View Full Version : Vector cam offsets

03-12-2001, 06:52 PM
When I offset in vector on any curves,arcs,fillets and even square corners the tool turns circles, usually once on a corner or fillet and continuously on arcs. Can anyone help

03-13-2001, 02:37 PM
Don't quite understand. There is a problem with shopbot software and real small arcs, sometimes cuts the large part instead of the small. If that is the problem go to Change-Break-Interpolate. Change arcs into line segments. Good discussion at support Vectorcam.com and click the vector forum. Search on elipse.

If that isn't the problem give a little more detail and I'll try to help


03-13-2001, 03:20 PM
I have the same problem as Greg. John, could you expand on the "problem" in shopbot software ?

03-13-2001, 04:14 PM
I had the same problem,when reorganizing a dxf
with vector, it would do circles where there were
none, and sometimes at the end of a line..
and the shopbot software didn,t come into play..

03-13-2001, 04:55 PM
The problem that John is talking about is not really a problem with the Shopbot software as such, but rather a problem with all digital CNC software. It is also found in RS-274D, G-code systems. When creating tiny detailed toolpaths, at some point the smallest step that is expressed to the machine becomes less than the last digit that is fed to the machine. If an arc has an end point of .12501 in X and .12502 in Y, but the controller is only programmed in .0001 increments the difference between another arc with an end point of .12500 X and .12504 Y cannot be discerned by the controller. By definition a G-code style arc that has the same endpoint as start point is cut as a full circle. When these two positions vary by less than the smallest programmed resolution and an arc is commanded, a full circle will result.

The solution suggested by John will fix the problem. These tiny arcs are developed because of offset curves to the inside of a concave or internal corner. Vector correctly offsets the inside curves by placing smaller arcs at the specified distance from the original geometry. Unfortunately it is always possible to create a situation in which the offset curve length is shorter than the programmed resolution.

Geometry constructed from raster to vector conversions, from Tru-type fonts, and from DXF files containing other complex spline curves, represented by a series of short lines, is most suceptable to this kind of problem. Things drawn as CAD data, consisting of lines and arcs, in the first place are less of a problem because it is less common to construct geometry using arcs with radius of less than .0001 inch. When fitting arcs to a spline curve and offsetting it is VERY common for this to happen.

Entering a duplicated arc command(CG) will cause the second one to cut a full circle. Even if the first one was just a partial circle.
Interpolating the arcs into short accurate lines is a reliable way to prevent this problem.

Anybody remember Shopbot version 1.4 that didn't support G-code style arcs at all?