@@matthiaswandel I'm agreeing with you here. There is certainly the practical aspect of the difficulty of producing a scarf joint in the home shop. Also, in a production environment a scarf joint 'wastes' more material compared to a finger joint. But the engineering aspects may give a scarf joint improved performance. With a point load (as in your test set-up) more of the joint is further away from the point load, where the moment and shear are reduced. But that particular rabbit hole is getting a bit far removed from your original question of the strength of finger joints.
@@briantaylor9266 If material waste is that much of a concern, just do a buttjoint. But really, waste is only guaranteed at the ends of a boards. With sufficient board length, each cut provides both positive and negative sides, i.e. no waste beyond the kerf. Regardless, people suggesting a scarf joint aren't expecting 15 to 5 degree angles. There's clearly a happy middle ground that balances strength to feasible cut angle. Personally, I'm more interested in seeing a combination scarf+V joint.
Most people say don't bother trying to glue end grain to end grain, the joint with straight fingers has half of the total area of the wood that is actually end grain to end grain. If you disregard that the maximum strength of the straight fingers is 50% of the total strength of the wood.
That's one of my favorite things about these random material test videos. I don't do a lot of woodworking so I don't have a good intuition on the strength of different joints and woods, so just having an idea of how they behave is super useful.
The joint, even with end grain pores cut at a suboptimal for bonding strength angle, did perform very well relative to solid wood, retaining near 50% of strength of a solid piece. This is plenty strong if dimensioned right.
I recall from my boat-building days many, many years ago, we used 8-to-1 scarf joints on plywood glued with epoxy. This was supposed to give the wood close to non-jointed strength. Although this may not be practical for what you are doing, it is a great way to get 16' lengths of plywood.
Matt, In the beginning of the video I was wondering about 10 degrees so thanks for reading my mind and answering my question before the end of the video!. Take Care, Virtual Step Dad
It would be interesting to see some of these tests scaled up (or down) in size a bit as well if only to see the relation between size and strength in each joint. Basically it’d be cool to see the workpiece/joint size vs strength curve on a graph. Great video series!
I used to work in building wooden aircraft. We used 1:15 ... 1:20 single sided scarf joints to join both birch plywood and pine. Joint was first primed with raw SP106 epoxy followed by thickened (fumed silica) SP106 as actual gap filler.
With all those glue failures I'm wondering if spraying a little water on the hardwood right before gluing would help the glue penetrate deeper into the wood.
I've been following this series with interest. For finger jointed lumber like mouldings and such sold in the building centres the results are directly applicable. But it occurs to me that for panels made up of several finger jointed strips, unless the finger joints line up the panel as a unit will fare better. It might be an interesting statistical exercise to make an estimate of the panel strength compared to individual strip strength.
Worth noting that the permenant bending you saw after a minute or twos load happens at around a third of the breaking strain, if applied for years. This limits the practical load limit in many cases to well into the glued joint region.
Most seasoned woodworkers just go by "longgrain to longgrain is the strongest" rule without ever testing it. Now that people outside material testing labs have access to affordable loadcells and can rig up a testbench to get somewhat repeatable results, there's more and more videos that put this belief to the test and as it turns out, with modern glues, the end grain to end grain joint is actually stronger than sidegrain to sidegrain for equal glue area, as the glue is stronger than lignin holding adjacent fibres together. As demonstrated and explained by excellent series of videos by Partick Sullivan called Glue Myths. However, apart from absolute strength You also have to consider mode of failure and glue area, as well as wood movement that weakens the joint over time. For a joint like this though, You can get both the strength of an end to end butt joint and the gradual failure of a longgrain joint if You add a scarf part to it. The total area of end to end pores would be the same, but the total glue are would increase and also include some longgrain contact that would fail without abrupt separation.
It would seem that the glue makes the fibers of the wood more brittle. I wonder if there's glue that matches the elasticity of wood more and might give similar strength as non glued?
If a wood piece is very knotty does that affect the strength of the entire piece or only around the knot ? Do you cut off and re-joint those sections? Is it better to use biscuits for jointing two load-bearing sections ?
@@matthiaswandel Would love to see a set of tests using resorcinol glue (typically used in adhering the plies of exterior and marine grades of plywood)
In einem alten (1956) Buch über Flugmodellbau wird auch das verlängern von Vierkantleisten aus Kiefer und Balsaholz behandelt, Dort steht, man schrägt beide Enden einseitig an (also nicht V förmig) und verleimt es zu einer längeren Leiste. Die Länge der Schäftung ist dabei die 10fache Materialstärke. Habe ich immer so gemacht und hatte nie einen Bruch.
Yes this makes perfect sense to me because both parts of the stock ha e equal thicknesses and no weak (thin) parts. The longer length of the joint halves will cause the joint to flex more before it fails.
Bu in practice, you would rarely ever use a single board jointed to another to extend the length, they're almost always used in laminated pieces with multiple offset pieces assembled to a single board. I'm not sure if your current testing apparatus can supply enough force for it, but a test of commercial glued laminated boards versus same measure of single-piece boards might be interesting. Another possibility could be OSB vs plywood vs glued laminated shelf stock.
A 1:10 thickened epoxy glued scarf joint used in boatbuilding may not be practical in most woodworking. I'd still love to see a test of that as the common claim is that such a joint is close to the strength of a solid plank.
Fascinating stuff as always! I'm wondering if you'll achieve different results on the V joints if you apply glue to both pieces and allow them to remain unjoined for several minutes...then apply a little more glue and join/clamp. The reason is end grain can pull glue out of the joint, leaving a starved glue joint. The extra step of allowing some 'soak time' may improve the bond.
When building the old stick and cloth airplanes, any scarf joint was recommended to be 30:1 meaning 30x the thickness in terms of length. So your 20mm thick spar would have a 60mm long scarf at whatever angle that turned out to be.
These tests are always fascinating. I've always wondered how some of those Japanese scarf joints would fare. But nobody's got time to make a batch of those.
Matthias, could You also redo the same test with a simple butt joint (end to end) for control on how end pore orientation affects strength of glue adhesion and then adjust the result relative to glue area. I expect the result to be surprising to most woodworkers that have been taught all their career that long grain to long grain gives the strongest joint.
A lot of those failures look like adhesion failures. You also showed that the type of glue used was significant to the results. I wonder if a construction adhesive like PL-8X would work on your V joints, even at lower angles. I have used construction adhesives and tried taking them apart after realizing there was a mistake only to end up destroying the wood in the attempt.
That looks like a lot of fun and everything you said sounds right on. As a structural engineer I'll say that I thought the long V joint would develop more strength. It appears to be limited by glue strength meaning you could try making it even longer. Also I wonder if epoxy would do better as an adhesive. Your comment about the straight fingers should be correct also- having half of the wood fingers in what amounts to end grain to end grain should limit the strength of that joint to about half of the total strength. Even if you had a good finger joint bit this could still be a problem because there is still a good percentage of end grain involved. I was surprised that the straight fingers came as close to the V joint as they did, the straight fingers do have a lot of glue surface area to help them.
Very interesting video! Did you notice any effect on the break point based on the loading condition? I noticed halfway though that the load was cyclic, this could induce the time dependancy on stress/deformation
Very interesting! The shallower the V, the more it's end grain, and the steeper V more side grain, so, next test: Do you really have to go twice as long to get twice the strength? Would a more flexible glue perform better - something that flexes about the same as the wood?
I'd like to see a "peg joint" test, where one end has a protruding peg (a couple inches?) and the other side has a drilled hole for the peg. Since there is some of the original solid wood, and a lot of surface area for glue, I think that would be a strong joint. Or maybe the strength would be only as good as the peg part. Might need a conical part at the bottom of the peg and a matching tapered hole for more strength and glue area. Only one way to find out. (Can't think of a quick and easy way to make the cone and tapered hole though.) Or maybe using just a dowel and drilled pieces on each side would be easier and probably nearly as strong. Hope I get to see if I'm right! If I didn't have to build some kind of pressure tester I might try it myself!
I have to wonder about a half lap joint test now, and whether increasing the joint size would increase strength. Also scarf joints from boats but they can be a pain to glue up. Pin nails? Or perhaps a small alignment dowel?
While in air frame school we were taught that a scarf joint with a 1:12 slope (minimum) with planed smooth surface and plywood glued over the joint seam (one on each end) to be an approved repair. Other trades that had to splice wood for strength also mentioned the 1:12 slope (this seems to spread the stresses out long enough to approach solid wood. Naturally this uses a lot of wood so for many folks they avoid (as in furniture making). And it ain't easy to do well. If memory serves Resorcinol glue was used.
It's a trade off of material waste vs strength. At 15 degrees, the glue area is substantially smaller, still retaining nearly 50% of strength of a solid piece, which considering how much deflection the solid piece experienced before breaking, would be more than enough for any practical application.
I love your testing, but I don't love the lack of epoxy. Epoxy basically never fails, the wood fibres always do. Do an episode on just that doing all the various tests again and showing it versus the lesser glues.
It's not that I dislike your scientific testing videos, I enjoy them quite a lot, but I really miss the old videos where you actually built stuff. It was so refreshing to see the video where your wife built that bookshelf with you. Wish you'd do stuff like that more often, reminded me of old times. Grew up watching your videos, and you're the main reason I got really interested in woodworking.
Have you ever used your strength testing setup to test the old trick of rubbing a bit of glue across the end grain and letting it sit for about 10 minutes (with tite-bond 2, anyway) to soak in before adding more glue and clamping up an end-to-end or butt joint? People swear it works, and I'd be curious to see what the numbers say!
I'd like to see you test different scarf joints (tabled, tensioned and stopped) at different ratios, 4:1 to 20:1 for instance. Perhaps a bit trickier to set up but it would be interesting to see the results.
do stress concentration points apply to wood like they do for steel? what are the gains if any from making sure that the joins have 0 gaps and or 0 sharp corners? basically either finishing with a chisel or making sure that your saw blade has a flat cut instead of the sort of M cut. Have you tested expanding glue or maybe adding a bit of wood filler in the gaps as you glue them?
have you compared tightbond 3 to tightbond 2? I believe 2 is supposed to be stronger, though it is less resistant to water. Since we are not in the habit of putting our wood joinery in water, tightbond 2 would be the proper glue to test with.
For what it's worth, in the airplane world, if you need to join two pieces end to end, the standard is a scarf joint with a 15:1 ratio of wood thickness to length of joint. IOW, if you're joining two 1"-wide pieces, the scarf joint has to be 15" long.
Hi Matthias, out of this topic, it would be nice if you try strength of wood with holes inside. Usually people use it for boards to make them lighter, f.e. for beds in caravan they drill lot of holes 5cm or bigger and it is still strong enough. But it will be interesting to measure bow much strength it looses by drilling those holes... Thank you
It appears that you are on your way down a rabbit hole with this various joint strength testing. Well, if you do proceed that way, I am curious to see where it goes. There is a possibility I might need to do some joints like this in the near future.
I’ve used scarf joints before. I read that they are common in aircraft manufacturing. They use a ratio of 1:6. If the stick is 1” wide, the joint needs to be 6 inches long. This is easy to cut on the table saw. Can you investigate the strength of this joint?
Did you try different glues like regular gorilla glue or epoxy i cant really think of anything else that makes sense maybe like some contact cement i guess like a couple min after joining and then whatever the recommendation is probably 24 hours
I seem to recall that Norm Abram seldom used clamps when gluing and relied heavily on his brad nailer. Could you test this approach if you have not in the past. PSReally enjoy all of your woodworking videos
the problem is you're using too inflexible glues. wood fibers can flex a pretty large amount, but once you put ordinary glue on it, it becomes too inflexible and thus will crack either the hardened glue or the wood around the hardened glue.
Ultimate strength would be the side by side wedge cut but one peak is deeper than the other so you don't get the perforation effect you got from the finger joints
@@matthiaswandel Sorry, i just meant in general, not necessarily related to this video. Regardless, practicality is still a consideration. :-) I just think it's a neat subject and I'm sure you'd have an interesting take.
@@matthiaswandel A Round dowel of hardwood embedded say 2 inches on either side would potentially lead to less stress concentration, especially combined with some PL 8x or a non brittle 5 to 15 min epoxy
Matthias I am not sure what the conclusion of this experiment was? There were two main variables here, the glue strengrh and the mechanics of the joint design. The problem with the v joint from a mechanical point of view is the thin end of the female v will fail first causing the rest to fail. I think the v joint performed better than the finger joints because of the longer crossover of thicker material. I think the strengrh therefore would increase as both the overall length of the joint was increased while having a design with the thickest material on both components. A thin part of a component will fail quickly reducing the overall load capacity of the piece. A v joint is therefore not the most optimal joint for strength although I agree that it is simple enough to make and still very strong. What this experiment has shown to me is that the mechanical properties of the glue are inconsistent with the properties of the wood itself and that results in the thinner parts of a join failing first especially when the joint has short overlap. In other words if the glue offered the same properties as the wood the results in theory should be the same for the solid pieces of wood as it was for the joined pieces. Wood joined at the cellular level is mechanically superior to synthetic glues.
Very interesting watch, like all the strength testing videos! A thought: at some point, could it work to make a pure analysis video that tried to draw general conclusions/rules of thumb based on your extensive testing? That kind of [trustworthy] information is very hard to come by!
Boatbuilders use the long scarf joint and they believe it's the best, especially for hulls. It should be easier to make by the way. What about long curing time epoxy ?
The 'W' shaped cut your considered at the end would not have any increase in glue surface to just a single 'V' shape since you're just flipping the tip of the 'V' over.
Now I kind of want to see what would happen with a v joint that spans the entire length of the wood. Totally pointless to test, but I'm curious anyway.
Does your test stand record deflection at detected break? I know you've done *a lot* of break tests of both soft and hard woods, but have you collated the data to show which woods are better for flexible or stiff applications? The amount of deflection/breaking force/wood species would be an interesting graph to see!
Stopped scarf joints are annoying to cut but easy to clamp. I would be curious to see what technique/jig you would come up with to make scarfs easier to cut. Finishing the stop cut by hand isn't too bad.
As for glue, try pl premium… go to the technical doc and they state the glue’s strength. I made a glue-lam column with pl, it was the only glue that offered stated strengths… nice video
@@matthiaswandelI agree a lot of length is lost. I’ve seen them cut a few ways. For a member of moderate height, it could be clamped to a table saw sled. For very wide members, a slanted router jig could be constructed. Wooden boats and aircraft both use scarf joints. Albeit, the slope on boats isn’t required to be 15:1. Even plywood is scarfed on boats.
@matthiaswandel Deinitely. That was a clever way you cut it in the video. Supposedly, the scarf joint should be even stronger, but I'm hoping you'll test it in another video
Thanks for the further testing! If I may bet my 2 cents and grains of salt, I'm convinced that half lap joints and scarf joints are both superior and look better. A simple enough fixture (couple stop blocs and some screws/cyano ) to glue multiple scarf joints can be made to offset the "difficult" glue up. I say "difficult" because the alternative is weak joinery with sharp visible joint lines that imitates cheap mass made boards, therefore mostly pointless to me, especially given a similar time investment for either methods anyway.
Half lap would give you about half the strength max. because you only have half the material going continuous through the joint. And if you bend it the wrong direction it will have 1/4 the strength.
@@billj5645 I disagree, the glue will replace lignin and the fibers are mostly intact for over 85% of the thicknress and are in the same "plane" we should have nearly identical strength and weaknesses as the original board. Fingers and box joints create localised stress points where a half lap has only a thin glue line interrupting the original woodgrain orientation. Figure a 2.00" x4.00" board (actual) with a 6.00" long x 2.00" wide joint.
i am not sure what the translation from german to english is, but would you like to test real "Schäfter"-joints. it's a long bevel cute which is calculated: cutlenght = thickness times 8 for example a 2x4 should have a wedge at the end with a lenghth of 16" that joint is used in theaters for 16m long 60mm x 30mm and i have never seen one broken
What about ripping the boards and gluing them back together with the joints staggered? I know it's a lot more glue, but I think it would be a lot easier for a novice than trying to make an accurate V notch.
