join vs cog

join and cog both are nouns.

Word	Noun	Adjective	Verb	Adverb
join	Yes	No	Yes	No
cog	Yes	No	Yes	No

As verbs, cog is a hyponym of join; that is, cog is a word with a more specific, narrower meaning than join:

join (noun)	cog (noun)
a set containing all and only the members of two or more given sets	tooth on the rim of gear wheel
the shape or manner in which things come together and a connection is made	a subordinate who performs an important but routine function

join (verb)	cog (verb)
make contact or come together	join pieces of wood with cogs
cause to become joined or linked	roll steel ingots
become part of; become a member of a group or organization
come into the company of
be or become joined or united or linked