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PM-940 CNC Conversion using Servos(questions)

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RADustin

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Hi all,

I posted in the PM specific forum, but I think I'm best to ask my questions here.

original thread-https://www.hobby-machinist.com/threads/pm-940-diy-cnc-or-turn-key-pm-940m-cnc-vs.72218/#post-606265

I'm trying to decide what servo motors and controllers would best power a PM-940 CNC conversion, and then compare the performance and costs of the servos to steppers and make a decision.

I'd build around the Acorn motion board. I've look mostly at the DMM servos, with the DYN4 controller. The DYN4 controller is appealing because it is AC input power, so no power supplies and it can run motors from 0.75 to 1kW. I like the DMM servos because of the very flat torque curves up to 3,000rpm. They seem to be flatter and more torque at RPM than comparable ClearPath servos.

I've spoken with arizonavideo and he is releasing a ball screw kit soon for the PM-940. He is suggesting stepper torque around 640 oz-in for X and Y and 900 oz-in for the Z. I believe the turn-key PM-940 CNC runs 640 oz-in for the X and Y and 1200 oz-in for the Z.

How do these stepper torques relate to servo torque? Am I looking for a servo that has rated torque at these values or peak torque?

DMM shows they have a NEMA34 case servo that is 0.75kW and does 340 oz-in rated and 1000 oz-in peak, could this be a satisfactory motor for x,y or even z?

I've read where some are using a brake on the z axis to hold the head. I can understand why this is required, and it seems like it would be required on all axis? why is a brake not used with servos on the x and y? is the nature of the ballscrew 'autolock' enough to keep things in place? when steppers are used, are they normally 'locked' and then unlocked and given pulse and direction, then relocked to make a move?

arizona video has mentioned he can mount other motors, like 60mm servos, but these seem to not make much torque. I'll have to ask him more about this.

I appreciate any and all insight. Links to build threads with servos is also helpful.

Thanks all!
 

JimDawson

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I'm happy with the DMM servos that I have used, I have 4 of their 1.8KW units on my lathe, and ordered 2 more for another project. There is nothing wrong with the Clearpath units either. I have also installed those on one project and just ordered 4 more for another project. The down side of the Clearpath units is that there is no possibility of closing the loop at the controller. Where the DMM units allow you to close the loop either at the drive or the controller. It depends on the application which is best. The Acorn board is only open loop so either Clearpath or DMM would work fine.

You will find that servos are an order of magnitude better than steppers. With steppers the torque drops off rapidly with RPM, servos have a pretty flat torque curve up to their rated RPM. You can normally get away with less max torque on the servos. Steppers and servos are held in place electronically, they don't ''lock'' when stopped.

A Z axis brake is almost mandatory because the Z will drift down when not under servo control, unless you counterweight the Z. Counterweighting the Z is not a bad idea in any case. The X and Y don't need a brake because they are not being affected by gravity. When the servos or steppers are energized, they hold their assigned position.
 

RADustin

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Thanks sir, I appreciate the feedback.

My goal would be to get the machine running on the DMMs and Acorn and later(much later) upgrade to a better board and bring the feedback all the way to the controller.

I'm familiar with how steppers work, and how multiple segments can be powered at one time to hold the motor in place. I wasn't sure if servos could perform a similar task. Good to know they can.

I guess I'm still looking for torque inputs. If anyone else has CNC'd a larger bed mill?
 

spumco

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Thanks sir, I appreciate the feedback.

My goal would be to get the machine running on the DMMs and Acorn and later(much later) upgrade to a better board and bring the feedback all the way to the controller.

I'm familiar with how steppers work, and how multiple segments can be powered at one time to hold the motor in place. I wasn't sure if servos could perform a similar task. Good to know they can.

I guess I'm still looking for torque inputs. If anyone else has CNC'd a larger bed mill?
Some anecdotal info to help with motor selection:

One of my CNC mentor's VMC's is an older 40x20 (full-size) mill with box ways. No Rulon or Turcite on the ways (no low-friction slides or coatings).

His X/Y/Z servos are 750W Yaskawas driving 5mm ( I think) pitch ballscrews. I don't know the amps or torque curves for the motors, but he has 600ipm rapids and a 10HP 6kRPM spindle. His Z-axis is counterweighted. I suspect the Yaskawas are under-rated - meaning they have quite a bit of extra short-term 'oomph' - but not grossly so.

I don't know the saddle or table weight, but it's at least 4x the weight of your PM-940. Unless you hand-scrape & fit the ways on your machine you will most likely have higher friction than his mill, but significantly lower inertia and cutting forces.

The DMM NEMA34 750W motors should be plenty, assuming you're going to use 5mm pitch (1605 or 2005) screws.

One thing to keep in mind regarding the DMM vs. Clearpath - connectors. When I was considering upgrading my steppers I looked in to this exact choice - DMM vs. Clearpath. Both appear to be very nice, well made systems. The one show-stopper against the Clearpath was the fact that the motor & encoder connectors were not remotely waterproof. Deal-killer in a full-enclosure flood-coolant mill, otherwise pick your poison.

The Acorn looks nice & easy, and getting everything from a single manufacturer is really attractive. But remember that it will be an expensive 'temporary' solution compared to some other options. Once you pay for the software with all the options you're getting close-ish to the kind of closed-loop Galil controller Jim mentioned. If you seriously plan to upgrade later you might consider geting it moving around with a cheaper control system: cheap 2-port BOB with on-board spindle PWM, ESS or other low-cost motion controller, and Mach3, UCCNC, or other PC-based software. Those will all drive the servos and spindle and have enough I/O for limit switches and a couple of relays for air/coolant.

There are, of course, other expenses and this isn't a straight apples-to-apples thing, but here's a rough estimate for a few motion controller systems

$814 - Acorn hardware + Digitizing level software (for rigid tapping, probes & more offsets)

$359 - ESS/C25BOB + Mach3 (minimal but adequte I/O, expandable)

$369 - UC300ETH/UB1 BOB + UCCNC license - $60 (6-axis with tons of I/O and expandable to 5 ports. Rigid tapping/spindle encoder built-in.)

$595 - MASSO all-in-one BOB + motion controller + software (5 axis, no PC needed, not expandable)

$1495 - Galil 4143 + Jim Dawson's software (4 axis, not expandable I/O, top shelf stuff)

$2295 - Galil 4043 + Jim Dawson's software (4 axis, expandable I/O, top-shelf stuff)

If it were me and I knew I was going to upgrade later, I'd go for the MASSO as that would be the easiest thing to physically pull out and install on a different machine. If I thought there was a good chance that I'd never get around to upgrading, then I'd go with the Acorn, UCCNC, (or Galil) depending on budget and skill level. Just my opinion, of course, and there are plenty of options I haven't mentioned here.

Hope this helps,
Ralph
 
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