Re: Re: Polar Coordinates CNC?

Hi Fred.

Well, yea. I can see the device but it seems to me that by the time you
build C with center bearings, drive, and some sort of support for
cutting forces around the outside of the suggested 4' diameter you'd have
more work into it than a conventional mill and all to save two slides, a
ball screw, and their supports.

I did recently see a mega lathe with a C axis on the flat like this. The
turntable (spindle) had to be 10 - 12 feet across. There was an X gantry
that looked more like a press brake spanning the whole wheel. Z with a
tool holder hung from one side of the gantry. The shop guy said they
found it in a farmers field and that it had been surplus from the Rock
Island Arsenal. It took several guys a few months to clean up and get
going. He thought it could hold 0.0005 on an 8' diameter wheel but he's
still looking for a micrometer that size.<g>

Ray

>    From: "Fred Smith" <imserv@...>
> Subject: Re: Polar Coordinates CNC?
>
> --- In CAD_CAM_EDM_DRO@y..., Ray Henry <rehenry@u...> wrote:
> > 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.  
> > 
>
> It's the machine that is simpler.
>
> Think of a turntable (like a lazy susan) with the part mounted in
> place on the turntable.  The rotation is C axis
>
> Now place a single horizontal linear axis parallel to the face of the
> turntable (X) above the work piece, and mount a Vertical Z axis onto
> the X. (R is not a valid CNC linear axis so I will refer to it as X.)
>
> The machine is simplified to a single linear X and a bearing for C.
> Instead of at least 4 precision linear guide components for a linear
> gantry style machine, you reduce it to 2 (shortest possible) plus the
> bearing (Actually only 1 if you use a dove tail arrangement like a
> Bishop-Wisecarver rail).  The Z axis is the same mechanism in this
> case.
>
> The intriguing part is how to accurately position the work piece
> because there are no straight edges to indicate, away from the
> central X axis.  I'm thinking of mounting a vise.  How to indicate it
> in, is a much different thought process if only one horizontal axis
> is linear.  I guess you could indicate one face, set zero degrees,
> rotate 180 degrees and tram for centrality, or rotate 90 degrees and
> use an edge finder to set the X distance and angle to a part edge.  I
> would guess that you would want to have an axis transform for the
> vise to avoid going nuts trying to mount it square to the X axis.
>
> Logically it's the same as setting X & Y, but it's still a mind
> twister.
>
>
> Fred Smith - IMService