RE: [CAD_CAM_EDM_DRO] Re: RoboFac / Stewart-Gough Platform / Hexapod Milling Machine

I think that as a group(s), much attention has been given to accuracy of
ballscrews and the like. Much has also been devoted to control wiring and
good practices in grounding, etc. Very little attention has been given to
the importance of rigidity of a tool. There is a reason why a Bridgeport
weighs 2,000 pounds plus and even a lowly Logan 10" manual lathe weighs
about 600 pounds. These guys are as interested in cutting costs as any
manufacturer, but the purpose of the weight is to ensure the rigidity of the
tool. Now of course, everything is relative. The forces exerted on the
tool by a jeweler working in wax are going to be orders of magnitude
different than someone taking heavy cuts in titanium.

I don't think I am talking out of turn by saying that the number of moving
parts and joints in a tool, as well as the length of unsupported linear
mechanisms and the accumulated backlash are all going to play a part in the
overall accuracy of a tool. A hexapod, for example, seems to exhibit a
challenge on all of these parameters. While the control (software and
hardware) seems to be what we all tend to look at, the mechanical design
seems to take a back seat. While we, as hobbyists, are on occasion, willing
to make compromises based on budget or ability to create professional
versions of what we envision, we should be aware of the consequences of our
decisions and a discussion of what constitutes a rigid design is not a bad
idea.

Jeff

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jacoby01" <BobJacoby@B...> wrote:
> Just checking with this group to see if anyone else is
> following the Servo magazine article series on RoboFac.
> The first article was published in the September 2005
> issue.

I have been following the series. The second article
came out and actually had some pictures of what he built.

> RoboFac is basically a triangular platform suspended from
> above by six linear actuators. The triangular platform can
> be equipped with a router, cutter, milling head, etc. The
> primary advantages appear to be (a) low-cost construction;
> and (b) 2 1/2 D milling capability.
>
> At any rate, I'm following the Servo article series and
> collecting the parts to build one. Is there anybody else
> on this list doing anything similar? Also, does anyone
> know of another group that might have active discussion of
> Stewart-Gough Platforms?

Is your goal to build one because it is neat? or to actually
build parts? If you want to build parts, you might want
to wait until after the 3rd article comes out. At the end
of the 2nd article he alludes to the fact that his design
has a lot of slop. Here is the last paragraph of the 2nd
article:

In Part 3, the final part of this series
I will explain the refinements I made to
turn Robofac into an accurate and useful
tool, including how I used software to
make up for inaccurate construction.
I will describe my first attempts at
making things with Robofac.

Whenever I read one of these CNC articles in a magazine,
the first question I ask is "How accurate is the machine?".
To date, the Robofac article has not discussed accuracy.
For the amount of money he spent building the thing, I
suspect he could have purchased a Sherline/Taig table top
unit and done a quite straight forward CNC conversion.
Again, it is hard to tell because he does not discuss
prices to any level of detail.

I don't want to come down too hard on his work, since
it really is quite a challenging project. But in the
end, the question is whether or not he is able to build
useful parts or not with his creation. So far, I have
my doubts.

My $.02,

-Wayne




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