Maharashtra HSC Engineering Drawing (Technical Science) common mistakes
Maharashtra HSC Engineering Drawing (Technical Science) · Understand the traps and how to avoid them.
Setting the pencil compass by pressing the needle into the paper and rotating the whole wrist, so the drawn circle comes out with an oval shape, a stepped radius, or a hole at the centre.
Candidates hold the compass like a pen and turn the wrist instead of twirling the knurled head between thumb and forefinger; the needle also sinks into soft paper, shifting the centre between the two arcs of a large circle.
What to do instead
Set the radius on the compass with a steel rule and lock the hinge screw, keep the needle shoulder just below the lead shoulder, and draw by rotating the head with the thumb and index finger while the wrist stays still. For circles above about 50 mm, draw two 180-degree arcs from the same punched centre and check the diameter with the rule before inking.
Drawing the border, title block, and initial construction lines in dark hard pencil, then being unable to erase construction arcs, leaving a smudged sheet with visible guide lines under the final view.
Candidates use one pencil for the whole sheet, so the layout stage and the finishing stage share the same lead grade and pressure; construction lines are drawn as heavily as object lines.
What to do instead
Use a 2H or H lead with light pressure for layout, centre lines, and construction arcs, and a sharpened HB for final visible outlines. Erase construction work with a soft eraser before inking, and keep a scrap sheet under the hand to prevent smudging.
In first-angle projection, placing the top view below the front view but drawing hidden edges of the rear face as visible continuous lines, or omitting hidden lines in the plan altogether.
Candidates memorise the position rule (top view below, side view to the right) but not the viewing direction, so they do not realise the plan is what is seen from above and therefore shows the topmost edges as visible and lower edges as dashed.
What to do instead
Before drawing, state the viewing direction for each view aloud: front view from the front, top view looking down, side view from the left placed on the right. Then classify every edge by whether it is nearest to that observer; draw it continuous if visible and dashed if covered. Cross-check that hidden lines in the plan align vertically with the corresponding visible edges in the front view.
Losing the alignment between a point or line and its projections because the projector lines are drawn freehand and not carried across with a tee square or set square.
Candidates construct each view independently and rely on eye judgement for the transfer of heights and depths, which accumulates error across the sheet and produces mismatched views.
What to do instead
Draw thin projectors from every feature with the tee square and 45-degree miter line for depth transfer, and keep the three views inside a single layout rectangle. Verify at the end that every vertex in the front view has a matching vertex directly above or below it in the plan.
Drawing the development of a cut solid by unfolding the outline only, without projecting the true lengths of the cut edges, so the development cannot be folded back to the given truncated solid.
Candidates treat development as a copy of the front view rather than as a separate true-shape construction; they do not first find the true lengths of slant edges or the true shape of the cut face.
What to do instead
First determine the true shape of the cut section (auxiliary view or rotation method), then develop the lateral surface using the true slant length and mark each cut point at its measured distance along the corresponding generator. Check that the developed cut line closes and that its total length equals the perimeter of the section.
Drawing an isometric view by measuring the true lengths along the 30-degree axes and also drawing circles as true circles, producing an object that looks stretched and has elliptical holes.
Candidates confuse isometric projection with isometric drawing and do not apply the 0.816 foreshortening or the isometric ellipse construction; they also draw the 30-degree axes without a 30-degree set square.
What to do instead
Decide at the start whether the answer is an isometric drawing (full scale along the axes) or an isometric projection (0.816 scale), state it on the sheet, and use a 30-degree set square for the axes. Draw every circle on a face as an ellipse using the four-centre method or an isometric ellipse template, and keep all three axes at 120 degrees to each other.
Dimensioning a machine part with a chain of dimensions from feature to feature instead of from a common datum, so tolerance accumulates and the drawing contradicts the fit requirement.
Candidates dimension in the order features appear on the drawing rather than in the order the part is machined or inspected; they also omit the datum identification.
What to do instead
Choose the functional datum (usually a machined face or axis) and dimension all locating features from it in parallel. Reserve chain dimensioning for non-critical spacing, add the overall dimension once, and check that the sum of intermediate dimensions never conflicts with the overall dimension.
Writing a fit as H7/h6 without stating the basic size, or interpreting H7/h6 as a clearance fit in every case and mislabelling the hole and shaft tolerance zones.
Candidates memorise the letters as a pair and do not read the fundamental deviation direction: capital letters describe holes, lower-case describe shafts, and the position of the zone relative to the zero line decides clearance, transition, or interference.
What to do instead
Always write the basic size with the fit, e.g. 40 H7/h6, and determine the fit type by comparing the hole and shaft limits numerically, not by the letter pair alone. Practise reading the tolerance table: H is a hole whose lower deviation is zero, h is a shaft whose upper deviation is zero, so H7/h6 is a clearance fit only because the hole minimum exceeds the shaft maximum.
Drawing a thread as two parallel lines with no crest, root, or thread-run indication, and using the same line weight for the minor diameter as for the major diameter.
Candidates simplify the conventional representation without knowing which line is thick and which is thin, and they omit the incomplete thread at the runout, so the drawing cannot be read as a threaded part.
What to do instead
Draw the major diameter as a thick continuous line and the minor diameter as a thin continuous line, with the thin line about three-quarters of a turn short of a full circle in the end view. Show the thread runout with the thin line continuing at 45 degrees, and add the thread callout (for example M20 x 2.5) rather than relying on the lines alone.
Drawing a building plan without cutting the walls at the sill level, or drawing the section through a window instead of through the wall and showing door swings in the section.
Candidates confuse plan, elevation, and section conventions: the plan is a horizontal cut about one metre above floor level, the section is a vertical cut through openings, and door swings belong only to the plan.
What to do instead
State the cutting plane before drawing: plan cut at sill level so windows appear as openings in the wall, section cut through a door or window so the opening is shown with the lintel above. Show door swings only in the plan, show levels and floor-to-floor heights in the section, and keep wall thicknesses consistent between plan and section.