A cog in the machine
Meaning: a small, insignificant part of a whole system
Often referring to a person who is easily replaceable in a company or workplace.
The idiom suggests the meaning relates to the assembly line system (whereby a person plays a small role in production), which began in factories during the Industrial Revolution.
This idiom is often mixed up with the idiom ‘a cog in the wheel’. This phrase means: a necessary and vital part of a whole, which has positive associations as it suggests that, however small, each part places a crucial role in the working of the whole. This idiom goes back to the 15th century so has a different origin entirely. This proposes the idiom refers to a communal role, probably in a much more rural society.
J March 