CNC Milling a shape that would benefit from tabs to hold it in position
If you were to machine the shape shown in Fig. 6-11 from thin sheet, you would find there was a danger that as the final cut was completed, the finished workpiece might fly out of the sheet, or at least might move slightly and be damaged by the
cutter. It is not possible to clamp this shape without the cutter running into the clamp at some point, so there is clearly a problem here. This is not an isolated in- stance because shapes like this all suffer from the same problem. Double-sided
tape, strong glue and vacuum tables are all useful in this situation, but lead to their own problems. The tape may not be strong enough and its grip will be affected by dust on the workpiece. The glue may be strong enough, but may be difficult to re- lease or to clean off the finished work. And vacuum tables require additional vac- uum pumps, as well as being relatively expensive.
Fortunately, Cut2D is capable of dealing with this situation by creating small ‘tabs’ that continue to hold the cut shape in place until machining has been com- pleted. These tabs can then be broken or cut quite easily to release the work. Al- though this does mean there may be a small amount of cleaning up to do, it is a good solution to this problem. Tabs can be created within the Profile toolpath panel. Experiment with placing four tabs around the edges of the shape. Once you are happy with the result, calculate the toolpath once again, preview it and then re- turn to the main Toolpath panel. Click on the Save Toolpath icon (the floppydisk symbol), and choose a postprocessor to suit your CNC software. This may be Mach3 Arcs (millimetres or inches) or EMC2 G61 (or LinuxCNC, which was for- merly called EMC2).
From within your CNC program: Load that file, preview it in the Backplot win- dow, set the work origin, position the cutter safely and run the program. The Con- trolled Point should follow the path shown in the drawing, but will be a short dis- tance away from the finished sides of the shape because the program takes account of the diameter of the cutter so that the finished sides end up the right size.
