Testing Joints?

[Dan Kirkland]

So I have an interesting query. I want to test the strength of a glued joint Mainly a glued scarf joint. I want to test it against a square end grain to end grain joint using the same glue as the scarf joint. I want to test both for strength and rigidity.

I realize this is an odd request, but I'm just doing research, any ideas? I had considered simply gluing the joints and setting up some clamps to steadily add pressure and see which one breaks sooner.

[Jerry Wright]

Fine Woodworking did joint tests a few years ago in an article. Check their archives.
Jerry

[pat warner]

This end to end connection: Nothing but a square butt joint, no tee's no gees, no laps or dovetails, nuthin'?
Then no reason to test.

[Lee Schierer]

Here's a link to a guy that built his own wood joint testing rig. Joint tester (https://woodgears.ca/joint_strength/pockethole.html)

[Andrew Hughes]

I quit testing joints long time ago.From what I remember we just licked the side of the paper and twisted the ends.No glue was needed.😋
Sorry I couldn't resist

Aj

[Charles Wiggins]

This end to end connection: Nothing but a square butt joint, no tee's no gees, no laps or dovetails, nuthin'?
Then no reason to test.

Agreed. End grain to end grain is the weakest glue joint you have have, whereas a scarf joint done properly should be stronger then the wood itself.

[Dan Kirkland]

Agreed. End grain to end grain is the weakest glue joint you have have, whereas a scarf joint done properly should be stronger then the wood itself.

That is absolutely true. What I'm doing is gathering information for a luthiery research project I'm doing. I do instrument repair for a living and I consistently see the same repairs repeated over and over again. And one of the most common I see is that people constantly try to graft in a new piece of wood for a guitar top (if it was damaged or missing a part of it's top) with no thought as to long grain orientation for the glue to adhere to. So inevitably they fail and they show up at my shop, and my job is made much more challenging by the previous guy's attempt.

My goal with this particular test is to give repair guys who don't understand the woodworking side of things some real ammunition so that these instruments can survive. It might not work out great but if I can do something to avoid having a nice instrument get severely damaged due a good intentioned but ill educated individual, then I'm ok with that.

Here's an example of this kind of repair, I think even if you don't know anything about guitars you can immediately identify the issue. This is also not a cheap instrument in the photo, worth well over 50k that has had this applied.

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So what I'd like to do, is show guys who don't know any better, that this kind of joint simply does not work for a guitar top, the strings on a 6 string acoustic guitar pull with about 130ish (maybe more) pounds of force. This particular repair is not one that is going to last since the end grain to end grain was used, it won't survive. I figure some video/written evidence of the difference in strength from an end grain to end grain joint compared to a good solid scarf joint might be enough to sway them in the right direction.

[Dan Kirkland]

I quit testing joints long time ago.From what I remember we just licked the side of the paper and twisted the ends.No glue was needed.
Sorry I couldn't resist

Aj

Haha! clever sir

[Jim Dwight]

I really like that Woodworking for Engineers website. The guy does interesting things and I think his results are more credible than many other sources. Wood, for instance, tested wood joints and made it appear that domino joints are inferior to pocket screws. The size of their workpieces and their decision to use a single domino dictated their result. I do not have a domino but I think it's pretty obvious that a well cut loose mortise and tenon should not be much stronger or weaker than a traditional mortise and tenon. They also got results which looked too high to me for butt joints. I blame that on the angle of their parts in the test rig. It wasn't a useless article, they demonstrated a tenon half the thickness of the wood was stronger than one 1/3 the thickness - at least for their test rig - but it wasn't as good as the linked piece IMHO. The Woodworking for Engineers also has some good information on dust collection. His conclusion is that it is not fine dust is not as great a health risk for most people as Bill Pence would argue. He has a doctor supporting his views and test results supporting his theories. Interesting stuff.

[Mel Fulks]

That's interesting,Dan. Here at SMC details are always of interest. I had a friend who did violin making and repair and I used to help him from time to time. He acquired a violin that he said the f holes had been recut too big. I patched them in so he could recut them. Patches terminated in two points at top and two at bottom. And ,of course, the rest of the patch was diagonal following the general direction of existing. After the recut he said it did sound better.

[mark mcfarlane]

I quit testing joints long time ago.From what I remember we just licked the side of the paper and twisted the ends.No glue was needed.
Sorry I couldn't resist

Aj

Those joints only had to last a few minutes, and could be easily repaired, or so I read.

[Dan Kirkland]

Mel, that's cool with the violin repair, I've done a few small things on those but I've not done that before, sounds fun.

So back to my original question, what do you guys think would be the best test to show the inferiority of the end grain to end grain joint vs a scarf?

[Mel Fulks]

Since test results are available, I'm guessing you want to demonstrate to a group. If that is right it might be more dramatic to add weight rather than break with a clamp. That's all I got.

[John Blazy]

When I started using PL Premium polyurethane construction adhesive for ALL my case joints in cabinetry (butt joint without any metal fasteners, no dado, nothing, just PL) whether plywood or melamine, I knew there would be skepticism, so I made test joints in all the popular configurations - dowel, biscuits, drywall screws, PVA glued Dados and tested them all against the PL butt joint (all were perpendicular butt joint samples - a 4" x 4" pc fastened to 4" x 4" pc) by placing the samples in an arbor press with a bathroom scale on the bed, and squeesed the sample recording the lbs of force at the threshold break point. BTW - the PL butt joint blew away all the other samples, holding for almost double the pressure. Since then I have made numerous custom kitchens and entertainment center cases using only PL with finish nails to hold til glue sets.

Point here is that maybe you can set up a press with the scarf joint bridging over two ledges, on a bathroom scale using the arbor press to add pressure til breakpoint and record the lbs of force.

As an exception to the rule that end grain to end grain joints are bad (which they are - not contesting that) I had to glue the coaming around my boat seat in solid mahogany as one pc in order to mill it, so the trick is to pre-heat the end grain with a heat gun, add the first coat of epoxy, and let the grain drink in the epoxy for about ten minutes til it thickens up from the heat, and stops soaking in, and then clamp. When the epoxy cured, the coaming (half cylinder) was strong enough to clamp in my tail vise for milling. All the end grain joints in my boat are filled so well with this method, that after machining and sanding, the finish coats never soaked into the gluelines - perfectly filled.
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[Dan Kirkland]

John, thanks for the post and information. I like the idea that you posted about using the scale and the arbor press, I'll see what I can come up. Thanks for the tips and ideas gentlemen