metzmail wrote:I am trying to find a way to drill a 4" flat-bottomed hole without a forstner bit. I thought about using a router, but I guess I would have to build a large round jig that the router would fit in. Does anyone have any other ideas?
Take a thin piece of ply 1/8-1/4 inch about ten inches long and about 1 or 2 inches wider than your router base. Remove the plate from your router base and place it on the ply, lined up at one end. Mark and drill the screw holes that hold the plate on, plus mark the center of bit opening and make a hole in the center at least as big as your router bit.
Now mount the ply to your router and mount the bit now measure from the outside of the bit back toward the other end of the ply 2 inches and put a nail there with the pointy end facing the same direction as the bit.
Now place the router on you work piece making sure the nail is in the center of where you want the hole to be and plunge the router in. push the nail into you wood enough to hold it from slipping. After you make a ful circle you have the outside of the perfect circle 4 inches in diameter. Now pull out the nail and use the router to clean our the rest of the circle.
Caution make sure you don't allow the router to dip into the circle or you will ruin the bottom. Perhaps you should leave the ply mounted to the router and just pull the nail out. The large ply will help you keep the router from dipping into the hole.
Another way requires a bandsaw. IF you have one mount a circle cutting jig,(board clamped to bandsaw table with nail exact distance from the side of the blade as the diameter hole you need aligned with the front teeth) and cut a circle the size you want minus 1/2 the diameter of a straight cutting patttern router bit. Now mount the cutout circle to the wood where you want the flat bottom hole and use it as a pattern to cut the outside of the hole you need. Again you will have to go back and clean up the inside by hand but as long as you keep the router flat on the wood and don't touch the outside edge you will end up with a perfect 4 inch hole.
Hope this helps