45.127XFigure 2-14.-French curves.Figure 2-14 shows an assortment of french curves.In such an assortment you can find edge segmentsthat can be fitted to any curved line that you needto draw.French curves should be cared for and stowedin the same manner as triangles.DRAWING INSTRUMENT SETSSo far we have discussed only those instru-ments and materials that you will need fordrawing straight lines (with the exception offrench curves). Many drawings that you willprepare will require circles and circular arcs. Forthis purpose, instruments contained in a drawinginstrument set are used. Many types of drawinginstrument sets are available; however, it issometimes difficult to judge the quality ofdrafting instruments by appearance alone. Oftentheir characteristics become evident only after theyare used.The drawing instrument set shown in figure2-15 is typical of those sets found in the standarddraftsman kit. The following sections describethese instruments. Some special-purpose instru-ments not found in the set will also be described.They may be purchased separately or found inother instrument sets.CompassesCircles and circular curves of relatively shortradius are drawn with COMPASSES. The largepivot joint compass (fig. 2-15C) is satisfactory fordrawing circles of 1 in. to about 12 in. in diameterwithout an extension bar. The pivot joint providesenough friction to hold the legs of the compassin a set position. One of the legs is equipped witha setscrew for mounting either a pen (fig. 2- 15B)or a pencil attachment on the compass. There isalso an extension bar (fig. 2-15D), which can beinserted to increase the radius of the circle drawn.The other type of compass found in thedrawing instrument set is the bow compass(fig. 2-15K and 2- 15L). Many experienced drafts-men prefer the bow compass over the pivot jointcompass. The bow compass is much sturdier andis capable of taking the heavy pressure necessaryto produce opaque pencil lines without losing theradius setting.There are two types of bow compasses. Thelocation of the adjustment screw determines thetype. The bow pen (fig. 2-15K) and bow pencil(fig. 2-15L) are the center adjustment type,whereas the bow instruments shown in figure 2-16are the side adjustment type. Each type comes intwo sizes: large and small. Large bow compassesare usually of the center adjustment type,although the side adjustment type is available. Thelarge bow compasses are usually about 6 in. long;the small, approximately 4 in. long. Extensionbars are available for large bow compasses. Bowcompasses are available as separate instruments,as shown in figures 2-15 and 2-16, or ascombination instruments with pen and pencilattachments.Most compasses have interchangeable needle-points. The conical or plain needlepoint is usedwhen the compass is used as dividers. Theshoulder-end needlepoint is used with pen orpencil attachments.When many circles are drawn using the samecenter, the compass needle may tend to bore anoversized hole in the drawing. To prevent theseholes, use a device called a horn center or centerdisk (fig. 2-151). This disk is placed over the centerpoint. The point of the compass needle is thenplaced into the hole in its center.2-12
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