Re: [CAD_CAM_EDM_DRO] Re: Z axis drive

Little Black Box on the Ball Screw is very likely a brake to guard
against free fall or drift of the axis in the event of a power failure,
drive or CNC fault, e-stop or simply shutting the machine down.

There is very possibly a balance of some sort hidden away in there and
as Peter mentioned it may be quite small. Remember that the area of a
1-1/4" cylinder is about 1.25 in2 and at 1,000 psi this would apply a
1,250# load to the axis. In fact on most axis counterbalances the
minimum cylinder diameter is limited by the rod diameter required to
resist columnar buckling rather the required load. Since these are
typically single acting ram type cylinders relying upon the weight of
the axis to retract the rod, the rod may be of size almost equal to the
bore of the cylinder tube. I have learned through that "Great Mother of
a Teacher", experience that there MUST be some clearance between the rod
and cylinder bore and a return fitting on the rod side of the piston.

Even with the best of seals and piston rings, given enough time there is
some leakage across the piston. If this is not vented back to the tank
with an open line, the ram may eventually be locked hydraulically and
not allow the axis to be raised. The first time this happened, the first
diagnosis was a failed servo motor pulling excessive current. After
swapping the motor with no joy, the next "for sure" fix was replacing
the ball screw thrust bearing packs even though the didn't look bad.
Finally, in the process of tearing down the entire axis to looks for way
damage, we found that the top of the cylinder was full of oil and
"locked in one direction. Added a return to the top head and went back
to work.

It has been my experience that the head needs to be weight biased in one
direction or the other (usually down) even if a counter weight or
counter balance is used. Exactly balancing the head at first would seem
to be a good idea but the "floating effect" seems to encourage the drive
to bounce between encoder limits. This seems to make the head less
stable and we cannot run the control error limit, control gain and drive
gain as tight as we would like without encouraging axis oscillation (an
unstable 10,000# axis prone to oscillation is not a pleasant experience,
things start to break). The lower gains tend to make the axis
electrically less stiff and more prone to deflect out of position during
a heavy cut.

There is another approach to counter balancing the head but it applies
only to the performance characteristics of the axis while under servo
control. Allen-Bradley and others have supplied servo drives that have
an electronic counterbalance option. When this option is enabled, it
applies an adjustable bias current to the servo to drive the motor up
even when the analog command voltage from the CNC is 0V. This way the
current and gains applied by the drive to the motor at -10V and +10V are
similar and added or subtracted to the constant bias voltage. This does
not however replace the the need for a brake and mechanical balance to
guard against problems that arise if the servo drive is not in control
of the axis but serves more as a "tuning tool". For example we have used
the electronic counterbalance to offset the added axis load caused by
adding a large boring bar or or tool block to the face a VBM Ram where
this tooling could add 500# or more. It is easier to tweak the drive
than adjust the hydraulics and then tweak the drive to compensate.



gary

skykotech wrote:

>>Are you sure there isn't a counterballance system hidden in there
>>somewhere? A hydraulic counterballance setup for a modest weight
>>
>>
>like
>
>
>>1,000# can be pretty small and easily hidden.
>>
>>Without a counterballance system of some sort, any machine with a
>>heavy head will cause a massive imballance in the servo load between
>>up and down which I would expect would hurt performance.
>>
>>As someone else noted, without a counterballance the servo could end
>>up driving in the up direction to move the head up, and driving in
>>the up direction (just less force) to move the head down as the
>>weight of the head backdrives the ballscrew.
>>
>>Pete C.
>>
>>
>>
>
>Ok, now I am curious. I will go check and see if indeed there is a
>hydraulic cylinder or something that I have not noticed before. I
>always assumed the little black box sitting on top of the ballscrew
>was some sort of clutch or brake. I will snap a picture, maybe
>someone will know from that.
>
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