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Circles and circular curves of relatively short radius are drawn with COMPASSES. The large pivot joint compass is satisfactory for drawing circles of 1" to about 12" in diameter without an extension bar. The pivot joint provides enough friction to hold the legs of the compass in a set position. One of the legs is equipped with a setscrew for mounting either a pen or a pencil attachment on the compass. There is also an extension bar which can be inserted to increase the radius of the circle drawn.
The other type of compass found in the drawing instrument set is the bow compass. 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 and bow pencil are the center adjustment type, whereas the bow instruments are the side 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" long; the small, approximately 4" long. Extension bars are available for large bow compasses. Bow compasses are available as separate instruments, 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 a horn center or center disk. This disk is placed over the center point. The point of the compass needle is then placed into the hole in its center.