PARALLEL STRAIGHTEDGE

Engineering Aid 3 - Beginning Structural engineering guide book
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29.275 Figure 2-8.-Parallel straightedge. Turn the T square over and redraw the line with the same edge. If the blade is warped, the lines will not coincide. If the blade swings when the head is held firmly against the edge of the drawing board, the blade may be loose where it is joined to the head, or the edge of the T square head may be warped. You can usually tighten a loose blade by adjusting the screws that connect it to the head, but if it is out of square, warped, or in bad condition, you should select a new T square. PARALLEL STRAIGHTEDGE Many draftsmen prefer to use a PARALLEL STRAIGHTEDGE (fig. 2-8) rather than a T square. The primary purpose of the parallel straightedge is the same as the T square. The parallel straightedge is a laminated maple blade with transparent plastic edges similar to those on the T square. The parallel straightedge uses a system of cords and pulleys so that it is supported at both ends by a cord tacked to the drawing board. You can move the straightedge up or down the board with pressure at any point along its length and maintain parallel motion automatically. It comes complete with cord, tacks, cord tension adjuster, and mounting instructions. Some straightedges, like the one shown in figure 2-8, are equipped with a cord lock on one end of the blade. The straightedge is locked into place by turning the cord lock clockwise. This permits use of the straightedge on an inclined board. It also prevents accidental movement when you are inking or using mechanical lettering devices. The advantages of the parallel straight- edge become particularly significant when you are working on large drawings. While the T square works well for small work, it becomes unwieldy and inaccurate when you are working on the far right-hand side of large drawings. STEEL STRAIGHTEDGE When drawing long, straight lines, you should use a STEEL STRAIGHTEDGE (fig. 2-9) because its heavy weight helps keep the straightedge exactly in position. The steel 45.677X Figure 2-9.-Steel straightedge. 2-9

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