Rigid tapping with only A and Z signals

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psz
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20 May 2021 01:50 - 20 May 2021 08:42 #209597 by psz

Rigid tapping with only A and Z signals was created by psz

Is it possible to do rigid tapping without B phase pin? I only have 2 pins left on my MESA card for spindle encoder. As I understand B phase is necessary for direction sensing, however direction is already known, because spindle is commanded right/left by the controller. It can be assumed that the spindle would not turn backwards by itself before direction reversal by the controller.

Last edit: 20 May 2021 08:42 by psz.

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PCW
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20 May 2021 14:23 #209652 by PCW

Replied by PCW on topic Rigid tapping with only A and Z signals

I would expect the problem to be that the spindle reverses considerably
_after_ the command. With 2 pins I would think using them for A,B
and having a simulated index component would be a better solution

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psz
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21 May 2021 04:16 - 21 May 2021 04:17 #209727 by psz

Replied by psz on topic Rigid tapping with only A and Z signals

Yes, the spindle will reverse after the command, but we already know its direction, so at this moment counting A pulses alone is sufficient (B becomes redundant). This is how I see it. I'm little confused, as it's stated everywhere in the docs/forums that index pulse is mandatory for rigid tapping

. With A/Z it should be possible to somehow 'force' the direction of the encoder in HAL to the known one (reversal of the spindle). As for the A/B, how to setup such a simulated index?

Last edit: 21 May 2021 04:17 by psz.

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PCW
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21 May 2021 13:38 - 21 May 2021 13:39 #209756 by PCW

Replied by PCW on topic Rigid tapping with only A and Z signals

The issue I see with no A is the you know the commanded direction but
when you reverse if you count A in the reversed direction you will break the
tap because LinuxCNC will start moving Z up while the tap is threading down

You might avoid this on a Index+A setup by delaying the counting
reverse until the spindle velocity gets very close to 0

You can simulate the index with a component. This has the disadvantage
that the index position is "volatile", that is, if you shut LinuxCNC down, you cannot
restart a threading/rigid-tapping session because there is no physical spindle
position reference. just math that keeps track of angular position of the simulated index
(that is the fractional parts of a full turn)

Last edit: 21 May 2021 13:39 by PCW.

The following user(s) said Thank You: tommylight

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PCW
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21 May 2021 19:12 - 21 May 2021 19:12 #209780 by PCW

Replied by PCW on topic Rigid tapping with only A and Z signals

Here is a generic index simulation component to allow threading or rigid tapping for
systems with spindle encoders without index or step/dir driven spindles:

component sindex "simulated index at angle 0";
pin in float fb_in "position feedback from sensor, scaled in turns";  
pin out float fb_out "position feedback to motion, scaled in turns";
pin out float fb_degrees "position feedback scaled in degrees";
pin out float angle "angular position for readout";
pin io bit index-enable "connect to motions index-enable pin";

function _;

variable double fb_frac = 0;
variable double fb_frac_last = 0;
variable double offset = 0;
 
license "GPL";
;;
#include <rtapi_math.h>

FUNCTION(_) {

    fb_frac=fb_in-floor(fb_in);

    if (index_enable==1) {
        if (fabs(fb_frac-fb_frac_last) > 0.50) {  // when we wrap
            index_enable=0;
            offset=trunc(fb_in);
        }
    } 

    fb_out=fb_in-offset;    
    fb_frac_last=fb_frac;
    fb_degrees=fb_out*360.0;
    angle=fabs(fb_frac)*360;
}

Last edit: 21 May 2021 19:12 by PCW.

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Aaroncnc
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21 May 2021 19:22 #209782 by Aaroncnc

Replied by Aaroncnc on topic Rigid tapping with only A and Z signals

I know you mention only having 2 pins open but there are ways to get a few more depending on how you have it setup
if you have 3 end stops/ limit or home you can put them all on the same pin.
so in stead of having 1 pin for each you can set it so they are all one. just wire the switches in series.
so 3 home switches that used to take 3 pins now uses 1 pins
The change to the machine is that it will have to home each axis 1 at a time and bounce off the switch.
3 limit switches that used to take 3 pins now take 1.
You could go further and combine limit and home and free up another pin.

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