Controlling a lathe with a stepper spindle

[TheMechanix08]

Hello,

first of all thank you for developing such a nice motion control software!
I'm building a small CNC lathe at the moment and am unsure, whether grblHAL supports my needs.

I am using a (closed loop) stepper as a spindle motor since I want to do light turning, threading and C-Axis indexing.
I've seen that there a stepper spindle is supported which is nice unique feature.

Does the stepper spindle allow the user to seamlessly use the stepper as a normal C-Axis? I was also planning to use the stepper to do threading without an extra encoder.
Would this setup work in conjunction with iosender and are there special requirements regarding the board to effectively use the stepper spindle?

Best regards!

[terjeio]

Does the stepper spindle allow the user to seamlessly use the stepper as a normal C-Axis?

Should be possible since the stepper spindle binds to an axis motor. Seamlessly might be an issue, it may require some code changes to sync the current position on a switchover. The stepper spindle uses a 64 bit position counter to avoid overflow since it may run continuously for a long time. The axis positions are 32 bit so a conversion must to be done on a switchover? Or C has to be homed before it can be used?
Another, for now, is that the stepper spindle binds to the highest numbered axis so you will ned a board or a board map with support for six axes. A and B can be "faked" in a board map by "wiring" their signals to the Y motor. Or I have to add specific support for XZC in the core - a bit like the LATHE_UVW_OPTION.

I was also planning to use the stepper to do threading without an extra encoder.

It should be possible to use standard gcode for threading since the stepper spindle provides the encoder signals - derived from the position.

Would this setup work in conjunction with iosender and are there special requirements regarding the board to effectively use the stepper spindle?

It should work but until I add the option to specify axisletters to use for axes > Z it will display positions for A, B, and C. And the 3D viewer is currently limited to displaying X, Y and Z. For board requirements see above.

Anyway, the functionality you require is within the scope of what should be supported and it would be nice if you would try it out/refine it together with me. I cannot do it on my own since I do not have a lathe with a stepper driven spindle to verify on nor do I know the finer details of how it should work.

[TheMechanix08]

Seamlessly might be an issue, it may require some code changes to sync the current position on a switchover.

As long as I don't need to rewire anything or reflash I firmware, I would consider this seamless enough for now!

Another, for now, is that the stepper spindle binds to the highest numbered axis so you will ned a board or a board map with support for six axes. A and B can be "faked" in a board map by "wiring" their signals to the Y motor. Or I have to add specific support for XZC in the core - a bit like the LATHE_UVW_OPTION.

Ah, that's sad. I was planning to use an Espduino or a Nucleo board with a cnc shield to allow me to use 36 V and maintain a smaller formfactor compared to a BTT octopus or similar.
Do I understand correctly that this plan won't work in any way currently?

It should be possible to use standard gcode for threading since the stepper spindle provides the encoder signals - derived from the position.

Do you mean that I would be able to use G76 or that I could create a custom gcode that links the rotational and linear movement through basic motion commands?

Anyway, the functionality you require is within the scope of what should be supported and it would be nice if you would try it out/refine it together with me. I cannot do it on my own since I do not have a lathe with a stepper driven spindle to verify on nor do I know the finer details of how it should work.

I would love to help with that! My machine is almost finished mechanically and is just waiting for its controller in the next few weeks.

[TheMechanix08]

Sorry for the second message!
Just as a quick sanity check:

Is there any reason against me using a Nucleo F411RE or a F446RE in a 4 axis configuration? I could just ignore the Y and use the A axis as the stepper spindle/rotary axis. As long as my gcode calls for movement of the A instead of C axis the naming should be arbitrary, right?

I do not plan on using an endstop on the rotary axis. Thus, pins should be enough or am I wrong?

[terjeio]

As long as I don't need to rewire anything or reflash I firmware, I would consider this seamless enough for now!

I believe I have to make some changes to the code, the main one could be how to handle switching from spindle to C control. Is there any manual workflow for that? Somehow align the workpiece after turning and before starting C controlled motion? Or just reset the C position to 0? If you have gcode with combined turning and C controlled motion that may help answering this. Or does the CAM output separate files forcing a manual step?

Do you mean that I would be able to use G76 or that I could create a custom gcode that links the rotational and linear movement through basic motion commands?

You should be able to use G76. I have not tested it yet though.

Is there any reason against me using a Nucleo F411RE or a F446RE in a 4 axis configuration?

No.

I could just ignore the Y and use the A axis as the stepper spindle/rotary axis. As long as my gcode calls for movement of the A instead of C axis the naming should be arbitrary, right?

That will work - but I would rather map the A pins to Y in a custom map file and use C since I understand that this is how it is normally done. BTW which CAM are you going to use?

I do not plan on using an endstop on the rotary axis. Thus, pins should be enough or am I wrong?

There are plenty of pins for this application on a Nucleo-64. And I would prefer that you chose a Nucleo board over an Espduino since I use aF446RE as my main development platform.

[TheMechanix08]

BTW which CAM are you going to use?

The main parts that I intend to machine should be basic enough for conversational/manual programming or gcode generation through a quick script.
I think I still have a valid student license for MasterCam somewhere. So in case I can help with CAM compatibility, we can figure something out.

Is there any manual workflow for that? Somehow align the workpiece after turning and before starting C controlled motion? Or just reset the C position to 0? If you have gcode with combined turning and C controlled motion that may help answering this. Or does the CAM output separate files forcing a manual step?

For my personal usecase I would be fine with just resetting the C position as I won't be doing multiple milling operations with intermittent turning. The C position would thus remain consistent through all milling operations.
The personal F360 license does not allow millturn operations for example (afaik not even machining using a rotary axis) so for most hobbyists, millturn operations would need to be created using a different piece of software or by just pasting different gcodes into one file.

Again, in case I can help out by doing some research, generating gcode or testing features, I would be I would be happy to do so.

You should be able to use G76. I have not tested it yet though.

That's amazing!

I use a F446RE as my main development platform.

Then I will order this board.

[TheMechanix08]

My 446RE just arrived! Do you have any tips on tests I could do/firmware to install while I'm finalizing my controller build?

[TheMechanix08]

I configured the firmware for the board today and installed it to try out whether the interaction with iOSender works as planned.

Everything mostly worked out, except for one thing:
G33 and G76 moves lead to an ALARM 14. Is there anything less obvious I need to change in order for the stepper spindle to work for threading?

[TheMechanix08]

Hi @terjeio ! :-)
I just hooked up one of my axis steppers as the C axis/ spindle for testing.

Unfortunately it shows somewhat weird behaviour:

• C Axis indexing works perfectly
• The motor does not move when the I turn on the spindle. When I try to move the C axis while the spindle is active the motor turns in a weird jiggery way.
• When I try to perform a G76, the motor spins up to the requested 100 rpm but stops after some time and throws an alarm 14.

I've tried to catch this in a quick youtube short:
https://youtube.com/shorts/gXMAofVLJzY?feature=share

Do you have an idea what I might have configured wrong?

[terjeio]

Sorry for the unclear communication. G76 works well, but a M3 command does nothing when setting $4 is enabled.

So G76 works and M3 does not with motors enabled (according to the $4 setting)? Or do you send M3 only when S is 0?
Anyway it works for me so you might have some error in your board map?

My settings:
settings.txt

[TheMechanix08]

Sorry for the late reply! I spent the last few days building the final controller for my machine.

This evening I was able to run all axes for the same time. After a bit of playing around with the settings everything (C-axis, spindle, threading) worked just as expected. I genuinely do not know what my previous issue might have been.
The synching between the spindle and the C axis seems to work as well!

I'll do some finishing touches on the machine next week and hope to be able to do a test cut by then and hopefully post a short video :-)
Is there anything that I could try out for you?

Thank you very much for your help!

[terjeio]

Is there anything that I could try out for you?

Not really, only post feedback about how your project is going. Even if everything works out.

[TheMechanix08]

Good afternoon! I had a it of time to test the machine further, so I wanted to giva quick update.

-I tried internal threading (M8x0.75), worked really well (up to at least 500 RPM)!
-I also tried external threading (M10x1.5) as well. This did not work for me. I think that the machine was not lining up the individual passes. I am not sure whether this might be because of lost steps in Z, even though I don't think so as I reduced the spindle speed down to 50 RPM.
-After setting up my tool library I noticed that the offsets only take effect when I send a G43 after each change. Is that supposed to happen? I have the probing set to "normal". I do not plan to touch of the tool after each change as I am using a quick change system.

[terjeio]

-I also tried external threading (M10x1.5) as well. This did not work for me. I think that the machine was not lining up the individual passes. I am not sure whether this might be because of lost steps in Z, even though I don't think so as I reduced the spindle speed down to 50 RPM.

Color the workpiece with a marker pen and let the tool just engrave through the color to check.

-After setting up my tool library I noticed that the offsets only take effect when I send a G43 after each change. Is that supposed to happen?

From the LinuxCNC documentation:

Warning
The tool length offset is not changed by M6, use G43 after the M6 to change the tool length offset.

[jollydodo]

Hello,

first of all thank you for developing such a nice motion control software! I'm building a small CNC lathe at the moment and am unsure, whether grblHAL supports my needs.

I am using a (closed loop) stepper as a spindle motor since I want to do light turning, threading and C-Axis indexing. I've seen that there a stepper spindle is supported which is nice unique feature.

Does the stepper spindle allow the user to seamlessly use the stepper as a normal C-Axis? I was also planning to use the stepper to do threading without an extra encoder. Would this setup work in conjunction with iosender and are there special requirements regarding the board to effectively use the stepper spindle?

Best regards!

Hey, I'm in the process of converting a mini lathe to a cnc lathe as well. Although I have quite a bit of experience with different CNC mills, I find that the information about lathes is very minimal. Thankfully projects like grblhal from @terjeio don't forget lathes (unlike fluidnc for example), but it would be great to find more practical information about the builds. I would like to use the BTT Scylla for example (STM32H7) and figuring out XZ is not a problem, but the spindle setup is still something I'm not certain about. Using a stepper as a spindle sounds great since the wiring would be extremely simple and there wouldn't be a need for an external encoder for simple threading (if there are no missed steps off course, but not everybody needs an extremely over engineered rigid machine. I'm more into small aluminium stuff for hobby work). What is your experience up until now? @terjeio support for XZC in the core would be really beneficial if this stepper approach works well. It would allow a CNC lathe to be much more approachable I think.

[terjeio]

support for XZC in the core would be really beneficial

It is on my todo list to allow assigning axis letters to the motors at compile time, perhaps I can get it done in the autumn.