Measurements Off in Mach3

[ScottBob]

I have a USB 6040 Chinese machine. I have cut several things with it and they have turned out ok, but they have all been small. Yesterday, I cut something that was 17.5 inches long. I had a box that was supposed to be centered in the material, but it was off. Then I noticed that although my piece measured 17.5 inches, if I jogged the axis from one end of the material to the other, it measured 20 inches in Mach3. I went back and calibrated my motors again and they were off very slightly. I fixed that, but my measurements are still far off by the same amount. I am telling Mach3 to move the axis one inch. I use my micrometer to measure. I am sure it is not NASA quality measuring, but it is very close. I don't think it could be enough off to cause 2 and 1/2 inches in difference in a 17.5 inch span. Should I go back and tell the machine to move 20 inches and see tell the machine it moved 17.5 and see if it fixes it? Is there some other variable I am unaware of? Any help would be appreciated!

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[ger21]

Should I go back and tell the machine to move 20 inches and see tell the machine it moved 17.5 and see if it fixes it?

Yes, tell the machine to move 20" and see if it actually moves 20"

[ScottBob]

That fixed it! Thanks. I wasn't thinking how only a fraction of an inch would add up the bigger the job was. So, I used the 20 inch and it only moved 17.5. I plugged that in to the box and it fixed it. I was using the small measurement because my micrometer only goes so big. I am sure it is not perfect now, but it is close enough for me to keep learning... Thank you again,

Scott