Lathe conversion - open loop vs closed loop

[bhowden]

I am looking at a 14 X 40 manual lathe with DRO and thinking about the best way to convert it to CNC. Most of the conversions I have found on the web seem to add aadditional zero (low) backlash screws and drive them with open loop stepper motors. I am wondering why I can't use the existing drive by adding pullys and stepper motors to the existing handles. There is plenty of backlash (say 5 to 10 thou +) but the DRO has an output so I would think you could get a closed loop controller and ignore the backlash. Am I missing something here? I do not see many (any?) examples on the net that do this. Any suggestions for controller software that understands closed loop feedback?

Brian

[pstockley]

Your problem will be getting a controller and software with feedback for less than $4000. Cheaper to fix ballscrews. Mach IV will support it eventually but don't hold your breath.

[RotarySMP]

Backlash is always a PITA for machine control. All the CNC control software has backlash compensation, but it is no substitute for backlash free motion. On a lathe it is not as critical as a mill, but you will stillget dwell marks on the work when an axis takes up backlash during a curved cut, and backlash is one of the big contributors to chatter on any machine.

Backlash on a closed loop machine is even worse than an open loop machine.

[balsaman]

If I get you right, you want to use the glass slides for the dro as feedback for the control. It is 10 times more critical that you have NO backlash in this type of system over say a system that uses motor encoders as feedback.

Eric

[wizard]

I'm not sure what you are asking for? In any event trying to run stepper motors closed loop is complicated. It is far easier to run conventional servos closed loop.

As to back lash I concur with the others you need to get rid of that no matter how you control the screws. That is not to say that yoou can alwys eliminate 100% of the backlash in a system. The reality is you can't do that, but gross mechanical slop has to be dealt with.

Dave

[bhowden]

Thanks for all your responses. When I posted, I was under the understanding that the DRO controller had an RS232 output and that the CNC software would be able to read the exact position of the caridge and cross slide. I have since found out that there is no output on the controller so it would have to read the actual sliders (pickups?). From your responses, it does not sound like a very practical way to proceed. Assuming I would like to maintain the existing manual control and power feed, am I better off converting the existing lead screw to a ball screw or somehow add a second lead screw to the the machine and disengage the existing one when runing CNC? Any thoughts on how to disengage the new lead screw when running manually?

Brian

[mike hide]

Originally Posted by bhowden I am looking at a 14 X 40 manual lathe with DRO and thinking about the best way to convert it to CNC. Most of the conversions I have found on the web seem to add aadditional zero (low) backlash screws and drive them with open loop stepper motors. I am wondering why I can't use the existing drive by adding pullys and stepper motors to the existing handles. There is plenty of backlash (say 5 to 10 thou +) but the DRO has an output so I would think you could get a closed loop controller and ignore the backlash. Am I missing something here? I do not see many (any?) examples on the net that do this. Any suggestions for controller software that understands closed loop feedback? Brian

Seems to me closed loop systems are sensitive to backlash as it introduces a nonlinearity into an otherwise linear system,which in turn can result in system instability....Again I thought the stepper motor was ideal solution for an open loop system being cheap simple and precise, providing it is not overloaded and operated within it's design parameters.

[titchener]

Originally Posted by bhowden Assuming I would like to maintain the existing manual control and power feed, am I better off converting the existing lead screw to a ball screw or somehow add a second lead screw to the the machine and disengage the existing one when runing CNC? Any thoughts on how to disengage the new lead screw when running manually?

Any reasonable controller (I use Mach from ArtSoft) will allow you to use an MPG (a knob type wheel) to manually run the machine. If you want to get fancy you can set the lathe up with a dedicated MPG for each axis. I'm cheap so I use a single MPG with pushbuttons to set it to the selected axis.

A "zero-backlash" (well, close to zero at least) ballscrew machine with an MPG control is very nice for manual work, its not worth the extra effort to try to keep 2 lead screws on the machine, do the ballscrew conversion and you'll be set.

Also, like every retrofitter finds out, once you have the machine running CNC style you'll find out that the time you use it in manual mode is a lot less than you thought it would be, especially with a flexible control like Mach.

Good luck-

Paul T.