Most trammel heads come in pairs and can accommodate a beam between 19-38mm wide. The length of their steel point(s) can be adjusted manually after the clamping nut is loosened. Once the steel points are at the required length, the nut is tightened to secure the points in place.
Flat-lying trammel heads
Flat-lying trammel heads have a lower centre-of-gravity and so they are easier to use than other trammel heads. They are self supporting, so the user does not need to hold the trammel heads rigidly in place in order to draw a perfect circle or arc.
Trammel points
Trammel points are a more precise type of trammel head. Their greater precision is a result of the size of the the tips of their steel points, which are smaller than those of other trammel heads.This means that they can be used to draw circles and transfer measurements with a greater degree of accuracy than other trammel heads.
When choosing a type of trammel head you should consider three things:
Accuracy
Capacity (the width of beam they can be used with)