Re: [CAD_CAM_EDM_DRO] Re: Open (i think) design for a parallel robot (reprap may be interested)

Ron Yost wrote:

>Is it just me, or are there others wondering what these machines are
>intended for?
>
>How can the apparent goal of self-replication (of a largely metalic
>machine) be
>accomplished by squirting layers of hot-melt plastics? Or carving foam??
>
>They're fascinating to watch .. but other than rapid-prototyping (making
>plastic
>models of things), and/or as a conceptual challenge, of what utility are they
>for getting -real work- done???
>

For some, it may be an intellectual challenge, only. For modelmakers
and RC airplane
folks, small lightweight parts may be perfecly usable as is. And, for
people with the
ability to do lost-wax casting, these parts may be the mold patterns for
either sand,
lost wax or investment casting.

>Things have to be really beefy to do heavy, yet precise, work in steel,
>and I don't see how eliminating mass is going to work at all for us home-
>shop types doing metal. Plastic bearings wouldn't last long at all, either.
>
>
>

Yes, and it is with constant dismay that I see yet another newbie coming
in and
saying he's going to build a machine with 1/2" Thompson shafting that
will mill steel
blocks a foot on a side.

>Yes, I know a tripod is a very strong structure. And I do get the concept
>of the things. But, there has to be mass somewhere, doesn't there?? How can
>a decent spindle holding real milling cutters hang off spindly legs?? Or
>is there a 'scale' thing here I'm not getting?
>
>
>

Well, mostly. Mass is a cop-out when sufficient stiffness can't be
obtained.
Really, stiffness is the first goal. But, looking at a Bridgeport
machine, for instance,
it should be pretty obvious that spindly struts can't maintain the
stiffness required
unless things are quite overconstrained. That's why the 6 legs of the
Stewart platform
help, when the adjacent struts are loaded against each other. Secondly,
it is rarely
necessary to use 1" diameter end mills (and larger) on smaller parts.
The only real
requirement for them is when reaching DEEP into a cavity. A smaller
cutting tool
spinning much faster requires much less stiffness of the machine.
But, if you want smooth cut surfaces on hard materials, spindly legs are
not likely
to help achieve that goal.

Jon