Jerky Z axis????

[bill south]

Has anyone experienced a problem on the z-axis (or I guess any other axis) where the head jerks on the column when going in the negative direction during manual rapid movement???? My lathemaster conversion does that and I'm wondering if the gibs may be loose or somehing. I set up the column on the bench and it worked wonderfully. I've tried adding more lubricant on the ways but it helped very little if at all.
I don't do a bunch of precision machining so it is still working OK. Just wondering if it is falling apart on me!!!
Thanks.
Bill

[Al_The_Man]

Is the axis stepper or servo driven?
Al.

[LongRat]

I had the same problem. Turned out to be caused by the backlash compensation in Mach3. It was winding the x axis through its backlash after half a rapid z move, then completing the final half of the z move. This showed up as a big jerking in z. I found it out by slowing the backlash move speed to 1%.
In the end I had to turn backlash comp off to fix it, as it was causing lost steps in z.

[bill south]

Thanks for the info. Al, I'm using servos. The jerkiness seems to be more mechanical than motor driven though. Also, I don't have any compensation programmed (I don't think). I'll have to check that to make sure I have not accidently turned something on. Again, thanks for the info.
Bill

[Ron111]

Two thoughts. Did you do the Mod to the Z-axis like Aaron suggest for his mill and the RF45s. I wonder if you have developed slack where the head bolts onto the carrage (I can't think of the correct name). I could see where slack here could make the head rachet down during rapid moves.

If you are using a counter balance or gas springs, I wouldn't thing that what I'm suggesting would come into play.

But, please do share what you come up with.

Ron

[bill south]

Thanks for the thoughts Ron!!! I did make the mod to the saddle (bushing) for the head and installed a larger bolt. I also anchored the bottom of the screw to make it more stable. BTW, I checked this alignement many times to make sure the screw was not binding.
The jerkiness is almost like the head itself is binding in the ways during negative movement like it's not getting any lube. Maybe a loose jib???? Is there a way to adjust the jib w/o taking off the head again. 250# monster.
Thanks for all the info.
Bill

[Pres]

Consider these remedies:
Put some zerk fittings in the slide area and pump some
heavy grease in there.
-or- a counterweight to offset the weight twist on the gibs.

[Al_The_Man]

The symtom of jerkiness in rapid Z- can appear to be a mechanical problem but I have also seen it due to the following error becoming too large due to the Z inertia at that speed and the Z axis control tying to remedy it causes a momentary rapid decel. the resultant noise or shudder, resembles a mechanical binding.
Al.