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 #Specify finish and accuracy no greater than are actually needed. If a part will adequately serve its intended purpose at some lower level of accuracy of machining than is technologically possible, then cheaper, simpler production processes may be used which make closure easier to attain. The specification of needlessly close tolerances and an unreasonable degree of surface finish invariably results in a low part production rate, extra operations, high tooling costs, and high rejection rates and scrap losses (Yankee, 1979).

5F.2 Selection of Basic Production Processes

A wide variety of fabrication processes is available using current technology, each of which is optimum for the production of one or more classes of parts or in certain specialized applications (see table 4.17). From inspection of table 4.10 it is reasonable to conclude that there are perhaps only 300 fundamentally distinct fabrication techniques in widespread use today. Ultimately, the LMF factory in production phase may be called upon to perform many if not all of these functions. However, most may be unnecessary for initial system growth or replication. indeed, optimum seed design should permit maturation to adulthood in the minimum time with the fewest parts using the fewest machine operations possible.

The team concluded that four basic processes - plaster casting, vapor deposition, extrusion, and laser machining are probably sufficiently versatile to permit self-replication and growth. These four techniques can be used to fabricate most parts to very high accuracy. Plaster casting was selected because it is the simplest casting technique for producing convoluted parts as well as flat-surface parts, to an acceptable level of accuracy. (A number of alternatives have already been reviewed in app. 4B.) The laser machining tool can then cut, weld, smooth, and polish cast parts to finer finishes as required. Vapor deposition is the least complicated, most versatile method of producing metal film sheets to be used as the manufacturing substrate for microelectronics components, mirrors or solar cells, or to be sliced into narrow strips by the laser for use as wire. The extruder is used to produce thread fibers of insulating material, presumably spun basalt drawn from a lunar soil melt as described in section 4.2.2.

5F.3 Casting Robot

The casting robot is the heart of the proposed automated fabrication system. It is responsible for producing all shaped parts or molds from raw uncut elemental materials. The moldmaking materials it works with are of two kinds. First, the casting robot receives thermosetting refractory cement with which to prepare (a) molds to make iron alloy parts, (b) molds to make iron molds to cast basalt parts (but not aluminum parts, as molten aluminum tends to combine with ferrous metal), and (c) individual refractory parts. Second, the robot receives hydrosetting plaster of Paris with which to prepare (a) molds to cast aluminum parts and (b) substrates for the vacuum deposition of aluminum in sheets. According to Ansley (1968), small castings using nonferrous metals (aluminum, magnesium, or copper alloys) may be produced using plaster molds with a surface finish as fine as 2-3 μm and an accuracy of +/-0.1 mm over small dimensions and +/-0.02 mm/cm across larger surfaces (a drift of 2 mm over a 1 m2 area).

Traditionally, the plaster casting technique requires a split metal pattern in the shape of the object to be cast. This pattern is used to make a hollow mold into which molten metal is poured, eventually solidifying to make the desired part. Alternatively, patterns may be manually carved directly into the soft, setting plaster, after which metal again is poured to obtain the desired casting.

The casting robot should have maximum versatility. It will have access to a template library located within its reach, containing samples of each small or medium-sized part of which the LMF is comprised. If the SRS seed is designed with proper redundancy, it will use the fewest number of different kinds of parts and there will be large numbers of each kind of part. Assuming that on average there are 1000 pieces of each type of part in the original LMF architecture, then the total template library has a mass of only 100 tons/1000 = 100 kg and there are perhaps a thousand different kinds of parts (see below).

In addition, the casting robot is equipped with shaping and carving tools which can create any desired shape in the slowly hardening plaster. (Pure gypsum plaster hardens in 6-8 min after water is added, but this setting time may be extended up to 1-2 hr by adding lime, CaO, to the emulsion. Setting time is also temperature-dependent.) The shaping tools may represent perhaps 100 specific shapes and sizes and should also include at least a dozen "universal" carving instruments.

To make a given part, the robot searches its template library to see if it has a convenient pattern already in stock. If so, it uses the pattern to form the mold; if not, it uses its many tools to carve out a mold of the appropriate size and shape. Plaster of Paris is a hydraulic cement - it sets with the addition of water. Refractory cement is thermosetting and has to be heated to 1300-1400 K in a kiln to set the mold.

Water used to make the plaster molds cannot remain liquid in the lunar vacuum. Thus, the casting robot plaster system must be pressurized, probably with nitrogen gas to permit the pouring of molten aluminum. The triple point of water (the bottom end of its liquid phase) occurs at 608 Pa, but a 1.3&times;104 Pa atmosphere (16 kg N2 to fill a 100 m3 working volume) prevents water from boiling off up to about 323 K.

Mass requirements for plaster molding are estimated by assuming that 10% of the volume of each mold contains a