Re: [CAD_CAM_EDM_DRO] re:Cnc router help

> Doug,
>
> So the long Z axis is "not used" most of the time? It just 'sticks
> up' in the air (adding moment load on EVERY move). And when yopu need
> it you "prop up" the gantry cars?

We talked about it some. I think when Doug saw the truly massive
construction required of a long z gantry( mine is 18 to 24 inches depending
on tooling) to maintain stiffness at the spindle he thought of alternatives
since his big stuff is foam.

You are correct that without the risers z would be sticking up
and creating a large inertial moment load. But due to the
stacking nature of a gantry machine Z is pretty light. It's like
a vertical mill that starts off with a huge heavy casting at the base but
ends up as a light small assembly near the spindle.

So removing the risers would not improve the inertial moments
so much but it would greatly improve STIFFNESS at the tool as most of z
would be floating up high in the breeze and not reducing spindle stiffness.

Sound reasonable?

We talked about rotating ballnuts some too. My machine is not
of that type (except for the ballscrew pitch correction thing)
but I thought about how a practical design would be configured and how much
it would cost. As you mentioned I sketched up
a hollow shaft with the nut inside. For thermal stability thrust
would be handled by a preloaded db angular contact clamped pair with a third
floating conrad type to locate radially.

I think timing belt or gears could be used (helical?) but as gears need an
oil lube system I think the belts would be simpler. I think KOMO et al use
this method.

Of course this is two motor with motors on the gantry.

The only other issue with rotating nut is ballscrew lubrication.
The manufacturers talk about "rotating nut rated" for their
products. I think this really has to do with slinging grease
out of lube vent holes etc with some ballnut designs if they are rotated. I
could see that as an issue. Active oil lube on the screw would seem to solve
that though.

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html