ITI/NCVT Mechanic Machine Tool Maintenance (AITT Theory) key terms

A formal written authorisation naming the task, its hazards, the isolation points and the precautions to be taken, signed before maintenance begins on hazardous plant or machinery.

A procedure in which every energy source is isolated, locked by the person doing the work and tagged with that person's name, so the machine cannot be restarted accidentally.

Items worn by the worker such as safety glasses, gloves, hearing protection and safety shoes, which reduce exposure to hazards but never replace guarding, ventilation or safe procedures.

A fixed or interlocked physical barrier that stops any part of the body from reaching a danger point such as a rotating shaft, belt drive, gear mesh or cutting tool during operation.

An unplanned event that caused no injury or damage but had the potential to do so; reporting near misses lets the hazard be corrected before a real accident occurs.

A measuring instrument with a main scale and a sliding vernier scale, typically reading to 0.02 mm, used for outside, inside, depth and step measurements on a workpiece.

A precision instrument using a calibrated screw and graduated thimble, usually reading to 0.01 mm, to measure external dimensions, with a ratchet or friction stop to control measuring force.

The smallest change in a measured quantity that an instrument can display; for a vernier it is the smallest main-scale division divided by the number of vernier divisions.

An instrument with a spring-loaded plunger and a graduated dial that shows very small displacements, widely used for checking runout, alignment, flatness and machine tool accuracy.

A heavy, accurately flattened cast-iron or granite slab used as a datum reference surface for marking out, inspection, scraping checks and precision measurement of components.

A wrench that indicates or limits the tightening torque applied to a fastener, ensuring bolted joints are tightened to specification without over-stressing the threads or the part.

A precision bar with two rollers of known centre distance, used with slip gauges on a surface plate to set or measure angles by simple trigonometry.

A fitting operation in which a hardened scraper removes very thin high spots from a surface, checked repeatedly against a mating surface or surface plate, until the required contact pattern is obtained.

A thin coloured paste spread on one surface so that contact with the mating surface reveals the high spots; used to check fits, alignment and the progress of scraping.

Setting two coupled shafts so that their centrelines coincide within tolerance, checked with dial indicators, feeler or taper gauges, or laser systems, to prevent vibration and bearing wear.

A key is a small steel piece seated in matching slots called keyways in a shaft and hub, transmitting torque positively while still allowing the hub to be removed for maintenance.

A power transmission arrangement using an endless belt running over two or more pulleys, giving quiet, flexible transmission with slip protection but needing correct tension and pulley alignment.

The ratio of the number of teeth on the driven gear to the number on the driving gear, which fixes the output speed and torque relative to the input.

A rolling-element bearing using hardened steel balls between inner and outer rings; it carries radial and some axial load with low friction but needs correct lubrication, alignment and fitting.

The small gap between rolling elements and raceways, or between shaft and bush, which allows a lubricant film to form and accommodates thermal expansion and slight misalignment.

A measure of a lubricant's resistance to flow; it decides how well an oil forms and keeps a film in a bearing, and it falls as temperature rises.

A flexible elastomeric ring with a spring-loaded lip running against a shaft to retain lubricant and exclude dirt; the lip must face the medium and the shaft surface must be smooth.

The study of friction, wear and lubrication between surfaces in relative motion, which guides the choice of lubricant, surface finish and bearing material in machine maintenance.

A pressure-limiting valve that opens to bypass fluid back to the tank when system pressure exceeds its setting, protecting the pump, hoses and actuators from overload damage.

A combined filter, regulator and lubricator fitted at the inlet of a pneumatic system that removes water and dirt, sets the working pressure and adds oil mist when required.

Setting a machine base so that its guideways and tables are horizontal and free from twist, using spirit levels, wedges, shims and anchor bolts before the foundation is grouted.

Checks of a machine tool's built-in errors, such as straightness, squareness, parallelism, spindle runout and pitch accuracy, made with precision instruments to prove the machine can hold tolerance.

The deviation of a rotating spindle surface or test mandrel from a true circle or axis, measured with a dial indicator; excessive runout causes poor surface finish and dimensional error.

Planned, scheduled servicing such as cleaning, lubrication, adjustment and replacement of worn parts, carried out before failure occurs to keep equipment reliable and cut unplanned breakdowns.

Measuring parameters such as vibration, temperature, oil debris and noise to detect developing faults early, so that corrective work can be planned before a functional failure occurs.

Recording and interpreting the frequency spectrum of machine vibration to identify faults such as unbalance, misalignment, looseness and bearing defects, so repairs can be planned before damage spreads.

Repair work carried out only after a machine has failed; it is simple and cheap for non-critical items but causes unplanned downtime and may damage surrounding components.

A heat treatment in which the metal is heated, held at temperature, then cooled very slowly to soften it, relieve internal stresses and refine the grain structure for easier machining.

Hardening heats steel above its critical temperature and quenches it for wear resistance; tempering then reheats it to a lower temperature to reduce brittleness and give the required toughness.

The total permitted variation in a dimension, being the difference between the upper and lower limits; it is stated on drawings and controls fit, function and manufacturing cost.

The system of specifying maximum and minimum sizes for holes and shafts so that assemblies give the desired clearance or interference, using standard graded tolerances and designated fit letters.

A drawing convention in which the object is imagined between the observer and the projection plane, so the front view is placed above the plan and the view from the left appears on the right.

A numerical measure of the fine irregularities left by machining, expressed as the arithmetic mean deviation in micrometres and shown on drawings by a surface texture symbol.

A workplace organisation method covering sort, set in order, shine, standardise and sustain, which keeps tools and areas clean and arranged so that work is faster, safer and easier to control.