I gave that some more thought as well. My initial apprehension was brought on by the fact that my top-bearing bits are all 1-1/8 ø x 1-1/2" tall. IOW...my thickness of the jig would have to be a minimum of
1-1/2"+ whatever to catch the bearing... After a couple of slaps to the forehead, I realized that I can get shorter bits than that as well, soooooooo I see all the benefits of the top-bearing set-up... bushings are a pain as they never centre properly although a PC base on a Milwaukee is damned close to perfect.
arrrgggh. use a little math. make a shallow cut with your bit and measure it. then measure the outside of the bushing. subtract and rip a slice of wood to the result. now when you slide the guides together around a sample of your shelf material include the piece you just made. mark and measure from the center of the dado and all's well.
I later felt and by suggestions of
it depends on how many you're making. pattern bits cost significantly more than plain 'ol straight bits and are limited in length- usually too long... this is a top bearing application for a sub-3/4" cut, which means the bit will have a 1/4" shank with all of the flex and runout issues associated with small shafts.
if you are just making one case and are unlikely to do it again for a long time, the pattern bit is cheaper than a bushing set and is easier to set up, though not either by much.
centering the bushing IS an extra step, but it's worth it and isn't too hard once you've done it a time or two. the larger diameter of the bushing vs the bearing will give more accurate results, especially over time and repeated passes as the template wears.
how you keep from dadoing the jig cross-pieces both in front of and behind the piece you wish to dado if they're the same thickness as the piece. Is there a trick or am I being too thick?
You do indeed cut into the "T" that squares the jig on both ends. Typically though you are only cutting 1/4" deep and the jig at 1/2" at that location and that part if the jig is the same height as the work. You only run the router the width of the board receiving the dado. Basically quit cutting after the work has received the dado.
At that point though, haven't you already "damaged" the jig cross-pieces? How can you "run the router the width of the board receiving the dado" WITHOUT touching the other pieces?
Reread what I said, you do indeed cut into the T's that square the jig. You damage the jig much like you damage a zero clearance plate on a TS the first time you raise the blade. If you are talking about the guides that guide the router, you use a top bearing flush cut bit and run the bearing along the arms.
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