sizing servos for a cnc lathe

I am fairly sure the lead screw is around 50 or 60mm, I haven't measure it, I did measure the pitch for setting up Linux CNC. I would be VERY happy with 200ipm as a top speed, I will have to test and see what the machine can handle reliably. I bought big drives, specifically to allow for strong accel and decel control.

Okay, so assuming your lead screw is 50 mm dia and 2 m long, its inertia should be about 100 kg×cm². (Twice that at D=60 mm!)

The inertia of the load as seen through the ballscrew depends on the ballscrew pitch. 1800 kg of load at 20 mm/turn becomes about 180 kg×cm². At 10 mm/turn it's only about 45 kg×cm². If my spreadsheet math is correct.

At 200 ipm (~85 mm/s, but let's say an even 0.1 m/s), a 1500 rev/min servo and a gear ratio of 5:1 I'm guessing your ballscrew pitch is about 20 mm/turn?

it's measured actual ratio I found was roughly 4.5 thread per inch. I was manually turning by hand just to get in the ballpark. The screw is totally enclosed and it's hard to get at it, and that all adds up as the rated speed of these motors is 1500rpm/ AFAIK the rapid speed of this machine is around 300ipm and max velocity of 150ipm factory. If you find better specifications let me know. it is a 1979 cincinatti milacron cinturn 12 u 22.5 swing x 60z in inches 

Yeah that makes sense. 4.5 threads/in is suspiciously close to 5 mm/turn which is pretty standard, so I'm going to assume for the moment that that is the correct value. That also gives very close to 300 ipm (0.125 m/s) top speed with a directly coupled 1500 rpm motor.

The reflected inertia of 1800 kg with a 5 mm pitch screw is only 11.4 kg×cm². IF your screw is 50 mm dia and 2 m long the screw inertia will swamp the inertia of the carriage at ~100 kg×cm². You mentioned 4-5 gees of acceleration earlier but that would require well over 500 Nm of torque with such a screw! 1 gee seems much more reasonable (taking just 13 ms from zero to full speed) but even that requires 134 Nm of torque. A 7.5 hp, 1500 rpm servo has ~35 Nm of torque so I guess you'll have to dial it down a bit more. (If your screw dia is 30 mm things look ok though at 1 gee.)

The inertia ratio looks fine at well below 3:1 probably (depending on the motor).

(I've updated my spreadsheet to separately calculate torque required for acceleration and machining.)

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 

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.