[Biagio]

@fishin&routin,
not sure why you have two(are they identical?). One may be for the parallel guide. The one you show, is for routing the edge of a board. In practice, the little bearing wheel should cover almost all of the bit, viewed end-on as you show it. Only the cutting edge of the carbide flutes should be exposed, and then not completely.

The bearing wheel needs to ride on the edge of the board (meaning you cannot use that guide if you want to machine the whole thickness of the edge). If more of the bit were exposed, it would be difficult to keep the router aligned at 90 degrees to the edge, leaving an uneven edge. Ditto if the bit diameter exceeds the bearing wheel diameter.

The guide allows you to follow a curve (convex or concave). For a straight edge. You might find a parallel guide easier to use.

 

[Biagio]

@LeftFinger ,
Agreed. This question came up in another thread in mid2020. I said at the time that I can not think of a use case today.

 

[Biagio]

@Semipro ,
in my hands that would be an invitation for sadness. I accept that someone more skilled could do better, but I would stick to a circle-cutting jig, or a two-point guide attached to the parallel guide (for convex shapes).

 

[Biagio]

@CharleyL,
I bow to the wisdom of your greater experience - one of the benefits of such a forum is exposure to the thoughts of people like you.
However, I am not sure I quite follow you. Any chance of a rough sketch to illustrate what you are saying?

Up to now, I have only used a parallel guide or bearing-mounted bit for laminate trimmimg, with acceptable results - but that was on straight edges of counter tops. Have also only used a straight bit, not one of the specialty laminate trimming bits that produce a slight chamfer.

Always keen to learn a better method.