Any molecular machine that can be programmed to build virtually any molecular structure or device from simpler chemical components using mechanochemistry. Also called a drexler.
Assemblers differ greatly according to function, composition, and design, but a typical hylonano assembler is a molecular-scale robot arm which is able to hold and position reactive atoms and molecules so that they react at precisely determined locations to build or dismantle a structure a few atoms at a time. It consists of about a million atoms, making up an arm that is roughly one hundred atoms long, together with devices to modify the tool mounted on the arm, a control system that can interpret instructions from a nanocomputer and a receiver for accepting molecules from a conveyor system. An average assembler arm is capable of positioning approximately a million atoms per second.
The first commercially available assembler was Neotek's "Universal Micro" in 195 A.T., but it was relatively limited in scope and required purified feedstock to operate. By 213 A.T. the Nanoscale Collective on Asimov Orbital had developed an advanced multipurpose assembler, the Genii 2 Matter Compiler, which was followed by even more efficient types.
Modern assemblers can work in tandem with a disassembler system to process raw materials, scrap or rubbish into finished products. Such a system is known as an autofab or nanofab; an autofab which can make copies of itself is known as a self-replicating system, generally known as a 'neumann'.
See also Assembler FAQ