Drop Bow Pen

Engineering Aid 3 - Beginning Structural engineering guide book
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to step off a series of equal distances, and to divide lines into a number of equal parts. Drop Bow Pen The DROP BOW PEN (fig. 2-16D) is not one of the standard instruments. However, for some jobs it is essential. It is used to ink small circles with diameters of less than a quarter of an inch. As the name indicates, the pen assembly is free to move up and down and to rotate around the main shaft. When using this instrument, hold the pen in the raised position, adjust the setscrew to give the desired radius, and then gently lower the pen to the paper surface and draw the circle by rotating the pen around the shaft. Maintenance of Compasses and Dividers Figure 2-17 shows the three shapes in which compasses and dividers are made: round, flat, and bevel. Figure 2-18 shows two types of pivot joints commonly found on compasses and dividers. When you select compasses and dividers, test them for alignment by bending the joints and bringing the points together. New instruments are factory adjusted for correct friction setting. They rarely require adjustment. A small jeweler’s screwdriver or the screwdriver found in some instrument sets (fig. 2-15H) is used for adjusting most pivot joint instruments. Instruments that require a special tool should be adjusted by skilled instrument repairmen. Pivot joint compasses and dividers should be adjusted so that they may be set without undue friction. They should not be so rigid that their 45.158X Figure 2-17.-Shapes of compasses and dividers: (A) Round; (B) Flat; (C) Bevel. 142.34X Figure 2-18.-Sections of pivot joints. manipulation is difficult, nor so loose that they will not retain their setting. Divider points should be straight and free from burrs. When the dividers are not in use, the points may be protected by sticking them into a small piece of soft rubber eraser or cork. When points become dull or minutely uneven in length, make them even by holding the dividers vertically, placing the legs together, and grinding them lightly back and forth against a whetstone. (See fig. 2-19, view A.) Then hold the dividers horizontally and sharpen each point by whetting the outside of it back and forth on the stone, while rolling it from side to side with your fingers (fig. 2-19, view B). The inside of the leg should remain flat and should not be ground on the stone. The outside of the point should not be ground so that a flat surface results. In shaping the point, be careful to avoid shortening the leg. Needles on compasses and dividers should be kept sharpened to a fine taper. When they are pushed into the drawing, they should leave a small, round hole in the paper no larger than a 2-14

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