Machinist ' s mate Machinist ' s mate ( or MM ) is a rating in the United States Navy ' s engineering community . Description . According to the Bureau of Naval Personnel ( BUPERS ) , the job of a machinist ' s maste is to " operate , maintain , and repair ( organizational and intermediate level ) ship propulsion machinery , auxiliary equipment , and outside machinery , such as : steering engine , hoisting machinery , food preparation equipment , refrigeration and air conditioning equipment , windlasses , elevators , and laundry equipment . Operate and maintain ( organizational and Intermediate level ) marine boilers , pumps , forced draft blowers , and heat exchanger s ; perform tests , transfers , and inventory of lubricating oils , fuels , and water . Maintain records and reports , and may perform duties in the generation and stowage of industrial gases . " Enlistees are taught the fundamentals of this rating through on - the - job training or formal Navy schooling . Advanced technical and operational training is available in this rating during later stages of career development . Machinist ' s mates ( submarine ) and machinist ' s mate ( surface ) generally fall into one of three roles : Propulsion mechanics . The core job of an machinist ' s mate is the engine room . A mechanic ' s job in the engine room is to operate the steam plant that provides propulsion , electric power ( along with Electrician ' s Mate s ) , potable water , and service steam to the ship . Machinist ' s mates operate the boilers that generate the steam and use it to operate main engine ( propulsion turbines ) , turbo generators , distilling units , and various auxiliary turbines . This job was previously performed by boiler technicians , however , the BT rating merged with machinist ' s mates in 2000 . Machinist ' s mates working in the propulsion plants are often referred to as " pit snipes , " " hole snipes , " or just " snipes " for short . Machinist ' s mates responsible for the propulsion plant are found in machinery division ( or main engine division , main propulsion division , etc . in some ships ) , commonly referred to simply as M - Division or MP - Division . On surface nuclear ships , propulsion mechanics are found in reactor propulsion division ( commonly still referred to as M - Div ) , reactor mechanical division or reactor laboratories division . was the exception , in that M - Division was still part of engineering department , as opposed to reactor department . " Navy Times " of 7 January 2013 reports the Navy will be realigning machinist ' s mate , and engineman ratings . EN whose primary skills are in maintaining auxiliary engineering will become MMs . EN aboard ship will now concentrate strictly on diesel propulsion which powers many amphibious ships , while all surface shipboard auxiliary machinery work will be under the MM rating . This will affect almost 2000 ENs . Auxiliary mechanics . The other major place to find machinist ' s mates is in the auxiliaries division of engineering department , often referred to as " A - gang " . Mechanics in this role establish training & work on mechanical equipment within and outside of the engine rooms . To include the operation and preventive maintenance of : air conditioning and refrigeration units , liquid oxygen and nitrogen units , hydraulic lifting and hoisting gear , hydraulic power plant components and mechanisms , chilled and fresh water , atmosphere control , ventilation , emergency diesel and corresponding sub - systems , ballast control , auxiliary drain , compressed air and gases , plumbing , trash disposal & other equipment as designated by the engineering officer ( chief engineer , or CHENG ) . Members of " A - gang " may also help damage controlman , gunner ' s mate s , or other ratings . Some listed attributes apply to submarine warfare to include SUBSAFE requirements for work authorization . All systems listed apply to a minimum of valve , pump maintenance repair . Nuclear mechanics . Some MMs are qualified to work on nuclear propulsion plants . They are designated as machinist ' s mate nuclear ( MMN ) , though the N is generally only used for the purposes of billeting and advancement exams . These mechanics go through a rigorous training program consisting of three months at the Nuclear Field " A " School , six months of Naval Nuclear Power Training Command and six months at one of four Nuclear Power Training Units ( prototypes ) . Two NPTUs are moored training ships ( MTS ) using S5W reactors located at the Naval Weapons Station in Charleston , South Carolina , and two additional NPTUs are land - based prototype units using an S7G reactor and an S8G reactor located in Ballston Spa , New York . This course of instruction is commonly referred to as " the nuclear - power pipeline . " MMNs earn a 3385 NEC ( surface ) or 3355 ( submarines ) upon completion of prototype . Along with the normal duties of a propulsion mechanic , they are responsible for the operation , maintenance , and repair of the auxiliary equipment in support of the naval reactors under their care . Later in his career , the MMN will qualify his / her senior in - rate watchstation ( chief reactor watch for surface MMNs in reactor mechanical division , chief machinery operator for those in machinery division , and engine room supervisor for submariners ) . Additionally , some MMNs are designated as engineering laboratory technicians ( NEC 3386 for surface , 3356 for submariners ) following successful completion of an additional 13 weeks of training . ELTs are water chemistry and radiological controls experts for their commands . After completion of required watchstation qualification , submarine ELTs may complete an additional supervisory qualification card in order to be assigned a supervisory NEC of 3366 . This designation is important insofar as it garners additional pay and consideration during advancement . Also , on submarines , some MMNs are designated as submarine nuclear propulsion plant emergency welders ( NEC 3351 ) . They go through a two month school learning welding , brazing , and soldering from hull maintenance technician senior enlisted welders . Each submarine is required to have at least one of them to go underway . They are on board to provide emergency repairs to the reactor plant if necessary to get the ship back to port where professional welders can complete a certified repair . Various machinist ' s mate ratings . The range of machinist ' s mate ratings include :