Because they say a picture is worth a thousand words...
_
That's a Freud brand router bit.
Often times I have the need to put a 1/8" roundover on 1/2" thick material and, as you can see from the photographs above, the screw holding the bearing on makes the router bit project down from the base about 5/8" overall. For a lot of stuff like this I like to route directly on top of a non-slip mat, but that doesn't work in this case.
My usual method involves putting something under the 1/2" material to raise it up off the surface or hanging it off the edge of the table. Both methods require clamping, which isn't idea and usually takes longer to position the clamps in just the right place not to interfere than the actual routing! But, I got thinking that maybe someone makes a bottom bearing router bit with with a countersunk hold-on screw? Or maybe there's some better method?
Well, I guess doing it on the router table would solve the problem... but, I like having a number of routers already setup with dedicated bits for various purposes (ie. round overs, chamfering, flush cutting) that I can grab-and-go without needing to mess around with setting it up each time.