RE: [CAD_CAM_EDM_DRO] Hexapod question

Your platform will rotate easily because the particular geometry amplifies
any play in your joints and flex in your outer frame.
In particular, all of the "legs" are more or less perpendicular to the
stress they encounter when you try rotating the platform, so the small
amount of radial play in the leg is experienced as a much larger play
tangential to the leg.

As a more concrete example, consider a 2-dimensional system with just two
legs:

o fixed pivot Y
\ ^
\ leg 1 |
\ +--> X
\
o mobile flex joint
/
/
/ leg 2
/
o fixed pivot

Assume each of the legs has, say, 0.005" of lengthwise play due to all the
joints, and that the fixed pivots are 40" apart.
When the mobile point is dead centre (both legs 20" long), the Y play of the
mobile point would be about 0.005", but the sideways (X) play would be a
whopping 0.9"! The actual X play in this case depends on whether the legs
are actually 20.000 +0.005 -0.000 (which gives the large play) or 20.000
+0.000 -0.005 (which mathematically gives zero play but in practice will
still have large play because you have a 3rd-degree polynomial relationship
between stress and strain instead of the usual first-degree polynomial).

As the legs lengthen and the mobile joint moves to the right (as
illustrated) the play in the X direction improves but the play in the Y
direction can get arbitrarily large (ultimately proportional to the square
root of the leg length).

There is a "sweet spot" (likely where the legs are 90 degrees apart) where
the play in either direction would be about equal to the leg longitudinal
play of 0.005" times sqrt(2)

Most commercial hexapods are designed to run with each leg more or less
perpendicular to the next one to stay in the sweet spot.
-Kevin Martin