Missing Projection Symbol: Boss Mirrored and Countersinks Reversed
A UK subcontractor receives an A3 print of a cast bearing housing from an overseas customer. The sheet is cropped along the top edge, so the projection symbol is missing. The front view shows the housing with an offset boss 30 mm from the left-hand end face and four base holes countersunk on the top face; a rectangular side view is placed immediately to the right of the front view. The title block says SCALE 1:2. The only height dimensions are a chain: 18 mm (base thickness), 42 mm (base top to boss centre) and 25 mm (boss centre to top face); there is no overall height. The machinist, trained only on first-angle drawings, set the job up so the boss finished 30 mm from the right-hand face and countersunk the base holes from underneath. Twenty castings have been machined. The customer's response is 'the drawing is correct; your interpretation is not'.
1. From the evidence on the sheet alone, what can and cannot be concluded about the projection system, and what check should have been made before cutting metal?
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What the placement tells you. In third-angle projection each view is placed on the side of the front view toward which the observer looks: the top view goes above, the right-side view to the right. In first-angle projection the view is placed on the opposite side: the top view goes below, and the view placed to the right of the front view is the left-side view.
So a side view sitting immediately to the right is the right-side view in third angle but the left-side view in first angle. Placement alone therefore does not identify the system - it only does so once you know the convention the sheet was drawn to. The machinist assumed third angle without evidence, read the view as a right-side view, and located the boss from the wrong end: a mirror image, giving 30 mm from the right instead of the left.
What does identify the system. The projection symbol, or an explicit note; failing that, internal consistency of feature correspondence. The boss centre is 18 + 42 = 60 mm above the base underside in the front view and must appear at exactly that height in the side view. The countersink hidden lines must sit at the top face in the front view. A mirror flips left–right but preserves heights and hidden-line depths, so height correspondence cannot catch the error - only an unambiguous left/right datum can.
The check that was missed. Ask the customer, or search the sheet for the symbol or a 'FIRST/THIRD ANGLE' note, before setting up. Never 'adjust' a feature to make a view fit.
2. Explain how the countersinks came to be machined on the underside, and specify the drawing change that removes the ambiguity.
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How the misread arose. The countersinks are on the far face in the front view, so they appear only as hidden detail: two short hidden lines and a wider hidden line for the countersink cone, drawn either just above or just below the base outline depending on which face is toward the viewer. With no section and no projection symbol, the hidden-line pattern is the only evidence, and it is exactly the evidence that reverses when the view is read in the wrong angle system. The machinist, having already mirrored the part, also mirrored the countersink face - one error propagated into the other.
Drawing changes
- Restore the projection symbol and the words 'THIRD ANGLE PROJECTION' on the sheet.
- Add a local or removed section through one base hole, cutting plane marked and arrowed, so the countersink is shown open on the correct face.
- Add a clear note: '4 HOLES Ø11 THRU, CSK Ø22 × 90° FROM TOP FACE'.
- Dimension the countersink depth or the land diameter, since a 90° included angle alone leaves the depth to be inferred.
Why a section settles it. A sectional view is unambiguous about viewing direction because the cutting plane and its arrows state it: the material shown cut is on the far side of the plane from the arrows. That residual ambiguity is absent from hidden-line work, which is why critical face information should not be carried by hidden lines alone.
3. Determine the overall height that the chain of dimensions implies, and state how it should (and should not) appear on the drawing, given the 1:2 scale.
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Value. The chain is continuous from the base underside to the top face: 18 + 42 + 25 = 85 mm overall.
How it should appear. As a reference dimension in parentheses - (85) - because it is derived from the chain. A reference dimension carries no tolerance and must not be used as an inspection requirement; the chain controls the part. Tolerancing both the chain and the 85 mm at the same time would over-constrain the part and could make it impossible to satisfy all four dimensions simultaneously. If the designer genuinely needs a controlled overall height, the chain must be broken and one link opened.
Why the scale must not be used. '1:2' describes the printed size, not the part. Measuring the print and doubling is unreliable because:
- paper and printer shrinkage of only 1% over a 170 mm image is 1.7 mm on the part - an order of magnitude worse than a normal machining tolerance;
- photocopied or PDF-fitted prints are routinely 2–5% off;
- the dimension figures, not the line lengths, are the specification.
The correct action on finding a missing dimension is to query it, not to scale it. The customer is right that the drawing was correct - but a chain with no overall dimension and no projection symbol is a drawing that invites exactly this error, and a note plus the symbol would have cost nothing.