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Issues When Programming From CAD
Written by Mark Boucher, CMM Quarterly   

Surface Normals

There are several issues that arise when bringing a CAD model into your CAD based programming software. One of these issues has to do with surface normals (surface vectors). You can bring in a model and the entire model or a portion of that model is visible, but dark (Figure1), or certain sections are not visible at all. This problem arises from the surface vectors pointing in the opposite direction than your CAD system views them. You are looking at the back side of the surface. You must reverse the surface normal (Figure 2). If your software has this capability, you are looking for something similar to ‘reverse surface normals’. This will flip the surface so the front faces in the correct orientation for your software to view the surface.

 

Figure 1

 

Figure 2

 
 All surfaces have a front and a back side; a CAD program must know which is which. How is this done?  The model must somehow include information to specify the front of a surface. This is done by surface normals. This is a line perpendicular to the front surface and beginning on that surface pointing away from the surface. Meaning it exists only on the side of the surface that is its front. The CAD system must have this information to shade the model properly. Those that use a CAD system need this to drive the probe normal to the surface.

Direction vectors have been covered extensively by Richard Clark, his three part series was featured in CMM Quarterly.  Suffice it to say that these surface normals are what give you the direction vectors from CAD models when programming. If you do not use a CAD model to program then you must calculate the normal vector. Contact rcmetrology@yahoo.comThis e-mail address is being protected from spam bots, you need JavaScript enabled to view it for a Direction Vector Calculator.

When picking a feature off a CAD model the software will extract the normal vector from the CAD surface. As mentioned above you may have the ability to flip surface normals or you may have the ‘view surfaces from both sides’ option. Care must be given to this selection because the surface will be visible but the vector may point in the opposite direction you need to probe the part, your probing direction vector. Just know when viewing the vector after feature selection, that it is correct.

 

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