius similar to carbide inserts. About 0.002" maybe.

[RICHARD ZASTROW]

Don't laugh till you've tried this. Spray PAM vegetable oil as a coolant/lubricant on the work zone.

Pam keeps things from sticking in a hot frying pan and does the same thing on a tool tip.

We had to turn chrome plated copper printing rolls. The tool was a PCD insert that could stand up to the thin chrome plating. The copper would weld to the insert. One of the lathe operators was familiar with the problem and showed us the PAM trick. Worked for us.

Dick Z

[fizzissist]

For copper, and I've done a fair amount of OFC, you need razor sharp tools, positive rake, dead on center, and ..... no sulfur based oils unless you like staining...

PAM sounds like a good idea. I've used WD-40, but found that 1/2 & 1/2 works great, especially for tapping (kudos to Guy Lautard). Copper seems to be happy when confronted with fatty lubricants.

I've found a .015" tool nose radius works fine, dabbing on the milk, running about 100SFM and .002-4"IPR should get you in the ballpark. Don't expect great tool life with copper and HSS, but you can get good finishes and tolerance if the tool is got a good polish, is sharp, and on center.

Spend some time taking test cuts and see what works for you.

[RICHARD ZASTROW]

Remember Castor oil in race engines? Worked great as a lubricant but had to clean the ring grooves in the pistons after every race. Gummed up badly. Thats a bean oil. They also use Jojoba bean oil as a lube and in machine cutting tool coolant.

Dick Z