routing a dado for a shelf

Use a straight edge to guide both sides.

-Jack

I assume you know to rout in the direction such that the rotation of the bit is forcing the router against the straight edge.

You can use an undersized bit and rout twice, once on each edge of the dado. Of course, if precision width is important, then fence alignment is critical (but can be accomplished more easily with a spacer to align the fence the second time).

Greg

George wrote:

Use two guides, one on each side. Use a 1/2" bit. Make one pass and then come back to do the other side and clean up any imperfections on the first side. Be sure to make the passes in the proper direction, but you already know that. Ed

Your bit pretty well centered in the guide? If so, make a plywood jig with some 1/4" ply, nailing two parallel strips of wood to it, as far apart as the diameter of guide you will be using. That way you can attach a fence to butt it against the pieces you're routing, make stopped dados by nailing a couple pieces of your spacer at either end of the parallel strips, etc. As the router is contained between the strips, no problems with jump. You might even get a little wacky and try routing a sliding dovetail - same jig.

Those guides are great items, though some tend to think of them as pattern routing aids only.

It is mainly a matter of practice, but...

If one side is less critical than the other, besure to put your straight edge on the critical side!

That sounds obvious, but it took me a major screw up to learn it.

Ed, What is the advantage of using a 1/2" bit vs. a 3/4 inch bit?

With a 3/4" bit you are plowing through the cut in one or two passes depending on the depth. With the smaller bit, you plow through the first time but on the second and any subsequent passes there is more room for the chips and I find a cleaner cut.

Works well for me, but it is not the only way. Ed

One very simple solution is 2 straight edges set the width of the router base apart.

John

Advantage of using a 1/2 bit to route a 3/4 dado is that you make 2 passes, and if the router jumps a bit, the 1/4in lets it move without gouging the far side, you just go back and make another pass to correct the spot where it jumped

John

If you rout on the correct side of the straightedge, you have to physically pull it away from the straightedge.

Very true but a wooden T-square takes about three minutes to make and has the advantage that it has a built in alignment pointer.