Figure 5-70.-Basic isometric forms.isometrically for a model, you should draw anoblique sketch following the basic steps shown infigure 5-69, view B.The first step is to draw a rectangle of the frontview (using light lines). Then, second, draw anoblique base line at a 45-degree angle starting atthe corner (intersection) of the horizontal andvertical base lines. Third, sketch the remaininghorizontal and vertical lines parallel to the otherbase lines. Fourth, erase any unnecessary lines,and fifth, dimension and darken the completeddrawing for easier reading. Remember, place thedimensions so they are parallel to the axis lines.The final step is to check the sketch forcompleteness and accuracy.In the above procedures for development ofpictorial sketches, a simple rectangular form wasused. All objects may be simplified to their basicgeometric forms. These forms are the firstconsideration in the pictorial sketch. Basicvolumetric forms are shown in figure 5-70. Bycarefully analyzing any object you sketch, you willsee one or more of the forms shown in figure 5-70.However, at times only a part of a form is present.Before attempting detailed sketches, practicesketching the basic forms. Then look for theseforms in the object you are about to sketch, andconcentrate on the basic form representation.Enclose the object in a basic form, or buildit up with a series of different forms, dependingon the nature of the object. Details are added or“carved” from these forms after shape andproportion have been determined.Overlay SketchesTo make OVERLAY SKETCHES, sketchfreehand on transparent paper placed over existingdrawings or other sketches. Sometimes when youmake overlay sketches, you merely trace, free-hand, objects or lines from another drawingor sketch. But more often you will prepareoverlay sketches by tracing and then addingsupplementary sketched lines or objects.Usually, when this type of sketch is prepared,only the prominent or desired features aretraced. Overlay sketches are primarily used forplanning purposes.A suggested procedure for using overlaysketches as a tool for planning is explained in thefollowing example:The drafting room is being relocated. You aretasked with developing a proposed furniture andequipment layout. You have the latest prints ofthe floor plan and an electrical plan, and you5-36
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