cog in the wheel
Also, cog in the machine. One who holds a minor but necessary post in a large organization, as in Frank knew he was just a cog in the wheel of this giant corporation. This term alludes to the role of the mechanical cog, one of the teeth on a wheel or gear that, by engaging other teeth, transmits or receives motion. Used figuratively since about 1930, it sometimes is put as small cog in a large wheel, emphasizing a person's lack of importance.