Re: Re: Polar Coordinates CNC?

Lee

I think that the essence of your analysis of the mechanical stuff is true
for a Puma robot type of thing but I can't see how that truth affects
whether it is programmed in polar or Cartesian coordinates. The EMC
could rather easily be configured to run such a device, it has Puma
kinematics in the source files, but current programming is with respect
to Cartesian space.

When we command a coordinated move like g1 f10 x1 y2 z3 what the
interpreter does is compute a vector and uses the f10 as the feedrate
along that vector so in effect it is a polar move. I can't for the life
of me see how specifying three angles and a distance would be more
efficient or would somehow alter the fundamental thinking about milling
or turning.

Ray

>    From: "wenger2k" <wenger2k@...>
> Subject: Re: Polar Coordinates CNC?
>
> The application would be the same as any other - i.e. router/plasma
> application (4'x8' as practical size for discussion purposes).
>
> I would think that Linear moves in polar space would be roughly the
> same and no more complicated than arc/circular moves in cartesian
> space - would they?
>
> I think that a polar machine could be a more cost effective approach
> given that you eliminate many of the most expensive components of the
> system (the long axis).  Parallelism is eliminated as a requirement
> which generally requires you to do "special" things to tie the
> parallel sides of a large gantry style machine together either via
> encoders or long belts if using multiple drives.  You lose a certain
> amount of rigidity with a gantry machine that either must be made up
> by stiffer slides or beefier components on the gantry itself.  All of
> these result in a large heavy gantry which of course requires a
> larger motor(s) to drive.  It looks to me like a lot of these issues
> can be avoided on a polar based machine as there is a single pivot
> point and therefore single drive point.  A very large machine would
> have to either have an extremely heafty arm or would have to be
> supported on the outer end of the boom.  So significantly larger
> sizes likely are not practical.  
>
> I think the biggest problem with a polar machine is one of
> accuracy... i.e. the accuracy will "expand" as you go out from the
> central pole.  That's ok as long as you can get sufficient accuracy
> in towards the pole/pivot point.  This factor would also likely
> dictate the largest practical size.  As a test application I was
> thinking of a 4x8 application using 2 quadrants.  4' x 1.414 (2*sqrt
> (2) ) makes a boom of 6' able to cover the entire area of a 4'x8'
> sheet.  
>
> Why hasn't this been done before?  I don't know - I expect that the
> cartesian basis of g-codes themselves have preselected this to a
> degree.  A Polar router has some similarities to a hexapod in my mind
> in that it requires ongoing trig conversion for each axis/movement.  
> But it's obviously been handled for hexapods which I think are
> significantly more complicated than the polar option.  So it seems
> doable...
>
> Lee Wenger
> Denver, CO