Ballscrew Mapping to Having an Effect

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21 Jun 2021 18:16 - 21 Jun 2021 18:17 #212574 by CNC4Life
So I'm trying to laser calibrate out the error on my ballscrew and I'm not having much luck. One of the quirks of my setup is I'm trying to maintain backward compatibility with the old control so my machine can run the same programs with little to no changes.

The machine's X/Y/Z are homed to the positive limit and all axis move in the negative direction from home. I can't put negative values in the first value of the octet or LinuxCNC gives me errors. So I tried moving the machine from home/limit+ to limit- and vice versa. The change in values after compensation are almost nothing. Like on the last value where the error is .00194" I get a change of maybe .0002". The results are repeatable over multiple runs both comped and not.

I'm trying to at least match the OEM controller that could get the machine within .0002" across 12".

Any help would be greatly appreciated. I'm running 2.8.1 of LinuxCNC
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Last edit: 21 Jun 2021 18:17 by CNC4Life.

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22 Jun 2021 00:10 #212623 by andypugh
If LinuxCNC can't accept negative numbers in the screw comp file then I think that LinuxCNC is wrong.....

What is the exact error message? (Then we can search the source code for it)

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22 Jun 2021 01:55 - 22 Jun 2021 01:56 #212632 by CNC4Life
joint 0: compensation values must increase (X)

It created an error for each entry. Which was crazy when I had 118 entries.

Octets were
target out back
0.0 0.000 0.0002
-1.0 -1.003 -1.005
-2.0 etc.....

As soon as I made them positive LinuxCNC didn't complain.
Last edit: 22 Jun 2021 01:56 by CNC4Life.

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22 Jun 2021 18:55 #212693 by Joco
I followed this thread in going down this path at one point:
forum.linuxcnc.org/38-general-linuxcnc-q...adscrew-compensation

The compensation file that resulted worked with linuxcnc. I eventually installed my old 5um scales to create a closed loop system so as to ge better tolerance on resulting parts. But that was more due to other limitations.

Cheers - j.

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23 Jun 2021 13:24 #212739 by andypugh

joint 0: compensation values must increase (X)


Ah, that is actually reasonable, as the system needs to search through them to find where it is.

But you need to start at the most-negative value and increase. So negative is OK, but the numbers need to increase monotonically.

ie:

-100 ...
-99 ...
-98 ...

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23 Jun 2021 19:59 #212762 by CNC4Life
Well, I feel sheepish...

I'll try that!

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