Laser cutting speeds typically are as much as 30 times faster than friction sawing (Yankee, 1979). Cutting accuracy is about 0.01 mm/cm under closely controlled conditions. All metals - including high-strength, exotic, and refractory alloys such as Inconel and titanium, as well as aluminum, stainless steel, and brass - and nonmetals such as diamond, ceramics, and plastics may be vaporized by laser beams. Hence, parts of these materials may be easily machined. Burr-free laser holes may be drilled as small as 10-100 μm. Lasers can also be used for pattern cutting, gyro balancing, insulation stripping, surface hardening, trimming, photoetching, measurement of range and size to 1 μm accuracy or better, scribing 5-10 μm lines on microelectronic wafers, flaw detection, marking or engraving parts, and impurity removal (e.g., carbon streaks in diamond). Laser beam machining is "especially adaptable and principally used for relatively small materials processing applications such as cutting, trimming, scribing, piercing, drilling, or other delicate material removal operations similar to milling or shaping" (Yankee, 1979).
Dunning (unpublished Summer Study document, 1980) has suggested a variety of space and lunar applications for laser machining, including flash trimming of cast basalt parts; engraving bar codes on parts to enable quick and accurate recognition by robot vision systems; drilling holes in workpieces an inch thick or less; internal welding of cast basalt joints, pipe, and structural members; impurity removal from lunar-produced semiconductor chips; cutting operations on gossamer structures (Brereton, 1979) in orbit; and case hardening of cast basalt or metal parts. Dunning has also suggested two potential major problems associated with the use of lasers in the context of a selfreplicating, growing lunar manufacturing facility: (1) the need for gas jets, and (2) the requirements of closure.
In normal industrial usage, vaporized workpiece material is carried away by a gas jet, usually oxygen (Yankee, 1979). The gas serves three functions: (1) to oxidize the hot working surface, decreasing reflectivity, (2) to form a molten oxide (i.e., the metal "burns") which releases a large fraction of the useful cutting energy, and (3) to remove slag and hot plasma from the path of the beam. There is no problem maintaining a moderate-pressure O2 atmosphere around the laser work area, as the beam penetrates air easily. In this case the usual gas jet can still be used. Or, the laser could be placed outside the pressurized working area, shooting its beam through a transparent window. If pressurization must be avoided, laser machining can be done entirely in vacuum and the ionized plasma wastes removed by a magnetic coil following the cut or weld like an ion "vacuum cleaner." However, it is estimated that up to 80% of the laser cutting energy comes from the exothermic oxidation reaction, so in this latter case laser energies would have to be on the order of five times the value for the equivalent O2-atmosphere machining.
The problem of closure is even more critical in a replicating autonomous remote factory. The materials closure problem is solved in large measure by resorting to CO2 gas laser technology. This gas is available in limited quantities on the Moon, whereas materials for solid state lasers such as yttrium, ruby, garnet or neodymium are generally very rare (although Dunning has suggested that spinel, which is plentiful on the Moon, might be substituted for garnet). Quantitative materials closure may be argued as follows. A typical CO2 laser uses three gases for high-power operation - carbon dioxide to lase, nitrogen to sustain the reaction, and helium for cooling because of its excellent heat conducting properties. Since oxygen is plentiful, the three limiting elements are C, N, and He. From appendix 5E, the LMF in one year can produce 400 kg C, 400 kg N2, and about 40 kg inert gases (at least 90% of which is He). This is sufficient to make 747 m3 (33,300 moles) of CO2, 320 m3 (14,300 moles) of N2 and 224 m3 (10,000 moles) of He, at STP. Even if the laser machining device requires several hundred moles of these gases (a few thousand liters at STP), still only a few percent of available LMF stocks of these elements need be diverted for this purpose, a negligible resource drain.
The problems of parts and assembly closure cannot be answered satisfactorily at the present time. However, it is often asserted that machining the laser end mirrors to high accuracy may be a major roadblock to automated manufacture of lasing devices. Nazemetz (personal communication, 1980) has pointed out that a laser is accurate enough to surface a rough-hewn mirror to the accuracy required for its own construction. In a pinch, concave mirrors could be hewn from solid metal or basalt blanks simply by sweeping the laser beam radially across the disks, applying higher power nearer the center so more material volatizes there, thus creating a perfect spherical or parabolic surface gradient. There appear to be no major unresolvable difficulties associated with the use of lasers in an autonomous lunar manufacturing facility.
After parts leave the laser machining station they may require some slight further treatment such as annealing or coating to prevent cold weld, though this latter function may be unnecessary if laser welding takes place in an oxygen atmosphere (a thin layer of metal oxide prevents the vacuum-welding effect). Once fabrication is completed each part may have one of three possible destinations: (1) assembly sector, where the part is given to a mobile robot for transport to wherever it is needed, (2) parts warehouse (which serves as a buffer supply of extra parts in the event of supply slowdowns or interruptions), where the part is taken to storage by a mobile robot, or (3) fabrication sector, when more fabrication must be performed upon an already manufactured "part" (e.g., solar cell aluminum sheets), where a mobile robot carries the part to wherever
