I always wondered how strong various finger jointed lumber is that you can buy, so took a few samples and broke them on my joint strength tester to find out.
Many years ago I worked in a lumber mill that made finger jointed 2x4 studs cut to standard lengths for 8ft and 9ft walls. I actually did quality control for the factory and the studs we made were only to be used vertically (structurally) They would fail tests in horizontal orientation where you put lateral stress on the glue joints. But under compression they were stronger than solid pieces. Mostly because we'd cut the knots and other weak points out of the material that we made the finger jointed studs out of. The finger joints and glue didn't make them stronger, the lack of knots and other inclusions did.
@@kronographer When fj studs came out, their selling point (up here in BC anyway) was that they were always straight with no wane. By the end they were worse than full studs. Also doesn't the glue melt in a fire, causing premature collapse? I think I heard that.
I've watched a number of videos on gluing endgrain and the important point that was made--and backed up here--was that end grain tends to soak up the glue into the fibres, which makes them much stronger, but tends to starve the joint, itself. I believe that you can see this in some of the samples, where there is barely any glue between the two pieces of wood. I suggest you first try adding a generous amount of thinned glue to the finger joints, leaving it for 5-10(?) minutes to soak in, then wipe away the excess and add the proper amount of glue, followed by clamping/etc. I suspect that you will find the joint to be *MUCH* stronger than just gluing the joint as you would with edge grain. Cheers!
What you say is true, but in my experience of restoration of ancient pieces and repairing old ones I noticed that if the glue joint is strong when is new the things change with time. To make it simple a table top made with recently glued boards nailed to the frame so it can not expand and contract with the seasonal change of humidity will develop a crack in the wood, the glue is stronger then the wood itself, but if you bring it to a dry environment after some years has passed is very likely that the glue joint and not the wood will fail. This is even more true when gluing endgrain to long grain or to other end grain as edge grain, end grain parallel to the growth rings and perpendicular to them have 3 different coefficents of expansion. Finger joints can be used to joint the heads of two pieces to make a longer one, but then that longer piece should never used in a place where loads are involved, even if precautions to don't starve the glue line are taken. Tests on freshly glued wood can be somehow useful, but can also very misleading if we build something that has to last, the more the time passes the more the glue cristallizes and becomes brittle and with time and seasonal movement micro cracks will begin, we can allow some movement fixing a table top to the frame to avoid problems, but we can not allow movement inside a glued joint between end and edge grain, it can seem to be strong if the glue has not starved, but it will surely fail with time, and with time I mean some years, not centuries.
@@andreachinaglia5804 "the more the time passes the more the glue cristallizes and becomes brittle and with time and seasonal movement micro cracks will begin [...] but it will surely fail with time, and with time I mean some years, not centuries." That's an excellent point, and probably why many Japanese (wooden) temples have withstood over a millennia of earthquakes/storms/etc.; they are built without glue or fasteners, just complicated, strong joints.
@@thedrunkenrebel "Sure, but nobody will do that industrially." True enough. "The point of joinery is to be easy to make and stronger than the wood, not complicated and strong" I have to disagree with you there. For commercial and newbie use, sure, but masters of the craft don't worry about 'complicated' when creating their works of art. Their focus is on doing the best job they can, not just fast and easy. Many Japanese (wooden) temples have withstood over a millennia of earthquakes/storms/etc.; they are built without glue or fasteners, just complicated, strong joints.
@@KeithOlson Because Japan actively restored these buildings for centuries. Many if not most of these buildings did not stand the test of time. The irony is that Japanese housing aren't built to last.
The joints you were testing with the Radiatta pine and rubber wood samples were 1/4” or 6mm joint lengths, these joints were designed to ‘stretch’ board length i.e. get longer pieces out of short scraps. They are designed to be strong enough but not structural by any means. Having said that I’ve handled and used pine molding up to 16’ length made from 4” and longer blocks using 1/4” joints that was plenty strong for the job. 18 years in the finger jointing business taught me that it is THE most exacting and difficult job in a molding or lam beam plant. Thanks for showing a very creative test machine and very interesting video.
@@fookingsog When he uploads - his videos are so few and far between now, but his back catalog is pretty impressive, especially for someone just dipping their toe into machining
Wow. This is one of the very few video's I watched from start to finish without my distraction-seeking mind wandering or even considering to look away. The ending came out of left field entirely. Amazing
For gluing end grain, I saw a vid by a long time cabinet maker who advised applying glue to end grain twice. He applied end grain glue to all end grain butt joint faces in a face frame first, then started at one corner of the face frame & applied glue to all joint surfaces hitting the end grain faces a 2nd time. Theory being it takes a few moments for glue to wick into end grain pores so 1st application won't do as well if that's all you have for the end grain face. Seems logical, but it needs to be tested to see if the theory matches practice.
It’s been tested in practice for hundreds of years, and nobody advises gluing end grain to anything. Maybe it helps, in addition to the long grain surfaces, but the general consensus is that it’s the long grain surfaces that count when you glue wood together. Was that guy perhaps talking about mitre joints? they’re surprisingly strong, and maybe that’s a technique that makes them even stronger.
It also could be compensating for the glue being used. Different glues and different woods have different rates of absorption, and glue which gets absorbed by the wood too quickly leaves the joint dryer and weaker. You can prove this by gluing two pieces of MDF together as it rapidly absorbs glue. On one joint apply and wipe the glue on both sides, leaving it to sit a bit until it's beginning to tack up. Apply another light coat then clamp. On the other test joint slobber it with glue and clamp. Once both have cured fully, the latter joint will separate cleanly at the glue joint with little provocation, while the first joint will be tearing apart the MDF itself from both sides and will take more force to break it. Softer woods will act more like the MDF with harder woods not so much, so harder woods will join better with one coat of glue.
Called gluesize, mostly useful with hot glue as it is really absorbed, not so much with white glue as shown in the video. Endgrain on endgrain never works anyway, except for compression.
Thanks for the video. Made round chair bases using veneer in a RF press for fast cure time for leather chairs. They where put together using finger joints. We did a test using the standard glue, and Lepage construction adhesive ( this would be 15 ago so different then you could buy today ) . When the test came back the standard glue we used failed at the joint. The Lepage glue tested had to stop machine limits where reached, ie the joint did not fail the machine was just destroying the part as a whole. For stupid reason the company did not use the Lepage glue, even tho Lepage wanted to give them a free system to dispense the glue from a 50 gallon drum.
I wonder if you would replicate the same type of finger joint as the manufactured ones and then glue them together with with tight bond three ultimate glue, which is a quality glue. I would bet that they would show better bonding results equal to solid wood.
Im 16 and have never done something with wood where i needed to join it using any method(never worked with wood). Yet, this is one of the most entertaining, watchable, informative, intriguing, fascinating and intresting videos on youtube, good work sir.
The mystery panel shown at 2:42 looks like rubberwood. Harvested from rubber plantations as trees age out of production. I pick up discarded tables just for the panels which make excellent project wood.
Good thing to keep in mind for dumpster diving woodworkers. About 70% of my projects have one of these finger joints somewhere in them just because the material I start with
We use fingerjointed construction lumber for rafters, floor joists etc. Same ratings as regular-non jointed timber. A bit strange to work with as usual wood has one bow direction but finger jointed can bend like a snake, so varying direction, but the bow is not as large as regular lumber though. Finger jointed can also be ordered in any length which is nice. We've ordered over 7 meter lengths.
I have been building boats, and we always said that a skarf had to be the thickness of the board times 12 - T x 12 - to be sure that it was strong enough. Both on the hulls and on masts.
Maybe it's because it's closer to end-grain-to-end-grain rather than long-grain-to-long-grain? Would be interesting to see if the strength changes with the angle of the fingers.
If you have a very long finger (small angle) you are basically gluing endgrain to endgrain. But you "waste" a longer piece of wood. Having more smaller steep fingers is probably harder to manufacture, so I guess the given pattern is the best compromise
@@greenjom Seems the opposite to me. with longer fingers there is more edgegrain surface to glue, where as with shorter ones it is more end grain. Up to the point where you use squared off box joints and the amount of end grain stops changing and the finger length just affects the edgegrain surfaces.
@@johngaltline9933 You are correct here. If we remember that wood's strength is in it's overlapping and intertwined lignins and fibers, then it's clear that having a longer glue joint parallel to them will bind them together better. This will also give a more gradual failure mode which is always desirable. The finger-joints we buy are mainly designed to maximize production and minimize waste of both wood and glue.
This is good to know. It was several decades ago when I learned, but IIRC in normal jointery I think the ratio called for a minimum or 6:1, preferably with a locking feature involved.
Sorry, but that doesn't make sense. The joint doesn't know where it ends up. So if it's just as strong, then location would not matter. It's probably considered at least strong enough for the intended purpose.
I had similar instructions in a wooden glider repair course. 8:1 generally, 16:1 on spars. We scarfed 1/64 plywood RC glider skins by chamfering their thickness over 1/4" (8:1) and never had a joint failure.
@@dutchhankI don't know the details, but perhaps it has to do with the traditional/normal shapes of those pieces. A different width/thickness profile might need a different scarf ratio.
My stair hand rail broke at the finger joint glue line. I re-glued it and it's noticeably weaker at the joint. I need to add a wall support at the joint before it breaks again. This was good information Matthias, to give relative strength % values to what we intuitively knew. Also great comparison info on hardwood vs softwood.
You can check out the Canadian SPS 1, Fingerjointed Structural Lumber - interchangeable with standard lumber. There is also SPS 3 (Vertical Stud use only) and SPS 4 - Machine Graded
Is it just one piece of wood glued end to end with another piece, it is more interlocking with multiple boards of different lengths glued together, where the finger joints are not in the same place. That's how I've seen it used in construction lumber and the seller claimed it has higher strength than regular solid wood. Which I believed, because different grain structure would mean it's less prone to cracking from humidity changes.
It's apparent that the finger joints for this type of use is only for utilizing short scraps from other manufacturers. When being sold to other businesses to make longer boards. Its very apparent that these joints have no structural bearing other than the structure needed to make a flat board.
I’ve never seen them as single sticks of wood, I only know them as laminated boards, table tops and stuff like that. Those are more than strong enough - although I really don’t like the scrap wood look.
@@brothyr It would be interesting to test these. I would also like to see a structural test of a regular wood beam vs. a Glu-lam beam (which is like a stack of finger jointed 2x4s or 2x6s stacked and glued together).
They sell finger-jointed hollow posts. They ostensibly do support a load ... for a while. If there's any moisture, they start peeling open and splaying out like an upside-down banana. Very economical.
I have a bookcase my parents got at some finish it yourself furniture store c1960. The middle shelf is adjustable and rests on four metal pins that fit into holes in the sides. The shelf is made of boards joined lengthwise right in the middle of the shelf! I never realized that until I removed an entire set of old World Book Encyclopedias off the shelf. Those things are heavy, but the shelf is still straight as a board! No sign of failure.
Love the test setup !! Timber joints were tested extensively back in the 40's and 50's when timber was used extensively in aircraft. Your roughly 50% value reproduces those results for simple joints with good shear area. The only way to get much higher values is by using the more complex stepped lap joints that manage the bond load and peel stresses at each step to get cohesive failure i.e half the adhesive is evident on each side of the laps after failure. In the 70' programs became available to analyse the joints in detail and versions of these are still used today for bonded joints in metal and composites.
It would be interesting to see a comparison of a section where the finger joints are in line with a solid piece Vs the two solid pieces glued together. As in the boards the finger joints are usually staggered.
I was thinking the same thing; most of the places I see finger jointed wood are in trim where strength isn't really a big factor compared to stability I suppose - and then in wider boards where the joins are staggered in strips so each join has solid wood on either side.
Marvelous video! No surprise you have such strong fan base :) Nothing excess or redundant talks - just business plus fascinating experiment - pure joy!
love seeing you test joints \ glues ect,, i've read so many claims from books and magazines that suggest one is better then the other, and even though it published,,, i believe what i can see over a written claim trying too sell a publication or manufactures brand , thank you for these videos, seeing is believing, and i was always weary of these joints,even on trim work ,i seen them fail while handling during a install .
You know, these days it's so easy to throw an opinion out there, and misinform people. It's nice to have superheroes like you actually testing this stuff. Thank you!
Glue is stonger then lignin, but glue is not stronger that the fibers. It would be interesting to see how strong end grain to grain is for the same wood. My Ikea replaces the broken slats without asking for a receipt, but I rather glue them and then add some long bamboo 6mm dowels from the side.
Something I've seen quite a bit of IRL is that the finger jointed material uses an adhesive that is not outdoor rated and so old pieces will fall apart very easily right at the joints.
My cousin recently bought a post hole digger and one of the handles had a finger joint about 8 in up from the bottom. You can imagine how long that lasted.
So really, the answer is that the finger joints are more than strong enough for application they are used for, especially when glued in a staggered pattern like on the large panel. Surprising that the partially seated joint was as strong as the fully seated ones. That doesn't make much sense to me.
There’s no structural wood sold around here with finger joints. All you find are 2x2s and 1x2s meant for strapping, etc. I like the idea of using up scraps in this way, like with LVL, OSB, and MDF, shelving/counters, etc.
You might want to look into 4 point bending tests, it gives a uniform bending force over a larger distance, negating to some extent localised stress effects. Also as you surely know, the dimensions of the fingers, quality of glue up etc. signifiantly affect the strength of the finger joint. Design Codes for structural glulam beam staves cover this sort of thing.
MicroLam or GlueLam beams use resorcinol glue which is far stronger, but it neither takes paint well and it's hardness makes for accelerated wear on shapers and cutters.
I suspect those joints suck up a lot of glue, and manufacturers don't want that kind of squeeze out. Very interesting the results you got from a V. I further suspect you're gluing technique is superior to the manufacturer, but maybe letting the glue soak in a bit longer would increase the strength of the joint. I'm looking forward to further testing with the finger joint router bit! And if you're taking suggestions, I'd be curious to see the difference in results from that V vs scarf joint with a similar glue area.
Glad to read this as I was about to bring up scarf joints Saw a video from Thomas Johnson fine furniture to lengthen a side rail using a scarf joint. as some may know side rails can be under stress at times
The strength of wood comes not from local hardness but instead from long contiguous stands of the grain that can spread the stress out, so it makes sense that no matter what joinery you use it caps or at around the same strength. Said another way, the joint will never be able to bend as far as unbroken wood and still be able to return to its neutral state. The harder woods get more out of joinery because they do less bending to begin with.
My wood tech professor in university invented that joint. I observed the testing. You are see a production quality issue. The joints, prepped and glued and cured with the designed equipment do not fail that way. Prof Talbot also invented Masonite, particleboard and truss joists.
Interesting, then where did the name come from if not from William H. Mason, as the Wikipedia claims? Also when did you have classes with that professor, since all of the technology you mention is now very old. Wikipedia says the first masonite-like product was already made in 1898.
When you think about it, since the fingers are cut at an angle you are getting a glue up of partial end grain. Of course end grain glue ups are extremely weak. So a weaker joint from this sort of joining process is to be expected.
Surprising but interesting results. I was under the impression that modern glue was better than this. I wonder how different types of joints or glue would impact the results… Thank you for doing the test🙏🏻
And this is why some highly skilled person is making big bucks some where, because they know what type of wood and what type of joints to use in any different situation.
Recently had a finger joint break. I thought it was just poorly glued, so I prepared, glued, and clamped it. The joint still split with surprisingly little effort.
@@matthiaswandel I've watched a number of videos on gluing endgrain and the important point that was made--and backed up here--was that end grain tends to soak up the glue into the fibres, which makes them much stronger, but tends to starve the joint, itself. I believe that you can see this in some of the samples, where there is barely any glue between the two pieces of wood. I suggest you first try adding a generous amount of thinned glue to the finger joints, leaving it for 5-10(?) minutes to soak in, then wipe away the excess and add the proper amount of regular glue, followed by clamping/etc. I suspect that you will find the joint to be MUCH stronger than just gluing the joint as you would with edge grain. Cheers!
I would be interested in a comparison of the angle joints VS. a box type joint. I believe the angle joint puts the glue more in tension and the box joint would put the glue more in shear resulting in a stronger joint closer to the strength of the wood.
I agree. I suspect straight cut finger joints would be stronger, as the glue surface would be a higher proportion of long grain to long grain, as opposed to a semi endgrain to semi ingrain joint with the V finger joints.
Nordic carpentry rule of thumb when joining softer woods is a 7:1 ratio on the finger length the reach near solid wood strength, assuming that you use proper glue.
I've seen finger-jointed studs used in home construction and often wondered if they are as strong as regular studs. I guess that would be compression strength instead.
I wonder how finger joints that use rectangular fingers instead of triangular fingers compare. The triangular fingers don't really have true long grain to long grain glue surfaces. Maybe a narrow kerf blade on your boxjoint jig would put up a better fight. Thanks for doing the hard work of empirical evidence gathering.
The code actually has two different ratings for finger jointing. They look the same but are very different. The finger joints used in glulam are rated to meet the minimum strength of the wood(including tension load). You can also special order dimensional lumber with the stronger finger joints. Other finger joints are only rated for compression gravity load or short term bending/tension loads. The strength in the code is based on the 5th percentile capacity. The 5th percentile strength of the joint has to be more than the 5th percentile strength of the unjointed wood. So even though the mean strength is less for the joint, you can still assume a full strength for the joint. I would also be curious what would happen to the strength of the v-notch if you also clamped perpendicular to the joint during glue curing. I think your glue joint would be stronger.
A practical application for Mattias's experiment. I have several paint grade finger jointed interior doors in my house (french style with 15 glass panels which makes them quite heavy) so far i've had to repair 3 of them because the door handles fell almost prefectly on the finger joint glue line. in evey case the failure was in the glue. In fairness more than 50% of the joint was removed for the mounting holes and it took almost 20 years to fail. I would by the same door again but would only accept ones were the door handle set falls on solid wood. P.S. there are several Manufacturing finger joints videos here on youtube.
Wood be interesting to see test cohorts of your in-shop joints, ranging in surface area from butt joints to finger joints. Or even more interesting, an emphasis on finger-length vs total glue surface area as factors
This is making me dig out my old fracture mechanics readings but a lot of the models aren’t making it easy to predict a 50% stress loss or a stress intensity factor of ~2…i haven’t had coffee yet though
Theoretically, wouldn't the strongest joint be a single dowel joint where the diameter of the dowel and hole are about half the side length of the wood square?
I would assume the strength of those joints very much depends on the manufacturing quality of the board. How well the fingers fit together, the quality of the glue that was used and so on
Not always. If you read the instructions and recommendations of the manufacturer, you'll see where they recommend that with dense woods you dampen the glue joint first, which helps the wood draw the poly glue in deeper for a stronger joint. I've seen several failures of poly adhesives where the glue didn't penetrate well enough. With your usual wood glues this is not a problem, and if you use them properly you can get a joint about equal to the strength of polyurethane where the wood fails with or before the joint does. Where poly excels is bridging gaps and regular wood glues do poorly with that.
@@P_RO_ for a moment there I thought you were comparing two things that started with the prefix poly- and distinguishing one from the other by shortening it to poly
Very interesting. I suspect that part of the reason is that finger joints are closer to end-grain than face grain. But I wonder if joint strength is really part of the rationale for commercial finger-jointed wood. Most of the material I have seen has been intended for decorative rather than structural applications (molding, etc) so strength isn't as critical. I really like the rigor that Mathias puts into these testing videos.
Always love your videos Matthias, hopefully caught you early enough to get your eyes on a Q: I've always heard that the glue is stronger than the wood itself, and that seems opposed to your results. What am I missing?
It depends - these are mostly glued end grain to end grain - probably the worst case scenario. If you glue side grain to side grain, then I think it's stronger, but go back and watch the other strength tester videos to be sure - he's done a lot of them now, and they each show you something interesting (just that I don't remember them all)
PVA (and really any glue) is stronger than the lignum in wood. Lignum is what holds the wood grain fibers together. Think of a handful of straws being the wood grain fibers that are held together with a rubber band. Lignum is the rubber band. Glue is never stronger than the wood grain fibers. A typical panel glue up is gluing the long boards together along the grain. So the glue is replacing the lignum that was cut when ripping the boards down to size but the wood grain fibers run end to end. In that joint, the glue is stronger than what the wood would naturally be without the joint. But that is not the case for the finger joint show in this video. Think about a scarf joint, the finger joint is like a bunch of really short scarf joint. The longer a scarf joint is the closer it becomes to being like long grain joint (like a panel glue up). So these finger joints are on a continuum between a long scarf joint and a butt joint. They will be strong than a butt joint but will never be as strong as a continuous piece of wood.
@@devinfisher5171 I think that glue can be stronger than wood, however it lacks one quality of wood: It is solid, while wood fibers can and do actually move and stretch under load. Ideally would be a glue which exactly matches that quality of each type of wood it's being used on.
I'd love to see how your homemade finger joints will fare. I've always wondered if they use cheaper, less effective glue than what a regular carpenter would chose.
glue them up and then test them again in 35 to 45 years. i've noticed after a good long while the glue to turns to powder but, the wood can often still be fine.
It seems like in some cases the glue was the weak point. I used to work at a furniture store that built furniture and we had a wood glue that was almost like super glue with sawdust. We used to make a batch everyday, scoop it out like flour. It was definitely stronger than the wood.
It would seem that since a good portion of the failures were glue failures, that looking into a stronger glue MIGHT give the joint a bit more strength.
Drawing stuff in ascii is my jam. Linux devs have been doing it for decades. Always makes me happy when I see something as simple as a little time taken to make plain old ascii have a bit of fun and flair.
Enlightening. I expected finger-joints to be as good as no-joint. I was wrong. Reasoning: grains are about 3x strong as glue, but Finger-joints have roughly 3 times the glue, so it cancels out
Doesn't really solve the weaker than the wood problem - the staggering largely just distributes the weakpoints more evenly. Though I would expect being surrounded by continuous wood grain that bridge the finger joint will help a little more than just the strength added by those fibres - distributing the load away from the weak point, and shifting the force directions and so how you are stressing some of the glue bonds may help an outsized amount.
I think the pine panels are made from the off cuts from the sawmill so they are probably not the best quality wood to begin with. The oak bench top looked pretty high grade though and made to a higher standard generally so that should have been better. I guess those joints are more for alignment than anything else though since you usually see them in places where they wont really be stressed from normal use. Would have been interesting if you hade cut the pine panel in the other direction though and tested wether the finger joint was stronger than the long grain butt joints.
I work in the engineered wood industry. I will tell you this. Of course a single jointed board is weaker. However, when lawyered (laminated) gluelam beams are significantly stronger than a piece of lumber if the same dimensions. Just look at some of the buildings that are constructed with mass timber.
Really interesting indeed, Matthias! Awesome testing! 😃 But yeah, it's better than an endgrain glue up I guess. Anyway, stay safe there with your family! 🖖😊
the horizontal finger joint should be less strong than the vertical just by how the glued surface is being stressed here. When put vertical, the forced act in a much more advantageous way to a glued connection than they do in the horizontal alignement
If you let the wood weather you don't need a stress tester, the wood self-destructs. Don't use this stuff externally, even painted. Finger-jointing is nominally used to producer straighter sticks, which is why it has been used in joinery, not for strength but straightness. Use internally only. Note also that it requires greater paint preparation.
