[endgrainguy]

On a new machine I"m building I'm getting missed steps on X axis, which seems to be caused by ball screw. I'm totally new to CNC, so if my questions sound stupid, I'd like to think its out of ignorance rather than simply stupididy. It's a 39" THK BLK 2020E KX-OI27, 20 mm diameter, 20 mm lead, four starts, with both its THK end mounts. I"m left to conclude that despite the smooth feel to me when turned by hand, the ball screw is causing missing steps.I should say that in motor tuning the axis sounds bad to horrible, especially at very slow speeds. The smoothest travel came from using highmicrostep values, and about 100 IPM setting at 1/25 or 1/32 microstep, but it's still usually a bit grumbly, and inconsistant in its smoothness, which I guess is a clue. It consistantly misses about 2.5% of its steps. i can put in a step value that will allow it to travel very close to the right amount, but errors accumulate after multimple jog commands, and I have determined the actual lead by turning the axis by hand with power off, and this corresponds to within a couple thousandths of the 20 mm one would expect from the THK screw.
I have an embarassing history with this ball screw. I initially planned to build not a CNC machine, but a manual XYZ template following joint cutting machine , a giant JDS Multirouter. When I started looking for the components to build it from I discovered CNCZone, which introduced me to linear bearings, and my first Ebay purchase, my big Ebay bargain, was a two axis table with THK rails on two axises, 39 inches of x travel, and a cross table with 11 inches or so-- all for $200. The X had this ball screw on it, which I knew nothing about, and it was in the way of what was going to be a lever pushed and pulled travel. In taking it apart, without even knowing it was a ball screw, I spilled the bearings. I did my best to pick them all up, but didn't understand the value and precision of the thing.
Then months later my CNC plan developed, and I went to put what I now realized was a very valuable ball screw back together. It's a THK BLK 2020E KX-OI27, 20 mm diameter, 20 mm lead, four starts, with both its THK end mounts. I saw I was missing bearings. I bought the smallest size in that category from the Ebay guy who sells them, .1237, as I measured mine at .124, and thought I would be OK if they were a little smaller. I only have a cheap, thousandths-measuring caliper, which i don't feel adept at using. I just now measured the smaller balls and they seem closer to a full thousandth smaller than the ones that were in the screw, so I guess the original balls must be closer to.1245 or so. I thought the smaller balls (about 10 of them)wouldn't create a problem, but perhaps because they ended up pretty much grouped together they are causing roughness and missed steps?
I now have the nut disassembled again. Are the smaller balls simply too small to be used in this screw, even just a few of them? Or maybe I need better/different lubrication? I put a little grease in when I reassembled,(to help me get those last few bearings in) but its pretty dry. Its also possible I put too many bearings in. I basically put in as many as I could. I just disassembled it, and they're a total of 215. Since it's a four start screw, one would expect a multiple of four, right? It seemed smooth, but snug. You could hold the 39" long screw in a vertical position by the nut and it will hesitantly at first, then faster, spin itself down by gravity. In one direction it even needs a little twist to get started. I seem to remember before I took it apart is was even smoother. Thanks for all your help. I wish I had just bought a working CNC machine and altered it for my needs, but it's way too late now.As ever, since I"m asking so much here, if anyone would rather talk than type, you can call me at 423 624-3048, or send me a time and number to call. Thanks, Al

[ger21]

Does the screw make the rough noises, or the stepper?

[ger21]

I'm guessing you're problems are PC or electronics related, or you're trying to go faster than your motors are capable of.
Do you lose steps at any speed, or only at high speeds. Run the same program with the acceleration and velocity cut in half. Does it make a difference?

Run the Mach3 driver test. Do you get a flat line?

Could also be noise on your step and direction wires. If the ballscrew turns smoothly, I doubt it's causing the problem.

[endgrainguy]

I think its the screw, but having limited experience with these things, I can't really be sure. But I"m pretty sure its the screw - it occassionally even "bumps". The travel discrepancy is actually worse at very slow speeds (tried 10 IPM last night). That's not surprising, since travel sounds smoothest at around 80-100 IPM.
Is this helpful: on Z axis, which travels correctly and very smoothly,I observe the following in motor tuning. At slow speeds up to about 40 IPM it's fine. Above this is stalls --awful screeching noise and no movement, --but above 55 or so, it rocks. I don't use this higher speed -I set it at 37, but is this normal behaviour? Is this "midband resonance?"
I talked with THK guy this morning, who was very helpful. I"m going to try reloading the screw, distributing the smaller bearings (15 of them) evenly spaced in the four starts, and with three fewer bearings than before. Will also run driver test for Mach. Will post results. Thanks for all the help!

[ger21]

Sounds like resonance on the Z axis, and you probably have it on the other axis as well. You could add dampers to try to minimize the resonance.
http://www.cnczone.com/forums/showth...639#post256639

Or get some Gecko drives with mid band resonance control built in.