What is a Ball Bearing Guide?
Ball bearing guides are used in concert with routers to create smooth and accurate cuts. The closed, wheel-like casing is attached to the router's shank or bit-spindle and then is put in contact with a jig or edge surface to secure the path of the router while cutting.
There is no standard size for this style of guide, as its size will be dependent on that of the bit. Ball bearing guides can be located below the cutting tip or above it. The former location is best suited for the creation of moldings or edges (as it can also act a gauge for horizontal depth, whereas the latter position is typically reserved for deeper, multiple-pass cuts.