Maharashtra HSC Mechanical Maintenance (Bifocal) key terms

A drawing method that shows a three-dimensional object as separate flat views, such as front, top and side, each projected at right angles onto its own plane.

A projection convention where the object sits between the observer and the plane, so the view seen from the front is placed below the front view on the sheet.

A single pictorial view in which the three axes are drawn 120 degrees apart, giving a quick three-dimensional picture without true perspective foreshortening.

A view produced by imagining part of the object cut away to reveal internal features; the cut surface is filled with uniformly spaced hatching lines.

A system of upper and lower size limits assigned to mating parts so that the intended clearance or interference is guaranteed during assembly.

The intentional difference between the maximum shaft size and minimum hole size, chosen by the designer to give the required clearance or interference.

The total permissible variation in a dimension, equal to the difference between the upper and lower limits of that dimension.

A heat treatment in which steel is heated, held, and then cooled very slowly in the furnace to soften it, relieve internal stress and refine grain structure.

Heating steel above its critical temperature and cooling it in still air, producing a finer, more uniform grain structure and moderate hardness.

Hardening quenches heated steel to form martensite; tempering then reheats it to a lower temperature to reduce brittleness while keeping useful hardness and toughness.

A treatment that enriches the surface layer of low-carbon steel with carbon so the skin becomes hard and wear resistant while the core stays tough.

A measuring instrument with a main scale and a sliding vernier scale that reads outside, inside and depth dimensions to a typical least count of 0.02 mm.

The smallest value an instrument can read reliably, found for a vernier by dividing the main scale division by the number of vernier divisions.

A precision instrument using a calibrated screw and thimble to measure small thicknesses and diameters, usually to 0.01 mm, with a ratchet to control measuring force.

A reading shown by a micrometer when its anvils are closed; it must be added to or subtracted from every measurement to correct the result.

A flat, accurately finished cast-iron or granite plate used as a reference surface for marking out, checking flatness and supporting components during inspection.

A block with a V-shaped groove used to hold round work securely for marking, drilling or inspection, often in matched pairs with clamps.

A hardened steel point held in the hand or a surface gauge to scratch layout lines on metal workpieces before machining.

A pointed punch struck with a hammer to make a small conical indentation that locates a drill or preserves a layout mark.

Bench work in which a file is pushed across the workpiece to remove small amounts of metal, correct edges and bring surfaces to shape or size.

A hand saw with a fine-toothed replaceable blade held under tension in a frame, used to cut bar stock, pipe and sheet to length.

Matching cutting tools: a tap cuts an internal thread inside a drilled hole, while a die cuts an external thread on a round bar or bolt blank.

Producing a round hole by rotating a fluted cutting tool called a drill while feeding it into stationary or rotating work.

A lathe operation in which the workpiece rotates against a single-point tool that is fed along or into the work to reduce diameter or face an end.

A machining process where a multi-tooth rotating cutter is fed into the work to produce flat surfaces, slots and complex profiles.

Scheduled inspection, cleaning, adjustment and replacement carried out at planned intervals to stop equipment failing unexpectedly during production.

Repair work done only after a machine has failed, restoring it to service with little or no advance planning.

Measuring parameters such as vibration, temperature and oil debris while a machine runs, so developing faults are detected before they cause failure.

A structured investigation that asks why a failure occurred repeatedly until the underlying cause is found, so the repair prevents recurrence rather than only restoring the machine.

A lubricant's resistance to flow; thicker oils have higher viscosity and suit heavier loads and slower speeds, while thin oils flow more readily.

A measure of how stiff a grease is, graded by NLGI number; higher numbers mean firmer grease that stays in place in slow or vertical bearings.

The small gap between a shaft and its bearing or between rolling elements and races, allowing lubricant film and thermal expansion without seizing.

The tightness of a drive belt, set so it transmits power without slipping yet does not overload shaft bearings or stretch prematurely.

A safety procedure in which an energy source is isolated, locked and tagged by the person working on the machine before maintenance begins.

Wearable protection such as safety glasses, gloves, helmet and safety shoes, selected for the task and used as the last line of defence.

Keeping the workplace clean and orderly, with materials stored, spills cleared and walkways free, to reduce slips, fires and tool losses.

A check made with a multimeter on a de-energised circuit to confirm that a conductor or switch provides a complete low-resistance path.

A portable instrument that measures voltage, current and resistance, and often continuity, so a maintenance technician can diagnose electrical faults quickly.

The resistance between an earthing electrode and the general mass of earth, measured to confirm that fault current will flow safely to ground.