Re: Hexapod question

Ray,

That is the only one that is going to be accurate without
compensation. As Mikkelson's machine shows, you can do without ball
ends. BUT every fullsize realworld machining oriented hexapod I've
ever seen or heard about uses ballends. (I don't know the details of
Till F's full size mill.)

Several commented that ball ends would rotate, making extension
impossible, or impractical. Just ain't so. Any more than an actuator
has wildly spinning ends because of the ball or acme screw turning
within...

Hope this helps,

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Raymond Heckert" <jnr@a...>
wrote:

> I think the best (only) usable joint is that each strut end must be
> hemispherical (a half-ball) that mates with the hemisphere of the

mating

> strut, constrained in a ball-socket.
>
> RayHex
>
> ----------
> > From: Kevin Martin <kpmartin@t...>
> >
> > 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