Editing boundaries is the most important step in your DXF file conversion. In this step, you have to think about the material properties of the various shapes you have drawn in the DXF file. Moreover, you have to connect edges together to form a closed boundary. Please note that all boundaries should be closed. In other words, if you start from one point on the boundary, you should be able to return to the same point by traversing the edges of the boundary. Furthermore, all boundaries should be simply connected. That means you cannot have shapes like doughnuts for your boundaries.
Let us consider the example Figure 3.1, “Magnetic Circuit” in more detail here. Here you will have four boundaries. Can you guess what they are? First you will have the outer boundary with the four outer edges. Next, you have the iron core, which will have 20 edges (see Figure 3.6, “Editing Boundaries”). The hatched area forms another boundary with four edges. Finally, the two conductors form another two boundaries with four edges each.
You will also notice that some of these boundaries will include other boundaries. For example, the outer boundary will include all other boundaries. When we define boundaries, we always start with outer boundaries and finish with small, inner boundaries that do not include any other boundary within themselves. There is a very good reason for doing that. When we generate the mesh, we have to find the boundary for each triangle in the mesh. Since some boundaries are included in other boundaries, some triangles will belong to more than one boundary. However, you will see that the correct boundary for each triangle is the smallest boundary that includes that triangle. Hence, it is important to think ahead when you edit boundaries. As a rule, we always start defining the outer boundary. If you do not do that, your input will not work.
The dialog window for editing boundaries will be like Figure 3.7, “Editing Boundaries Dialog”. By clicking the left hand side radio buttons, you can select any boundary for editing. The fields hollow, mu, and rho are described in the input file format the section called “Boundaries”.
The first button on the last row is for adding a new boundary. The next button is for adding edges to the the selected boundary (using radio button). The final button is for closing the dialog.
We should elaborate on adding edges to a boundary. Once you click on the Add Edges button, the edges belonging to that boundary will be highlighted as seen in Figure 3.6, “Editing Boundaries”. The edges that are coloured are the edges on the boundary being edited. Just by clicking on any edge, you can add it to this boundary, or you can remove it from the boundary if it is already included. Note also that the edges need not be in any order. The ordering is done by the program. However, the program will warn you if a boundary does not close.
Once you have edited the boundaries, and if everything is fine, the final step will be just saving it as a coordinate file. Just use the popup menu and you are good to go.