Re: Is this a silly idea

Chris, the method that you propose will indeed work pretty much as
you envision and I've used it a few times myself although for totally
different reasons. My Sherline 5400 mill measures about 3 ½ inches
from the column to the spindle. So I can cut a 3 ½ inch maximum
diameter part using the normal X-Y setup, but can cut a 7 inch
diameter part(or anything that will fit inside a 7 inch circle)using
your method. And if I offset the rotary in the Y-axis then that is
even larger, (but you can't cut to the center). For example, I cut
the tracks and numerals on a 10-inch clock dial using your method in
a single setup. Only needed access to the outer 1-½ inches in this
case and it worked fine to extend the range of the Sherline.
But there are many reasons why you wouldn't want to use this as your
primary setup. As you noted, the software to convert X-Y Gcode to
polar is a pain. Straight lines must be broken into segments since
they will be plotted as spirals, (except in the special case of a
radius). A new feedrate must be provided for every segment. Then
there is the case where a line crosses the rotary center. (It must
stop and do a 180 then proceed). And of course the resolution varies
with the distance from the center. There are other gotchas as well.
So again, yes it works, but is NOT a viable substitute for an X-Y
table IMO.
Hope this helps,
Al

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Chris Cain"
<chris.cain@d...> wrote:

> I am thinking of building a small CNC mill and looking at X-Y tables
> construction is difficult and good ones are expensive.
> One though I had was that rather than have an X-Y table as the base

of

> the mill what about a X - theta table.
> i.e. a rotary table on a cross slide, no Y movement at all.
>
> As far as I can see this would be a lot simpler to make as small

rotary

> tables can be purchased relatively cheaply.
> With CNC control it should be possible to duplicate any cutting move
> that could be done on an X-Y table as well
> As a number that would be difficult to do on a straight X-Y table.
>
> About the only down side I can see is that I would probably have to
> write the software myself to control it ?
>
>
>
> Chris Cain
>
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> [Non-text portions of this message have been removed]