Router Compass

[joe greiner]

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

[Jerry Allen]

Nice design, Joe.
I love allthread tech. I recently made an extended rod for tensioning my bandsaw tensioning using 3.8". Quick and inexpensive.

I don't have the greatest skills and often have problems with small pilot holes in metal that is tougher than aluminum drifting. As a result I rarely start with a bit smaller than .093" which tends to flex less than a .062 bit.
I had to put a pin through some 3/8 allthread and was having trouble getting a good hole. So, I put a couple of nuts on the allthread, and ground a small flat on one side of the allthread. After the hole was drilled, I was able to just run one of the nuts over the hole to clean the threads. In the case of your design, it looks like that might be an alternative to the coupling nut. Just leave the nuts on to sandwich the pin.

(Looks like something similar to a Longworth chuck that you are routing there.)

[joe greiner]

Correct about the Longworth chuck, Jerry. I've used it only two times, so I haven't finished assessing my "improvements" to the design; although I've decided on two revisions already. That said, the Longworth chuck is the best way I've found for finishing the bottoms of bowls; haven't tried a vacuum chuck, but as attractive as they seem I think there'd be centering problems.

Putting the pin through the all-thread itself, with just the two nuts pinching it, would be simpler. But not best for me. By the time I use it next, I would have long since used the all-thread for a connection of some sort, and I'd need to drill a new one. With all the custom components secured, I'm now at liberty to cut old or new all-thread to suit my next radius cuts (when/if).

I agree that grinding a flat is the best way to drill anything with an awkward entrance condition.

Joe