Sheet layout and line convention check for a first-angle drawing
A candidate is preparing an A3 landscape sheet at 1:2 for a cast mounting bracket. The sheet has a 10 mm border all round and a title block in the lower-right corner, but the outline of the bracket runs into the 20 mm filing margin on the left. Every line on the drawing is 0.5 mm thick; centre lines are continuous thin lines, and hidden detail is shown as short dashes that cross the centre lines. The projection symbol is missing. Dimension figures are 2.5 mm high and some are placed inside the hatched area. The part will be machined in a European workshop that uses first-angle projection, but the client's drawing office requires third-angle projection. The candidate asks whether the sheet can be patched or must be redrawn.
1. List four non-conformities in the sheet and explain, for each, the drawing-standard reasoning that makes it a non-conformity rather than a matter of taste.
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1. Drawing enters the filing margin. ISO 5457 gives A3 sheets a 20 mm filing margin on the left and 10 mm on the other sides. The drawing area is inside the border; extending the outline into the filing margin defeats filing and reproduction.
2. All lines are 0.5 mm. ISO 128 expects a hierarchy of line widths. Visible outlines should be the thick group (for example 0.5 mm), while hidden and centre lines should be the thin group (for example 0.25 mm). A single width loses the distinction between visible and hidden geometry.
3. Centre lines are continuous thin. Centre lines should be long-dash-dot and should extend 3–5 mm beyond the outline. Continuous thin is reserved for dimension, leader, projection and hatching lines.
4. Projection symbol is missing. ISO 5456 requires the first-angle or third-angle symbol so the reader knows how the views are arranged. Without it, the top view could be misread as being on the wrong side of the front view.
A fifth problem is dimension text inside hatching; ISO 129 expects dimensions to be placed outside the hatched area, or the hatching to be interrupted where text must go.
2. The client requires third-angle projection, but the workshop uses first-angle. Recommend what must appear on the sheet and how the views should be arranged. Explain why simply adding the client's symbol is unacceptable.
Read the worked answer
The projection symbol is a statement about the layout, not a decoration. If the sheet carries the third-angle symbol, the top view must be above the front view and the right-side view must be to the right of the front view. If it carries the first-angle symbol, the top view is below the front view and the left-side view is to the right. Adding a third-angle symbol to a first-angle layout is a false statement and could cause the part to be machined mirror-image.
For the client, redraw or re-layout the views in third-angle arrangement and put the third-angle symbol in or near the title block. For the workshop, issue a separate first-angle sheet with the first-angle symbol, or agree that the workshop will work from the client sheet with the correct symbol. Patching is acceptable only if the views are actually moved and the title block is revised; changing the symbol alone is not.