The Corner Shelf That Wouldn't Fit: Working Drawings at 1:5
A student is preparing an A3 working drawing at 1:5 of a wall-mounted corner shelf unit for the staff room. The client's freehand sketch shows an overall width of 900 mm, two side panels marked 400 mm, a shelf depth of 250 mm, and the note 'fit tight into the corner'. On site, the student finds the corner measures 88 degrees with a sliding bevel, and the left wall leans out 8 mm over the 1.2 m height of the unit. The first draft shows an overall width of 860 mm, arrived at by adding the two 400 mm sides to a 60 mm back rail, and the side panels are drawn square to each other. The shelf depth has been measured off the printed outline with a steel rule and written as 90 mm. There is no scale bar, and the title block plus parts list will not fit on the sheet with the three views at 1:5.
1. Identify three faults in the student's draft and explain the practical consequence of each for the person making the unit.
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Fault 1 - the width chain does not close. 400 + 400 + 60 = 860 mm, but the sketch states 900 mm overall. Adding the back rail as if it were extra, rather than part of the internal width, leaves a 40 mm shortfall. Consequence: the unit will not span the opening, or the maker wastes material recutting. The drawing must nominate one controlling dimension (the site opening) and derive the others from it.
Fault 2 - square sides in an 88 degree corner. The unit as drawn will touch at the back and gape at the front by roughly 2 degrees over a 250 mm return - a visible gap and a rocking fixing. It needs a scribe fillet or an angled back rail, dimensioned as an angle, not assumed.
Fault 3 - the 90 mm depth. At 1:5, a 90 mm measured length represents 450 mm true size. The dimension as written is wrong by a factor of five and would be built as drawn if the maker trusted it.
Other faults worth naming: no scale bar (the sheet is unreadable if photocopied at 95%), and a scale chosen without checking it fits the sheet - 1:10 or two separated views would solve the layout.
2. Explain how the student should resolve the 900 mm versus 860 mm conflict and the 88 degree corner, and what should appear on the drawing to communicate both.
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Resolve the numbers first, on site, not at the desk. Re-measure the opening at three heights and record the smallest figure as the controlling dimension; sketch the actual corner with the bevel, marking 88 degrees and the 8 mm lean.
Then rebuild the size chain from the controlling dimension:
- overall width = 900 mm (controlling);
- back rail width = 900 - (2 x 400) = 100 mm if the sides are full height and the rail sits between them;
- if the intended arrangement is rail behind sides, the overall becomes a different sum - the point is that the drawing must show which part laps which, with one stated convention and all parts summing to 900 mm.
For the corner: draw the unit with a straight back and add a separate scribe strip 20 mm wider than the measured gap, marked 'scribe to wall on site'. Alternatively draw the true 88 degrees and dimension it as 88 degrees with a tolerance note.
For the 8 mm lean: show a packing/scribing allowance at the top fixing and note 'walls not plumb - pack or scribe, do not force'. Add a general note: 'verify all site dimensions before manufacture; do not scale from this drawing.' A construction line showing the 900 mm opening and the ceiling/floor datums makes the intent unmistakable.
3. A classmate argues that measuring the printed drawing is fine 'because the scale is written in the title block'. Explain why that reasoning fails, and what the scale bar and the 1:5 ratio actually mean.
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A scale of 1:5 states a relationship, not a guarantee of accuracy on paper. It means every true size is drawn at one-fifth: a 450 mm shelf is drawn 90 mm, a 900 mm opening is drawn 180 mm. Three things break the relationship the moment it leaves the printer:
- Reproduction. Photocopiers and printers shrink or stretch by 1-5%, and the sheet may be trimmed. A 90 mm line on a 95% copy is 85.5 mm - a 22 mm error once scaled back up.
- Paper and pen. Paper moves with humidity; a 0.5 mm pen line at 1:5 represents 2.5 mm of real material, so measuring to the nearest millimetre on paper is meaningless at this scale.
- Practice. Measuring a drawing is unreliable even when accurate, because it bypasses the designer's intent - the maker cannot tell whether a feature is 89.5 or 90.5 mm, or which face is the reference.
Correct convention: dimensions always state true size, regardless of scale, and are placed once, on the view that shows the feature most clearly, from a stated datum. The scale bar is a graphic ruler drawn on the sheet; it shrinks and stretches with the sheet, so a bar reading 0-100-200 mm still reads correctly on a 95% copy. It is a checking aid for a visual read, never a substitute for written dimensions.