so I have a few jobs coming in that are (for me!) large diameter workpieces. I'm looking at machining some D2 and 4140 PreHard round bars at 9" diameters (these are ABS plastic extrusion workpieces).

Are there any things I need to be aware of when working with heavy large diameter pieces.. the width is probably < 5" so I'm thinking the tailstock won't be necessary.



Can I use CSS on the lathe or should I turn it off or limit max rpm to a certain limit? ie.. I'm not sure I'm comfortable spinning a 9" diameter workpiece at 3400 rpm when the tooling gets close to the centerline. Are there some guidelines in regards to max rpm per workpiece size & weight?

cheers!
Paul

Use CSS but have a G50 command to clamp the maximum spindle speed at what you feel comfortable with.

I am not aware of any guidelines for max rpm based on workpiec size, other than those related to the cutting speed.

For doing large diameters I have used two approaches to ensure a good grip:
One for manual machines is to tap the work and put a length of threaded rod through the spinlde with a flange and nut on the end to act as a draw bolt so the work is fimly pulled into the chuck.

The draw bolt idea is not workable on a CNC with a hydraulic chuck so I have machined custom step jaws with a recess toward the back and prepped the work with a groove so there was a mechanical interlock when the part was gripped. A variation on this which I have used is to machine the ID of the jaws with grooves that have a standard thread profile (but no lead) and mating grooves on the workpiece. This gives a very secure grip.

excellent - appreciate the input.. I'll just keep the big pieces at slow rpm and take it easy.. someone had posted a video of a crash in a lathe with a big chunk chucked up and when the tool got to the center of the piece the jaws opened up (centrifical force pulled them outwards) and the piece went flying! that is some thing I do not want to repeat!

cheers!
Paul

Is CSS constant surface speed?


Yea, I am guessing it is. Yall confused me there a bit.


What size chuck do you have on your 30? I ran one with a kittawach chuck 18" chuck I believe and 9" wasn't a problem.

Also, I would run css and I would definitly run a g50 command like previous said before.

It's a 10" chuck. I'll just make sure to keep it slow and clamp the upper limits on the rpm via G50.

Cheers