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Author Topic: Weird Calibrating Issues!!  (Read 3111 times)

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Weird Calibrating Issues!!
« on: September 10, 2015, 02:02:01 PM »
We have a simple X/Y plasma table on servo motors, chain driven on THK rail.

We calibrated steps/per at 44in.  When doing a 4in square we are .15 under.  If we jog out to 44in, we are dead on.  If we go out to 68in we over .187.   So its acting like   0- - - - under- - - 44+++over+++68 .  No idea of what to do.  Please help :)
Re: Weird Calibrating Issues!!
« Reply #1 on: September 10, 2015, 09:51:59 PM »
If the chain is the foundation for all distance traveled, that is a poor basis. The error in the chain will be accumulative. You would be better served with a rack gear on the table, and the servos driving a gear pinion to move the X and Y axis. Rack gears are made with a uniform pitch, and the rack is cut with precise distance tooth to tooth.

For example, if each link was 1/2", but to be more precise if might be .502 from pin to pin, +/- the manufacturing tolerance. If you pulled the chain tight and measured 10 links, your total distance would be 10 X .502=5.020 inches. The actual distance is .020 over 5", not very noticeable.
Now take 100 links of your same chain, and do the same test.  100 x .502 = 50.200 inches, or almost 1/4" over 50.000 inches. Very noticeable.

I hope this makes sense.

John

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Re: Weird Calibrating Issues!!
« Reply #2 on: September 11, 2015, 12:03:57 AM »
Variation over travel might not be constant, ie error linear with distance. You might be able to using screw mapping to map and eliminate the variation.
Re: Weird Calibrating Issues!!
« Reply #3 on: September 11, 2015, 08:12:31 AM »
I'd think you could get it closer than that, using new chain.
This is a good place to use the auto cal. feature.
I would run the auto cal. in the mid third of the axis and check again.
Russ