45.132EFigure 2-19.-Divider maintenance: (A) Evening the legs ofdividers; (B) Sharpening divider needlepoints.pinhole. Since the same center is often used forboth the compasses and dividers, it is best thatneedles on both be the same size. If the compassneedle is noticeably larger, grind it until it is thecorrect size.To make a compass needle smaller, wet oneside of the whetstone and place the needle withits shoulder against this edge. Then grind it againstthe whetstone, twirling it between your thumb andforefinger (fig. 2-20). Test it for size by insertingit in a hole made by another needle of the correctsize. When it is pushed as far as the shoulder, itshould not enlarge the hole.The screw threads on bow instruments aredelicate. Because of this, you should take carenever to force the adjusting nut. Threads mustbe kept free from rust or dirt.If possible, it is best to keep drawinginstruments in a case, since the case protects themfrom damage from falls or unnecessary pressures.Then, too, the lining of the case is usually treatedwith a chemical that helps prevent the instrumentsfrom tarnishing or corroding.To protect instruments from rusting when theyare not in use, clean them frequently with a softcloth and apply a light film of oil to their surface142.35Figure 2-20.-Shaping a compass needle.with a rag. Joints on compasses and dividersshould not be oiled. When the surface finish ofinstruments becomes worn or scarred, it issubject to corrosion; therefore, a knife edge oran abrasive should never be used to cleandrafting instruments.Beam CompassThe BEAM COMPASS (fig. 2-21) is used fordrawing circles with radii larger than can be seton a pivot joint or bow compass. Both the needle-point attachment and the pen or pencil attachmenton a beam compass are slide-mounted on a metal45.134XFigure 2-21.-Beam compass.2-15
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