Re: Hexapod question

Simon,

You're very close. BUT...

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)

By using the u-joints, you've added a lever length PAST the joint
movement area. This lever is what is allowing the table to tilt. Ball
joint design is one of the toughest "nuts" to crack on hexapods. You
need a LOT of rigidity, and as much angular freedom as possible.

two sources for inexpensive ball end joints are an R/C hobby store-
but these will be smaller than what you likely need; and rod ends
used in aircraft controls (and some hot rodders)

You may even try an auto scrap yard. Think yugo, or other small light
car, and look at the steering gear... 3 cars should get you there<G>
(these will be a bit big, but cheap)

Ballendo

P.S. An experiment to try: drill into the EDGES of your lower plate.
Insert rod sized to fit your u-joints, and glue. Cut these six rods
so that the existing u-joint slips over the end and touches the edge
of the lower table. (what this does is place the joint on the plane
of the table, which might just firm things up. It also makes the
table "bigger" effectively. Since the lever length I spoke of above
is now in plane with the table, the geometry is closer to what is
needed for a hexapod. Please let me/us know if this helps.

(what I expect is that in some positions it will be nearly perfect,
but at other will have some flex-like you have now, but less, due to
the non "circularity" of the u-joint, when compared to the ball
joint. which means... That middle piece in the u-joint must be zero
length for the lever effect to be completely eliminated in all
positions.)

BTW, The lower table "should be" an equilateral triangle, with each
corner cut off just enough for the 2 ball joints to have freedom of
movement... Having your upper struts equally spaced is not a problem,
IMO.



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Simon M. Arthur"
<chroma@m...> wrote:

> I've started building a hexapod on the cheap. Some photos:
> http://www.tinyplanet.com/images/hexapod-portrait-reduced.jpg
> http://www.tinyplanet.com/images/dscf0011-reduced.jpg
>
> I noticed that the platform can be rotated and tilted more or less
> freely, even with power going to the steppers. Translational

movement is

> a lot harder. This makes me think that I misunderstood completely

how a

> hexapod is supposed to work. I have a universal joint on the frame

so

> that the motor can tilt, and another on the platform.
>
> Can this setup be made to work? Why doesn't my hexapod lock the

platform

> in place?