The Templet Collar Guide (Guide Bush)
Collar guides are one or 2 piece rings that surround the router bit. They’re often screwed to the router subbase, or in some cases, to the base casting. Most are sandwiched (PC/DW) on a subbase. In the example shown, the nut is on the router side of the subbase. They’re usually of brass or steel, fabricated or completely machined. Porter Cable brand collars come in 15 different sizes. Their extensions vary from supplier to supplier as do their dimensionality and machine quality.
They act very much like a shanked bearing pattern bit. The ring (collar) is pulled against the templet as the router is slid-along the pattern (templet). All the while, the cutter is extended below the collar and cutting sideways into the stock beneath the pattern. The pattern is duplicated on the work but since the collar is always larger in diameter than the cutter, the work is always larger than the pattern, usually by no more than 1/2″.
Consider them armor; they do their business and protect you from most of the cutter at the same time. A shanked bearing guided trimmer, the exact analogy of the collar/cutter ensemble, has to be completely exposed beneath the subbase for it to engage its templet. Another advantage of the collar guide bush is its independence from the cutter. A guide bush deployed plunge router can plunge though its entire plunge range without interfering with the collar. What that means is safety and application. Materials otherwise impossible to rout are rout-able with collars. 2″ thick material can be wasted in 1/16″ increments with just 1 or 2 horsepower. A cut that rigorous can break a 2″ trimmer or burn out a motor if done in one shot. Moreover, that much waste in one pass is dangerous business. Try wasting 1.5″ of Acrylic or MDF in a single pass if you don’t believe me.
Its isolation on the subbase is another advantage. Bearing guided cutters, for example add a third bearing to the armature system of a router. That usually adds some measure of chatter/vibration to the work. The bearing, whilst pulled against the work or templet, may emboss the work as it bounces along the mill frequency of the jointer or planer marks. This vibration is transferred to the cutter & then to the work. With a collar, all of the lateral pull against the templet is transferred to the subbase, not the cutter. As a result, cuts are cleaner, cutters & motors suffer less stress, and last longer.
The device is not without adversity, however. They must be centered to the cutter for close work. Dovetail and box joint templets, e.g., are intolerant of off centered collars. And general templet work, of all sorts, express collar eccentricity as the router is rotated, even slightly. As such, your cuts can vary, especially if part A is cut from the right side of the collar and part B from the left