polygon effect, and chain use in CNC was Re: Plasma cnc

Jens,

Mind if I try to explain?

What Jens calls polygon effect can be easily seen by thinking of a
chain sprocket with 3 or 4 teeth. In order to have a consistent
movement for a given rotation of the sprocket, the radius of
the "pulley" must remain constant (i.e.; a circle). Any deviation
from circularity will result in uneven distance and speed. (As the
radius changes)

Going back to our 4 tooth sprocket, we can see that the radius at
the "corners of the square" is much greater than at the "flats"
between the corners. This means that the motion will be uneven, in
both distance and speed. (It will follow that SIN/COS relationship I
wrote about a few posts ago, when talking about the "scissor" Z axis.)

In actual practice, chain can work well for CNC. Size 25 chain was
used in several hundred commercial CNC router machines produced by
Cam tech. Users (mostly sign makers, woodworkers, and plastic
fabricators) were VERY satisfied. BTW, the output sprocket of the
Camtech machines was 12tooth, making it equivalent to a 12 sided
polygon. The motors were run through a 4:1 chain reduction before
this output sprocket.

FWIW, There is some of this same effect with timing belt drives. It
gets worse as the diameter goes down,for a given pitch of teeth (same
as chain.) With timing belt drives the term "cogging" is used, and
you have the additional detail of being sure that the "teeth" of the
timing belt "go into" the recesses in the pulleys cleanly. This is
one reason the trapezoid shape and parabolic shape are used for the
teeth of timing belts.

Gears have developed a few different tooth shapes to deal with the
same problems. Poorly shaped or meshed gears can also exhibit cogging
and/or what Jens calls polygon effect.

Hope this helps,

Ballendo

P.S. I just thought of a possibly simpler explanation. Think of a
fishing reel with a square line holder instead of round. The fish
would be pulled in in jerks... (at least until the line had "rounded"
the square1<G>)



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Jens Swales
<jipeess2000@y...>" <jipeess2000@y...> wrote:

> hi
>
>
> i too thinking of build a plasma gantry with an "open

end "toothbelt

> drive. i would not recomend to use a regular chain because of the
> polygoneffect (i wont explain what that results in, my english isnt
> that good ;))
>
>
> js
>
> ps thanks for all links regarding artcam/bmp-gcode converters