Re: [CAD_CAM_EDM_DRO] Re: Counterweighting the Z axis ?; Power off position holding ?

On Tuesday 08 July 2003 11:32, Kim Lux wrote:

> The problem with springs is that their force varies with where the head
> is. Maybe they could be adjusted for zero force in the home position.
> That wouldn't help for an EStop though.

There are constant force springs, they look like a coil of flat steel
strapping material (such a spring is used in clock movements). I've seen a
Sherline mill head counterbalanced by two of these type springs, one on
either side, but I don't think they're available in a size large enough for
your application.

> A regulated air cylinder could give a constant force with any position,
> but I'm worried the air dynamics would mess up the head motion.

Bridgeport used this method to counterbalance the knee of their Series 2
mills. The trick that makes it work is that the volume of air displaced by
the movement of the cylinder through it's travel has to be small in
comparison to the total amount of pressurized air. This assures that the
total pressure bearing against the face of the piston doesn't change much as
a result of the change of the cylinder's volume. One way to achieve this is
to use a cylinder with a bore & stroke suitable for the task, then attach an
air tank to add volume.

> So far the best solution seems to be to counter weight it and/or apply a
> bit of drag to the ballscrew drive.

Major machine makers agree, they almost all use counterweights. The
disadvantage is the added mass that has to accelerated and decelerated.

Matt