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Plotting a 12 m house plan at 1:50 from a model-space drawing set up at 1:20

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Plotting a 12 m house plan at 1:50 from a model-space drawing set up at 1:20

A drafter in a college CAD room draws a single-storey house plan in model space at full size in millimetres. The building measures 12 000 mm × 8 000 mm. The sheet is A3, 420 mm × 297 mm, with a 10 mm margin all round and a title block drawn in model space 180 mm wide. The plot dialog is set to scale 1:20 with 'fit to paper' ticked, units in millimetres. Text is created 2.5 mm high in model space and the dimension style uses 2.5 mm text. Layers are set up with 0.50 mm lineweight for walls, 0.25 mm for dimensions, and a centre-line layer using a DASHDOT linetype at 12 mm pattern. On the printed sheet the plan runs off the right-hand edge, the title block prints only 9 mm wide, dimension figures are unreadable, all walls print at the same thin weight, and every centre line prints as a continuous line. The drafter blames the printer.

1. Diagnose the plot problem with numbers, and state the settings needed for a correct 1:50 A3 plot including the model-space text height.

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Diagnosis

  • At 1:20 the 12 000 mm building would print 600 mm wide, against a 400 mm printable width (420 − 2 × 10). It cannot fit; 'fit to paper' then silently rescales to roughly 1:30, which is why the plan still overruns and nothing on the sheet matches the stated scale.
  • The title block is drawn 180 mm in model space and printed at 180 ÷ 20 = 9 mm - a title block drawn at paper size but never placed in a layout.
  • Text 2.5 mm in model space prints at 2.5 ÷ 20 = 0.125 mm, far below the 2.5 mm minimum for legible drawing text.

Correct settings

  1. Plot scale fixed at 1:50, 'fit to paper' unticked. Check: 12 000 ÷ 50 = 240 mm and 8 000 ÷ 50 = 160 mm, both inside 400 × 277 mm, leaving room for dimension strings.
  2. Model-space text height = paper height × scale denominator = 2.5 × 50 = 125 mm. Set it in the text style or use annotative text with 1:50 as the annotation scale.
  3. Dimension arrows, extension-line offsets and gaps scale the same way: a 2.5 mm arrow becomes 125 mm in model space.
  4. Draw the title block in the paper-space layout at 1:1, or if it must live in model space, draw it 180 × 50 = 9000 mm wide.
  5. Plot area set to the layout or to the sheet limits, origin at 0,0, plot in millimetres, and preview before sending.

2. Explain why the walls printed thin and the centre lines printed solid, and give the settings that fix both.

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Lineweights

  • Lineweights in CAD are paper-space widths, applied at plot time. A 0.50 mm wall line prints 0.50 mm only if lineweight plotting is enabled and the lineweight scale is 1:1.
  • If 'scale lineweights' is ticked, the 0.50 mm is multiplied by the plot scale and the line collapses toward a hairline - which is what has happened to every wall.
  • Check, in order: the plot dialog's 'plot with plot styles' option, the selected CTB/STB table (a monochrome table may map everything to one weight), the 'scale lineweights' checkbox, and the object or layer lineweight override. Lineweights do not necessarily show on screen, so a correct screen is no proof of a correct plot.

Linetypes

  • The DASHDOT pattern is defined in drawing units, 12 mm here. Printed at 1:50 that pattern is 12 ÷ 50 = 0.24 mm - shorter than the pen width, so it reads as a solid line.
  • Two working fixes: set PSLTSCALE = 1, so linetype patterns are generated in paper units and are correct at any plot scale; or leave PSLTSCALE = 0 and set LTSCALE = 50 (or 50 × 12 mm equivalent) so model-space patterns scale up with the drawing. Set MSLTSCALE = 1 if the model-space display should match.
  • Apply the same logic to any hidden or dashed layer, and keep linetype scale on the layer or the object consistently, not mixed.

After both fixes, re-plot and check a known dimension with a scale rule on the paper - if 12 000 mm does not measure 240 mm, the plot scale is still wrong.

3. Recommend a layer and drawing strategy so that the same model can be plotted at 1:50 for the client and 1:100 for a planning submission without redrawing.

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Draw once, at 1:1, in real millimetres. Model space holds true geometry so that lengths, areas and dimension values are correct and the same file can be plotted at any scale.

Layer strategy

  • Walls and structural elements: 0.50–0.70 mm, one colour.
  • Openings, doors, windows, fittings: 0.25–0.35 mm.
  • Dimensions: 0.25 mm, separate layer, non-plotting? No - plotting, but its own layer so it can be frozen for the planning sheet if the client wants a clean drawing.
  • Text and annotation: own layer, 0.25 mm.
  • Hatching/poche: own layer, thin, with a separate hatch scale for each plot scale.
  • Centre lines and grid: chain linetype, 0.25 mm.
  • Title block and border: paper-space layout only, never in model space.

Scale strategy

  1. Use annotative objects (text, dimensions, symbols) with 1:50 and 1:100 added as annotation scales; the same text then prints 2.5 mm on both sheets without editing.
  2. Alternatively keep non-annotative text at 2.5 × 50 = 125 mm for the client sheet and use a second text style or layer state for the 1:100 sheet.
  3. Use named page setups and layer states - one for A3 at 1:50, one for A2 or A3 at 1:100 - so switching between submissions is a matter of restoring a state, not rebuilding the drawing.
  4. Adjust hatch scale with the plot scale (hatch appears half as dense at 1:100 if the pattern spacing is fixed in model units), and check that dimension text does not overlap at the smaller scale.
  5. Keep the model associative: dimensions attached to geometry update when the design changes, which is the whole point of not redrawing at scale.