Choosing and Correcting a Pictorial for a Maintenance Manual
A maintenance manual for a water-treatment plant needs one illustration of a clamp block: 80 mm long, 50 mm wide and 30 mm high, with a Ø20 mm through-hole in the 80 × 50 front face, a 15 mm wide slot milled 6 mm deep into the top face at 30° to the long edge, and a 5 mm × 45° chamfer on the top front edge. The illustrator has supplied a cabinet oblique with the receding axis at 45° but drawn at full depth, and has drawn the Ø20 hole as a true circle on the front face. The editor says the block 'looks a third too deep', and the workshop says the slot angle cannot be taken off the picture because it 'doesn't measure 30°'. The illustrator argues that a circle is a circle and that the angle is dimensioned in the accompanying plan view, so both complaints are wrong.
1. Diagnose the depth complaint and adjudicate the dispute over the circle.
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Depth. Oblique projection has three standard forms. Cavalier draws the receding axis at full length; cabinet draws it at half length; both usually use 30° or 45° to the horizontal. The illustrator has drawn a cabinet-scale picture but used a full-length receding axis, so the 30 mm depth is plotted as 30 mm instead of 15 mm - the block reads twice as deep as it should. The editor's 'a third too deep' is consistent with that: the visible depth on paper is roughly double the intended cabinet depth relative to the true-shape front face.
Fix. Either plot the receding edges at 15 mm along the 45° line (cabinet, the normal choice for small parts in manuals) or keep 30 mm and label the picture cavalier. Mixing the name with the construction is the real error.
The circle. The illustrator is right on this point. Oblique projection leaves the face parallel to the plane of projection in true shape and true size, so a hole in that face is drawn as a true circle. That is oblique's chief advantage over isometric, where the same hole must be an ellipse. The only caveat is consistency: the hole is correct because it lies in the plane of projection, and the illustrator must not then draw a hole in a receding face as a circle. The complaint about the circle should be dismissed; the complaint about depth should be upheld.
2. Explain why the 30° slot cannot be measured off the pictorial, and give a set-out that removes the problem.
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Why angles distort. Only lines parallel to the plane of projection keep their true angle in oblique projection. The top face is a receding plane: one of its edges is drawn at half scale along a 45° axis while the other is drawn full size horizontally. An angle measured on that face in the drawing is therefore a transformed angle, not the 30° true angle, and no amount of careful measuring on the sheet will recover it. Linear dimensions are equally unsafe on receding edges.
Set-out that works. Locate the slot by offsets measured along the two isometric/oblique axes, not by angle:
- slot start: 20 mm along the length from the left end, 10 mm across from the front edge;
- slot end: 40 mm further along the slot direction, so 40 cos 30° = 34.6 mm further along the length and 40 sin 30° = 20 mm further across;
- so the end point is at 54.6 mm along and 30 mm across.
Plot both endpoints on the top face, join them, then offset 15 mm across for the slot width. Everything is then measured parallel to drawn axes, where scale is known.
Better still. Add a true-shape plan view with the 30° dimensioned there, and keep the pictorial as an aid to recognition rather than a measuring document. A pictorial should never be the only place a manufacturing dimension or angle appears.
3. The editor now wants a single picture that shows the form of the block clearly. Compare isometric and oblique for this part and, if isometric is chosen, specify how the Ø20 hole is drawn.
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Comparison.
- Oblique keeps the front face true shape, so the Ø20 hole and the 80 × 50 outline are exact and easy to read; it is quick to draw with set squares. Its weakness is that it shows only one face properly and the receding faces are distorted - precisely where the 30° slot lives.
- Isometric sets the three axes at 120° and foreshortens all three equally, so all three faces are shown with equal weight. It gives a far better read of the slot and the chamfer, at the cost of ellipses for every circle.
For a manual illustration whose job is to show overall form, isometric is the better choice here; oblique would be preferable if the front face and its hole were the whole message.
Drawing the Ø20 hole in isometric. A circle on any face becomes an ellipse. Using the standard isometric factors for a drawing (not a projection):
- major axis = 1.22 × Ø20 = 24.4 mm, perpendicular to the hole axis;
- minor axis = 0.71 × Ø20 = 14.2 mm, along the hole axis;
- the hole axis lies along one of the 30° isometric axes, so the major axis is set at 60° to the horizontal, perpendicular to that axis.
Construct it with a four-centre ellipse or an ellipse template of the correct size, and note that all three 30° axes in an isometric drawing are plotted at full length; the 0.816 foreshortening applies only to a true isometric projection and is normally ignored, which is why the picture must be labelled as a drawing.