Re: [CAD_CAM_EDM_DRO] XYYZ Gantry

Peter Chen wrote:

> XYYZ GANTRY
>
> Good morning,
>
> Asking for opinions on XYYZ gantry. I believe this
> configuration will involve having 2 lead/ballscrew and
> 2 stepper motors for the Y axis. Suppose one of the
> stepper on the y axis lose step. Does this mean that
> the x and y axis will no longer be perpendicular to
> each other? With this arrangement how does one ensure
> squareness?

This is what the CNC industry calls a "tandem" axis. It
is often used in gantry bed mills, with the pattern usually
XYZZ. In other words, there the tandem axis is the
vertical columns. There is a beam across the columns,
and the Y axis is the movement of the spindle (or fixed
tool for a vertical boring mill) along that beam.

Anyway, as best as I can tell, this is done by having a
bearing at one end of the cross-beam, and some mechanism
that constrains the other beam end vertically, but allows
some small length variation where the beam is attached
to the vertical axis. This allows the two Z axes to get
a little out of sync without breaking parts or binding the
axis slides. They have separate limit and home switches
on both Z axes, and once homed, the CNC control
keeps them in sync.

Using separate stepper drivers and a CNC program that is
unaware of the dual nature of this axis setup, it is not to
easy to do this properly. I can see some fairly simple
setup of gates on the step and direction lines that will
allow the tandem axes to be homed together, and the
circuit will shut off step pulses to which ever driver
gets to the home position first. The other driver will
continue getting pulses until it reaches home. I think
you'd need a manual switch to engage this 'homing mode'
circuit. Once homed, you'd then turn it off.

Jon