CNC daydreaming

[rabler]

Rabler - you should know stepper controls do NOT take feedback from the servos. They just issue commands and expect the machine did it right. I put this concept on rebuilds in the 90s. After ruining only the high dollar parts, I went out and found a servo control with position feedback. My opinion - stepper control is for low end hobby use only. maybe I am a snob. But its a well earned opinion.

I'm aware that steppers can miss a step if overloaded, and that this can cause real problems. I'm aware that servo motors are considered the solution, but I'm not sure which software solutions are able to take advantage of this, or if it is handled in the hardware servo driver. Some learning to do.

 

[rabler]

I spent an hour this morning skimming Charlieman22's thread.
I have an oscilloscope, DVM, and a Keysight 3 channel DC supply, so the basics for testing.
At this rate I'm going to end up with a VMC before I have a shop to put it in

 

[JimDawson]

Say Jim, what control would you suggest for the OP? My Camsoft and your custom controls are a bit over the top for a first timer. Mach used to be suggested, sounds like that one is not so much any more. linux CNC?? Centroid?? I really have not kept up here.

@rabler suggested he would like to use LinuxCNC since he has experience with Linux. Mesa or Galil products are compatible with LinuxCNC and both have +/- 10V analog control capability. There may be others also.

I do like those DMM servos you turned me on to. They come with a step driver for the low end controls. After ALL YOUR HELP, getting +/- 10 volt analog servo mode going, they are wonderful. But I do not know a control to suggest that can use this.

I have been happy with my DMM servos, they have been running my lathe for the last 3 years, running almost every day. I did just replace my spindle motor with a 7.5kW Delta (Automation Direct SureServo2) servo motor to have full C axis capability. Very happy with the Delta system also.

 

[JimDawson]

I'm aware that servo motors are considered the solution, but I'm not sure which software solutions are able to take advantage of this, or if it is handled in the hardware servo driver.

The servo loop can be closed at the drive or at the controller level. All modern servo motors have a built in encoder that is connected to the drive. The drives also have an encoder output that can be connected to the controller.

Normally if the loop is closed at the drive, the system is commanded by step & direction signals just like a stepper system. The drive does its best to move the motors to the commanded position. This works very well.

Closing the loop at the controller is done by connecting either the drive encoder output or an external encoder to the controller and commanding the drive with a +/- 10V analog signal. My preferred method of doing this is to use a magnetic encoder attached to the load (table) and connect that to the controller. This automatically compensates for any backlash (within reason) and any ball screw error.

Basically any software solution can use one or either of the above systems, just depends on your preferred software.

It may be possible the use the existing servo motors and drives on any VMC you choose to buy. Just depends on the installed hardware, and what documentation is available for that hardware.

 

[Karl_T]

@rabler suggested he would like to use LinuxCNC since he has experience with Linux. Mesa or Galil products are compatible with LinuxCNC and both have +/- 10V analog control capability. There may be others also.



I have been happy with my DMM servos, they have been running my lathe for the last 3 years, running almost every day. I did just replace my spindle motor with a 7.5kW Delta (Automation Direct SureServo2) servo motor to have full C axis capability. Very happy with the Delta system also.

Rabler, The Galil card is a SUPER DUTY real time controller. If LinuxCNC works well with it, that would be a top end solution. Both Jim and i are sold on using the Galil products for machine control.

 

[rabler]

Rabler, The Galil card is a SUPER DUTY real time controller. If LinuxCNC works well with it, that would be a top end solution. Both Jim and i are sold on using the Galil products for machine control.

Yes, I saw that mentioned in Charlieman22's thread and caught that Jim said it was compatible with linuxcnc. I also found threads on the linuxcnc site showing it was supported.

While a far cry from completely understanding it, at this point I'm comfortable enough with the linuxcnc->galil interface and C library support that I'm going to focus on VMC specs and prices to see what makes sense in terms of a physical machine.

It may be possible the use the existing servo motors and drives on any VMC you choose to buy. Just depends on the installed hardware, and what documentation is available for that hardware.

It would be nice to reuse the existing servo and drives as they're not cheap. Anything I should look for or avoid in this department? I'm browsing VMCs to get a feel for cost, weight, power requirements, size, and capabilities to see what makes sense to me personally.

 

[rabler]

Since the spindles run on a VFD, is it reasonable to use an appropriately oversized VFD to run one directly off single phase 220V? Looks like the rest of the electronics is mostly a matter of AC->DC conversion, that's easy enough to manage on single phase? I would imagine the servo's need a bit of amperage.

I have a 20HP RPC and am not opposed to upgrading that, but am not sure there is a need.

 

[JimDawson]

It would be nice to reuse the existing servo and drives as they're not cheap. Anything I should look for or avoid in this department? I'm browsing VMCs to get a feel for cost, weight, power requirements, size, and capabilities to see what makes sense to me personally.

The existing controls would not be a deciding factor for me. Overall mechanical condition is much more important.

Post 1987 Fanuc red cap motors/drives are basically not compatible with anything but a Fanuc controller. The Fanuc yellow cap (DC) motors/drives may be compatible with other controls. Most other controls use more conventional servo motor control. Availability of documentation for the existing motors/drives is also important.

 

[JimDawson]

Since the spindles run on a VFD, is it reasonable to use an appropriately oversized VFD to run one directly off single phase 220V? Looks like the rest of the electronics is mostly a matter of AC->DC conversion, that's easy enough to manage on single phase? I would imagine the servo's need a bit of amperage.

I have a 20HP RPC and am not opposed to upgrading that, but am not sure there is a need.

The existing spindle MAY run off of a VFD if it is not a servo spindle (the line between a VFD and a servo drive is a little blurry today). While it is possible to oversize a VFD and run it off of a single phase supply, since you already have a substantial RPC, I see no reason to do that. If the installed spindle motor is a more or less standard 3 phase induction motor, then it would run fine on a VFD. If the installed axis drives are brushed DC motors, then running those on single phase is no problem. If they are AC servos, then most likely would need 3 phase. New axis drives could most likely be run on single phase.

Just to put this in perspective let's take the case of my lathe. 7.5kW (10HP) servo spindle, four 1.8kW servo axis drives, one 2HP hydraulic pump, and other misc. hardware all happily running on a 15HP RPC. Prior to upgrading my 7.5kW Fanuc spindle motor, I was running it off of a VFD. Interesting enough, my power usage actually went down after replacing the original spindle motor with the servo system.

 

[Karl_T]

Yep, just look at the color of the fanuc servos, red=bad, yellow=good.

I agree with Jim, machine condition is FAR more important.