I seem to remember a similar situation some time ago where the fix was to reverse the step low/high in config/ports and  pins. (Then toggle the reverse setting in config/homing/limits to keep movement in the proper direction). It's a long shot, but will only take a minute to check out.
Al

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Maybe a daft question but it's not backlash?

Brian

Hi Bobby, you are not alone in this.

Since yesterday I have the same kind of problem. The Z is moving down twice the distance that it moved up. So when I zero Z and jog up till 70, and than jog down again the Zero point is at +35.  So after each Z move up, it diggs in half of the lift.
Spindels and all are clean and move without friction. I tried pushing up and pulling down the Z during movement, always same results. Stepper makes the same sound in both directions.
Replaced the microstep driver.
 
I will post here again if I find something.
Frank

I found my problem and solution. It was interference from the computer. I replaced the PC and all problems are gone.
Frank

my first thought is with the z, as i am finding out, correct counterbalancing is pretty important...   have you experimented with different weights or even have it counterbalanced???   bill

hi,i had similar problem with y-axis,after i finished job  ,the tool was  always few milimeters off the zero,my problem was the rubber conection  that was holding screw and motor together.i realized that after someone suggested me to run my y axis at extra slow speed,and there it was,motor was running but  tool didnt.

Hi
I am having the same problem, have you found a solution ?
This is how It occurs on my machine....
I have purchased aspire 3 upgrade, last week. I have run preliminary tests: The z-axis is diving down while cutting, it progresivly lowers its posision while cutting a 3d image, not by much but it is steadily working its way down into the table. I have found a thread about this on the aspire forum but there was no resolution to this problem, I have taken all the steps sujested but I am still having the same problem with the z axis.

I am pograming on a computer running vista, 6gb ram - then exporting to a computer running xp via a memory stick, into Mach3, this computer runs my cnc router, I have sucsesfully used V-carv pro on this machine before Aspire with no such problems.

Please advise....

From what I have read on other forums this appears to be an Aspire problem (within the GCode generated).
If your Z axis DRO shows the increase in depth then there is a more than good chance it is in the GCode and not your machine.

Tweakie.

My Z does the same thing, But it is always high by about .25 in at the end of the program.  It is a constant.  I have tried different parallel ports and different motors.  Nothing seams to work. My Y axis used to do the same thing only on a much larger scale. It would be off by inches after a few moves.  Then it stopped and is correct again. But the Z axis is now doing it.  I have even switched the x and y outputs but the result is that the x axis would now be off.  I have tried several different breakout boards with the same result.  I am at a loss.

My Z does the same thing, But it is always high by about .25 in at the end of the program.  It is a constant.  I have tried different parallel ports and different motors.  Nothing seams to work. My Y axis used to do the same thing only on a much larger scale. It would be off by inches after a few moves.  Then it stopped and is correct again. But the Z axis is now doing it.  I have even switched the x and y outputs but the result is that the x axis would now be off.  I have tried several different breakout boards with the same result.  I am at a loss.

Lower your Z axis acceleration by half. It's losing steps on the complex 3D moves.

I had exactly the same problem. I had been using my machine to do 2.5D cutting for months with no problems, but when I started cutting 3D parts, I had the same problem. Slow down the Z accel and the problem went away.