How to map a ball screw?

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12 Dec 2018 23:40 #122332 by Ross77
I'm looking fro help as to the best way to map a ball screw? I understand the comp file structure of commanded vs actual and backlash but want to check what the best way to do the whole process of checking was?

I have some varying errors in my mill, which may be binding in the jibs etc so I need to test the whole system and start by setting the scaling. My suspicion is that it is just the center part that has worn as the ends seem to ok when checked over a small distance using a 10mm travel DTI.

I have now borrowed a 5mic glass scale and DRO as a standalone system for checking. Assuming it is the ballscrew I was going to do an initial sweep in one direction and check the full travel for the scale check and then check again in small increments and log the variations for the mapping file

My question is , Do i need to do it in both directions as full sweeps or move back and forth?

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14 Dec 2018 13:36 #122417 by andypugh
Replied by andypugh on topic How to map a ball screw?
I think I would log all the way from one end to the other in 10mm steps, then step back all the way the other direction.
And then probably repeat this a few times (the number of times being related to how much I had managed to automate the process)
If you can log the DRO values to a PC while running a G-code file that steps backwards and forwards then you could do an infinite number (10) of repeats.
Thinking about it, there might be an argument in favour of making the sample steps an integer multiple of the leadscrew pitch. Or maybe good reasons to _avoid_ that. Certainly something to consider.

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20 Dec 2018 22:46 #122661 by Ross77
Replied by Ross77 on topic How to map a ball screw?
Thanks Andy
Averaging lots of samples seems like the best option, I think that's were I'm going wrong buy using too small a sample and introducing more error

Can i log it with linux cnc or would I need a a separate pc and program? any pointers as I've not done any data logging yet.

cheers

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20 Dec 2018 23:09 #122664 by andypugh
Replied by andypugh on topic How to map a ball screw?
You should be able to log with LinuxCNC.

See section 16:
linuxcnc.org/docs/2.7/html/gcode/overview.html#gcode:comments

To get the scale position in to G-code use one of the "analog inputs"
linuxcnc.org/docs/2.7/html/gcode/m-code.html#mcode:m66

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20 Dec 2018 23:44 #122667 by Ross77
Replied by Ross77 on topic How to map a ball screw?
Seems easy enough. famous last words :oD

I'll have a play and see how I get on.

Initial questions are where does "print" print to?

and "The current value of the input is stored in parameter #5399" where is that

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21 Dec 2018 00:24 #122668 by andypugh
Replied by andypugh on topic How to map a ball screw?
LOGOPEN, and LOG, (section 16) is the way to go.

(LOG, #<_X> : #5399)

Should do what you want.

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21 Dec 2018 03:34 #122672 by skunkworks
Replied by skunkworks on topic How to map a ball screw?
I think you can only have 256 points per axis.

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03 Jun 2019 16:55 #135761 by AndiT
Replied by AndiT on topic How to map a ball screw?
This seems to be right. The question is, how does linuxcnc handle the positions between these 256 values?
Is there an algorithm that avarages the space between or is it stepwise (linear compensation before/behind the values) until the next value is reached?

Does anybody know something about it? (have not found anything)

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03 Jun 2019 19:40 #135786 by andypugh
Replied by andypugh on topic How to map a ball screw?
Linear interpolation
The following user(s) said Thank You: AndiT

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