With an engineers square or try square, trammel heads can be used as a substitute for a mortise gauge. Mortise gauges are used to draw one or more lines parallel to the edge of a part.
Step 1 – Install pencil in your trammel head
Install a pencil or other scribing device in the body of each trammel head.
Step 2 – Attach trammel heads to engineers square or try square
Attach the trammel heads onto the blade of the square. The distance between the trammels should be the same distance as you would like between the parallel lines. They should be clamped a set distance away from the edge of the stock of the square.
Step 3 – Run the square along the edge of the workpiece
Position the square on the edge of the workpiece. Run the square along the edge of the surface you will be marking so that the scribing tools installed in the trammel heads mark two parallel lines down the length of the part.
The lines you have drawn will be parallel to each other and to the edge of the workpiece.