Router Pilot Bit Offset

[Vince Shriver]

I have several pilot bits (in a set) that the bearing diameter is slightly larger than the cutting diameter. I'm assuming that is for leaving laminate with a small "overhang" over the sub straight so adjacent laminate can be fitted up against it at 90 degree. Is that the reason?

[Frank Pratt]

That kind of work flow makes no sense to me, but I'm far from a laminate pro. They sound like some kind of special purpose set to me. I'm curious too.

[Jamie Buxton]

Perhaps it has to do with the manufacturing tolerance of sharpening carbide cutting edges. The bearing OD is extremely precise, but maybe they can't hold that precision in grinding the cutting edge, and they think users would rather have the overhang you describe than an "underhang".

[Vince Shriver]

That's a thought, Jamie, but these are premium bits, and in the same set as precisely aligned bits. These have been deliberately sold that way for a specific reason I believe.

[Ken Andersen]

I can't quite imagine the bit you are describing. Could you post a reference to it? (manufacturer, model). Thanks.

[Vince Shriver]

I can't quite imagine the bit you are describing. Could you post a reference to it? (manufacturer, model). Thanks.

I was mistaken in my original description of the offset bits; after measuring the diameters it turns out the bearings are 1/8 larger than the cutting diameter (CD). Some years ago I purchased a book about routers, written by the late Pat Warner. He recommended several tool manufacturers that made router bits. Among them was as company in my neck of the woods that has since gone out of business, Paso Robles Carbide. I bought a "set of five" pattern bits from them: 2 of which have an 1/8" greater diameter bearing than the CD - 5/8 & 3/4 and 1" & 1/8. I do a bit of plastic lamination and I can see how they could be useful in that regard. I've just never run across any other like it. Maybe someone on the board here knows.

[Ken Andersen]

Thanks for the clarification. Now I understand. You have "pattern" bits, not "pilot" bits, and the bearing diameter is "larger" not "smaller" than the cutting diameter. With this, as you say, the result will be an "overhang" of the pattern you are following. This can be useful for example in rough cutting laminate and other materials to size/shape before final finish sizing/shaping. On profile cutters, changing the size of the bearing can often be useful in modifying the profile produced.