Why Index and Phase for lathe threading?

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10 Aug 2015 00:46 #61245 by russtuff
From what I have been reading, LinuxCNC requires two signals from the spindle to do the threading. Is this true, and if so, why? It's my understanding that Mach3 only requires one signal, and I would just like to understand why we need two here.

I already have a C3 Pulse Index Card hooked up, so it sounds like I need to get another one (which is not a problem). When running two cards, I use Index and Phase A, but not Phase B. Correct?

The second question I have is about the number of pulses required on the Phase encoder. It sounds like more is better (makes sense), and some threads are saying go with anything that divides evenly into 360, and be over 100. This leaves 120 and 240 as being ideal. Is this correct?

Thanks everyone.

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10 Aug 2015 01:21 #61247 by cmorley
Because the developers in linuxcnc decided that one pulse per rev is not accurate enough.
Too much time between pulses for the spindle to change speed do to load etc.

IIRC you can actually use one sensor by tying the index and phase A together and invoking 'counter mode'

IFAIK there is no particular preferred # of pulses.

Chris M

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10 Aug 2015 01:54 #61251 by russtuff

Because the developers in linuxcnc decided that one pulse per rev is not accurate enough.
Too much time between pulses for the spindle to change speed do to load etc.

IIRC you can actually use one sensor by tying the index and phase A together and invoking 'counter mode'

IFAIK there is no particular preferred # of pulses.

Chris M

Thanks Chris. I see your point, however, I know Mach3 users have no problem threading with just one pulse per rev. Additionally, if you have an encoder with any number of pulses, it still makes no sense to me why you would have a second encoder with a single pulse. Isn't that the idea of having Index and Phase? Two sensors and two encoders (one with one pulse, one with many).

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10 Aug 2015 02:20 #61253 by cmorley
I am not sure you can say _all_ mach 3 users have had no trouble with threading.
I am also not saying it can't be done. - particularly if the spindle is big/heavy and a powerful motor running it.
What I am saying is that be using multiple pulses you have good threads within a large set of variables.

The general idea was to use one quadrature encoder that includes an index pulse.
inside these actually have three sensors in them.

If you make your own then you can get away with one sensor tied to phase A and index inputs (and set linuxcnc into counter mode) IFAIR
even better use two sensors - one for the pulse track one for the index track.
or best use three sensors two for the pulse track (quadrature) one for the index track

linuxcnc (and mach) need an index to know the beginning of the rotation.

There has been many ways to do this besides adding an self contained encoder.
proximity switch mounted close to a gear could work for the pulse track.

Chris M

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10 Aug 2015 02:54 - 10 Aug 2015 03:13 #61254 by russtuff
No I'm sure you're right, that not all mach users can thread with one pulse.

OK that makes a lot of sense, and I appreciate the thorough explanation. I am going to try the counter mode and see if I can get away with tying index and phase A together. If not, I'll cut a new encoder with a second track and get another C3 to provide a second pulse.

Thanks again :)

EDIT: I just had another thought. Can Phase A and B read the same source? For instance, if I mount two sensors to read teeth from the same gear, then have the index pulse read it's own ring?
Last edit: 10 Aug 2015 03:13 by russtuff.

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10 Aug 2015 16:07 #61260 by cncbasher
phase a and phase b are used to be able to read direction when threading , i.e threading a blind hole for example , you need to reverse direction accuratly and quickly
if all your doing it threading through holes then tie a phase to index is fine , as you dont need to read direction , and overrun is permitted

you'll find being able to accurately adjust speed of the spindle to the speed shown on the gui and breaking of the spindle , perhaps using a brake resistor to dampen current etc
is probably more important dependent on your control

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11 Aug 2015 01:43 #61280 by russtuff
That is helpful, thank you.
I haven't figured out how to tie index and phase A together yet, but I may bite the bullet and go with three sensors anyway since I know I'll want these other capabilities in the future.

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11 Aug 2015 08:25 #61284 by cmorley

phase a and phase b are used to be able to read direction when threading , i.e threading a blind hole for example , you need to reverse direction accuratly and quickly
if all your doing it threading through holes then tie a phase to index is fine , as you dont need to read direction , and overrun is permitted

you'll find being able to accurately adjust speed of the spindle to the speed shown on the gui and breaking of the spindle , perhaps using a brake resistor to dampen current etc
is probably more important dependent on your control


I assumed he meant single point screw threading rather then ridged tapping.

Chris M

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11 Aug 2015 08:26 #61285 by cmorley
use your c3 to input the single sensor then use HAL to connect that to the phase A and index of the encoder component.

Chris M

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12 Aug 2015 09:13 #61305 by russtuff
I currently have:
net spindle-phase-a <= parport.0.pin-15-in

and I get errors if I try to also have:
net spindle-index <= parport.0.pin-15-in

I've tried to assign index to phase-a too but that doesn't work either.

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