RE: [CAD_CAM_EDM_DRO] Re: Hexapod question

I may not be able to explain exactly *why*, but I have to disagree with you,
ballendo...
If the two cross-axes of the U-joint are in the same plane the joint can
flex in any direction without any change in "length", which is the same
motion as you would get with a ball joint whose centre was where the U-joint
cross-axes met.

What a U-joint does *not* give you is smooth transmission of *rotary* motion
as one shaft of the joint rotates the other.

And of course, a U-joint it really two joints in series, so has twice the
play of a ball joint built to the same clearances.

It is difficult to tell from the photos whether the U-joints satisfy the
above geometry; at first glance they appear to.
I am actually not convinced that having extended u-joints (where the
cross-axes are not coplanar) produces more degrees of freedom either,
although it does complicate the positioning math, and may increase the
effects of play and frame flex. I have certainly seen photos of commercial
hexapods which do not use ball joints (not that I would be able to find one
now).
-Kevin Martin

-----Original Message-----
From: ballendo [mailto:ballendo@...]
[...]
The u-joints you've used are creating the problem. You need a ball
joint at the end of the struts. Definitely at the table end; I'm not
positive if you'll need them at the upper ends (but I am positive
that without them at the upper ends you'll lose some accuracy)