Lathe feed rates and RPM questions

[starr256]

I understand feed rates for turning operations and how to determine them. But if you are tuning off a lot of material, shouldn't you adjust the RPM as you reduce the diameter of the material? What about facing operations? Shouldn't the RPM be increased as the cut progresses toward the center? Or is this all of no real consequence once basic RPM and feed rates are determined?

 

[Rodneyk]

Ideally the RPM would increase as the cutter moves closer to the center, and on large diameters you can see the effect while making facing cuts easily.

Some high end machines actually make this adjustment for you to maintain the cutting rate constant, but most machines don't.
I am also new to this but have seen examples of machinist do it both ways. It also depends on the tools you are using to machine the part with. For example, Carbide inserts prefer an aggressive cut compared to HSS.

Having a variable speed lathe with a VFD has allowed me to do exactly what you are saying, I slowly increase the RPM as me cuts are closer to the center.

 

[G-ManBart]

For facing operations I don't think it makes much difference, but if you're turning something from 4" down to 1" you're probably going to want to increase the RPM at some point. I'm also still new at this, but often use one style tool/insert for roughing and just try to get in the ballpark, then switch tools/inserts, and often increase the RPM to get a nice finish near the end. It probably adds a few extra measurements since it's a new zero but that doesn't seem to bother me as I tend to stop and measure frequently....good steal isn't cheap and I don't want to waste it by not measuring enough.

 

[K30]

As a thought experiment, yes it should, that would be ideal. With a caveat: your ideal speed increases to infinity as you get close to center. So yeah, in a perfect world it would, but in real life, you can live with a speed. If you're facing something really large in diameter, you might pick a speed that's a little too high on the outside diameter, and perfect in the middle, but it'll still be "too slow" by the time you get to center. Experiment!
If you're turning something way down, it's pretty common to change speed in the middle somewhere if you think you need to.

 

[Shotgun]

I've found a super simple way to find the perfect SFPM (surface feet per minute) for whatever material, DoC (depth of cut) or cutter combination your using. Put in a large diameter piece and make a facing cut.

The cut will start with a crappy finish, slowly progress to shiny perfection, then turn gross again. Take the diameter of the pretty section, combine it with the RPM, and you've got your perfect SFPM at that DoC.

If you're doing a HUGE diameter change (12" down to 3"), you might want to have some RPM changes in there; but, for most cutting, you can get away with setting the RPM to what it should be for the target cut. Once you're within the book marks of "not burning tools" and "getting it done this decade", having the SFPM dialed in is really about good surface finish. Unless you're worried about the finish on the chips in your tray, those roughing passes aren't much of a concern.