Re: CNC Plasma cutter is resolution moot?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lee Studley"
<indigo_red@q...> wrote:

> Hi,
> I've heard people harp that they are striving for
> thousandths resolution on a table for CNC plasma
> cutting, but in my very limited experience with one,
> I found the arc wandered some, and height variances
> affected kerf width etc.
>
> So I've almost concluded that about a 1/32 to a 1/16
> is what to expect. Does this sound in the ballpark
> or near the surrounding parking lot :-)
> Thanks
> -Lee

Well, yes and no. (don't you love those kind of answers). While the
flame wanders and the width will vary based on the height above the
cut and the "age" of the tip being able to place a cut precisely on a
large sheet comes in handy. Let me give you an example. My pieces
are ones we design and are considered "artsy-fartsy" by most but they
consist of lots of cuts that start and stop. I cut my designs in
stages so that normally the straight line (on-line) cuts are done
followed by the smaller inside cuts (where pieces fall out) followed
by larger inside cuts (where I might have to worry about a piece
flipping up and hanging the head up) and then the outside perimeter
cuts to finish the piece. Each one of those steps is a separate g-
code segment with a return to 0,0 after. I depend on the
repeatability of my machine (typically + - .003) to keep everything
in register and not have starting divits overlapping closeby cuts.
Can I place two parallel cuts closer than about .125...no. But if I
make them .25 apart I want them to stay that much because of
the "floppy flame" syndrome. I guess what I am saying is that just
because the cutting "tool" is inaccurate does not mean the
positioning mechanism needs to be that inaccurate. Do you
need .003? Probably not. Can you live with .050? Depends on what
you are cutting.

I made my machine accurate to use with a router for deep carving but
there are times having that accuracy is a plus for plasma. I think
rack and pinion is plenty accurate for plasma but technologies that
have a lot of inherent backlash can cause problems. Plasma likes
nice constant cutting speeds (CV) and you can't do backlash
compensation with CV so it just builds and builds...

PS: Think THC and the height to flame width issue is reduced.