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General Mach Discussion / strange step loss.

« on: January 23, 2010, 11:12:37 PM »

I have constructed a new mill and have come across a strange problem. My z drive has been losing steps. It was losing them in the negative way and the bit just gets deeper. I tried slowing the drive down. I am running at 300mm per min with an acc of 6.8. still the same. I swapped a motor from my y to the z. Still the same. Now if I swap the wiring on the z, so it is set up to run the opposite direction then set motor outputs to be "dir low active" unchecked. I then loose steps on the up stroke. So I wrote some code, g01 z0, g01 z-30, g01 z-35, g01 z-30, g01 z-35 etc till I had 100 up and 100 down movements. Then went back to zero and was within .002mm so no lost steps. I cant see it being a motor problem as I changed it. I cant see it being a mechanical problem as when its wired in reverse then sent fwd with software the step loss follows the software. What am I missing. I am using a dell laptop d610 that is exclusively for the mill. The motor tests are great. I am using univelop 4/5 axis drivers. The particular code that is giving me problems is a converted jpeg, so there is tons of minute z movement.There's a total of 30,000 lines of code. About 50% of the lines have a z directive in them. After the program runs the z drive is out by about 6mm or some 3700 steps. Some of this movement is smaller than my resolution. I presume that if the command is less than the resolution then there is just no movement. I also presume that mach keeps tally of the numbers though and still calculates them into the next line. Is that correct? What should I try next?
Many thanks Bazza.