Re: [CAD_CAM_EDM_DRO] Re: Hexapod design drawings

rab@... wrote:

>
> Doug,
>
> How are the joints for backlash ?

The socket universals that I purchased, have no backlash.
And of course being designed for high torque (a hundred or
more foot pounds of torque) they are up to the task (for a
small machine).

> Can you pre-load the axis on a hexapod design ?

That is the basis for a hexapod's phenomenal rigidity at
such a light weight!

Of the six actuators, three are always in tension, acting
against three in compression. Coming to a triangular
mount point above, the pairs

AB CD EF


instead are paired differently at the bottom (tool platform):

BC DE FA


so if AC&E are in compression, and BD&F are in tension at
the top, then each actuator under tension is matched with
one under compression. So it is also at the bottom (ie
B under tension is paired with C under compression, etc).

Keeping this state of affairs of course is one of the
constraints of the hexapod controller software, and
hence very difficult.

- - -
Some of you are no doubt thinking that under great tool
pressure, all of the members will be under compression.
In fact (or should I say, I understand) that the whole
structure would then relax (causing positioning error)
when the compression is relieved.

Instead, the tensions and compressions are pre-set to
far overwhelm the tool pressures, and hence there is
insignificant relaxing (dimensional change) when the tool
pressure is relieved (helping maintain tolerances).

Or so I am led to believe......

Doug Fortune
http://www.cncKITS.com