Assemblers were robots that constructed furniture, including the UNIT ZX419 that the Cylinder wanted from Generios 8. Banto Zame read up on the planet, saying it was a thriving society until the Assemblers turned on the humans, killing them all. The Assemblers had giant pincers on the end of their arms. They felt they were superior to the "organic humans".
They wanted Mel and Banto Zame to build the Shelves of Infinity in thirty minutes. They thought it was impossible. When Mel and Banto tricked them into thinking they assembled it, they removed it from their memories. (AUDIO: The One Doctor)