Magical though it seems, a simple geometric principle is the secret behind this fixture. With it, the bit can easily be centered across the workpiece.
Centering a hole or a mortise or a groove on the edge of a workpiece is always tough. Measuring and scribing layout lines isn't error-free. Setting up a guide can't be error-free. You can get the cut dose to center, but it is seldom really on center.
Here's a simple baseplate that can be made in a short time. And it will almost magically center a cut on a work-piece. The baseplate has two pins projecting from it. You set the router atop the workpiece, with a pin on each side. Twist the router to bring both pins in contact with the work; the hit will automatically be centered on the work.
The trick is simple geometry, lite pins must be equidistant from the bit axis and must be on a straight line that passes through the bit axis. The bit will always be equidistant trom them, regardless of the angle from which you measure.
As in all magic, precision apparatus used with well-practiced skill is die key to success. In making the baseplate, you must be precise. Lay out and drill all the ncccs-sirv holes carefully. Before using the baseplate on good work, practice. Use of the baseplate requires steadiness. Hie fixture is firmly attached to the router, but it doesn't fix the router to the workpiece. Keeping the router square on the work and the pins in contact with the work is the operator's job.
I've seen this son of baseplate fitted to a fixed-base router, but there's more sense. I think, in using it on a plunge router. You can't make a through cu; with it. simply because both pins must be in contact with the work. Depending upon the pin placement, you must begin and end any cut 2 to 3 inches from the end of the work. Stopped cuts like these are the stuff of the plunge router, not the fixed-base router.
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