If the exact stop mode test fail, What I'll do is dismantle the Z-axis -> inspect & measure the parts -> re-assemble it the correct way.

I suspect the plate V isn't parallel enough. tight at top loose at bottom or vice verse. This can't be fix by turning the eccentric bush...

On the Z-axis, all the rollers must be on the same flat plane, and the slide plate itself must be flat.

Then the eccentric bushes must set the roller pressure quite tight (a lot of "springy" preload). If the holes for the bushes were drilled incorrectly (too far apart) in the spider, then the holes on one side need to be welded up amd re-drilled about 2mm closer to the other side.

Hi Gerald,

I think we'll be looking/dismantling the z Slide tomorrow now as we have only just finished getting the racks fixed back on strait this afternoon. They are certainly much better now anyway. Hopefully that is another issue with the machine ticked off now.
http://i1108.photobucket.com/albums/...1-10170128.jpg
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After a couple of attempts at re drilling through the rack and rail/angle section we decided that we would just try the tape on its own as this way we would not have bolts not drilled and tapped perfectly square twisting the rack. As you have suggested previously Gerald this is actually quite adequate on its own with out the bolts. In fact we have actually just used a good quality automotive tape that we use for quite a few things and to be honest unless you cut it off it is not moving on its own. We did make sure we prepped and cleaned both the rack and bottom of the angle section very thoroughly with both acetone and IPA before applying it then clamped down its length to give a decent amount of pressure to the join.

Sean (or anybody else) - Any chance you can explain the difference between the setting you have noted? They are not ones we have used or altered previously. And I really have feeling that the distance traveled / steps per mm issues we are having could be a software or calibration issue.

Forgive me stating what you may have already done, but have you tried the automatic steps calculation from Mach 3? Put a pen in the spindle and move the gantry 2m a few times, measure the actual distance moved and let Mach calculate steps (or you could fit your fancy meter at the end of the 2m stroke to get an even more accurate measure). I did a few rectangles 2mx1.5m for precision (repeatability) purpose.

Remember, you may be able to calculate to 3 or 5 decimal places, but the decimals are only accurate to the number of places you did your measures to. Wikipedia significant figures entry quote:
Ithink this means that, if you can measure 2000.0 (150.0)mm, then you can have confidence in xx.xxx (xx.xx) steps.

Only tape holding my racks on for a year now. No signs of weakness, even though the motors only run over the racks once a week.

Hi Red - Thanks for replying but we have been using the calibration setting and taking measurements over 980mm with a good quality set of engineering vernier calipers.
I am hoping that having the racks straitened/squared properly will help with this incremental error but haven't had a chance to really check it properly as we are working on the z slide at the moment.

Gerald - The holes for the V rollers (W3) are drilled at a spacing of 138mm and the Z slide measures roughly 99.3mm across from point to point. There are captive bolts welded on the back of the slide. On the Drawings it says the spacing should be approximately 132mm depending on v rollers and z slide.

Sean - we had a look at the Mach 3 manual regarding those settings and they appear to only to refer to the post processing reference/origin points. Ie one references the origin for all cordinates from 0,0 the other takes the origin as the previous point. Our post processing is set up for all points taken from origin.

Gerald dont worry just found your post regarding this subject http://mechmate.com/forums/showthread.php?t=878:o