Re: [CAD_CAM_EDM_DRO]A question of size managenent.

Hi Bill:
I think I am misunderstanding something here.
As far as I know, LINES versus ARCS are completely different entities in a
CAD system.
They are handled differently mathematically.
There is no such thing as "line segments of arcs" (correct me, anyone , if
I'm wrong here!)
When the CAM portion of a CAD-CAM system recognizes an entity that is an
arc, it will apply a G02 or G03 move to it, even if it is a bunch of little
tiny arcs strung together.
Arcs, by definition , must lie completely on a single plane.
Splines and Surfaces cannot be handled by most controllers directly.
So, the CAM system throws out a series of short lines that approximate the
surface or spline.
You can tell the CAM system how far the lines are allowed to deviate from
the contour of the surface or spline.
Any curved contour that does not completely lie on a single plane is a
spline, even if it looks circular when viewed from "above".
I'm not sure, without a picture, what you were trying to cut.
If you got your toolpath from a surface though, it will be composed of short
linear moves EVEN IF THE SURFACE WAS ORIGINALLY CONSTRUCTED FROM A PAIR OF
ARCS!!
The CAM system does not know or care (I'm anthropomorphizing here) what the
original constructor was, and does not recognize the sequential line
segments as approximating an ARC.
Instead, it recognizes them as approximating a SURFACE.
It no longer cares about the start or end point of the arc, or its origin.
The only way I know, to get back to the arcs that you wanted, is to
reconstruct them on the surface, and you need to construct an arc for every
level that you want the tool to follow.
You then create your toolpaths to follow the ARCS rather than the SURFACE.

I agree, it would be nice if a means existed to take line segments that
approximate an essentially circular surface or spline and convert them into
arcs; Mastercam can do this, but only to a very limited extent.
The problem is recognizing when the deviation from circularity represents
the approximation, and when it represents the beginning of a direction
change in the spline or surface.
The CAM system will never know for sure just where the start and end points
of the arc really are, and exactly what the radius is. This is true even if
you constructed the surface or spline from arcs originally.

So, to get back to the nub of the issue:
If you want G02 and G03 (arc commands) output, you MUST be following arcs
when you set up your toolpaths.
If you follow surfaces or splines, even when they were made from arcs... you
will always get G01 (linear) moves.

Cheers

Marcus
-----Original Message-----
From: BillDarby <ddarby@...>
To: CAD_CAM_EDM_DRO@egroups.com <CAD_CAM_EDM_DRO@egroups.com>
Date: Wednesday, November 29, 2000 9:58 PM
Subject: Re: [CAD_CAM_EDM_DRO]A question of size managenent.

>Hi Again Marcus
>
>Well,,,, I suspect that every owner of these Cad programs is faced with

this size problem and if their CNC will not handle an arc

>command they are simple forced to live with it. However my controller is

conversational and does utilize the arc command and

>would benefit from having a short conversion program that would convert

line segments of arcs into arc commands. I don't imagine I

>am alone with this problem. I would expect that thousands of guys are

living with it as we speak.

>
>Bill Darby
>
>