Where To Go

Metalwork

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.