A beam compass is a compass

Compass (drafting)

A compass or pair of compasses is a technical drawing instrument that can be used for inscribing circles or arcs. As dividers, they can also be used as a tool to measure distances, in particular on maps...

 with a beam and sliding sockets or cursors for drawing and dividing circles larger than those made by a regular pair of compasses. The instrument can be as a whole, or made on the spot with individual sockets (called trammel points) and any suitable beam.

Trammel points

Trammels or trammel points are the sockets or cursors that, together with the beam, make up a beam compass. Their relatively small size makes them easy to store or transport. They consist of two separate metal pieces (approx. 2.5" X 5" X 1/2") that are usually connected by a piece of wood, The wood timber is not included in the purchase of the trammel points. It can be ripped on a table saw. A lumber yard or woodworking store should have a piece readily available to fit the opening also ,metal, or pipe. They work like a scratch awl.

Scribing a circle

The beam compass is used to scribe a circle

Circle

A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

. The radius can be adjusted by sliding the metal across the wood (beam) and locking it by turning a knob at the desired location. The threaded machine rod is similar to the bolt

Screw

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...

. The only limitation is the rigidity of the wood being used. Longer pieces tend to get floppier depending on the species of wood used. Metal can be used as an alternative but also has length limitations. Trammel points score a precise cut out line with the sharp point of the rod. When the circular knob is turned, it micro adjusts the radius of the circle. The spring locks the mechanism at the precise desired location. Turning clockwise decreases the radius

Radius

In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

while turning counterclockwise increases the radius slightly.

Transferring measurements

A beam compass can also be used to make a series of repetitious measurements in a precise manner. Each point is rotated 180° and this process is repeated until the desired measurement 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.