Not sure if you are still looking for the bearing to make a "mortises" jig.
You can get them from http://www.grizzly.com
Parts number(s) G3847,G3845 and many other sizes listed in the Cat.(replacement bearing for router bits)
You can make a quick base plate with a 1/4" clear plastic stock 8" x 8" and hole saw or a 1 1/2" Forrest Bit.
Cut the stock Sq. and on ea.corner drill a hole to fit the bearing I.D.(on center with the 1 1/2" hole) then take your base plate off your router and line it up with the center hole and drill the mount holes to it to your router motor.
The best way to line it up is to use a 1 1/2" plug with a 1/4" hole in the center of it and then snap it in the router base plate,then drill a 1/4" hole in the new base plate,this will line the parts up on center, then drill the 1 1/2" hole in the new base plate.
If you take your time you can you can use Forrest bits&hole saw and put in a 1 3/16" hole in the base plate to hold the brass bushing guides ( H3143 ) then just put the stock base plate back in the box for your router.
The 8 " x 8" base plate works great for many other jigs and gives you more control over the router.
I'm wondering if Fancyfull might actualy be looking for a baseplate that centers the bit on a workpiece edge (rather than a baseplate centering set-up)?
If so, take a look at this link, I believe it accomplishes what you're after - http://www.woodworkingtips.com/etips/etip032301wb.html
The stock you want to put the mortise in is locked in jig with a slide wedges and the router base is set on top and turned all the way to the right until the outer bearings rub on the stock this centers the bit on dead center, then just plunge down and pull then router to you ,it takes a time or two if you are using 3/8" X 1.250 deep slot.
It can be used for 1/4",3/8",1/2" mortise slots and they come out right on center every time.
You can use the router table also but it's a bit tricky to set it up for center when you use 3/4" to 2" stock and plunge it down but this way it's done in a flash .
And it works great and it's quick and easy.
The out side parts of the jig need to be the same height as the stock, this helps the slot stay at a 90deg. to the stock and a place to support the router.
If you are using a 1 1/4" x 2" stock it will take a space in the jig to offset the stock so the slotts come out right, and right and left side thing.
I don't recall when Bob made this jig but it's a great one.
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