Advice on an odd type of pivot or hinge?

Hi fam! I need suggestions on something I’m building. It would be an adjustable guide for a router, for cutting dados and such. Imaging two narrowish boards, say 1" x 3", set down parallel, then two shorter planks laid on the ends to make a parallelogram, and then attached with pins or pivots to the longer planks, so that the parallelogram can be made narrower or wider. The router would slide between the two longer boards. I could just clamp two boards to a workpiece to do the same thing, but the two shorter boards keep them parallel automatically, and also hold them steady so they’re easy to adjust.

My question is what hardware, if any, to use for the pivots. I could just drill holes and insert bolts, but I’m not sure I could make the bolts fit the holes tightly enough, and stay tight enough with use. I think a hole in bare wood would gradually get larger with wear.
The two guide boards need to stay parallel pretty accurately.

I could put two biggish nylon or brass washers between the two boards to reduce friction, but there’s still slop in the pivot to worry about.

I think ball bearings on a shaft are overkill, and anyway I don’t really know how to mount them securely.

Incidentally, I guess the boards don’t have to be wood. They could be shallow aluminum C-channel stock.

Any ideas? Much appreciated! I have access to a drill press, wood lathe and the other usual woodworking tools, plus taps and dies.

I think you could use bolts so long as you were using good hardwood and you sized the length of the bolts so that only the smooth unthreaded portion of the bolt was in contact with the wood. You could even go so far as to use “shoulder screws” which are special bolts with an especially smooth portion of shank designed to be used as a pivot.

Another option would be to drill oversize holes and then glue some harder bushings into the holes. There’s lots of options for this, you could make your own fancy bushings on your wood lathe from something like lignum vitae or you could buy cylindrical or flanged bushings aka “plain bearings” aka “sleeve bearings” in bronze, plastic, etc.

Thank you sir! I’d imagined something like shoulder screws but I didn’t know what they were called. Sleeve bearings are another great idea. I think nylon will be more than durable enough.

But now I’ve gotten ambitious, and am thinking about making the joint lock in place by tightening a knob on the shoulder screw. So I can separate the two pieces of wood with a wave spring washer (which I discovered on McMaster), which would be compressed when I tighten the knob, squeezing the two pieces together.

But there are two issues. One is that the spring washer has thickness even when compressed flast. So either the spring washer needs to be fitted in a very shallow counterbore so it lets the two pieces of wood contact each other when the spring is compressed, or it can sit flat on the wood or on a flat washer, and be surrounded by largish rings of some high-friction material, thick enough to accomodate the spring, almost like a clutch. I am not sure I can drill such a shallow counterbore, so I’m considering the “clutch plate” idea. Any thoughts on that? What could the rings or plates be made of? Rubber? Very thin plywood? I have some 1/16" ply handy.

The other issue is when I tighten the shoulder screw at one end, either by turning the screw or the nut, how do I keep the other end from spinning? I suppose I could put a t-nut in one piece of wood, and thread the shoulder screw into it, then attach a knob to the screw somehow.

Any thoughts, anybody? Thanks again!

If you want to be able to put knobs on the pivots to clamp things together I’d suggest this:
On two of your boards drill oversize holes and epoxy in some nylon bushings (or similar). On the other two boards tap in some Knurled style carriage bolts with a hammer (McMaster has these). Then use some knobs to clamp it together. I don’t think there is any need for spring washers or high-friction materials, the knobs holding the wood against itself should be plenty. I do think it is wise to put a thick fender washer under the knob so that the clamping force is spread out over the wood and isn’t just concentrated in the middle where the bushing is glued in. So from one end to the other:
Knurled carriage bolt - Wood - Wood (with bushing glued in) - Fender washer - Knob
If you do want to use spring washers put them under the knob instead of between the boards, so:
Bolt - Wood - Wood with bushing - Fender Washer - Spring Washer - Knob.

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Brilliant! That will work!

It’s kind of amazing how the prices work. I can get 25 3/4" long, 1/4-20 knurled carriage bolts for $35, but I can get 25 1 3/4" long bolts for $10. (And anyway I only need 4.) But the 1 3/4" bolts have a 0.625" knurled section, so I guess it would call for 5/8 to 3/4" plywood, which is thicker than what I was planning. Weird that these prices might end up determining my ultimate design.

Aha! I can just use ordinary carriage bolts! The square collar will sink into the wood and keep the bolt from spinning. Also McMaster-Carr has lock washers for square-neck bolts (at reasonable prices and amounts) that will absolutely keep the bolt from spinning (the washers have a square hole to fit carriage bolts, and teeth that dig into the wood to keep the bolt from rotating.)