By Mark Boucher, CMM Quarterly
I want to cover some basics about CAD models that might help us understand what is happening with some model features when you program from a CAD model using your coordinate measuring machine. By understanding surfaces we can better evaluate any anomalies we may encounter when we import a model into our CAD base CMM software.
There are several model types and we will cover two of the most common ones you would come across today, solid models and surface models. To be more accurate, they are parametric models and freeform surface models.
Parametric Models
Parametric models are created from features that are defined by parameters, or dimensions. These dimensions can be changed and the feature moves with the change. Prior to parametric modeling if a change was made then the feature was recreated, extruded, trimmed, etc…, in the new position and the old feature was deleted. Parametric modeling maintains the relationship of part creation, assembly, to output of the blueprint and a change anywhere along that process will update the model at every level.
Parametric models are referred to as a solid model, as opposed to a wireframe model. A wireframe model is made up of lines that represent the part but have no surfaces on them and makes 3d viewing somewhat tedious.
Parametric modeling revolutionized the CAD industry and allowed more affordable CAD software to become available to anyone. You can now pick up parametric CAD programming software up at your local Best Buy right off the shelf.
Freeform Surface Models
The second model type I want to cover is the surface model. With surface models curves are used to define the surface area and surfaces are applied between the curves then they are trimmed and merged, to make a solid. The problem with this method is to make sure all the voids between the surfaces are filled in. The surface definition changes as the need requires. Let’s say, you have a plane that requires basically four lines to define the boundary of the plane. A chamfer merging into a radius requires a greater amount of defining to create this type of feature. While creating the surfaces you may end up with a small void as you try to fill in the feature. Point placement from your CMM program will be dependant on where it sees the plane boundaries and a void will not be inclusive in this plane so the boundaries are redefined not giving you a true representation of the surface. In parametric modeling these types of transitions are automatically resolved.Some CMM CAD based software have a ‘healing’ or ‘repair’ functionality that will mend some of these errors. It is always advisable to use healing when using this type of model. If your software does not include this functionality there are third party softwares that do the job for you.SurfacesCreation of surfaces begins with a spline, aka curve. Splines are single lines that make up the shape of the surface. Imagine points that make up the shape of your surface and spline will fit through these points. These splines are then used to create the surface through a method known as ‘swept’ (using the curves as a guide rail) or meshed (lofted) through. A ‘swept’ surface follows the shape of the curve line. If you had a helical curve the swept surface will follow that helical shape as it extrudes the surface.
If your engineering department does any sort of reverse engineering they will ask for a series of curve files that they can import into their CAD system. The curve files are then used to ‘mesh’ or ‘loft’ the surfaces. The density or frequency of the curve lines along the surface will depend on the complexity of the surface being scanned. For flat planes only several are needed but scanning the chamfer to radius transition we discussed before would require a greater amount of curves to define the feature.
Another method is direct creation of the surface with manipulation of the surface control points. Points are created along the curves that can be grabbed and drawn in any direction to create a new surface shape. This inherently will create new surfaces to fit the new configuration.
It is important to note that the majority of CMM software in the market today do not have true CAD functionality and thus do not have the ability to manipulate surfaces as described above but it is important to know what is happening during model creation.
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