joe greiner
3rd June 2007, 11:34 PM
I recently had occasion to rout some arcs in a thick plywood sandwich. To use a conventional router compass attached to the base plate, it would have been necessary to mount the bit with reduced purchase in the collet. I prefer to shun that, and longer bits are hard to find.
I made a beam compass to attach directly to the router base. The base has transverse holes normally used for attaching a fence accessory. A 3/8" piece of all-thread fit the holes perfectly. At the pivot end, I attached a coupling nut and a jam nut. I drilled a hole through the coupling nut to accept a brad with the point sharpened. For each of many candidate locations along the coupling nut, there were three opportunities to drill the hole correctly. I got it right on the second try; on the first try, the drill bit found an internal thread on the opposite side of the nut and drifted off perpendicular. I filled the erroneous hole with epoxy so I wouldn't use it by accident. Except for assembly, these two steps were the only manual labor required. I added a short bolt to pinch the brad against the end of the all-thread.
At the router, I used a bushing and a nut to position the all-thread at each end of the hole. This provides the radius adjustment. The bushing places the nut where I could tighten it. I measure the radius directly with the router inverted; a gage stick would be more effective than the tape measure, though.
It isn't necessary for the compass to lie along the line between the pivot and the bit. The 3/8" all-thread is stiff enough to prevent flexing. Except for the all-thread, which can be any length needed, all the components fit in a small pill bottle.
Joe
I made a beam compass to attach directly to the router base. The base has transverse holes normally used for attaching a fence accessory. A 3/8" piece of all-thread fit the holes perfectly. At the pivot end, I attached a coupling nut and a jam nut. I drilled a hole through the coupling nut to accept a brad with the point sharpened. For each of many candidate locations along the coupling nut, there were three opportunities to drill the hole correctly. I got it right on the second try; on the first try, the drill bit found an internal thread on the opposite side of the nut and drifted off perpendicular. I filled the erroneous hole with epoxy so I wouldn't use it by accident. Except for assembly, these two steps were the only manual labor required. I added a short bolt to pinch the brad against the end of the all-thread.
At the router, I used a bushing and a nut to position the all-thread at each end of the hole. This provides the radius adjustment. The bushing places the nut where I could tighten it. I measure the radius directly with the router inverted; a gage stick would be more effective than the tape measure, though.
It isn't necessary for the compass to lie along the line between the pivot and the bit. The 3/8" all-thread is stiff enough to prevent flexing. Except for the all-thread, which can be any length needed, all the components fit in a small pill bottle.
Joe