Re: Lathe Hexapod

[Lukas du Plessis <lplessis-at-eng.up.ac.za>]

Brian Pitt wrote:

> being able to lock the main spindle and do some milling with live tools on
> the platform realy opens up the posibiitys for lathe work as well (no more
> 2nd opps  :-)  )

We have designed and built a test model of a planar 3dof hexapod.  It consists of
a moving platform connected to a fixed base via 3 linear actuators.  The linear
actuators have ball screws driven by stepper motors.  Our planar mechanism has 3
dof:  2
translations and 1 rotation. I have sent Ray two pictures that he will put in the
drop box.

What makes our planar hexapod unique, is that it has an adjustable geometry.  The
idea is to determine the optimum geometry for a specific tool path, and then
adjust the hexapod accordingly.  This way we can avoid singularities.

The original idea was to retrofit a "non-CNC machine" with our "planar 3dof
hexapod", so that the workpiece to be
machined can be  attached to the moving platform of the hexapod.  The machining
operation is then done by controlling the
hexapod.  The test model was built to test the feasibility of this idea.  The
workpiece, in this case a perspex plate, is
mounted to the moving platform of the test model.  The tool, in this case a
stationary pen, is mounted to the frame of the
test model.  The pen must then draw the required toolpath on the perspex plate by
manipulating the hexapod.

By mounting the tool on the planar hexapod, I think that it can also work for the
lathe hexapod idea.  Is there any advantage in having 3 dof to position and
orientate the tool for a CNC lathe?


> I like the basic idea  ,its got me thinking of what problems might pop up and
> trying to imagine the simplest ways to solve them
> I'm sure there are probably several ways to make it work
>
> Brian

The way I see it, one of the biggest problems is the "out of plane stability" of
the planar hexapod, but I think that with "sliding guides", one may get the
mechanism stiff enough to do light machining.  The German company WZL RWTH AACHEN
has a 2 dof machining center called the Dyna M, and they report a minimum "out of
plane stiffness" of 60 Newton per micron.

Lukas