Pancyprian Machine Elements T.S. (Practical) common mistakes
Pancyprian Machine Elements T.S. (Practical) · Understand the traps and how to avoid them.
Reading a limit dimension from a technical drawing as a nominal size and then selecting a tolerance class from the ISO 286 tables instead of using the limits printed on the drawing.
Candidates memorise the fits table (H7/g6, H7/k6, etc.) and assume every drawing dimension must be looked up. When the drawing already states explicit upper and lower limits, or states a fit designation such as 40 H7/g6, they mix the two systems and calculate a shaft limit from the wrong nominal size or the wrong fundamental deviation sign.
What to do instead
Before computing anything, classify each dimension on the drawing as nominal, limit dimension, or fit designation. For a fit designation, write down nominal size, hole class and shaft class separately, then look up each deviation in the table and always keep the sign convention: for holes, EI is the lower deviation and ES the upper; for shafts, ei is lower and es upper. Check the result against the rule that clearance fits give positive minimum clearance and interference fits give negative clearance. Practise ten mixed examples where some dimensions are already toleranced and some are given as fits.
Taking a reading with a vernier caliper or micrometer without first checking and recording the zero error, and without applying the correct least count to the vernier scale.
Under exam time pressure candidates grab the instrument, measure the part and write the number. They forget that a caliper can have worn jaws or a displaced vernier zero, and they misread which vernier graduation lines up, especially on 0.02 mm calipers where the difference between adjacent lines is small. Micrometer readings are also misread by one revolution of the thimble (0.5 mm) when the sleeve datum line is not checked.
What to do instead
Build a fixed routine: close the jaws or bring the micrometer anvils together, read and record the zero error with sign, then measure. For the vernier, count the main scale millimetres before the vernier zero, then find the coinciding vernier line and multiply by the least count (0.02 mm for a 50-division vernier). For the micrometer, read the sleeve in 0.5 mm steps first, then add thimble divisions times 0.01 mm. Always state the reading with the zero correction applied and quote the instrument resolution.
Selecting a material or heat treatment by name only, without linking it to the required mechanical property, and proposing a treatment sequence that is metallurgically impossible or in the wrong order.
Candidates remember labels such as 'harden and temper' or 'case harden' but not what each does. Typical errors are specifying through-hardening for a low-carbon steel that cannot reach the required hardness, tempering before quenching, or asking for a hardness that exceeds what the chosen steel can achieve. The link between carbon content, hardenability and the achievable surface or core hardness is missing.
What to do instead
For every material question, write three lines before answering: required property (wear resistance, toughness, core strength), carbon content of the steel, and the treatment that delivers it. Low-carbon steels (around 0.2 % C) need case hardening or nitriding for a hard surface with a tough core; medium-carbon steels (0.3 to 0.6 % C) are quenched and tempered; high-carbon and tool steels are hardened and tempered at low temperature. State the sequence explicitly: preheat, austenitise, quench, temper, and note that tempering always follows quenching. Verify that the specified hardness is achievable for that carbon level.
Setting cutting speed, feed and depth of cut on a lathe or milling machine by habit or by copying a number, without converting surface speed to spindle rpm for the actual tool and workpiece diameter.
The formula v = pi times D times n divided by 1000 is known but not applied, so candidates use a spindle speed suited to one diameter on a very different diameter, or use a high-speed steel speed with a carbide tool. They also ignore the effect of depth of cut and coolant on tool life and surface finish, and cannot explain why a finish pass differs from a roughing pass.
What to do instead
Always compute spindle speed from the chosen cutting speed and the actual diameter, and show the calculation. Then separate roughing (large depth of cut, moderate feed, lower speed) from finishing (small depth of cut, fine feed, higher speed, sharp tool). Learn approximate cutting speeds per tool material and workpiece material and state the source of the value. Check the result against machine limits and rigidity, and mention coolant where it matters for the material.
Measuring bearing clearance, shaft runout or coupling alignment with a single reading, or with a dial gauge that is not zeroed and not mounted rigidly, then declaring the machine aligned.
Alignment and clearance questions look simple, so candidates take one feeler gauge or one dial indicator reading and stop. They omit the second plane, do not rotate the shaft through a full turn, and do not distinguish radial from axial readings. Magnetic bases that are loose, sagging indicator arms and dirty seating surfaces all produce errors that are then reported as fact.
What to do instead
Use a defined procedure and state it: clean all mating surfaces, mount the indicator on a rigid base, preload the plunger, zero on the reference, then rotate the shaft and record readings at top, bottom and both sides. Take readings in two perpendicular planes for alignment and correct for indicator sag if the arm is long. For bearing clearance, measure at several angular positions and compare with the manufacturer's specified clearance range. Report the measured value, the method and the acceptance limit together.
Treating a rolling bearing as a single item when answering fitting, mounting or removal questions, ignoring the fact that the rotating ring must be an interference fit and the stationary ring a loose fit, and using the wrong removal method.
Candidates learn 'bearing fits are tight' as a slogan. They then specify an interference fit on both rings, which preloads the bearing and blocks axial float, or they specify a loose fit on a rotating inner ring, which allows creep and fretting on the shaft. Removal is answered with 'press it off' or 'hit it', without regard to which ring carries the load.
What to do instead
Decide first which ring rotates relative to the load direction. The ring that rotates relative to the load takes the interference fit; the other ring takes a locational clearance or transition fit to allow axial movement. Then choose the mounting method: press only on the ring being fitted, use an induction heater for larger bearings, never transmit mounting force through the rolling elements. For removal, use a proper puller acting on the ring with the interference fit, and state that the bearing is usually replaced rather than reused after a tight removal.
Answering lubrication and maintenance questions with a lubricant name or a generic interval, without specifying viscosity grade, quantity, method of application and the condition that triggers a change.
Candidates recall 'use grease' or 'change oil regularly' but cannot connect lubricant choice to speed, load, temperature and sealing. They also confuse grease consistency class with base oil viscosity, and give a calendar interval where the correct answer is condition-based, driven by contamination, water ingress or vibration measurement.
What to do instead
Structure every lubrication answer around four items: viscosity or consistency grade and why (speed, load, temperature), quantity and fill level (for example, half-full housings, or oil level at the centre of the lowest rolling element), application method (oil bath, circulation, mist, grease gun with a defined quantity), and change criterion (running hours, oil analysis, contamination limit). Add the seal type that protects the lubricant and state what happens if the seal fails. For maintenance, link the task to a measurable check such as vibration trend, temperature or oil sample result.
Drawing or interpreting a technical drawing without applying the projection convention, section rules and dimensioning standards consistently, so that hidden edges, sections and datum references contradict each other.
Candidates mix first-angle and third-angle projection, omit centre lines and centre marks, draw a section without hatching the cut material at the correct angle, or dimension from different datums on the same view. The result is a drawing that cannot be manufactured even though each individual element looks acceptable.
What to do instead
Fix the convention at the start: identify whether the sheet uses first-angle or third-angle projection from the symbol, and keep it throughout. Use hidden lines only where a view cannot be shown in section, hatch cut material with thin lines at the standard angle and leave shafts, bolts and pins unhatched when cut longitudinally. Choose one datum per direction and dimension from it, avoid duplicate and closed dimension chains, and check that every feature has exactly one defining dimension. Review the finished drawing feature by feature against the part.
Answering assembly and dismantling questions with a sequence that ignores preload, locking and tightening order, and skipping the record of the original setting before dismantling.
Candidates focus on naming the parts rather than the procedure. They tighten a bolted flange from one side in a circle, omit the locking device, forget to mark the position of an adjustable component, and cannot state how preload is set or verified. On dismantling they do not record shim quantities or match-mark components, so reassembly cannot restore the original adjustment.
What to do instead
Use a written procedure for each answer: record and match-mark before dismantling, note shim packs and settings, clean and inspect, then reassemble in the reverse order. For bolted joints, state the tightening pattern (diagonal or star for a flange, spiral from the centre for a cover), the stages (snug, then torque in steps) and the final torque value with its source. State how preload is set and verified, for example by torque wrench, feeler gauge, dial indicator or measured bearing friction. Name the locking method explicitly.
Giving safety and quality answers as general statements about being careful, without naming the specific hazard control, the PPE item, the isolation step or the quality record that the question is testing.
Candidates treat these questions as free marks and write broad sentences. They omit lock-out and tag-out before working on a machine, do not identify stored energy such as springs, hydraulics or gravity, do not mention extraction or ventilation for cutting fluids and dust, and do not link inspection results to a recorded acceptance criterion.
What to do instead
Answer with the control hierarchy: eliminate, substitute, engineer (guards, interlocks, extraction), administrate (procedure, permit, training), then PPE as the last layer. For any maintenance task, state isolation and lock-out, release of stored energy, and verification that the machine cannot start. For quality, name the inspection stage, the measuring instrument, the acceptance limit and the record that proves conformity, and state what happens to a non-conforming part. Keep answers specific to the machine and material in the question.