The Binding Router Arm
One of the more critical requirements when cutting the channels for guitar binding and purfling is that the cut be made vertically. This usually means in line with the sides of the guitar. In normal lumber, the face of a board is usually at right angles to the side. So the foot of the router ensures the blade is cutting the side of the board at right angles to the face.
But since the face of the sound board or back board of a guitar may not be at perfect right angles to the guitar side around its full perimeter, the foot of the router cannot be relied upon to keep the router blade in line with the guitar side.
What is required is a way to allow the router to be moved through three degrees of freedom; 1) forward and backward, 2) sideways right and left and, 3) up and down while holding the router perfectly vertical with the guitar held perfectly horizontal.
This router arm draws from many other designs. It allows for the three degrees of freedom while an adjustable bed holds the guitar body such that the facing surface is reasonably horizontal.
The guitar remains stationary on the bed while the luthier moves the vertically held router around the perimeter edge. This is a much preferred method to moving the guitar under a stationary vertical router.