Glue-joining of Cutoff Pieces for Lumber Recovery

[edflorence]

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...

Steve...

What I was thinking of is the Plain Scarf joint, which is simply a long bevel on one end of each of the two pieces to be joined.

plain scarf.jpg (29.53 KiB) Viewed 19879 times

Although the strength of a scarf is hard to determine precisely, the internet says that with a slope of 1 in 12, and using modern adhesives, you can expect something like 80-85% of the strength of a solid board of the same dimensions.

Here is a description:

Scarf, Plain
A plain scarf joint connects two pieces of wood end-to-end using overlapping, diagonal cuts without additional interlocking features. This slanted connection increases the surface area for bonding, offering a more robust attachment than a simple butt joint. However, because of its lack of mechanical interlocking, the plain scarf joint's strength heavily relies on the adhesive used or, occasionally, supplementary fasteners. Common in woodworking and carpentry when extending beams or planks, it is essential to ensure proper alignment during assembly. The plain scarf joint exemplifies the idea of simplicity in design, prioritizing efficient use of materials and a smooth, elongated bond.

Plain Scarf Wood Joints have full widths and scarf angles between of 10-30 Degrees. Typically used in wood members with widths between 1.5”-3.5” (3.8-8.9 cm) and depths between of .75”-3.5” (1.9-8.9 cm).

[algale]

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.

I'm not sure I would trust either the Freud finger joints or long box-type joints to make load bearing beams unless I glued another length of wood, similarly constructed, on at least one side (face) of the joint and staggered all the joints, resulting in a beam twice as thick as the original wood, which requires even more wood.

By the way, at first I did not understand your reluctance to use a scarf joint. But now I realize we are talking many short cut offs. With it's 12:1 ratio, a scarf joint of two 12" long cut-offs will still be a single 12" long board, resulting in no gain in length. Scarfs are used to join two boards, neither of which is long enough for the needed length but which, when combined are at least a foot longer than needed so that they can be scarfed and still have enough length.

[davebodner]

You ask, "is it worth the time and effort?"

I think the answer is clearly, no. But, is that really the measure? Most of us are woodworking as a hobby, not as a profession. It's fun to accept a challenge and succeed--or fail. Who cares if it's worth it?

Personally, I would probably be frustrated by the process, and trusting the load-bearing capacity would be a challenge, too. But, you might enjoy it. Go ahead and try. Let us know how it comes out. You might even try some destructive load-testing, just to see what happens.

[thunderbirdbat]

I am not sure what you are planning to build but if gluing up a panel, then staggering the joints may also help. Check out how this youtuber builds a panel for his projects starting just after a minute into the video. Not sure how much strength the panels have but he builds some beautiful pieces that way.

[SteveMaryland]

Thanks Dave and Brenda. I appreciate everyone's input. Up the learning curve we go...

As for the economics of lumber splicing, depends on the scrap volume, the lumber market price, and the labor involved. Who knows how expensive lumber will get?

As for the labor involved, the combination joint I showed above is maybe not the best. So let's spring for that Freud finger joint cutter and do a revised combination joint, as shown below.

FINGER JOINT DESIGN STUDY 4.JPG (49.17 KiB) Viewed 19730 times

FINGER JOINT DESIGN STUDY 4a.JPG (39.58 KiB) Viewed 19730 times

I am thinking that a hardwood spline would provide added strength, more strength than SPF. And yes, the interdigitated "fingers" above and below the spline, if well-fitted, glued and clamped, should be nearly as strong as the original fibers. It is the outer fibers that take most of the bending stress.

As for projects, next year I expect to replace a T&G front porch deck, which will involve much lumber-milling and cutoff-generating, and I will have a lot of cutoff left over, enough for at least 10 8 foot 2x8's, which I can then use in other structural projects, but only if the joints are strong.

[JPG]

My money is on correctly created finger joints.

[edflorence]

My money is on correctly created finger joints.

I think so too...the spline seems like "belt and suspenders"

[JPG]

They ARE strong and require little sacrifice of length.

However glue is the possible weak link. Using 'interior' glue outdoors WILL result in separation.

[edma194]

I think the finger joints will be fine. Of course the glue must be right for the application.

I think there are problems with the proposed modification to include a hardwood spline in the joint and not enough benefit to justify the extra work.

[JPG]

+1