Slant-Bed CNC Lathe Build

[JimDawson]

Servos are normally very quiet, steppers are annoyingly whiny, especially those drives without resonance damping.

I can't address the service life of the ClearPath units except to say that I installed a set of the 750W units on a router retrofit a year or so ago for a customer and they are still running fine. I installed one 20 AMP, 70V power supply to run all 4 motors.

The loop is closed at the servo so no additional feedback is really required, as long as your lead screws have near zero backlash, the system should be fine without linear encoders. Because of the way they work, you really can't lose steps.

If you haven't already looked at the DMM servos, they might be worth a look also, I'm using their 1.8KW servos on my lathe axis drives. https://store.dmm-tech.com/

 

[macardoso]

Those Animatics servos are a pretty cool architecture, kinda wish I'd known about them before diving in since it's an interesting alternative to a centralized motion controller. From what I gather, a computer generating motion commands instructs the servos, which each contain their own controller, driver, motor, and encoder, so the first control loop never leaves the motor housing.

This is actually the way that most control architectures for servos work nowadays. Not necessarily the motor + drive in one package, but the distributed control over a network. It began back in the day by sending commands over Modbus, ControlNet, Devicenet, CAN, etc. where a single network cable drop could save thousands of feet of I/O cables. This allowed the processor to be placed in a central location, with drives located near their motors. Modern controls take advantage of the high speed networks we have today like Ethernet/IP, EtherCAT, Profinet
, and a few others. For us as hobbyists, they are significantly less useful because we don't have a way to interface with this network easily.

 

[macardoso]

I'll make one last attempt & ask the Aminatics folks if the brains can be bypassed, and run like a motor/encoder pair. There's a pair on Ebay that look really good for this application. Those Clearpaths look nice as well. In either case I'd have to bypass the servo (or stepper) amp boards and run them directly off my controller & an external amplifier since they are brushless.

My recommendation would be to stick with the motor and drive combo that the servo manufacturer recommends. Trying to mix and match servos (especially AC brushless) can really get you into a mess.

 

[barnbwt]

It must be old-school brushed DC servos I'm thinking of then. You'd hear a whistle every time they'd spool up to 8000rpm (think movie-style robot noises).

I've decided I need to review my basic design criteria before making a decision on motors. According to a Kennametal calculator, it appears that for my ~1" diameter envelope, both steel & speed aluminum turning would impart about 50lbs of tangential cutting force, and about 20 axial (along the bar). Now that's just the max cutting forces; is there a rule-of-thumb for how much more powerful the ballscrews' holding force needs to be for solid control of the cut? At only 60lbs and lathe-speeds, I'm certain the moving mass of the slide isn't the driving factor for the axis motors like it is for big mill tables & routers.

 

[JimDawson]

Now that's just the max cutting forces; is there a rule-of-thumb for how much more powerful the ballscrews' holding force needs to be for solid control of the cut? At only 60lbs and lathe-speeds, I'm certain the moving mass of the slide isn't the driving factor for the axis motors like it is for big mill tables & routers.

I'm sure there is a rule of thumb that real machine tool engineers use, but I have no idea what it might be. A guess would be about 3X. Normally I look at what others have done to have an idea of what works. When in doubt go bigger, you can always easily torque limit any motor you decide to use, but it's really hard to get more power out of a under powered system.

I think you're correct, at about 60 lbs, the moving mass is probably not a factor.

 

[Karl_T]

Let the pros do the design work. Just copy what they did. Your machine is most similar to a hardinge CHNC or a Omniturn gang lathe. they both have 1/4 hp. servos. that's about 200 watt.

 

[barnbwt]

Let the pros do the design work. Just copy what they did. Your machine is most similar to a hardinge CHNC or a Omniturn gang lathe. they both have 1/4 hp. servos. that's about 200 watt.

LOL, fair enough; that's how the pros designed their stuff in the first place, after all (copying others' successful parameters)

FWIW, the 'ideal' formulas for cutting force suggest ~50W is needed to control the cut, so 200W would be a safety factor of 4; that's pretty rigid, which is what I want, here.

 

[macardoso]

Something to think about. Power is a function of torque and speed. Your rigidity is purely dependent on torque. Make sure your holding torque is sufficient. A 200W servo rated at 2000 rpm will have more continuous torque than a 200W servo rated at 5000rpm.

 

[barnbwt]

Something to think about. Power is a function of torque and speed. Your rigidity is purely dependent on torque. Make sure your holding torque is sufficient. A 200W servo rated at 2000 rpm will have more continuous torque than a 200W servo rated at 5000rpm.

Yes, but the higher-speed servo will (need to) be geared down for my desired rapid speeds (about 700rpm IIRC), and per some comparisons, they end up a lot closer to parity (because both are so efficient). The faster motor can react more quickly, but I think that aspect is damped out by the presence of the belt/geartrain vs. a direct drive arrangement with the stronger motor.

It seems Tamagawa Seiki has a line of 48V DC brushed servo/encoders in this power range that are around on the used market, sometimes for reasonable prices. That seems about right for the 200W supply, though it'd be nicer if the voltage were closer to the driver's capacity of 60V.

I'm not sure what motors this Galil driver board was designed for; 60V @ 3.3A (200W) DC brushed doesn't seem very common at all (48V, 60V, and 100V versions are far more common, across multiple manufacturers). Perhaps they were not really intended to operate right at max (volts or amps) and are commonly paired with 48V & 60V motors, with some headroom.

 

[Karl_T]

Galil will run any servo that takes a 0 - 10 volt signal.

I play fast and loose with the volt ratings. No way will the insulation break down if you go 20 volt higher. They just don't want the extreme RPM the motor would spin up to at the higher voltage. That never happens under servo control anyway. This may shock some of the more conservative in the crowd.