retrofit of Servosource Servo-II CNC knee mill

15 Apr 2023 05:40 #269059 by dog
Greetings all.New guy here. 

I have a CNC knee mill with a dead controller.  It's probably repairable but I've decided to go with linuxcnc for many reasons that y'all can probably guess.  The machine was produced in the late 80's/early 90's by Servosource, using their DOS based Servo II system that was designed for retrofitting bridgeport knee mills etc.  They are still selling their conversion kit, by the way, but for a while they were also selling complete machines using a fixed head knee mill that I'm guessing they got from Taiwan and putting their kit and branding on it. Unfortunately the motors on this mill are variable reluctance stepper motors (yes, steppers from a company called Servosource) so I don't think I'll be able to use them. I've been going through the wiki and the forums and learning lots but I still have a lot of questions.  I'll avoid asking all of them right away.  It looks like the first thing I need to do is source a set of servo's and servo amps. 

I'm still in shock at the price of servo motors, even used ones. From what little I know there's not a lot to go wrong with electric motors unless they've been abused.  Bearings and brushes can be replaced so I'm leaning toward used motors (ebay etc) if I can find the documentation for them (including what connectors I need to make the cables).  Am I asking for trouble looking for used servos?  As an alternative I have to ask, are those low cost new Takasawa motors on ebay from China for real?

Per servo amps, I prefer open source solutions so I'm very intrigued by the STMBL servo amp, is this a viable solution for a project like this?  From what I can tell it's still alive and I really like the DIY aspect of it but I need something that's a real solution.  If not is there a preferred solution?  I've seen lots of servo amps on ebay, again not cheap and again I'd want to make sure I can get the documentation.  From what I think I've learned mixing servos and amps from different sources is doable but is it smarter to look for a matched set?

That's probably enough for now, plenty of time later for the rest.  Thanks for any guidance you can offer.

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15 Apr 2023 08:41 #269072 by andypugh
If the drives are good then there should be no problems using the existing stepper motors, even if they are a little unusual.
If you are trying to replace the drives then some work would be needed to figure out if conventional drives can be used.
I have a vague recollection that VR steppers tend to be quite powerful, but then things have moved on and maybe replacing them with closed-loop steppers (such as those from Leadshine) is an option.

If you prefer servos, then people do report good results from some of the inexpensive Chinese servo and drive combinations, but be sure to get matched pairs.

I have always used second-hand servos from eBay, but ignore brushed types and concentrate on the brushless ones. Those have very little to go wrong. LinuxCNC can work with either encoders or resolvers, so see what you can find before committing to a control scheme. All my machines use resolvers as resolver-equipped motors are less sought-after and cheaper.

The options are basically:

(current motors / steppers / servos ) * (encoder / resolver / open-loop) * (step-dir / analogue voltage / digital-serial / ethercat) so quite a big configuration space before you even start to think about the control hardware.

It probably makes sense to see what you can find that will fit, once you have decided what of the current hardware it makes sense to keep.

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