eccentric turning

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29 Nov 2018 06:57 #121578 by Peter_Cassar
Replied by Peter_Cassar on topic eccentric turning
I've spent some more time today trying to get the setup correct. I believe I have a lot more work to do regarding the C axis Joint 2 config in the hal file. I read something in the docs that mentioned the basic_sim.tcl file generates a cmd.hal file that configures the C axis in the hal . This is where I get lost, I ran the sim on the controller and picked up opa_cmds.hal that it generated and moved it to my CNC lathe config and referenced it in the HALFILES section in the INI I've attached the files here if someone can tell me what else I need to do I would greatly appreciate it.

I suspect the Joint 2 following error mentioned previously is related to the fact that the system does not receive the signals when the external_offsets mode kicks in. The X axis positions the tool and then stops.

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29 Nov 2018 10:18 - 29 Nov 2018 10:19 #121589 by andypugh
Replied by andypugh on topic eccentric turning

Peter_Cassar wrote: I've spent some more time today trying to get the setup correct. I believe I have a lot more work to do regarding the C axis Joint 2 config in the hal file.


I don't think you want a C axis, and I think that the Tcl files from the sims might be very much complicating the situation.

For what I think you want all you need is the opa component loaded and connected to the spindle position with the output connected to the X axis external offset.

Do you still have the original working 2-axis lathe config? I would start with that, add the opa.hal and opa_panel.hal files and change this line in opa.hal
net E:angle          <= axis.c.pos-cmd
net E:angle          => xo.angle
to
net spindle-revs          => xo.angle

Your current config is an untidy hybrid of a sim config (with simulated spindle) and a real config with closed-loop spindle speed (which you might not want either)
Last edit: 29 Nov 2018 10:19 by andypugh.

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29 Nov 2018 10:51 #121590 by Peter_Cassar
Replied by Peter_Cassar on topic eccentric turning
Thanks Andy, I still have the 2 axis config backup . I'll redo it tomorrow morning and let you know how it works out.

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30 Nov 2018 05:11 #121625 by Peter_Cassar
Replied by Peter_Cassar on topic eccentric turning
I have something working, I cleaned up the HAL file and changed the line to link the encoder position to the offset angle but it seems I need to scale the encoder position to degrees. I find that the raw position is giving x axis motion that is too slow to produce the desired
pattern.

My encoder is a 4000 count device (1000LPR) so the encoder scale is set to this value. The position value is derived from encoder counts divided by scale so this is proportional to angular position . How do I create a signal in HAL to scale the position x 360 (degrees) and limit it in the range of 0 to 360?

I'm wondering if the eoffset-per-angle component does a modulo 360 internally . I think it might and that would solve one problem.

As an experiment I changed the encoder scale to 11.111 which (4000/360) which suggests the correct amount of movement per rev of the spindle but I'm no expert at HAL. I need to preserve the encoder scale at 4000 to ensure my spindle sync moves work.

I realize spindle needs to turn slowly <100 rpm for this to work, back gear would be nice.

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30 Nov 2018 16:11 #121640 by andypugh
Replied by andypugh on topic eccentric turning
Yes, the eoffset_per_ angle wants to work in degrees:
github.com/LinuxCNC/linuxcnc/blob/master..._per_angle.comp#L313

Easy thing to try, ask for a 360 * N polygon, it might work.

Alternatively, pass the angle through a scale comp
loadrt scale
addf scale.0.servo-thread
net spindle-revs scale.0.in
net spindle-degs scale.0.out => xo.angle
setp scale.0.gain 360

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30 Nov 2018 16:28 #121641 by andypugh
Replied by andypugh on topic eccentric turning

Peter_Cassar wrote: I realize spindle needs to turn slowly <100 rpm for this to work, back gear would be nice.


I have been considering a sub-spindle:

second-from bottom 2 pictures here: www.lathes.co.uk/holbrook/page4.html
And a home-made one: www.practicalmachinist.com/vb/bridgeport...experimental-207139/

An important point from that second link is that you don't need a set of spindle bearings, you can use a live-centre in the driven spindle, and arrange for a drive-dog plate to simply rotate relatively slowly around it while the live-centre body spins at full spindle speed.
The following user(s) said Thank You: Peter_Cassar

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30 Nov 2018 19:56 #121646 by Peter_Cassar
Replied by Peter_Cassar on topic eccentric turning
Andy, That's done the trick. Just for reference below is what I added to my main hal. The signal spindle-degs is referenced in oap.hal because for some reason the system couldn't find xo from the main hal. The motion appears correct now that the encoder position is scaled to degrees. Thanks for your help and I'll try turning a hex nut today.

<--hm2_stepper.hal-->
loadrt scale
addf scale.0 servo-thread
net spindle-revs scale.0.in
net spindle-degs=> scale.0.out
setp scale.0.gain 360

<--opa.hal-->
net spindle-degs => xo.angle


About the slow spindles. I was thinking of an auxiliary worm drive off the rear of the headstock spindle that I could engage/disengage and driven by an ac motor or even a servo would provide plenty of torque . The current spindle arrangement on the lathe has not much torque below 100rpm.

I have a 16inch lathe like the one you linked to in your last message. It's got a reduction gearbox before the spindle drive and the lowest backgear speed is 15RPM . The planetary drive is a great idea although I would have to add an encoder to the output shaft which wouldn't be too much of a challenge.

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30 Nov 2018 20:17 #121648 by andypugh
Replied by andypugh on topic eccentric turning

Peter_Cassar wrote: I have a 16inch lathe like the one you linked to in your last message. It's got a reduction gearbox before the spindle drive and the lowest backgear speed is 15RPM .

That might be slow enough.

The planetary drive is a great idea although I would have to add an encoder to the output shaft which wouldn't be too much of a challenge.

If it is geared then the ratio is exact, and just scaling the normal spindle encoder differently will do the trick.

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30 Nov 2018 20:42 #121652 by Peter_Cassar
Replied by Peter_Cassar on topic eccentric turning
OK that makes sense. I'll put some thought into gearing down the spindle. toothed belt reduction might even be the way to go on this machine, perhaps 2:1 or 3:1 is enough. The larger lathe could be converted but would need big motors!

The cutters I'm planning to make are only 3/4 to 7/8" diameter for watch and clock work so not as much torque is needed for machining these.

Another neat thing to try with the eoffsets would be to pump both x and z axis simultaneously to perform Guilloche or Rose Engine turning.

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30 Nov 2018 20:52 #121653 by andypugh
Replied by andypugh on topic eccentric turning

Peter_Cassar wrote: OK that makes sense. I'll put some thought into gearing down the spindle. toothed belt reduction might even be the way to go on this machine, perhaps 2:1 or 3:1 is enough.


What model of lathe is it? There are possible solutions.

See, for example the epicyclic back-gear here: www.lathes.co.uk/latheparts/page4.html
Or this solution:

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