Machines fill the floor of the Exploratorium, San Francisco’s beloved interactive science museum. Over there is a contraption called Bicycle Legs, in which visitors manipulate air pumps to replicate muscles we use when pedaling (it’s trickier than it sounds). A few hundred feet away is a perennial favorite, the Wave Machine, which demonstrates transverse waves with the turn of a crank (even I can manage that one).

“I told her she can give them to us if she wants, but we won’t take care of them.”

But the machines used to build these machines, along with the rest the Exploratorium’s 600 or so other interactive exhibits on display, are equally arresting, which is why everything from a late-model, high-tech CNC (computer numerical control) machine to a number of World War II-era lathes are given places of prominence on the museum’s floor.

These tools are not here because they’re valuable antiques, although some of them probably are, but because they help the Exploratorium’s dozen or so exhibit builders work their magic (or science, if you’d prefer) here and at science museums from San Diego to Oklahoma City. You’ll find them just across from the Tinkering Studio about halfway down the length of Pier 15 on San Francisco Bay, where the Exploratorium has been housed since leaving the Palace of Fine Arts, or PFA (a drafty relic of a 1915 world’s fair), in the spring of 2013.

“The views into the shop are the same as they were at the PFA,” says Tom Tompkins, a mechanical engineer who had some machine-shop experience when he became a member of the Exploratorium’s elite corps of exhibit builders in 1974. “The wood shop is enclosed because of the dust, and the welding shop is too. But the rest of the shop is wide open. We’re separated from the public by a low fence—that’s it.”

With thousands of people streaming by every day, you’d think Tompkins might be fed up with working in such a fishbowl, but mostly he enjoys it. “There are days when I’m either on deadline or feeling a bit grumpy, so I just ignore them,” he says of the clusters of people who at any given moment are loitering along that low fence to see what Tompkins and his colleagues are up to. “But then there are days when I have a little extra time and I’m feeling gregarious, so I’ll bring a few people in and show them around, talk to them, things like that. I like the contact in the shop.”

Even visitors who must content themselves with the view from behind the fence can get a good look at the shop’s workhorse machines, many of which were acquired during the 1970s from long-since-shuttered, local military facilities such as the Naval Air Rework Facility (known by its fun-to-say acronym, NARF) in Alameda, California, and the San Francisco Naval Shipyard at Hunters Point.

“Many of the original woodworking machines—the bandsaw, the jointer, and the planer—were here when I arrived,” says Tompkins. “Some of those machines may have come from the Naval Radiological Defense Laboratory at Hunters Point, but I don’t know if it was that lab or another one. The radiological lab makes sense because of Frank’s history. He would’ve known people there.”

“Frank” is physicist Frank Oppenheimer (1912-1985), who founded the Exploratorium in 1969, almost 25 years after helping his older brother, J. Robert, build the atomic bombs that were dropped on Hiroshima and Nagasaki at the end of World War II. I ask the dumb, obvious question: No, Tompkins assures me in reply, the bandsaw, jointer, and planer are not radioactive.

“By the time I got to the Exploratorium,” he continues, “we had National Science Foundation [NSF] grants for exhibit development. Because we were an NSF contractor, we were able to get our hands on ‘federal excess personal property’ through the General Services Administration [GSA]. During the Carter presidency, after the war in Vietnam had wound down, the West Coast was awash with machinery. The GSA was willing to shuffle this stuff off to educational institutions whenever they could—they sent us weekly catalogs of everything that was available. Essentially, we got a lot of this equipment just by paying the freight, which was a great deal, except for a couple of heavy mistakes.”

“Heavy” as in weight, not “bummer, man,” although in a few cases, there may have been some of that.

“We had ordered an impedance bridge,” Tompkins recalls with a sigh, “which is a standard piece of small laboratory equipment to measure resistance and conductance in circuits, that sort of thing. What showed up, though, were three racks loaded with equipment. It was an impedance bridge, but it had been built in the 1930s for measuring the impedance of telephone lines. It was nice to get the racks, but the rest of it was just garbage. The whole thing probably weighed about 1,500 pounds. A few years later, I ordered a 16-inch rotary table to fit our precision milling machine, but I didn’t check it out thoroughly. When it came in, it weighed something like 1,000 pounds, which was probably more than the machine.”

Since shipping costs were calculated based on weight, these were expensive mistakes for the fledgling organization to make, especially at a time when, as Tompkins puts it, “we were struggling to find money for payroll every two weeks.”

The key to avoiding such errors was for Tompkins to go out and take a look at things himself before he ordered them, which meant places like Alameda and Hunters Point became his stomping grounds. “I spent two or three hours a week over at the Alameda Naval Air Station crawling through their warehouses, which were really filthy. Since World War II, they’d been accumulating dust.”

Upon his return, he’d compare what he had seen with the description in the government’s catalog. “The catalog gave everything a condition code,” Tompkins says. “The condition was a letter followed by a number. For example, R meant repairable, N meant new, and the number was supposedly the condition, from 1 to 4. But these assessments were written by warehousemen, forklift drivers, so you’d find stuff in perfect shape that had an awful condition code, or you’d see something that was never going to work again that would be listed as N1.”

Which meant that sometimes Tompkins got lucky. “We got this giant Cleereman drill press from the Naval Air Rework Facility in Alameda,” he says, grabbing the handles, known as the horns, of the machine looming over us. “It can hold a 6-inch-diameter bit and drill through steel. The thing is gorgeous, and with only 225 hours on its Hobbs meter,” he says, in reference to the counter on the machine’s back, “it was essentially brand new when we got it in the ’70s,” (40 years later, that number is still under 280, which is still nothing for a Cleereman). “It was listed as ‘repairable with the cost of repairs exceeding the cost of replacement,’ but there was nothing wrong with it. We also got a huge number of twist drills for it, going up to two-and-a-half inches in diameter. When you are drilling even a two-inch hole in a piece of steel,” he adds with a grin, “the whole floor trembles.”

Not that I doubt his word, but Tompkins wants to show me the NARF sticker. “I love NARF stuff,” he says. “There it is.” Tompkins points to a diminutive, red metal rectangle on the side of the machine. “OVERHAUL & REPAIR DEPT, NAVAL AIR STATION, ALAMEDA, CALIF” it reads, a reference to NARF’s moniker before its name was changed in 1967. “The thing came to us with the ‘PEANUTS 10 CENTS A BAG’ sticker on it,” he adds, referring to the blue DYMO label located between a tag that identifies the drill as being “U.S. NAVY PROPERTY” and another pegging its date of manufacture to “8-8-45” in Green Bay, Wisconsin. “Somebody over at NARF had a good sense of humor.”

“When you are drilling even a two-inch hole in a piece of steel, the whole floor trembles.”

As with every exhibit at the Exploratorium, every machine in the museum’s shop has a story. “This drill spent the aftermath of World War II in Los Angeles,” says Tompkins of the Cleereman. How does he know? Well, the Cleereman label identifying it as a product of Green Bay also reads “50 CY,” as in 50 cycles per second, or 50 hertz. “The drill speeds are slightly higher than marked because during World War II, electricity in L.A. was at 50 hertz rather than 60 like in the rest of the country,” he says. “They didn’t change over until a couple of years after the war. People had to switch a huge amount of stuff—clocks, record players, drills—from 50 hertz to 60 hertz to get in sync with the U.S. electrical grid.”

We pass the non-radioactive DoAll bandsaw that Tompkins had told me came from the Naval Radiological Defense Laboratory at Hunters Point. I find myself keeping my distance anyway. “This had a U.S. Atomic Energy Commission sticker on it, but I rewired it a while back, so the sticker may be under that electrical box,” he says, pointing to a gray cube that is apparently filled with wires. “It would work fine on wood, but we try to keep sawdust out of this part of the shop and grease out of the wood shop, so we only use it for metal.”

Just beyond the DoAll is a quartet of machine lathes, which are barely a broom’s-length away from the public part of the Exploratorium. “These are the Monarch double Es,” he says of the first two, “the Rolls Royces of engine lathes. The only complaint I have about them is that they are a little short,” which is a reference to the length of the material one can work. “This one is dated December 1944, the other one is 1943.”

One of the Monarch lathes is also a NARF score. “It was in really good shape, except for a broken cog, which we fixed. It’s probably the most accurate lathe we’ve got.”

And it was cheap. “I had to have the drill press delivered,” he says of the Cleereman, “but for the lathes, we just went over and loaded them into a staff member’s pickup truck.”

One of the Monarchs has been retrofitted to make it a variable-speed lathe like its cousin. “Listen to this,” Tompkins says as he turns the thing on. The machine whirs to life. “That’s a five-horse, three-phase motor driving two generators, which drives a DC motor to achieve variable speeds. Very simple.” For Tompkins, perhaps, but I’m lost as he manipulates the machine’s various handles and levers to send its shaft spinning. The other Monarch, he tells me, has a three-phase motor with an inverter controller on it. “That’s the modern way to get variable speed. The inverter drive is quieter than the motor-generator setup.” He turns that one on, too. Sure enough, it is.

Though Tompkins has obvious affection for his Monarchs, the Exploratorium’s other two lathes (one of which is a Le Blond Dual Drive) are more versatile. “This one is immediately postwar,” he says of the Le Blond. “It came to us from UCSF; they closed their shop so we got it. The other one I would bet is around 1953. These two have larger throws than the Monarchs, which means they can turn a larger diameter of material, 16 inches instead of 12, and they are much longer.”

Passing a mid-20th century sheet-metal shear made by Pexto, we come upon a small orphan of a machine, whose roots are entirely different from the naval-surplus lathes and drill press of which Tompkins is so fond. It’s a Hammond Glider TrimOSaw, and Tompkins is soon off to the races with the story behind this one.

“This little puppy here is a composing saw,” he smiles, giving it an affectionate pat, “which is not very widely known around machine shops. It was typically used to trim the slugs from linotype machines,” he says of the technology invented by Ottmar Mergenthaler and used by newspaper publishers from the end of the 19th century until the late 1970s to compose type for press. “All the calibrations on it are in points,” which makes it useful for measuring and spacing type. “We use it to trim aluminum and Plexiglass. Linotype machines are crazy,” he adds. “You could do a whole article on them.” Noted.

The free-equipment gravy train came to a halt during the Reagan administration, when the rules governing what the GSA was able to list as excess property were changed. “Before Reagan became president, we could get machinery. After Reagan became president, GSA determined that items that were considered ‘common’ wouldn’t be listed. Common items turned out to include almost all forms of machinery.”

Which is not to say that Tompkins quit thumbing the pages of the GSA catalogs for deals. “I was still able to get some bizarre optics stuff. I got anything optic I could lay my hands on. Lenses, fancy mirrors, that kind of thing. One time they had listed a bunch of infrared lenses. I couldn’t figure out what the lenses were used for, so I called GSA to get the name of the manufacturer. When I called them to find out, they said, ‘We can’t tell you, it’s classified.’ So I called the GSA back and said, ‘Hey, do you know this thing’s classified?’ Suddenly the lenses were withdrawn—somebody had excessed them by mistake.”

These days, when Tompkins needs a tool, he can pretty much get anything he needs by going online, but in the early decades, it was catch as catch can. “We were very opportunistic,” he says. “If Frank was walking home one afternoon and he passed a garage sale with an old photo enlarger, he’d pick it up and it would become part of an exhibit. I got a call one afternoon from a staff member downtown who said, ‘Hey, there’s a debris box down here with a bunch of angle iron in it,’ so I took a couple of kids and a staff member’s pickup truck and we went down there and got it. We did a fair amount of dumpster diving.”

Though he calls himself a “tool freak,” Tompkins does not treat his treasures like, well, treasures. Hence, it makes no difference to him that the U.S. Atomic Energy Commission sticker on the DoAll bandsaw was covered up by an electrical box—rewiring the machine properly was more important than its history, however storied. Similarly, Tompkins and his colleagues are unsentimental about the equipment people occasionally try to foist upon them.

“As a museum,” he says, “we’re a teaching institution not a collecting institution. We don’t take care of things like a collecting institution would. Years ago, a woman in San Francisco really wanted us to have her collection of scales. It was an amazing collection, with scales going back to ancient Assyria. So I sent her in to talk to Frank. Afterwards, when I asked him how it went, he said, ‘I told her she can give them to us if she wants, but we won’t take care of them.’

“Another guy gave us an old bucksaw,” Tompkins continues, “a one-lunger, gasoline saw with a blade that went back and forth, a forerunner to the chain saw. It had a two-cycle engine whose crankshaft bearings were worn out, so it wouldn’t run. So we took it, machined up some brass bearings, bored out the babbitt bearings, got it up and running, gave it a coat of red spray paint, fixed the wooden frame that had kind of rotted, and gave that a spray-can paint job, too. Then we sawed a piece of eucalyptus with it and put it out on the floor. Eventually the guy came back, saw what we had done, and said, ‘You just destroyed a valuable antique!’ And I said, ‘Well, that’s sort of what we do.’”

(All photos by the author except as indicated in captions. For more information about the Exploratorium, visit exploratorium.edu.)