sizing servos for a cnc lathe

I was just making a example of accelerations involved, I hadn't bothered to calculate the actual forces. my screws are not directly coupled, but they are at a 1:1 ratio on a belt drive. my initial thought was a 1500rpm rated AC servo with a top speed of 8000rpm, geared down 2:1 or 3:1 to replace these DC servos, with half the rated HP, but having 3x the speed capability. The weight of my carriage isn't known, but my monarch had a 1400lb carriage, and it was 1/3 the size of this carriage assembly. So I might be underestimating the weight, my monarch was  the same swing and slightly longer Z, and it weighed 6750 lb, my Cincinnati weighs 21,000

I have been thinking though, the reacting inertia from the carriage the screw and the rotor are all dynamic, probably not very easy to calculate either. we can always slow the machine down, but as we speed it up, we start having a need for more motor TQ than rotor inertia and mass. in fact what we would ideally want, is the lightest rotor we could get, with infinite TQ reacting as quickly as possible, but I guess we will have to settle for the rotor that can handle the power input first !. AC servos definitely win here !

RotarySMP wrote:

This thread lacks photos. Please post photos of the lathe. Everyone likes photos of lathe.

This is too much focus on rated power in the selection of the motors. That tends to be a pretty unimport secondary parameter, in that once you have sized the motor for the inertial loads, and necessary torque, the resulting motor will have some rated power (typically way way more powerful than you would expect).

I did a course on this at the TU, and summarised the process they teach here:

The expectation of a manufacturer would be for a very rigid, highly dynamic drive system. For your purpose, you are prepared to compromise in the machine dynamic performance, and also the cutting power, but when you run through the inertial calculations, you may find you need a pretty beefy AC motor, to get it to work at all, especially if it has a long,  large diameter Z ball screw, direct driven. On a lathe, the inertial load of the Z ball screw and motion of the saddle can be significant, with the sort of light cuts you are willing to accept maybe buried in uncertainty of those loads.

I'd love to see photos of that beast.

I'll post more later, forum attachment size limits require compression


I've got a lot of machines in this slace, will be moving 2 of the mills in the future once the new garage goes up. 

That's a 5 gallon bucket btw for scale 

The swing-up tailstock is pretty wild.

it is ain't it, It is 100% hydraulic and I got to be honest, I am thinking about converting it to servo. Apparently if you run the tailstock pressure to high, it'll break itself. Talked to prior owner. comning with that massive steady rest was pretty sweet though. 

Even wilder, the machine had just under 1000 hrs on it.