For my final project -- a pinball machine themed on an opera whose libretto is written by monkeys on typewriters -- I want to have a central chorus pit with five or six monkeys on typewriters. This seems like a perfect application for the molding and casting unit. My wife (Jill Colinan) is making the monkeys, so I thought I'd cast the typewriters for this assignment.

I wanted to base the design of the typewriter on this drawing that my friend Sue Riddle drew almost a decade ago. I know I have a larger version around somewhere, but this was the only image of the drawing I could find (everything was much smaller on the Internet in the twentieth century). I modelled the thing in Sketchup but ran into a problem where the STL plugin we are using only outputs ASCII STLs, where the stl2png.py script only accepts binary STLs. Rather than muck with the plug-in code I decided that the mold was simple enough to draw each projection in 2D using Inkscape. (Note that we have since solved this ASCII/binary problem by first bringing the ASCII STL into MeshLab).

I sketched out what the mold projections might look like and mapped the grey values "by hand." Note that this is a three-piece mold; left, right and one for the angled keyboard.

I drew the three projections in Inkscape and brought them into cad.py for tracing. I used the 1/8" bit for the left and right sides of the mold. The typewriter keyboard required a finer bit to get around the keys, so I did that in two passes; one with the 1/8" bit and one with a 1/16" bit for the detail (which could not be more that 1/4" deep due to the difference in size between the tool tip and the shank). I'll write more about this approach in the tutorial.

After milling from the machinable wax, I cast a mold using the two-part Smooth-On rubber urethane. I realized it would be difficult to align the keyboard mold within the two side molds. Rather than re-do my mold with a key for the third mold, I cast a strong magnet into the piece. I was hoping to pull it into place with another magnet on the outside.

I had to break the wax to get the molds out:

and the magnet hack seemed to work well:

I cast the first couple of typewriters in hyrdostone, which did not require a mold release. I later tried to cast in plastic with mixed results. We're still trying to figure out a "soap scum" mold release method that works well without leaving bubbles in the material. Unfortunately I didn't choose a great location for the fill hole, which made it difficult to fill the keys of the keyboard all the way to the top without bubbles.

When you're done with the machinable wax it can all be melted back down and cast into a new block. The wax melts at about 230 degrees and vaporizes at about 575 degrees. The method that worked best for me was to bake the wax at 350 degrees in an old bread pan for twenty minutes or so, then pour it into a project box that was about the same size as the original block. The block on the left was heated at 250 degrees, and cooled down too quickly as it was poured: