Glue-joining of Cutoff Pieces for Lumber Recovery

[SteveMaryland]

We have all seen commercial wood molding that is made up of small pieces of wood butt-glued together along a wavy finger-joint type of profile.

Somewhere I have seen the rotary cutter that cuts this type of profile, but I do not recall where nor do I know what this technique is called, so I can't search for prior discussions of it on this forum. It is not a "scarf" joint.

Anyway, I have short cutoff lumber that continues to pile up and I can't bear to throw the material out. If I could just economically join all these pieces, I could re-use much of this material and also free up much space. And save some money and some trees also, I guess.

Needed is a butt-glued joint geometry that gives nearly full strength but is also easy to cut with tools at hand.

See attached illustration. I have not tried it yet. All cuts using two dado widths (3/8" and 3/4").

Of course, I cannot expect to obtain a usable stick of lumber by just butt-gluing, so I would also need to joint all edges and plane all faces.

What have others tried that works? Is it worth the time and effort?

[jsburger]

It is called a finger joint.

https://www.amazon.com/finger-joint-rou ... router+bit

https://www.rockler.com/finger-joint-router-bit

[edma194]

This video shows the use of a finger-jointing router bit. Tight fitting joints don't need much extra hold although the one you show with a cross piece is interesting. Finger joints of these styles are usually stronger than the original wood due to the strength of the glue and the high surface area contact. The angled fingers permit a very tight fit that is difficult to achieve with simple box jointing which needs very precise cutting. Simple box tenons need to be slightly smaller and shorter than the mortise they fit into while the angled fingers are wedged together until tight.

[jsburger]

FWIW, go to a big box store and look at the paint grade solid wood moulding. Window and door trim and even jams are or used to be all finger jointed from short pieces.

[edma194]

Mills cut wood on big bandsaws to get every last useful piece of wood out of them. The operators identify flaws in the stock like knots and voids and then the wood is cut around those parts. The resulting boards are fed into a finger-jointing machine like this. Imagine what common lumber would cost if they still threw away those boards.

[SteveMaryland]

OK, the "finger joints" made by the Freud tool and by the molding mfgrs are "wavy", whereas a "real" finger joint (or "box joint")is made up entirely of straight-line cuts, parallel and perpendicular to the grain. End-to-end grain glued interfaces are weak, and practically "don't count" for strength, making a "real" finger joint weak in bending - not good for lumber joining.

For higher strength, I want a joint that minimizes end-grain interface area while maximizing face-grain area, and also can be cut with a circular saw or dado.

By introducing the cuts shown in step 2, and by introducing the separate "fingers" shown in step 4, I think this joint will be quite strong - much stronger than a routed finger joint interface could be, because the face-grain interface area will be doubled. Plus the tongue and groove of step 2 adds even more face-grain. And I don't have to buy special tooling to obtain this result.

So has anyone used the Freud cutter for lumber recovery?

[edflorence]

Just curious...why not consider scarf joints? With a 12 to 1 ratio the simple scarf is easy to make, nearly invisible and should be plenty strong. Finger joints are great...but to join lots of cut offs into a few long pieces, scarfing would be quicker and not require any special jigs or tools. Disclaimer: I do not work for the Scarf Joint Industry, this is just me wondering.

[SteveMaryland]

Ed, not sure which sort of scarf joint would work. See attached illustration. Which one did you have in mind?

Looks like if I took two scraps and joined them with a scarf, I would wind up with a piece not much longer than the original scraps. Lots of glued area however...

[jsburger]

All the commercial finger joints I remember are not "wavy". They are triangular shaped so not end grain. What do you want to do with this stock? Glue joints of all kinds are stronger than the surrounding wood. Unless you are joining 2X pieces for planks for scaffolding what is the issue with any joint? The industry has been using finger joints (not box joints) for well over 50 years for all kinds of applications. If there was a problem with them they would have changed.

[SteveMaryland]

Well, I just don't want to buy the Freud tool if I can do a better joint without it. Using a finger joint to make ornamental molding is one thing, but I want to get material that is load bearing.

OTOH, the Freud tool would save a lot of work. But can't use it on a Shopsmith without a speed increaser or a router. The joint I described could be made without either.

Essentially, the joint I have illustrated above amounts to a combination of a tongue-and-groove joint + a splined joint. Each strong separately, and likely much stronger in combination - stronger than any other joint in the same volume.