Tawjihi Vocational Engineering common mistakes
Tawjihi Vocational Engineering · Understand the traps and how to avoid them.
When a drawing shows a dimensioned feature with a tolerance (for example 40 ±0.2), the candidate reads only the nominal value and ignores the tolerance band when answering what sizes are acceptable, or omits the limit dimensions entirely.
Candidates memorise 'the number next to the arrow' and treat tolerances as decoration. In the workshop the nominal size is only the target, but in the classroom the tolerance is the actual answer being tested, so the habit of reading only the big number causes systematic loss of marks.
What to do instead
Before answering any tolerance question, write the two limits explicitly: upper = nominal + plus deviation, lower = nominal − minus deviation. Practise converting between bilateral (±), unilateral, and limit-dimension notation until you can state both limits in under ten seconds for a given callout.
Misreading orthographic views: placing a feature in the wrong view (for example drawing a hole in the front view when it belongs in the top view), or failing to project hidden edges as dashed lines because the candidate assumes all edges are visible.
Candidates answer from a mental 3D picture instead of applying first-angle or third-angle projection rules systematically. Without a fixed projection convention the views get shuffled, and hidden detail is forgotten because it is not obvious in the given pictorial sketch.
What to do instead
Adopt a fixed routine: identify the projection symbol first, then locate height in the front view, width in the top view, and depth in the side view. For every feature, ask which views it is visible in, hidden in, or absent from, and mark hidden edges as dashed before finalising the answer.
Confusing material properties: treating hardness and toughness as interchangeable, saying a hard material is tough, or recommending a high-carbon steel for a part that needs shock resistance without explaining the trade-off.
The vocabulary is learned as a list of adjectives rather than as measurable, sometimes opposing properties. Candidates also skip the link between property and application, so they pick the 'strongest' material for every job.
What to do instead
Build a two-column table pairing each property with its test and its typical application: hardness with Brinell/Rockwell and wear surfaces, toughness with Charpy impact and hammers or axles, ductility with tensile elongation and wire drawing. For each answer state the property required by the service condition first, then choose the material.
Resolving forces incorrectly: adding magnitudes directly when forces act at an angle, forgetting to resolve into perpendicular components, or omitting the reaction force at a support when drawing a free-body diagram.
Candidates default to arithmetic addition because it worked for parallel forces in earlier schooling. Free-body diagrams are skipped as an unnecessary step, so missing reactions and unresolved components go unnoticed until the final number is wrong.
What to do instead
Always draw the free-body diagram first and label every reaction and applied force. Resolve each inclined force into horizontal and vertical components, then apply ΣFx = 0 and ΣFy = 0 separately. Check the answer by comparing the resultant with the largest individual force: it can never exceed their sum.
Mixing series and parallel circuit rules: using the parallel current-division formula for a series loop, adding resistances directly in a parallel branch, or assuming voltage is the same across components in series.
The two configurations are memorised as formulas without the underlying invariants (current common in series, voltage common in parallel). Under exam pressure the candidate grabs whichever formula looks familiar.
What to do instead
Anchor each rule to its invariant: in series the current is common and voltages add; in parallel the voltage is common and currents add. For any network, redraw it, mark the common quantity, then compute equivalent resistance before touching currents or voltages. Verify with a power balance or by checking that the largest resistor drops the largest voltage in series.
Choosing machining or joining processes by name rather than by capability: specifying turning for a square internal pocket, or selecting arc welding for thin sheet where distortion would ruin the part, without justifying the choice.
Candidates learn process names alongside one example each and generalise that single example to every case. They rarely compare processes on tolerance, surface finish, material thickness, or production volume.
What to do instead
For each process, record four attributes: achievable tolerance, surface finish, suitable materials and thicknesses, and economic batch size. When answering, state the requirement, eliminate processes that cannot meet it, then justify the survivor on those four attributes.
Ignoring instrument resolution and systematic error: quoting a measurement to more decimal places than the instrument can support, or treating a zero error as random and averaging it away.
Candidates equate 'more digits' with 'more accurate' and are not trained to read the least count of a vernier or micrometer. Zero error is confused with scatter, so a consistent offset is averaged instead of corrected.
What to do instead
State the least count before reading any instrument, and round the final answer to that resolution. Check and record zero error at the start of every measurement, then apply the correction with the correct sign. Distinguish systematic error (offset, fix by calibration or correction) from random error (scatter, reduce by repeated readings and averaging).
Treating safety as a list of slogans: answering 'wear PPE' for every hazard scenario, or failing to identify the specific hazard, its energy source, and the control that removes it at source.
PPE is the easiest control to recall and the last line of defence, so it crowds out the hierarchy of controls. Candidates also answer the generic question instead of the scenario actually posed.
What to do instead
Apply the hierarchy explicitly: elimination, substitution, engineering controls, administrative controls, then PPE. For each scenario name the hazard and its energy source first, then choose the highest-level control that is feasible, and mention PPE only as the residual layer.
In troubleshooting questions, replacing parts or adjusting settings before establishing what the fault actually is, and never confirming that the repair restored correct operation.
Candidates treat troubleshooting as a recall test of 'what usually breaks' rather than a logical sequence. The exam rewards a defensible diagnostic order, and guessing skips the evidence-gathering steps that earn marks.
What to do instead
Use a fixed sequence: gather symptoms from the operator, inspect safely, isolate the fault to a subsystem, test to confirm the cause, repair, then verify by running the machine through its full range and recording the result. State each step in your answer, including the confirmation test.
In costing and planning answers, omitting overhead, labour, or contingency from a cost estimate, or producing a schedule with no critical path, float, or dependency logic and no allowance for risk.
Candidates equate cost with material price and planning with a list of tasks in order. Overheads and contingencies are invisible in a simple exercise, and without dependency logic every task looks equally urgent.
What to do instead
Build estimates from four lines: materials with wastage, labour with hours and rate, overhead as a percentage, and contingency for identified risks. For schedules, list tasks, state predecessors and durations, identify the critical path, and note float on non-critical tasks. Show the calculation, not just the total.