DRAWING INSTRUMENT SETS
So far we have discussed only those instru-ments and materials that you will need for drawing straight lines (with the exception of french curves). Many drawings that you will prepare will require circles and circular arcs. For this purpose, instruments contained in a drawing instrument set are used. Many types of drawing instrument sets are available; however, it is sometimes difficult to judge the quality of drafting instruments by appearance alone. Often their characteristics become evident only after they are used.
The drawing instrument set shown in figure 2-15 is typical of those sets found in the standard draftsman kit. The following sections describe these instruments. Some special-purpose instru-ments not found in the set will also be described. They may be purchased separately or found in other instrument sets.
Circles and circular curves of relatively shortradius are drawn with COMPASSES. The large pivot joint compass (fig. 2-15C) is satisfactory for (fig. 2-15K and 2- 15L). Many experienced drafts-men prefer the bow compass over the pivot joint compass. The bow compass is much sturdier and is capable of taking the heavy pressure necessary to produce opaque pencil lines without losing the radius setting.
There are two types of bow compasses. The location of the adjustment screw determines the type. The bow pen (fig. 2-15K) and bow pencil (fig. 2-15L) are the center adjustment type, whereas the bow instruments shown in figure 2-16side adjustment type. Each type comes in two sizes: large and small. Large bow compasses are usually of the center adjustment type, although the side adjustment type is available. The large bow compasses are usually about 6 in. long; the small, approximately 4 in. long. Extension bars are available for large bow compasses. Bow compasses are available as separate instruments, as shown in figures 2-15 and 2-16, or as combination instruments with pen and pencil attachments.
Most compasses have interchangeable needle-points. The conical or plain needlepoint is used when the compass is used as dividers. The shoulder-end needlepoint is used with pen or pencil attachments.
When many circles are drawn using the same center, the compass needle may tend to bore an oversized hole in the drawing. To prevent these holes, use a device called ahorn center or center disk (fig. 2-151). This disk is placed over the center point. The point of the compass needle is then placed into the hole in its center.
Figure 2-15.-Typical drawing instrument set.
Figure 2-16.-Bow instruments: (A) Bow pen; (B) Bow pencil; (C) Bow dividers; (D) Drop bow pen.
DIVIDERS are similar to compasses, except that both legs are provided with needlepoints. The instrument set (fig. 2-15) contains two different types and sizes of dividers: large 6-in. hairspring dividers (fig. 2-15A) andsmall center adjustment bow dividers (fig. 2- 15J). The large pivot joint compass (fig. 2-15C) may also be used as dividers. As with compasses, dividers are available in large and small sizes, and in pivot joint, center adjustment bow, and side adjustment bow types. Figure 2-16C shows small side adjustment bow dividers. Pivot joint dividers are used for measurements of approximately 1 in. or more. For measurements of less than 1 in., bow dividers should be used. Dividers are used to transfer measurements, 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.