Step 5: CNC/Mach3 Operation
I just line up the tip of the mill bit with the top of the foam. Then I zero all the axes in Mach3. If you want it to cut a little into the foam on the top you can set the z position to 0.1 or something like that. It now thinks that it is 0.1 over the surface and it will cut into the foam.
Up to this point we haven't done anything to set the scale of the mold, it is just whatever the 3D model happens to be. We can see the dimensions now in Mach3 and there are a few ways to proceed.
The way I did it was to use the preview in Mach3 to take some measurements and determine the scale factor I needed. Of course we could also have done that earlier in the 3D modeling step, that probably would have been smarter. Well, that is why we do this, to learn.
Now simply add a scaling command in your gcode. G50 resets the scaling. And for example, if the scale was 2.5 use:
G51 X2.5 Y2.5 Z2.5
Now the mold is scaled to your face.
One final trick. Freemill wasn't sophisticated enough to cut the mold out in layers. It just cut all the way down to my nose in one go. This is a problem since I don't have a 3" long bit. I got around this problem by cutting a few times but with different Z scales. This allowed it to slowly carve out the mold. It is important to check that your Z-axis wont hit the top of foam when cutting all the way down. Fortunately the face is a gentle curve, so the dremel itself (my router for foam) wont hit the foam when cutting.
My bit was long enough to cut this out in two steps. So I first set my Z scale to 1.25 and let it run. This was only to remove enough material so that I could cut all the way down on the next pass. You may or may not have to do this.
So now we have the foam mold. Next we will pour the plaster mold.