April 7th, 2009

The most prominent feature of the clock is the filigreed brass metalwork. The brass parts are both functional and decorative. The two large counter-rotating disks which display the date and time are actually giant gears. Hidden behind the 4 decorative corner-plates are smaller brass gears which mesh with and drive the date and time gears. Covering the silver corner-plates are brass grilles which are purely decorative.

To cut and etch the metalwork, I had several options. I could have used laser-cutting, CNC machining or Photo-Chemical etching. Due to the large size and amount of detail, I went with Photo-Chemical Machining (PCM).

Photo-Chemical Machining is a process similar to the process used to etch the copper surface of printed circuit boards. A resist mask is created photographically from artwork and applied to the metal. Then the metal is placed in a chemical bath which selectively dissolves the exposed metal. You can achieve a stunning level of detail using PCM.

However, unlike the inimitable Jake Von Slatt, I didn’t have a large workshop where I could safely mix large vats of Copper Sulfate (such is life in a studio apartment). So, I took the easy way out and used a fabrication service called Acu-Line Etch. I had a good experience with them on a smaller, simpler project and they came through for me again. Acu-Line uses Photo-Chemical Machining to cut, etch and chemically darken brass and copper sheet metal directly from a digital file. They also applied a brushed metal effect ( using an actual brush, I think ) and a clear lacquer to help prevent tarnishing. It turns out the brushed-metal effect was very important, as it hid imperfections in the metal and made the metal literally sparkle in the sunlight.

Not insignificantly, they can also apply a chemical darkening to the etched areas which I used to blacken the printed text. This saved me from having to print the lettering on the brass in a second pass or from darkening the metal somehow myself. It also proved to be a serious design constraint, as they can only darken copper alloys. My original design incorporated a stainless-steel disk displaying the constellations. In the end I compromised by using the brushed stainless steel only for unprinted elements.

Probably the biggest challenge I had in creating the artwork for the brass components was generating the involute gear profiles for the large and small gears. If I didn’t get the math right, the gears wouldn’t mesh correctly. Also, because I was using Adobe Illustrator to create the artwork, I somehow needed to get the outline of the gears imported to Illustrator as a single polyline.

Lets just say my solution was less-than-optimal.

I found that the free CAD program from E Machine Shop has a feature for generating involute gears which you can export to Autocad format. Converting the DXF file into a usable Illustrator file is a long, sad story involving multiple conversion programs and some hand-made Perl scripts.

Suffice to say, next time I’ll use a commercial CAD program.

Here are some low-res images of the artwork which shows the layout of the brass parts and some close-ups on the brass parts themselves.

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