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Interesting video experiment, not sure of any Lutherie applications......
https://www.youtube.com/watch?v=m7HxBa9WVis
Comments?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Colin, thanks for sharing this one. A nicely done experiment and very worthwhile to watch. I'd be real interested to see a comparison of the different glues that we typically use. I had always just accepted that we "just don't glue end grain". I can take advantage of this new information.

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Steve Smith
"Music is what feelings sound like"

What we saw was actually the force required to break the wood. The end grain joint was the only one to fail at the glue line.

The grain direction in the endgrain joint was aligned on its strongest axis as well, so it's no surprise that the glue joint broke before the wood did, even if it took more force to do so than the sidegrain joints.

IMO he proved exactly the opposite of what he thinks he did...

I agree with Ed, In his test an endgrain glued board broke at less than half the force as an unglued board. It failed before the wood!!! The test he left out would be to glue two boards the double length but half as wide together with a side glue seam. The combined board would be as strong as an unglued board. He just showed glue is better than nature at gluing fibers together. In fact that board would be stronger than making the same size board with using endgrain as long grain is harder to break.

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I don’t think it is fair to say that the video proved that endgrain joints are stronger than side grain joints (something I feel like he implied In the way he presented his findings), but I also don’t think it is fair to say he proved the opposite either.

He proved that end grain joints will fail at the glue line. In that sense, endgrain glue is weaker than the wood itself. Sidegrain glue joints will fail at the wood. The glue joint is stronger than the wood.

But. . . The glue took more force, in these particular tests, to fail in the endgrain that the sidegrain wood did.

So, wood is stronger along the grain than across it. I think we all knew that intuitively.

Sidegrain glue joints are stronger that the wood around them. We have all been saying this when we talk about glue strength.

Endgrain glue joints take more force to break, than sidegrain wood. This is somewhat new information. Though it does stand to reason that the glue joint still is stronger than sidegrain wood but weaker than the much stronger long grain wood.

We all believe that end grain glue joints are bad. Apparently they are stronger than we thought. I’m not sure what that really means for us though. No one is going to glue two boards together end to end instead of using 1 long board. But it will make me worry less about the cutaway section glued to my heel blocks.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

You can probably beat that strength by quite a lot with hide glue. Warm the wood and coat the endgrain with glue so it soaks in and plugs the pores. After it dries, lightly sand the surface back perfectly flat and then do the actual glue-up.

Endgrain joints are usually glue-starved because it continues to soak into the pores after clamping and squeezing out. HHG is the only glue I know of that will fully bond to its own dry residue, enabling the pore-plugging procedure.

I should do some failure tests to get a better idea of exactly how strong it is. A 30 degree scarf will be a lot stronger than 90 degree butt joint, and only needs a little more wood length. I tried it on some giant sequoia soundboard offcuts years ago and the first glue-up without pore plugging broke at the seam. Then I sanded it smooth and reglued, and it broke elsewhere. But giant sequoia is even softer than western redcedar. I need to repeat the test on spruce, and if the glue still wins then try walnut.

Very interesting video. The point he proved was that the glue joint was strong than the wood fibers on side grain, which we all can see. But to prove that end grained glued joints were stronger than side grained joints, he would need a different test setup that isolated the force just to the glued joint. He never said that.

Scarph joints that are used by boatbuilders to lengthen boards are essentially end grain glue ups. The scarph is used to increase the gluing area over what a simple butt joint would have.
The scarph joint we sometimes use for peghead to neck joints is end grain to long grain. Again the flatter the angle the longer the glue surface. Lutes use a very steeply angled peghead. Their end grain/long grain joints don't have a lot of surface area, but they still seem to hold together.

The FAA specifies long scarf joints when joining plywood sheets for aircraft use; iirc about 12-times the thickness of the sheet. That's getting pretty close to a side-grain joint...

It is an age old existential question. At what point does an angled cut cease to be endgrain and become side grain. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Actually, if it was solid wood I think it would be a very long end grain joint......

Wouldn’t a 12 to 1 scarf joint be less than 5 degrees? That sounds like a long grain joint to me.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Hi Bryan,
Although it is an imperfect model, I visualize wood as an assemblage of straws glued together with lignin. If you glue the ends of the straws together that would be an end grain joint. If you cut the ends of the straws at an angle you would still be gluing the straws in an end grain fashion. Angling the ends of the joint provides more gluing surface, but doesn't change the way the individual "straws" meet at the ends.
I could be seeing this wrong.

I see what you are saying Clay, and mostly agree with the model. But, it isn't exactly straws. Straws are mostly empty space with very little wall thickness. If you start cutting angles away from perpendicular to the straws, you barely increase the straw surface exposed. you are still looking at mostly empty space. That just doesn't make sense with wood. It that model held true, any joint that isn't precisely along the grain line of a perfectly straight board cut alone the line of the split face, would still be an end grain joint. Even if it were perfectly cut, the straw analogy would have you gluing a thin wall of straw to the hollow space of another cut straw unless you were lucky enough for all the edges to align.

That is just not how we think of wood joints. If I have two boards and the grain runs a few degrees out of the edge of the boards, I'm going to visualize that as a side grain joint. If cut 93 degrees across the grain and glue those surfaces I'm going to say that is an end grain joint. If I glued two 45 degree cuts together, well, that still feels like an endgrain joint to me. A 15 degree scarf at the peghead? Well, that kind of feels like the one side is endgrainish but I don't really worry about the joint the same way I used to with endgrain gluing.

It is just an interesting thought experiment for me. Where is the line?

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I am trying this experiment. While I have glued end grain in the past it is indeed a rare occurrence in high end furniture and in guitar building all end grain is usually glued while supported on a block. On a cutaway the end grain is glued to the side so we do use the end and side grain. It makes for an interesting chance to change a paradigm

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hi Bryan,
I think we are talking about two different joints. The scarph joint I (and Alan) spoke of is where two boards are joined end for end to extend the length of the board (his in plywood, mine in solid wood), and not the peghead scarph used to join the peghead to neck shaft at an angle. The one is end grain to end grain and the other is end grain to long grain. They are different joints but in both cases I think the reason for the scarph is to increase the gluing surface to increase the strength of the joint.
As wood workers we sometimes can't avoid gluing end grain. Keeping it in low stress situations when possible may be the best strategy.

I have a question about the video. I might be interpreting it wrong.

In the video he shows the end grain glue joint taken to failure and says the end grain glue joint failed at a much higher load compared to the side grain joint. That’s not right though because the glue joint on the side grain example didn’t fail, the wood did. So we don’t see what it would take to make a side grain failure at the glue joint itself because the wood gave out first. Right?


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Correctomundo.
The way I look on it is the glue joint itself is a similar strength in all cases, it's the wood 's "breaking point" which is different.
The wood is stronger along the grain than across it, glue bond breaking point is similar (for a well fitting joint) - ?

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Thanks. And now that I read the whole thread I would have had my answer already.

I recently joined a few Facebook groups about making cutting boards. I thought luthiers were the only ones with an obsession over glue joints, but I was way wrong.


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Hi Clay,
Yeah, I think I had rambled a vita don’t presented a focused point. I’m not talking about any specific joint. I’m talking about grain orientation. I guess what I’m saying is that cutting at an angle to the endgrain (at some point becomes more like aside grain than endgrain orientation. In Alan’s 1:12 scarf example, the scarf is there to greatly increase the surface area but it also is such a shallow angle that it doesn’t feel to me like endgrain anymore. Of course, in this example, we have to ignore that it is plywood since the end drain would only be in some of the layers.. .

Anyway, it is just interesting to think about. It isn’t really going to change much in the way I work in the shop. It reminded me of an episode of the Woodwright’s shop were Roy joked with Christopher Schwartz about whether he should use a rip saw or a cross cut on a miter and when to change.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

For end-grain-to-end grain and end grain-to-long grain joints such as on cutaways, it seems like the most common issue is glue starvation of the joint due to the increase in common water-based glue absorption in end-grain. On cutaways, we always applied a full coat to the heated neck block's end grain first, then a full coat to the long grain, and finally, a second full coat to the end grain just before closure.

On scarf joints, the Forest Products Laboratory (older work focused on glues and finger-joints is Tech Bulletin 1512) has published general numbers on retension of tensile strength for scarf joint, with 1:12 or higher retaining about 90%, 1:10 at 85%, 1:8 at about 80%, and 1:6 about 65%. A butted joint retains about 10% of tensile strength, with failure in the adhesive. A 10 degree scarf joint would be a 1:4.25 and a 15 deg about 1:2.8, and the FPL data I reviewed did not cover those steeper angles.

The FPL scarf data looks as though it assumes both pieces have the same grain angle and joined by the angled-grain face, so a face-grain to sort-of-end-grain joint like a scarfed peghead joint would be expected to exceed those percentages for tensile strength. Given that this sort of scarf joint seems to hold up pretty well in service, it appears to me that even at steeper angles, strength is sufficient for our purposes if the joint is well made and well joined.

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