Screw Accuracy

[hardrocker]

A bit of a strange one, but enough experts here, someone should set me straight, or least have an opinion! [First two paragraphs are the background, after that I get to the point]

Went to look at a factory re-man VMC last week - beautiful! Looked great, pretty much all new components, and best of all, the laser report [for electronic componsation] showed great screws (supposedly new), with nothing but an occasional +.0001, -.0001, tallying a total of .0004 over 30inches and the last 10 inches finally giving another micron or so per inch to total .0008 over 40 inches; even better in the Y; 300mm ballbar looked great [just over .0001 each way, and neither on an axis].

Now it gets quirky, it had a controller we didn't need, but another customer saw the machine and wanted it for that exact reason [but we had first chance]. Anyway, the dealer called while we were deciding [only a day, but a day non-the-less] and said they'd give us a better deal on another one [with a different controller] they had coming, if they could sell the first one. I said that sounded like a win-win situation: if it was as good of machine.

The report comes up on my email today, and the X is creaping .0001 per inch, sometimes as much as .0003, but totaling .0045 over 40. Identical on the Y. Ballbar is nearly identical to the other machine though [actually .00001 worse total, but min & max almost perfectly equal, so I'm not splitting hairs]. This one has reground screws, obviously the grinder was off feed cal by .01% or the temp was wrong or something [if I interpret the +/- correctly the screw would be .0045 short @ test temp]. The factory says that it is in spec [no mechanical error over .0003], and the electronic compensation takes care of it anyway. However, I'm looking at some nasty temp swings [mostly high temps actually], so I'm a bit worried that the compensation won't follow with that much mechanical error....or with the high temps does it actually play to my favor?

Regardless, I feel like an idiot trying to make everyone a winner, and letting a machine that impressed me slip away. What do you guys think?!

[hardrocker]

I didn't notice until this morning that the ballbar errors are at the axis, thus in my puny mind it probably has some slop in the nut/screw too; again 1.5tenths I'm splitting hairs, but doesn't that lead to quicker wear?

Of course, if I add up all the time I've spent trying to find a great value on a good machine [actually machines; each time I found something that should work, the machine was sold while somewhere here had to ponder about it] and the value lost to projects I should have been working on instead, we probably could have budgeted for a new machine in pretty much any flavor!