CAD Plot Scale and Annotation in a Workshop Plan
A candidate drafts the plan of a single-storey workshop, 8.0 m × 5.0 m externally with 230 mm walls, in CAD model space using millimetres. Dimensions are inserted in model space with text height 2.5 mm, arrow size 2.5 mm, extension-line offset 0.6 mm, and DIMSCALE = 1. The drawing is plotted on A3 at 1:100. On the print the plan measures 80 mm × 50 mm, the dimension text is invisible, and the wall lines, dimension lines and centre lines all print at the same weight. The candidate proposes to fix this by exploding the dimensions and scaling the text up by 100.
1. Diagnose the print problems with calculations and explain why exploding and scaling the text is the wrong fix.
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The model is in millimetres, so 8 m = 8000 units. At 1:100 the plotted length is 8000/100 = 80 mm, and 5000/100 = 50 mm, which matches the print. The dimension text of 2.5 model units plots at 2.5/100 = 0.025 mm, far below the 2.5 mm needed, so it is invisible; the arrow and offset are reduced in the same proportion.
DIMSCALE = 1 tells CAD that annotation is at full size, but the plot applies a 100:1 reduction, so every annotation must be multiplied by the plot-scale factor. Exploding the dimensions and scaling the text by 100 destroys associativity: every later wall movement would require manual editing, and scaling each block about a wrong base point distorts arrowheads and text positions. The line-weight problem is separate: if everything is on one layer with a default 0.25 mm weight, walls and centre lines cannot be distinguished.
2. Recommend the plot scale and the DIMSCALE, text and arrow settings, and support the choice with an A3 sheet-fit calculation.
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At 1:100 set DIMSCALE = 100, so 2.5 mm plotted text is 250 model units, arrows 250 units, and the 0.6 mm offset is 60 units. But A3 inside 10 mm margins is about 400 mm × 277 mm; at 1:100 the plan occupies only 80 mm × 50 mm, leaving the sheet empty and the detail too small.
At 1:50 the plan is 160 mm × 100 mm, which leaves room for a section, notes and a title block on the same sheet, so 1:50 is the better choice. Then DIMSCALE = 50: text 2.5 × 50 = 125 model units, arrows 125 units, offset 30 units. Assign layers and plot weights: walls 0.50 mm, dimensions 0.25 mm, centre lines 0.18 mm, hidden lines 0.25 mm dashed, set in the plot style table rather than by scaling linework.
3. Compare model-space annotation with DIMSCALE against paper-space annotation, and state one reason for keeping dimensions associative.
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Model-space dimensions with DIMSCALE matched to the plot scale are convenient when the same model is plotted at several scales; each layout can use its own dimension style. Paper-space dimensions are drawn in the layout at true paper size, so the text stays 2.5 mm whatever the viewport scale, and the viewport is simply set to 1:50; this is often simpler for a single plot. The essential point is that the annotation scale must match the plot scale, whether that is achieved by DIMSCALE or by drawing in paper space.
Keep dimensions associative because the wall positions will change during design. Associative dimensions follow the geometry automatically, whereas exploded or overridden dimensions become stale and can issue a wrong building plan. If the plot scale changes, change DIMSCALE or the paper-space text height, never explode the dimensions.