Basic Model Requirements
It is a common but true dictum that the accuracy of a result depends on the accuracy of the data used to determine the result (or more plainly: garbage in, garbage out). Using a rough approximation of the machine work envelope will reduce the effectiveness of the simulation. There is little comfort knowing that a collision probably did not occur. However, using a rough approximation of the machine enclosure (or omitting it entirely) is perfectly valid, if you do not intend to test for collisions against the roughly defined object.
The first step is to know what to model and what not to model. The answer is quite simple:
Only model components of the machine that are to be collision tested.
You should carefully and accurately model the table and any devices that are normally attached to the table. You should model the spindle face and any devices that are attached to it if they can get in the way of the work. If your work area is enclosed and you are concerned with collisions, then you should model this area too. Modeling the tool changer, the stand-alone power supply, chip conveyor, and so on bring a certain degree satisfaction, but they add nothing to the accuracy of the result you are trying to achieve, which is to detect collisions.
VM models are built up from components, all attached together on a kinematics chain. Simple rectangular, cylindrical, conical, spherical, revolved and swept volume components can be created with VM’s construction features. Components can also be imported as STL objects from a CAD system. VM is not a CAD system; VM relies on your CAD system to prepare model components, which can then be imported as STL files.
Curved components are represented in VM as a series of closely meshed triangles. Even simple objects, such as a cube, are built of many triangles (a cube requires 12). The more triangles a component is constructed with, the longer it will take for VM to perform collision testing, and also the longer it will take for the component to be drawn in the simulation windows. This leads us to the next important maxim:
Use reasonably coarse faceting for cylindrical and STL components.
Models with hundreds of thousands of triangular facets will have poor rendering (drawing) performance and might have poor collision performance. VM surrounds each component with a simplified box that it uses in a form of pre-test for collisions, and only looks at the triangles inside the box if necessary. A model with components that are defined localized (not spread out) will have much better collision testing performance than one whose components are not. For example, modeling the entire enclosure of the machine in a single STL will ensure that every triangle of the enclosure will be tested on every motion (and in fact, many times for each motion). It is better to model the enclosure in sections: left, right, top and so on.
If a computerized model of the machine cannot be obtained from your machine tool builder, then you will have to create the model components yourself. If your machine can be modeled using the simple primitives described earlier, then you will not need the assistance of a CAD system. Complex curved surfaces can only be created with the assistance of a CAD system capable of exporting an STL file.