ITI/NCVT Tool & Die Maker (AITT Theory) common mistakes
ITI/NCVT Tool & Die Maker (AITT Theory) · Understand the traps and how to avoid them.
Marks water as the correct extinguisher for an electrical switchboard fire or an oil fire because 'water cools and is always available'.
Candidates memorise extinguisher colours or labels but do not link extinguisher type to fire class; water is familiar and seems universally effective.
What to do instead
Build a fire-class-to-agent table and rehearse it: Class A ordinary combustibles can use water; Class B flammable liquids use foam, CO2 or dry powder; live electrical fires use CO2 or dry powder, never water; Class D metal fires use special dry powder. Read the question for the material burning and whether the power is still on before choosing.
Writes 'heat to red hot and quench in water' for every steel, including high-carbon die steels and alloy steels, and skips immediate tempering.
The simplified school version of hardening is 'heat and quench', so the quench medium and tempering step are treated as afterthoughts rather than part of the process.
What to do instead
Link quench severity to hardenability and section: plain carbon steels generally use water or brine; alloy steels use oil; air-hardening steels use forced air. State that quenching produces martensite and that tempering must follow immediately to relieve stresses and adjust toughness. Practise identifying the steel from its designation before selecting the medium.
Reads a 0.02 mm vernier calliper by taking only the main-scale reading, or treats the least count as 0.01 mm and ignores zero error.
The reading procedure is rushed under exam timing; candidates confuse the 50-division vernier with a 0.01 mm micrometer and forget that the vernier coincidence must be added.
What to do instead
Use the fixed sequence: note zero error first, take main-scale reading at the vernier zero, find the coinciding vernier division, multiply by least count, then add. For a 0.02 mm calliper, least count = 1 main-scale division / number of vernier divisions = 1 mm / 50 = 0.02 mm. Practise reading both 0.02 mm and 0.05 mm callipers until the addition becomes automatic.
Calculates spindle speed with cutting speed in m/min but leaves the workpiece diameter in millimetres, producing an answer 1000 times too small or too large.
The formula N = 1000V/(πD) is memorised without tracking units; candidates plug V in m/min and D in mm directly into the denominator.
What to do instead
Write the unit conversion explicitly every time: convert diameter from mm to metres by dividing by 1000, so N = 1000V/(πD) with D in mm. Sanity-check the result: for mild steel at about 25–30 m/min with a 25 mm drill, the speed should be roughly 320–380 rpm, not 0.35 rpm or 350,000 rpm.
Sets the compound rest swivel to the full included taper angle instead of half the included angle when turning a taper.
Candidates remember 'compound rest is set to the taper angle' but do not distinguish the included angle of the workpiece from the angle the tool must swing through.
What to do instead
Draw the taper triangle: the compound rest swivel equals half the included angle, because it turns along one side of the taper. Use tan(α/2) = (D − d)/(2L) and label D, d and L on a sketch before calculating. Then verify whether the question asks for the included angle, the half angle or the swivel setting.
Treats 'jig' and 'fixture' as interchangeable and describes a fixture as having a drill bush that guides the tool.
In the workshop the words are used loosely; candidates learn a one-line definition but do not attach the defining feature to the device.
What to do instead
Use the presence of tool guidance as the decision rule: a jig locates and clamps the work and also guides the tool, usually through drill bushes; a fixture locates and clamps but does not guide the tool, so the tool position comes from the machine setting. When a question mentions drill bushes, bush plates or a drill jig, choose jig; when it mentions clamping only, choose fixture.
Uses one clearance value for blanking and piercing, or applies the clearance to the punch when it should be applied to the die opening for piercing.
The clearance concept is reduced to 'gap between punch and die', so the direction of clearance and the part-size rule are not separated for the two operations.
What to do instead
State the rule before calculating: for piercing, the punch determines the hole, so punch size = hole size and die opening = punch + 2 × clearance per side; for blanking, the die opening determines the blank, so die opening = blank size and punch = die opening − 2 × clearance per side. Clearance depends on material type and thickness, not on a single remembered number.
Writes G02 for a clockwise arc without checking the active plane, or uses I/J incorrectly because the sign convention is guessed.
Candidates memorise 'G02 is clockwise, G03 is anticlockwise' but not that clockwise is defined looking down the positive axis of the active plane; I and J are incremental from the start point to the arc centre.
What to do instead
For G17 (XY plane), view from +Z: G02 is clockwise, G03 anticlockwise. For G18 or G19, rotate the viewpoint according to the right-hand rule. In I/J format, I and J are the signed distances from the arc start point to the centre, not the absolute centre coordinates. Confirm with a dry run or graphic simulation before accepting the block.
Places the top view above the front view on a drawing that is actually in first-angle projection, or identifies the projection symbol backwards.
Some reference material uses third-angle examples, while Indian workshop drawing follows first-angle; candidates remember 'top view above' from third-angle diagrams without checking the symbol.
What to do instead
Recall first-angle projection physically: the object is between the observer and the plane, so the top view appears below the front view and the left-side view appears on the right. Read the truncated-cone projection symbol: in first angle the cone's small end points towards the observer in the side view arrangement shown. When the symbol is absent, look at the view placement before labelling.
Calculates simple indexing as 40/N and then writes a fractional turn such as 13⅓ without converting the fraction to an available hole circle.
The formula is known, but the practical step of selecting a hole plate is skipped; candidates do not connect the remainder to the hole circles supplied with the dividing head.
What to do instead
After 40/N, keep the whole turns and convert the fraction to holes on an available circle: 13⅓ turns = 13 full turns plus 1/3 turn; on a 15-hole circle that is 5 holes, on an 18-hole circle it is 6 holes. Check the standard plate hole sets and state the plate used. For angles, use 40/N where N = 360/angle.