Web site created by Donato Borello, Amsal Karic, and Randal C. Nelson


Mechanical Housing Details

In some sense, the mechanical housing was the core of the project, as it had to hold everything else together, and was the primary means we had at our disposal to affect durability. The electronic components, for example, were commercial, mass-produced products. We could select for quality and try to protect them against known failures, but we could not design and manufacture our own semiconductor elements. The housing, on the other hand was largely fabricated from bulk materials (metals, glass, etc.) giving us more flexibility to design for the ages.

Materials

The main body of the artifact needed to be of a material that was strong, durable, and formable with reasonable effort. We had access to ordinary machine shop resources (saws, drills, mill, lathe, grinders etc.) but not to specialized fabrication facilities. We evaluated advantages and disadvantages of a number of material classes.

  • Wood. Advantages: Easy to shape, readily available, moderate strength, attractive. Disadvantages: Not waterproof, strength is non-isotropic (it splits), can decay, softer and less durable than many other materials.
  • Plastics. Advantages: Waterproof, relatively strong, easy to shape resistant to decay and corrosion. Disadvantages: Relatively soft, and not as strong as metals or ceramics, Most photodegrade and lose their strength over a period of decades when exposed to light and air, most soften at relatively low temperatures. No long-term (centuries) experience with durability except for buried amber and petroleum products, the existence of which suggests that some plastics could hold up if protected from light and oxygen.
  • Stone, Glass, Ceramics. Advantages: Hard, durable, strong in compression, waterproof, fireproof, decay and corrosion proof. Disadvantages: Brittle, with a tendancy to shatter under impact, difficult to shape in general due to combination of hardness and brittle properties.
  • Artificial composites (fiberglass, carbon fiber, etc.). Advantages: Very strong, lightweight, waterproof, decay and corrosion resistant, impact resistant. Disadvantages: Plastic resins photodegrade over several decades, non-isotropic strength, special fabrication techniques required.
  • Metals. Advantages: Very strong, durable, waterproof, fireproof, decay-proof, non-brittle, no photo-degradation, shaping tools widely available. Disadvantages: Heavy, many are expensive, some are subject to corrosion, more difficult to shape than wood or plastic.

The combination of strength, resistance to brittle fracture, and lack of photo-degradation led us to choose a metal housing. The most easily available metal, steel, is unfortunately subject to severe corrosion (rust) in environments with any moisture. The second most available, aluminum is relatively soft, and is also subject to severe corrosion in certain environments. Gold, silver, and other durable precious metals were well out of our price range and likely to be stolen and melted down if used. Workable options included (316) stainless steel, titanium, nickel alloys such as monel and inconel, and certain copper-based alloys (bronzes). Stainless, titanium, and nickel alloys are all difficult to machine (compared to ordinary steel), and since we are not expert machinists, we decided to go with a copper-based alloy.

Specifically, we selected 954 aluminum bronze , an alloy of approximately 84% copper, 11% Aluminum, 4% Iron, with small amounts of manganese and other impurities. This material is as strong as medium steel, has excellent corrosion resistance, and is an attractive gold color. It also turned out to be somewhat difficult to work, especially tapping threads, (but easier than 316 stainless).

The artifact also required externally exposed screws and flanges for assembly and framing of transparent ports. We used stainless steel for these components as screws etc. are readily available in stainless (as opposed to 954 bronze), and behaves behaves well in contact with bronze (no strong tendency towards galvanic corrosion or contact welding).

The solar panels, viewer, and LED beacon required transparent ports. We considered various materials, including plastic, glass, and more exotic materials such as quartz or sapphire. Plastic was rejected as being prone to scratching and susceptible to photo-degradation. Quartz and sapphire were expensive, and difficult to shape, though more scratch resistant than glass. We settled on using glass ports, protecting against brittle fracture by using pieces as thick as practical, in this case 1/4 inch.

Tempered or chemiclly toughened glass would be preferable, and could increase the fracture resistance by an order of magnitude. Unfortunately, we did not find a source for tempered pieces of the sizes and shapes we needed at reasonable cost. It is possible to carry out chemical tempering on a small scale by immersing components formed from ordinary soda-lime glass in molten potassium nitrate for 6-24 hours. This process has certain hazards, and we did not get around to attempting it. Specifically, molten potassium nitrate has a tendency to react violently with any combustible material it comes in contact with, including many metals. Also, its melting temperature, (334 C = 633 F), is uncomfortably close to its decomposition point (400 C), so good thermal control is needed. Decomposition could produce extremely caustic particles of various potassium oxides (i.e. lung-dissolving smoke).

Design and Fabrication

The artifact as designed contained the following components.

  • Viewer/microscope
  • Solar panel
  • LED Beacon
  • Electronic circuitry (board)
  • Energy storage ultracapacitor

The bulkiest component was the viewer, which placed some lower bounds on the size of our artifact and precluded a flat design. As very amateur machinists, we chose a simple rectangular prism (an oblong box) as the simplest shape to make. For the same reason, we chose to fabricate the box from flat panels joined together with stainless steel machine screws rather than attempting to cut all the ports and support structures into a single solid piece of metal. The downside was more seams that needed to be sealed against air and water. We used 3/8" bronze plate for fabrication to facilitate drilling and tapping holes for screws. This is a bit heavier than needed for durability. 3/16" walls would probably suffice, even for 1000 years, with a monolithic design. Thicker metal could be restricted to regions where it was needed for screw insertion. This is standard professional design protocol, and generally the walls would be thinned in non-essential areas even with plate fabrication. Our fabricators, however, had never used machine tools before, so considerable weight was attached to easy-to-machine design.

The plates were band-sawed oversize from rough bar stock, and then face-milled on all six sides to final dimensions. Screw holes were made in two stages. First a small number of screws were inserted to tack the object together. Tap-sized holes were drilled through the plate and into the substrate in one operation to ensure alignment, with the plate holes enlarged for thread clearance in a separate operation. Alignment for drilling the initial holes was maintained by careful clamping of the pair of parts being joined at each step. Once the tack screws were in place, the remaining holes were drilled on the partially assembled body, which now kept its own alignment. The body was disassembled for tapping and thread clearance drilling.

In standard practice, all this manipulation can be avoided by accurate measurement and tool placement during milling and drilling operations. As amateur machinists, however, we found the above process faster and less error prone than getting the measurements right. As a final step in the error correction process, the metal parts were screwed together in sequence after all the drilling and tapping was completed, and any surfaces that were supposed to be coplanar given a finishing mill cut to ensure that they actually were.

The viewing scope was mounted in a machined aluminum block, attached with machine screws to one end of the box. The scope required an end port for viewing, and four side ports to admit external light to illuminate the printed material. The ports were constructed by placing a 1/4" thick glass disk in a shouldered hole bored in the body, and securing it from the outside with a stainless steel washer attached with six stainless button-head cap screws. The disks were cut from plate glass sheet glass using a diamond hole saw in a drill press with slow speed and lots of water. Disks had to be cut from both sides to prevent chipping on breakthrough. The edges were finished by careful grinding on a wet wheel or a wet belt sander. We found that the trick with glass is to go slow enough and use enough water to prevent heating and thermal chipping. Water also prevents the formation of airborne glass dust which is NOT GOOD to breathe.

Sealing the Artifact Externally

Much degradation of human artifacts is due to exposure to water, light, and oxygen in the air. The electronic and graphic contents of the artifact were selected to be as water, oxidation, and light resistent as possible, but there are limits. Metal conductors in active electric circuits exposed to moisture will corrode due to electrolytic effects even if the conductors are normally corrosion resistent. Centuries of exposure to room-level light will damage most organic materials. Consequently we made substantial efforts to seal the artifact against moisture, air, and where consistent with its operation, light.

The seams between the panels of the artifact were sealed by applying High-temperature automotive silicone gasket material between contacting surfaces during final assembly. The specific product used was Permatex Sensor Safe Ultra Copper High-temp RTV Silicone Gasket Maker. The material is advertised as being non-corrosive, and resistant to (intermittant) temperatures up to 700F (371C). A secondary bead was laid down in internal corners. This system is not as robust as possible because the panels were not machined with a gasket groove, and thus most of the sealant squeezed out when the machine screws were drawn tight. Machining a shallow gasket groove to hold additional material would probably be a good design modification.

The transparent ports have a double seal. A primary seal was formed by compressing an o-ring between the external washer and the glass disk. The same silicone gasket material was used to create a secondary seal between the glass disks and the bronze body. The tapped holes for the machine screws extended through the body plates, and these also were sealed with automotive gasket maker during assembly. A better design would be not to use through holes, but the bronze material turned out to be difficult to tap, and we did not have tools for creating bottomed threads. The order was reversed for the LED port because the retainer securing the glass disk was attached from the inside rather than the outside.

The solar array window is mounted behind a machined flange, and retained within by overlap from the side panels. This is not the best system mechanically. It was used because size constraints on already purchased components did not permit a separate retainer to apply positive pressure as with the other openings. The contact between the window and the flange, and the gaps around the window and between it and the retaining side panels were sealed with the silicone gasket-maker material. This is probably the weakest seal in the system. Sufficient inward pressure on the window might distort the gasket material enough to cause seal separation, though ordinary handling should not cause a problem.

Protection of Internal Electronic Components

Because of the likelihood of eventual failure of the elastomeric sealing components, a second method was employed to protect the electronic components long-term from the effects of air, moisture, and also certain shock and thermal effects. The approach was to fill the entire electronics chamber with paraffin wax, embedding all circuit components. This procedure has a number of benefits. The wax effectively excludes air and moisture from the circuit. It immobilizes the components, protecting against shock, and preventing chafing, rubbing, or any other movement that might produce a short circuit. It is an excellent insulator, and prevents the production of spurious connections that sometimes result from long-term corrosion and electrolytic effects in soldered circuits. It is almost entirely inert, and will not react appreciably with oxygen, water, metals, or any other material present in the artifact at ambient temperatures at time scales of a few millennia. It is self-healing in that cracks will eventually rebond at room temperature (over months at 70F, and hours at 90F). It provides protection against component overheating by melting locally and providing a liquid cooling environment. (The only component in the system where this might happen is the zener diode that protects the energy storage capacitor against overload, and only after an extended period in full sunlight.)

We used a food grade paraffin that melts just below 50 degrees centigrade. It was melted and poured into the electronics chamber of the artifact, which was completely assembled and sealed except for final attachment and sealing of the bottom panel. Pouring was done in several layers to facilitate final electronic component adjustment (mostly of the connecting wires) and minimize the effect of paraffin shrinkage on solidification (about 10%). The molten wax was carefully cooled to 50 C before pouring to avoid any chance of thermal damage to the electronic components, some of which are rated only to 60 C. Because the artifact, left in the sun, might possibly reach 50 C, we left 10% void space for the wax to expand if it melts, rather than forcing its way out through the weakest point of the artifact. We filled this space with foam polyethylene cut to shape to keep the initial voids where we wanted them.

Results

The final artifact is approximately 5-7/8" long by 2-1/2 wide, by 2-3/8" high, and looks like a steam-punk sarcophagus. It weighs 6.16 pounds (2.80 kg). Heavy metal. It does (marginally) fit in a pocket. But you wouldn't want to carry it there for long... Final assembly and sealing took place on Sunday, June 17, 2012. In north window exposure, the red LED flashes between approximately 50 and 100 times per minute, depending on the time of day and how bright it has been. Oddly enough, the rate is about the same as a human heartbeat.