Pioneers as Eric Drexler proposed as far back as the mid-1980's that nanoscale mechanical computers could be built via molecular manufacturing through a process of mechanical positioning of atoms or molecular building blocks one atom or molecule at a time, a process known as “mechanosynthesis.”
Once assembled, the mechanical nanocomputer, other than being greatly scaled down in size, would operate much like a complex, programmable version of the mechanical calculators used during the 1940s to 1970s, preceding the introduction of widely available, inexpensive solid-state electronic calculators.
Drexler's theoretical design used rods sliding in housings, with bumps that would interfere with the motion of other rods. It was a mechanical nanocomputer that uses tiny mobile components called nanogears to encode information.
Drexler and his collaborators favored designs that resemble a miniature Charles Babbage's analytical engines of the 19th century, mechanical nanocomputers that would calculate using moving molecular-scale rods and rotating molecular-scale wheels, spinning on shafts and bearings.
For this reason, mechanical nanocomputer technology has sparked controversy and some researchers even consider it unworkable. All the problems inherent in Babbage's apparatus, according to the naysayers, are magnified a million fold in a mechanical nanocomputer. Nevertheless, some futurists are optimistic about the technology, and have even proposed the evolution of nanorobots that could operate, or be controlled by, mechanical nanocomputers.