ITI/NCVT Mechanic Machine Tool Maintenance (AITT Theory) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Fire class D

Combustible metal fires such as magnesium, aluminium swarf and sodium are Class D; they need dry sand, graphite or special powder extinguishers, and water or CO2 reacts violently and must never be applied.

RCCB 30 mA

A 30 mA residual current device trips when earth-leakage current reaches 30 milliamperes, a level below the fibrillation threshold for a healthy adult, and is required for portable tools and damp locations.

Vernier least count

Least count equals one main scale division minus one vernier scale division. For a 1 mm main scale and a 50-division vernier spanning 49 mm, the least count is 0.02 mm.

Micrometer least count

The screw pitch divided by the number of thimble divisions. A 0.5 mm pitch screw with a 50-division thimble gives a least count of 0.01 mm and is read on sleeve plus thimble.

Sine bar formula

For a sine bar with roll centre distance L and slip gauge stack height h, the angle satisfies sin θ = h divided by L; it is most accurate for angles below 45 degrees.

Tapping drill size

For metric coarse threads the tapping drill equals major diameter minus pitch; M10 × 1.5 requires an 8.5 mm drill, and standard charts give exact values for all sizes.

Prussian blue scraping

High spots transfer as blue marks when the workpiece is rubbed on a blueed surface plate or straight edge; each cycle scrapes only the marked points until the required spots per unit area appear.

Angular shim correction

To correct angular misalignment, the shim thickness added under one foot equals the coupling face gap multiplied by the foot distance from the coupling, divided by the coupling diameter.

Gear module

Module is the pitch circle diameter divided by the number of teeth, expressed in millimetres. Gears of the same module mesh correctly, giving circular pitch equal to pi times module.

Belt velocity ratio

For an open belt drive, driven speed equals driver speed multiplied by driver pulley diameter and divided by driven pulley diameter, neglecting the small effects of slip and creep.

Self-locking screw

A square-thread screw is self-locking when the friction angle exceeds the helix angle; the load then cannot drive the screw in reverse even without an external brake.

Bearing bore code

In bearing designations the last two digits give bore: 00, 01, 02 and 03 mean 10, 12, 15 and 17 mm, and from 04 onward the code is multiplied by five.

Rating life L10

L10 is the life in million revolutions reached by 90 percent of a group of identical bearings under rated load; L10 = (C/P)^p, with p equal to 3 for ball and 10/3 for roller bearings.

Lip seal orientation

The sealing lip must face the medium being retained, with the garter spring on the oil side. Shaft hardness above 45 HRC and roughness near 0.4 micrometre Ra extend seal life.

Hydraulic power formula

Hydraulic power in kilowatts equals pressure in bar multiplied by flow in litres per minute and divided by 600; this gives the fluid power delivered to a cylinder or motor.

Accumulator precharge

Bladder accumulator precharge is normally set to 70 to 90 percent of the minimum working pressure. Pressure must be released before any maintenance and the precharge rechecked after repairs.

Precision level sensitivity

A sensitivity of 0.02 mm per metre means the bubble moves one division when one end of a 1000 mm base is raised 0.02 mm; it is used for machine bed levelling.

ISO 230-1

ISO 230-1 defines geometric accuracy tests for machine tools, covering straightness, flatness, squareness, parallelism, runout and pitch accuracy, measured unloaded at a 20 degree Celsius reference temperature.

OEE formula

Overall equipment effectiveness equals availability multiplied by performance multiplied by quality. Availability is running time over planned production time, and 85 percent is regarded as world class.

MTBF and MTTR

Mean time between failures is total operating time divided by the number of failures; mean time to repair is total downtime divided by the number of repairs. Availability equals MTBF divided by MTBF plus MTTR.

BPFO

Ball pass frequency outer race equals (z/2) times shaft speed times (1 minus d/D cos θ), where z is ball number, d ball diameter and D pitch diameter; a rising peak indicates outer race defect.

Annealing

Annealing heats steel above its critical temperature, holds it for uniformity, then cools it very slowly in the furnace to soften the metal, relieve internal stress and refine structure.

Nitriding

Nitriding diffuses nitrogen into a steel surface at about 500 to 550 degrees Celsius in an ammonia atmosphere, producing a hard wear-resistant case without quenching and with very little distortion.

EN31 steel

EN31 is a high-carbon, high-chromium bearing steel of about one percent carbon; it is through-hardened and used for bearings, rolls and dies that need high wear resistance.

Cutting speed formula

Cutting speed in metres per minute equals pi times diameter in millimetres times spindle speed in rpm, divided by 1000; rearranged, it gives the rpm for a recommended cutting speed.

Taper angle formula

The half taper angle satisfies tan α = (D minus d) divided by 2L, where D and d are the large and small diameters and L is the taper length along the axis.

Three-phase power

In a balanced three-phase circuit, power equals the square root of three times line voltage times line current times power factor; dividing by efficiency gives the required motor input power.

First-angle projection

In first-angle projection, used in India and Europe, the object lies between the observer and the plane; the front view appears above the top view and the left view appears on the right.

H7/g6 clearance fit

H7/g6 is a hole-basis clearance fit in which the hole lower deviation is zero and the shaft lies below nominal size, allowing free sliding or rotation with a small controlled play.

Datum feature symbol

A datum is a theoretically exact reference identified on the drawing by a capital letter in a rectangular frame, attached to a filled or open triangle on the datum feature.

Factories Act working hours

Indian Factories Act and OSH Code prescribe a maximum of 48 working hours per week and 9 hours per day, with a rest interval after five hours and limited quarterly overtime.

5S workplace

5S means sort, set in order, shine, standardise and sustain; it removes waste from the workplace, shortens tool search time and supports safe, predictable maintenance work.