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Router table construction problems - routing the rabbet for the plate
Ok so I've wrestled with this for a couple of days and done some experimenting..and failed..so now I thought I'd avail myself of the collective wisdom of the Knots group. Here's the deal:
I got some nice 3/4" birch-faced plywood, and a Triton router, and I want to make a router table. I bought the Bench Dog Pro-Plate router table plate to mount in the table top, and from which to hang the router. I've got a 1/2" x 1/2" router bit with a roller bearing at the top.
I want to route a ledge in the 3/4" plywood to place the mounting plate. I need to set the height of the bit so that when I put the plate on the ledge, the top of the plate is perfectly in line with the table top.
I tried it on some scrap. I set some straight edges (3/4" ply) around the plate using carpet tape. I then set the bit height by setting the Bench Dog Pro-plate on top of the straight edge, putting the router on top of that, and "plunged" the router down until the bit touched the table top. I was concerned that the sharp bit would dig into the wood when I pushed the router down, so I did it gently.
This resulted in a rabbet that was too shallow: I routed out the ledge and cut away the waste. I set the plate on the ledge and the plate fit perfectly around the perimeter BUT it was just a smidge too high. I could feel my fingernail catch on the plate as I ran it from the table top to the plate. I tried to take down the ledge with sandpaper but that didn't work.
In subsequent attempts, I set the bit height by setting the router upside down, putting some 3/4" ply scraps on the base and then the router plate. I set it such that running my fingernail from the bit to the plate and in the opposite direction didn't "catch" my fingernail. I looked at the setting with a magnifying glass and it looked right. This method does not allow for the thickness of the carpet tape (which is pretty thin stuff).
So my questions are:
1) why is the bit too high?
2) How can one set the bit height to get good results? In order to test fit the plate, you have to cut away the waste. Once you do that, I found it very hard to keep the baseplate of the router flat to the table top since less than half of it was on the table top - the router wanted to rock in. So my attempt to sneak up onto the right depth failed. I tried putting the waste back in so that the router had something to rest upon but it wasn't the same level as the table top. Plus I got skeert thinking that since it could move it would hit the bit and go flying.
I tried test settings on a piece of scrap only routing the edge but I found that testing the fit of just 1/2" edge of the plate was hard to do.
3) Is this REALLY the way to set the Pro-Plate (or any plate) into the table? I've looked over several articles where people did this, but I don't have the same success as they did. The articles go into great depth as to how to make the template but give very little clue as to how to set the bit height.
Did they find a way to get the bit height set EXACTLY right so that the plate is level with the table top? Or did they take that 'fingernail catch" error and live with it?
Any advice would be appreciated.