Machsupport Forum
Mach Discussion => FAQs => Topic started by: miamicontrols on September 10, 2015, 02:02:01 PM
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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 :)
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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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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.
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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