Selecting a scale and sheet layout for a machine-bed drawing
A candidate must draw three orthographic views of a machine bed measuring 3,600 mm long, 2,400 mm wide and 800 mm high on an A3 sheet (420 mm × 297 mm). The sheet has 15 mm margins all round and a title block 180 mm × 40 mm inside the lower-right corner. The permitted scales are 1:5, 1:10, 1:20, 1:50 and 1:100. At 1:10 the top view overlaps the title block; at 1:50 the drawing occupies only a small part of the sheet. The candidate uses HB pencil for visible outlines, 2H for hidden lines and 6H for centre lines. Visible lines are 0.5 mm thick and hidden lines 0.7 mm thick. The two holes in the bed have no centre lines, and the dimension lines are drawn through the object rather than outside it. The candidate asks how to choose a scale that uses the sheet well without violating standard conventions.
1. Determine the largest standard scale that will fit the three views and the title block. Show the usable drawing area and a check for 1:10 and 1:20, and explain why 1:50 is poor practice.
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Usable area after 15 mm margins: 420 - 2(15) = 390 mm wide; 297 - 2(15) = 267 mm high. The title block occupies 180 mm × 40 mm in the lower-right corner, so a rectangular group of views placed above it has at most 390 mm × 227 mm.
At 1:10 the top view of the bed is 360 mm × 240 mm. The 240 mm height exceeds 227 mm, and rotating it gives 240 mm × 360 mm, which also exceeds 227 mm in height, so 1:10 cannot fit without overlapping the title block. At 1:20 the views are: front 180 × 40, top 180 × 120, side 120 × 40. With, say, 20 mm gaps, the group is about 320 mm wide and 180 mm high, leaving room for dimensions inside 390 × 227. At 1:50 the bed would be only 72 mm × 48 mm, wasting the sheet and losing detail. Therefore choose 1:20. The scale is a standard ratio; do not invent 1:15.
2. Specify line types, line weights and pencil grades for visible outlines, hidden details, centre lines and dimension/extension lines. Correct the candidate's assignments.
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Visible outlines: continuous thick lines, 0.5-0.7 mm, HB or H pencil. Hidden details: short dashes, thin lines, 0.25-0.35 mm, 2H pencil. Centre lines: long-short-long chain lines, thin, 0.25-0.35 mm, 2H or 3H pencil. Dimension and extension lines: continuous thin lines, 2H pencil.
The candidate's HB for visible lines is acceptable, but hidden lines at 0.7 mm are thicker than the visible lines, reversing the required hierarchy. 6H for centre lines is unnecessarily hard and may score the paper; use 2H. Add centre lines to both holes, extending 2-3 mm beyond the circle.
3. Identify two convention violations beyond line weights and state the correction.
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Two violations are:
- Dimension lines pass through the object. Dimension lines should be placed outside the outline, with extension lines starting 1-2 mm from the object and extending 2-3 mm beyond the arrowheads.
- The two holes have no centre lines. Centre lines are required to locate holes and should be drawn as thin chain lines, extending beyond the hole outline.
A further issue is that the hole locations and diameters are not fully dimensioned; each hole needs a diameter and a locating dimension from a datum, not just a centre mark.