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Author Topic: Axis calibration  (Read 1746 times)

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Axis calibration
« on: December 07, 2016, 10:07:16 PM »
I wonder if I am missing some setting...
As I mostly do text based wooden signs with my router, I really never put much effort into getting the axis's super accurate, I just used a 24 inch ruler and estimated...I recently rebuilt the machine to gain Z distance and increased stiffness...anyway...(I have done this several times)...I can get the X and Y axis to repeat +/- .001" measured by an electronic digital caliper, over a 5" range...now when I cut a 5" circle (gcode generated by VCarve)...the circle ends up about  +.032" across the X and +.044 across the Y...not really sure if something else is going on...tomorrow, I will cut some squares and measure...

A couple of questions...I read somewhere that you MUST re-home an axis after you use the calibration routine...True?

Also, If I can cut say a 5" square and end up with (tool diameter compensated for) 5.030", can i just go into the axis calibration and say to move 5", then enter 5.030? THis is what I will try tomorrow, just did not know if there was some other config setting that may be creating the circle errors...

Thanks
Leslie

Offline RICH

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Re: Axis calibration
« Reply #1 on: December 08, 2016, 06:25:36 AM »
Leslie,
Make sure that you remove any backlash BEFORE using the axis calibration by making a move in the direction you move for the calibration.

It is best to calibrate the axis over the longest distance possible, say 24", since any error would be small in a short distance.
BUT
That would assume that the screw / components are linear over the 24" distance. That may not be true as the actual movement
may vary over that distance. Once calibrated  over the long distance, you should try different sections of the screw to check accuracy
or better yet make a plot of movement to accuracy.

 
Quote
can i just go into the axis calibration and say to move 5", then enter 5.030?
Yes, just remember that, any axis calibration is only as good as the info given. Above comment would still apply.

RICH

Offline Katoh

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Re: Axis calibration
« Reply #2 on: December 08, 2016, 07:39:22 AM »
I agree with Rich, the longer the better.
The way I do it is move the machine to 0 (zero) in X or Y makes no difference, now move the axis your going to calibrate to +0.1", that should remove any backlash. Now make that point your "0". OK time to calibrate, tell the machine to move the maximum it can before hitting it's maximum, (e.g if that’s 60" go 59") and see how it goes that should give you the best calibration you can get. after doing one axis repeat on the same method on the others.
For backlash you can read the manual tells you how to measure it on each each axis, or watch https://www.youtube.com/watch?v=dJ6eadoJJqo then simply put in your findings in backlash compensation and your good to go.
Cheers
Katoh

Offline RICH

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Re: Axis calibration
« Reply #3 on: December 08, 2016, 08:55:44 AM »
Quote
then simply put in your findings in backlash compensation and your good to go.

Just a comment,
Different parts of a screw can have different backlash.
So if you are using  part of the screw then you can refine the backlash settings to that part and when cutting a circle
is can be of better accuracy. Remember that BC is not perfect and has it's limitations in use.

RICH

Offline Katoh

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Re: Axis calibration
« Reply #4 on: December 09, 2016, 06:32:40 PM »
Rich

Just a thought, could you not take a backlash reading from say 3/4 different places of the screw and average,  Or maybe they could look into screw mapping, but I thought that was more for turns per moment rather than backlash.
Cheers
Katoh

Offline RICH

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Re: Axis calibration
« Reply #5 on: December 09, 2016, 07:37:48 PM »
Backlash, screw mapping, whatever, is just a software attempt  at making something mechanically inferior right.
I never used the screw mapping feature and don't know how well it works. Most can't measure basklash  let alone
profile a complete axis movement. It's tedious work ..... spent a day profiling  a 40' long mill bed be one time!
Took a week to level the stupid thing.


You could use whatever value that gives you an improved accuracy. An average value would just blend the extremes of the backlash.
If your going to use a bunch of different measurements then square each value, add each squared value ( sum), and take the square of the sum.

One has  what they have, and if it's not to their spec, then they just need to improve the components or make the machine right.

RICH


 
Re: Axis calibration
« Reply #6 on: December 09, 2016, 11:57:13 PM »
Thanks for all the thoughts...I have a belt drive system...got a 5 inch square to repeatably cut 4.999 to 5.001...circles still +/-.030...only using 5 inches because I am checking with a digital caliper...will get a long accurate ruler and try...

Leslie