[rybov]

Hi,

I am researching concepts for a new mill design I am working on. On my travels on the internet, I have seen designs for large routers that used two stepper motors to drive the x-axis. One of each side of the table. It is my understanding that the drive signals simply is wired to both motors.

I have also seen designs with two servos driving an axis. See attached picture. My question is how this is done? Servos control signals are not the same for the two servos, as is the case with two steppers.

Does it require the CNC controller to manage an additional axis? Does this design require absolut position eencoders on both servos to work?

Any input is hugely appriciated.

Best regards
Jacob

[mike_Kilroy]

........
I have also seen designs with two servos driving an axis. See attached picture. My question is how this is done? Servos control signals are not the same for the two servos, as is the case with two steppers.

Does it require the CNC controller to manage an additional axis? Does this design require absolut position eencoders on both servos to work?
......

Jacob, there are many ways to get there from here with servos. some examples how it is done:

1) yes, cnc has 2 completely independent axes and commands X & X' so it does the control with 2 independent servos

2) ONE step/dir command sent to both X & X' axis drives: being a position command, the axis drives will maintain the coordination from 1 cnc command.

3) one analog speed command from cnc to one drive called master: then it outputs command to slave drive to follow it. 1 cnc axis to do this too. master is velocity drive, slave is position drive

[rybov]

Hi Mike,

Thanks for your reply.

I am actually surpriced there are more routes to go, but then I am totally new to servos.

Can you comment on which approch to chosse if I am build a mill for milling aluminium and plastics with a focus on precision? Or are they equally valid and depend on the technology you choose?

Best regards
Jacob

[Al_The_Man]

On the Galil Motion controllers I use, electronic gearing is used, this gears a slave axis off a master axis encoder at any ratio you wish and is changeable on the fly if you ever wanted to.
Galil now have a 'Gantry' command that works the same way.
For this however, you need a CNC control that closes the loop back to the CNC controller, in systems such as Mach, the CNC control does not close the loop, this task is passed over to the independent drives.
Al.

[underthetire]

Hi,

I am researching concepts for a new mill design I am working on. On my travels on the internet, I have seen designs for large routers that used two stepper motors to drive the x-axis. One of each side of the table. It is my understanding that the drive signals simply is wired to both motors.

I have also seen designs with two servos driving an axis. See attached picture. My question is how this is done? Servos control signals are not the same for the two servos, as is the case with two steppers.

Does it require the CNC controller to manage an additional axis? Does this design require absolut position eencoders on both servos to work?

Any input is hugely appriciated.

Best regards
Jacob

Well, since that picture is of a Mori Seiki NVD, obviously you can slave servos. It's done with 2 motors and 2 servo amps per axis. The NVD also has 2 sony scales per axis. They can measure not only position, but pitch and yaw as well. We have 2 of them.

[rybov]

On the Galil Motion controllers I use, electronic gearing is used, this gears a slave axis off a master axis encoder at any ratio you wish and is changeable on the fly if you ever wanted to.
Galil now have a 'Gantry' command that works the same way.
For this however, you need a CNC control that closes the loop back to the CNC controller, in systems such as Mach, the CNC control does not close the loop, this task is passed over to the independent drives.
Al.

Hi Al,

Thanks you for your answer.

1. The Galil controller looks interesting. I have had a brief look at their website and they even have a local distributor here in Denmark. Have you had any experience with driving an axis with two servos? I like the idea of electronic gearing / 'gantry' command, but with small differences in the two balls screws, won't that eventually lead to twisting or even locking up the axis?

2. Will LinuxCNC (formerly EMC2) be sufficient as a CNC controller to close the encoder feedback loop?

Best regards
Jacob

[Al_The_Man]

With the Gearing and/or the Gantry mode, the ratio can adjusted to suit any mechanical discrepancy, it is intended to run two or more axis completely separate mechanically, not the motors to be connected to the same shaft.
I have used it for two separate R&P's on either side of a Gantry.
I believe EMC has the equivalent, but not positive on that.
Al.

[rybov]

Well, since that picture is of a Mori Seiki NVD, obviously you can slave servos. It's done with 2 motors and 2 servo amps per axis. The NVD also has 2 sony scales per axis. They can measure not only position, but pitch and yaw as well. We have 2 of them.

Hi underthetire,

You are absolutly right, the diagram is of an Mori Seiki NVD. I included the diagram as a sketch of a two drive per axis system. The NVDs are extremely complex machines and I can't even start to imagine what goes in to making such a machine. I am however, very interested in finding concepts that will lead to less flex in the axis, especially the z-axis.

I heavily suspected that servos could be slaved, so maybe the thread is a bit misnamed. I started the thread to learn more about how it can be done in practice.

When I was thinking about the two drive concept, I thought that it would need two absolute position encoders, just as you mention the NVDs have. I have never heard of a system that also measures pitch and yaw. What do they use that information for?

Best regards
Jacob

[underthetire]

They will use that to keep the axis straight, so there is no side saddling going on. Ive seen it done different ways over the years, back in the 70's diamond turn machines used laser feedback to compensate. The nvd machines are pretty stiff machines. We don't push them very hard, but I know they will take a hell of a cut without any vibration. Mori does actually have a version called the nv that is faster with the same construction, just no scales and higher pitch screws. They call it dcg or box in a box construction. The guys that run them do some pretty tight work, and our smaller nvd's I've seen .003" end mills in them. The surprising part is they can actually tool change them and not break it off!

[rybov]

Al and undertehtire,

Thank you for your help. Guys like you make the forum a very valuable place.

Thanks
Jacob