A scale model of the Voyager spacecraft shares a display case with replicas of a human skull, a cactus, George Washington's right leg and a convincing Swiss-cheese-on-pumpernickel in the model shop of the Smithsonian's Exhibits Central on North Capitol Street.

The staffs of the welding, plastic, woodworking and model shops, off-limits to the public, tackle a range of glamorous to dully exacting projects. The cheese sandwich, for instance, designed for a Museum of American History Automat exhibit, is made of soft plastic and feels cafeteria-fresh.

The average age of the craftsmen is 45 and, according to production chief John Widener, it takes at least five years on the job to show whether one is really meant for the model shop; a couple of young women apprentices are finding out.

In the same warehouse workroom, beyond a pile of mannequins on the floor, are fiberglass replicas of two elephant tusks, replacement parts for the 20-year resident of the rotunda of the Museum of Natural History, whose cellulose-acetate ones are deteriorating. The new set, to be installed after the tourist crush, weigh eight pounds apiece; the original ivories weigh in at 94 each.

Nearby, Walter Hock works from sketches and a plastic-foam mock-up, sculpting a clay model of a bisexual daisy, racing an August 30 deadline for "The Natural History of Sexuality" traveling show.

And Reginald "Buddy" Sayre, at 72 a 22- year veteran of the shop, sculpts miniature figures of archaeologists for a scale model of a dig site, part of the Smithsonian Institution Traveling Exhibition Service (SITES) show, "Ban Chiang: Discovery of a Lost Bronze Age," which will open in Washington in late 1983 after a year on the road. For Ban Chiang, staffers made and smashed ceramic pots, placing the shards just as excavated, accompanying life-size skeletons cast in plastic for an eerie hands-on exhibit.

The highest-profile creation is the pterosaur, with its 40-foot wingspan, overseeing Natural History's dinosaur hall. The flying reptile took three years from conception to completion, including time for research and for building three scale models in balsa wood and fiberglass. Widener says he's never bothered to view the beast in its new home on the Mall. He'd just as soon talk about his staff's bracket work and crate- building.

Some 65 percent of the shop's work is for SITES, which has 130 exhibits on the road at any given time. "We have to figure travelability into everything we do," Widener says. The cabinets, display cases, packing materials and text panels created here serve as prototypes for museums around the world.

Widener expresses almost as much pride in the packing jobs on items like Ban Chiang's pots from the third millenium B.C. as in the pieces that end up on display. Eileen Harakal, a SITES staff member, boasts, "Nothing has been broken in transit -- no glass, nothing. One time a crate fell 14 feet off a loading platform and the contents weren't at all damaged."

Isn't she tempting fate by talking about it? Well, she adds, Lloyd's of London handles the insurance.