I wonder. Hope more likely that maybe I just need to adjust the gibs. I
have never adjusted them, and to be honest I never considered whether or
not the previous owner ever did since I bought the machine from a third
party on behalf of his widow.
Now how to adjust the gibs. Hmmm... The X axis seems pretty straight
forward. There is a visible screw at each end of the gib. Not sure how
to get the feel for it though. On my little machines I can disconnect
the lead screw and push the table around by hand, but the table on this
thing is significantly heavier than that. Then there is the Y axis. It
looks like the gib and adjustment screws must be hidden under the wipers.
Well I ran across a reference that said to adjust the gibs based on
current draw of the servos. I wanted to try it and see if it made
sense. The post where I found it said to adjust he gibs so the motors
(different Hurco mill) so the motor draw between 3 and 5 amps. Ok. I
can atleast see what the current draw is.
I have done the X-Axis so far. It started out drawing about 2.65 - 2.8
amps across the range of travel with the ends being closer to 2.8. That
makes sense since the middle of the ways and gib will see the most wear.
I started tightening down the gib and it took a bit of adjustment
before I started seeing much change. Almost a full turn. At first I
thought that meant there was a lot of wear, but the change was pretty
linear until I started getting really tight. With a peak of a hair over
4 amps at the ends of travel its at about 3.7 amps in the middle of
Now remember, this machine originally ran on 90VDC and I have it running
on 78VDC, so I would expect higher current at the same rapid speed.
The fellow who described this process (I don't recall the location, but
I think it was on Practical Machinist) said his machine had near two
amps in current draw from the tight ends of the travel to the loose
middle. I'm seeing less than half an amp difference. That tells me my
machine is probably not as worn as his. I figured bracketing the 4 amp
mark was as good of a place to start as any. Its dead middle of the
suggested range, and well within the 35 amp capability of my servo
controllers. It will take some significant cutting loads to cause any
I don't know if all of that was a good idea, but I have this machine way
detuned from the speeds and acceleration I originally tested it at. It
ran at over 450 IPM in testing and I currently have it set for 30iss
with a 100 count following error, but the diganstic software only shows
less than 50 on a hard reversal. With a larger following error I tested
at over 100iss. with peaks of error on screen of 150-200ish. Still
better than the best setting I had on my old Geckos.
I think somebody must have actually loosened the X-Axis gib at some
point, or maybe it had never been adjusted in its previous service life.
Anyway, I think it will be a good place to run for a while. Now to see
if I can find the Y axis gib adjustment.
Adjusted loose (I loosened it to make sure) the Y-axis drew almost 3
amps. Across the range of travel with less variance from ends to
middle. Both of those were expected since the Y likely sees less travel
than the X its likely to wear less. Since its heavier its loose gib
current should be higher. I adjusted it to bracket 4 amps as well,
although due to the heavier weight it might have needed to be a little
higher. Its 4 amps median current across the range of travel its got
less than a 1/4 amp of variation.
I don't know if this really is the correct way to adjust the gibs on a
CNC Mill, but it sure makes sense. Obviously a lighter machine would
require less current and a much heavier machine would require much more,
but the principle makes sense.
Now to see if the chatter has improved. Well, after I make a new rear
Y-Axis way cover. I still think its likely a problem in the X -axis
lead screw assembly, but I have my fingers crossed. Especially since I
have a few more 4140HT parts to machine for the current job.
One thing seems odd. I see less dither on the servos. I think the
looser gibbs might have been causing it to over shoot from the mass of
the table and saddle. Dither was never really bad (even on the scope it
seemed acceptable), but sometimes I'd hear it for a for tiny fraction of
a second on a stop with the spindle off. During testing right now I
didn't hear it even once.
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