they also have length limitations. Some trammel sets include a support roller for attachment at mid span of the beam or rod, to take out the sag. Trammel points score a precise line by using a sharpened point, or draw a line using a lead clutch, or an ink nib. When the circular knob is turned, it micro adjusts the radius of the circle. On some, a spring and screw mechanism locks the compass at the precise desired location. Turning clockwise decreases the radius while turning counterclockwise increases the radius slightly. Transferring measurements A beam compass can also be used to make a series of repetitive measurements in a precise manner; the same as using a divider. Each point is rotated 180° along a straight line or large circle, and this process is repeated until the desired measurement or division is reached. The indentation created by the sharp point of the trammel is easily seen and makes a precise point to reference to the next location. Variants The circle cutter is a basic variation of the beam compass. There are many types of circle cutters. This cutter is used primarily to score a circular pattern in the drywall to fit over recessed lighting in the ceiling. The tool consists of a square shank with a sliding pivot that is locked into the desired location with a turn knob. The shank is graduated into 16 units and each unit is further divided into increments of one quarter. One end of the shank has a fixed cutter wheel that scores a fine line in the drywall. See also Technical drawing tools References Notes Bibliography . Metalworking hand tools Woodworking hand tools