Correct spelling for the English word "millcog" is [mˈɪlkɒɡ], [mˈɪlkɒɡ], [m_ˈɪ_l_k_ɒ_ɡ] (IPA phonetic alphabet).
There seems to be no etymology available for the word "millcog". It is possible that "millcog" is a coined term or a specific jargon used in a particular context with limited usage, hence lacking etymological information.