Remove the outer casing on the Monroe PC-1421, a 1964 mechanical calculator, and this is what you’ll find. Courtesy of Kevin Twomey

Mark Glusker had heard rumors about the mechanical calculator, a Monroe PC-1421, pictured above: that it was one the most complicated devices of the sort ever built; that it was powerful but notoriously difficult to keep running; that it was at the pinnacle of an effort to compete with the first electronic calculators.

“It’s kind of a holy grail machine for me,” says Glusker, a mechanical engineer and collector of early calculators (he has about 100). “When you’d read the specifications, you’d think, ‘That’s just crazy.’” Once he finally got his hands on the 40-pound behemoth from a retiring professor at the University of Iowa, it proved just as intricate as he’d imagined.

To learn more about the calculator, Glusker tracked down a former service manager for the manufacturer who “recalled the machine with a mixture of awe and dread,” says Glusker. Whenever the company came up with an improved part for the calculator—which contained thousands of pieces and frequently broke—it would send out a service bulletin, then dispatch service managers who would disassemble, reassemble, and recalibrate the entire device in order to incorporate the new part. And service bulletins came out every couple of weeks.

With a price tag of $1,175—at the top end for a mechanical calculator—the Monroe PC-1421 debuted in 1964, right as electronic calculators were overtaking mechanical ones. To stay relevant, manufacturers fought to improve the speed of their mechanical machines by modifying existing features and adding new ones. For instance, while early mechanical calculators required an operator to turn a crank by hand in order to add, subtract, multiply, and divide, later models, such as the Monroe PC-1421, turned automatically with a motor for each operation.

The Monroe PC-1421 had a “mechanical memory”—it could store an intermediate result during a lengthy calculation—and could print on paper, which most electronic calculators at the time couldn’t do. It was also one of few mechanical calculators with a decimal point button (many others required that users keep track of decimal points in their head). Dials controlled tiny cables—one of which wove directly through the heart of the machine—that set the decimal places for whatever numbers a user was computing.

“If that cable ever snaps,” says Glusker, “you’ll have a hell of a time threading it back through.”

Kevin Twomey, a commercial and fine arts photographer, took these portraits of the Monroe PC-1421, along with many more mechanical calculators in Glusker’s collection. He had been doing research on calculators for a different project and became curious about the variety, complexity, and bold industrial design of the mechanical devices from the ’50s and ’60s. That’s when he got in touch with Glusker.

When Glusker removed the outside casing from one of his machines for a demonstration, Twomey instantly knew that he had a compelling subject matter for a new series. “There’s so much going on inside there,” he says.

Twomey took multiple pictures of each calculator using varying focuses and lighting techniques, then melded those images together into a composite to make sure every interconnected part was in high resolution detail. “These chains, levers, and gears were almost reminding me of how ligaments and joints are working together,” says Twomey.

Glusker is also drawn to the diversity of designs and the ingenuity needed to put together mechanical calculators, which manufacturers started producing at around the turn of the 20th century. There was no standardization, he says, and so the machines “evolved completely differently, with different mechanisms.”

While engineers primarily used more complicated mechanical calculators like the Monroe PC-1421 (indeed, many in Glusker’s collection came from thrift stores near national labs like Lawrence Livermore and Los Alamos), shopkeepers and financiers relied on the simpler devices.

Despite the efficiency mechanical calculators provided, however, electronic ones eventually edged them out in the end—the new machines were faster, quieter, cheaper, and ultimately more reliable. These days, collections like Glusker’s and photographs like Twomey’s are all that remain.

*This article was updated on December 17, 2014, to reflect the following correction: An earlier version stated that the first mechanical calculators appeared in the 1930s. According to Mark Glusker, the first mass produced mechanical calculators were made at around the turn of the 20th century. The 1930s reference was to specific models of machines.