CNC and dirty geometries

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CNC and dirty geometries

Sometime a shape appears good to the human eye, but it isn't accepted by a Cam application for your CNC machine.

For example let's take a look at this simple rectangle:

Looks like good, but probably it won't be accepted from a Cam application.
If we take a look at the bottom left corner, zooming it, we can see this:

The corner is opened, because the vertices have slightly different coordinates, they don't match each other!

This is one of the most common cause of dirty geometries.
The rules for a geometry to be a valid contour are:

The vertices must be exactly overimposing (matching)
The contour must be closed
The contour cannot have autointersections

You can see below an example of a contour with an autointersection, and so not clean:

To make this geometry clean you must draw it as 2 triangles.

Clean geometries and Regions

To verify/validate the geometries to be clean DevFoam uses the Region object.

A Region is an element with

1 external clean contour
0, 1, 2 or more internal contours, the islands
an added rule: all those contours cannot intersect each other

Here below an example of a Region with 2 islands:

Region can always be cut as Parts. That' why we suggest to work with Regions.
To create a Region you can use these commands:

REGION It converts an object that encloses an area into a region object
REGION_AI It adds an island to a region by selecting its defining objects
REGION_BY_IP It creates a region clicking on an inner point of an enclosed area
REGION_DI It deletes an island from a region
REGION_DIP It deletes an island from a region and it converts it into a polyline

See also this help page about Regions

Hot to create clean geometries

We suggest:

to work with closed Polylines, and not with single Lines and Arcs
to take confidence with the use of Object snaps. In this way you can match vertices with precision
to create Region from Polylines before starting the cutting phase

Hot to clean dirty geometries

If you already drew a dirty geometry, or if you imported it from an external drawing (for example a DXF file), you can clean it.
You can adjust problems caused by unmatching vertices selecting the geometries and moving the BLUE control points of the vertices.
See below:

You can adjust intersection or autointersection problems by moving the vertices or breaking the geometries, see the FULLBREAK command.

A good piece of new: may be devFoam can help you in cleaning geometry

As the cleaning of dirty geometries is a rather annoying and frustrating job, we developed a special feature to help you.
You can try to select the dirty geometries then use the 'Cut - Try automatic Parts generation from selected elements' menu.
This function will try to:

Join together the unmatching vertices , using the max tolerance value set by the 'Cut - Set tolerance for automatic Part generation'. You must define here a value compatible with the error you can evaluate in vertices matching.
Split the contours with autointersections in more not autointersecating contours.
Create Regions and add inner islands if detected.

Please note: Region and Parts are about the same thing. Tecnically a Region is a Cad Object defined as help page about Regions.
A Part is a Region that you select and go to Cut. This because Part is the usual term for an object to cut in Cam applications.

Here below an example of very dirty geometries:

Applying the 'Cut - Try automatic Parts generation from selected elements' the result is:

As you can see valid Regions are created as expected. Also Regions inside Islands have been created.
The original dirty geometries are untouched, but the function creates new Regions.

Enabling the menu 'Cut - Use automatic Parts generation for Cutting', when you select dirty geometries and go to cut them, they will be automatically converted (if possible) in Regions/Parts. You won't see them in the Cad documents, but only among the Parts to cut. See Simple cutting and Advanced cutting

I suggest also to take a look at this page, to better understand some important concepts of devFoam: DevFoam FAQ and important concepts