Re: Z axis drive

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
wrote:

>
> Pete,
>
> Both of these get used alot. Under the knee is a very dirty and
>abrasive environment. Doesnt seem a pneumatic cylinder would last
>long.

I'm not sure it's really that bad, shock absorbers on cars get more
abuse and still last a good while.

>As far as the travel that is another issue, these have almost 14" of

programmable Z. Not much room for a cylinder of that stroke. These
are the knee type with just a hole for the stanchion. Not an open
casting. Another consideration when counterbalancing is the alignment
of the force relative to the Z axis. The leadscrew or jackscrew is in
perfect alignment, dead center of the ways. Mounting an apparatus off
to one side of center may impede accuracy and may definitely increase
friction, i.e. sideload on ways, binding motion. May be easier on the
axis motor but may be damaging the machine over long periods.

The gas springs get a 16" stroke in an 18" package so they should fit
just fine. Put one on either side of the X screw and it should be
pretty well balanced.

A pair of 250# gas springs would be about $40 +tax/shipping and
relieve the Z screw of 500# of load which should help a lot and not
hurt backlash.

>
> I've looked at several of these methods of counterbalancing the
>knee and none of them seem very practical. Cables, gas struts,
>springs, cylinders, counterweights, etc. Also, have to remember,
>storing that much energy as in springs, is very dangerous.

250# each in two gas springs shouldn't be too dangerous, not any
worse than a car hatchback.

>Actually the weight of the Z removes all backlash on the leadscrew.
>The closer it comes to zero balance, the closer it is to an axis
>with a ton of backlash.

You want to leave some weight, but as noted, that
knee/saddle/table/servo mess is blody heavy even without any work on
the table. Taking a decent chunk of that load off should improve
performance enough to be worth the $40 in gas springs and an
afternoon mounting them.

>
> Alot of things to consider.

Yep, a good CNC machine is not simple.

>
> I've opted to just use large servos and forget counterbalancing.

60 ipm on Z is ample. No cylinders or airlines or tanks to maintain.
No springs to fly across the shop. I've set three different knee-
mills up this way and no counter-balancing was used.

>
> Ron
>

That certainly does work, but since a great many commercial CNC do
use counterballances, it's worth considering them.

Pete C.