ITI/NCVT Mechanic Machine Tool Maintenance (AITT Theory) common mistakes
ITI/NCVT Mechanic Machine Tool Maintenance (AITT Theory) · Understand the traps and how to avoid them.
Reads a vernier caliper by taking the main-scale value at the vernier zero instead of the division just before it, and adds the vernier coincidence without applying the sign of the zero error.
The formula is remembered as MSR + (VSD × LC), but the 'just before zero' rule and the sign convention are not practised on the actual instrument; under time pressure the candidate grabs the nearest main-scale number.
What to do instead
Close the jaws and record zero error with its sign first. Then take MSR as the last main-scale division before the vernier zero, add the coinciding VSD × least count, and subtract the zero error algebraically. Do ten practice readings on gauge blocks and write the sign in every step.
Chooses a water or foam extinguisher for a live electrical panel fire or for a hydraulic-oil fire in the machine shop.
Fire classes are memorized as letters without mapping them to shop fuels; candidates remember that water cools but not that water conducts electricity and spreads burning oil.
What to do instead
Map the fuel to the extinguisher before answering: electrical and gas fires use CO2 or dry powder; oil and hydraulic fluid use foam, dry powder or CO2; wood and paper use water or foam. State the burning material and whether electricity is present.
Writes the belt-drive speed ratio as N1/N2 = D1/D2 and the gear ratio as T_driver/T_driven, so a speed reduction is reported as a speed increase.
The ratio is memorized as a single fraction without labelling driver and driven; the candidate does not check the physical result.
What to do instead
Label driver and driven first. For pulleys, neglecting slip, N_driver × D_driver = N_driven × D_driven; for gears, N_driver × T_driver = N_driven × T_driven. Sanity-check: a smaller driver pulley or driver gear speeds up the driven shaft.
Calculates L10 life as L10 = (C/P) × 10^6 revolutions for every bearing, omitting the life exponent p = 3 for ball bearings and p = 10/3 for roller bearings.
The formula is memorized in a simplified linear form; the exponent is treated as a small detail and is not linked to the bearing type.
What to do instead
Write the full equation L10 = (C/P)^p × 10^6 rev, state p before substituting: 3 for ball bearings and 10/3 for roller bearings. Keep C and P in the same units, then convert to hours using L10h = (10^6/(60n)) × (C/P)^p.
Says a pressure relief valve controls actuator speed and a flow control valve limits system pressure.
Both valves appear in the same circuit and their symbols look similar; candidates learn the names without the function.
What to do instead
State the function first: the relief valve limits maximum pressure and protects the pump (normally closed, pilot-operated); the flow control valve sets actuator speed by restricting flow, and a pressure-compensated type keeps speed independent of load. Draw the standard symbol beside each answer.
Describes lathe levelling as tightening foundation bolts until the machine 'looks level' and does not mention checking bed twist with a precision spirit level in both longitudinal and cross directions.
Levelling is confused with ordinary building level; installation and geometric accuracy tests are studied as separate lists rather than one sequence.
What to do instead
Use the sequence: foundation, packing and shimming, coarse levelling, grouting, final levelling with a precision spirit level on the bed in both directions, then geometric alignment tests such as straightness, squareness and parallelism using a dial indicator and test mandrel. Explain that levelling removes bed twist before alignment.
Diagnoses all vibration as bearing failure and does not distinguish 1× running speed, 2× running speed, and high-frequency broadband or impacting signals.
Condition monitoring is studied as a list of instruments; candidates do not practise matching a fault frequency to the component that causes it.
What to do instead
Learn the frequency map: dominant 1× RPM indicates unbalance; 2× and harmonics indicate misalignment or looseness; non-synchronous high frequency with shock pulses indicates a bearing race defect; gear-mesh frequency indicates a gear fault. Confirm with trend, temperature, oil debris and noise.
States that tempering increases hardness and that hardening relieves internal stresses, and mixes annealing with normalizing.
The four heat-treatment processes are memorized as a sequence of names; purpose and cooling rate are not linked to each process.
What to do instead
Build a table: annealing uses slow furnace cooling to soften, relieve stress and improve machinability; normalizing uses air cooling to refine grain and give moderate strength; hardening uses austenitizing and quenching to increase hardness but adds brittleness and stress; tempering reheats below the critical temperature to reduce brittleness and relieve stress, with a slight drop in hardness. Answer with purpose, not just the name.
For a taper, calculates (D − d)/L and calls it the taper angle in degrees, or uses the included angle where the compound rest must be set to the half angle.
Taper ratio, taper angle and half angle are not separated; the candidate skips the trigonometric step.
What to do instead
Write the definitions: taper ratio = (D − d)/L; half angle α = tan⁻¹[(D − d)/(2L)]; included angle = 2α. For compound-rest setting use α. Check by sketching the taper and marking D, d, L and α.
Reads a sectional view as if it were an external view, ignores hatching as cut material, and cannot identify the datum or fit from an assembly drawing.
Orthographic and section conventions are memorized as line types without practice in reading assembled drawings; the title block and bill of materials are skipped.
What to do instead
Before answering, read the title block, views, section labels and bill of materials. Treat hatched areas as material cut by the section plane; hidden lines are normally omitted in sections unless needed. Identify datum surfaces and fit notation such as H7/g6 from the drawing, and practise one assembly drawing each week by listing its parts and fits.