I just got a CNC mill setup from deepgroove on eBay, with the 280oz steppers... well, price was ok.
I've had time to set it up and run tests and such and I keep having trouble with the Z-axis binding up on the last few inches. It's unable to lift consistently at rates >10ipm even with low acceleration values. I even saw it unable to drop consistently early on but that seems to have gone away. Everything is nicely oiled. I switched the stepper to another axis on the driver and the problem is unchanged when jogged at the same speeds.
I had this question over whether it was binding in the leadscrew or the slide. I ended up unscrewing the pillowblock/motor mount from up top so I could just manually lift and lower it, the leadscrew and motor lift with the headstock. I do think I feel binding there consistent with the known problem area. So, it's the slide not the leadscrew. And being only on the bottom end of the travel I think I can rule out a problem with the headstock.
This problem appears to be a poor tolerance on ONE part of the slide. But I can't find one. I ran the axis all the way up which should have exposed the prob. I get 1.254" on the center rectangle the brass gibs deal with. I locked the caliper there and slid it down... it does not bind. I checked the sides both are 0.352" and again I locked the caliper and it does not encounter resistance the rest of the way down. Which leaves the possibility of the side edges which are not machined as contact surfaces, so contact seems unlikely and I see no marking- and overall bending or torsion of slide seems like my last possibility which frankly sounds the most plausible but I have no way to measure it.
Or... wait, when I tested that I didn't really try hard at measuring to the very inside corner on either side of the center rectangle. Are the surfaces used all the way into the corner or does the headstock's mating surface chamfer away from it?
I did set a program to run the axis up and down over this problem area at the fastest it would still work consistently, which was still quite slow (2-6 ipm), with hopes of "breaking it in". It does not seem to have had any effect, at least not in a couple of hours.
I saw the prior thread:
http://www.cnczone.com/forums/showth...535#post171535
And most important Nick Carter's gib adjustment instructions:
http://www.cartertools.com/millset.html
So I am considering adjusting the gib. But here's my question: Is this wise? The gib sets the loading all up and down the travel. It's only too tight at one point and it would be too loose everywhere else. Nor have I even determined if this is the problem, the calipers don't think it is. That prior thread has "Andy Fritz" (not a current forum user) saying he tried the gib adjustment and it never really fixed things, while "az7733" (also not a current user) said he had this problem and fixed it by getting a replacement axis from TaigTools. So it almost sounds like a waste of time and might just make things worse??