Plywood Scarph Joint Ratio

[Landlubber]

"I used to look for old tools in antique stores but unfortunately they have become collector’s items."

....so true, there is no new tools like the better quality older ones, but as you say, the bloody collectors make even the simplest tools expensive for those that wish to replace their tools...it is a pain we have to bear I guess if we want quality....

....still, it is cheaper and better to buy used tools than the new ones anyhow.

[ancient kayaker]

[quote=Landlubber;357564... still, it is cheaper and better to buy used tools than the new ones anyhow.[/QUOTE]

-very true, thanks to the Chinese! I am just glad my job wasn't making tools. I had a workshop fire three years ago and lost some treasured old friends. I had to replace almost every tool. I was amazed how little it cost: even the insurance adjuster was surprised! The price I paid for a right angle electric drill, variable speed and reversible was about the same as I paid for a basic two-speed drill in 1973.

However, when it comes to quality hand tools of the unpowered persuasion, , the Chinese don't (yet) deliver the quality needed. I got a lovely set of Japanese chisels at the same time to replace some ancient ones. Not cheap, but they take, and keep and incredible edge. I miss those beautifully shaped wood handles, the sense of history (some were well over 100 years old) and the patina, though.

There are some nice tools out there to be had if you have enough money. I salivate over the Veritas hand tools every time I visit my nearest Lee Valley store, but I would need a mortgage to fully equip myself with those!

[ancient kayaker]

This is from my post #13 way back in 2008:

[quote=ancient kayaker;216217]... So far I have used butt blocks for ply edge-to-edge joints in my boats but a few months ago I butt joined several test pieces of 3 mm and 4 mm ply using epoxy, no butt block. I found the joint strength was between 55% and 80% of the wood strength. ... QUOTE]

That was BS1088 so for the 3mm ply at least, the veneers would have been all the same thickness. I did some math on these results, based on strength being proportional to the square of the thickness. I assume that only the face veneers, which had grain parallel to the joint, were glued successfully, the end-grain joint of the core veneer had zero strength, and the wood strength across the grain is about 1/6th of its strength with the grain. I don't have data for Okoume so I averaged data for several other species.

The joint strength is equal to the outer veneers’ strength or (3^2 - 1^2)/6 = 1.33 (arbitrary units, with-grain strength set to 1)
The uncut ply strength equals (3^2 - 1^2)/6 + 1 = 2.33

This would predict a joint strength about 1.33/2.33 = 57%.

That supports the low-end results I got by experiment. The high-end results indicate the glue adhesion to the core veneer was partially successful, about 50% of the wood strength with the grain.

Now if I do the same math for the case where the face veneer grain is across the joint, assuming the glue performance is 50% on the face veneers since I was able to add glue when it was absorbed by the end grain, I get joint strength equal to (3^2 - 1^2) x 0.5 + 1/6 = 4.17 compared with uncut ply (3^2-1^2) + 1/6 = 8.1666: this leads to 51% ply strength. Again, this is similar to what I got by experiment.

For the 4 mm ply with 1 mm face veneers and 2 mm core, the results should be:
face veneer grain parallel to joint: joint strength/ply strength =
worst case: {(4^2 - 2^2)/6} / {(4^2 - 2^2)/6 + 2^2} = 33% (I did not reproduce this in practice)
best case: {(4^2 - 2^2)/6 + 2^2 x 0.5} / {(4^2 - 2^2)/6 + 2^2} = 67%
face veneer grain normal to joint: joint strength/ply strength =
{(4^2 - 2^2) x 0.5} / {(4^2 - 2^2) + 2^2/6} = 47%

Once more, reasonably close to experimental results.

Now I have a theoretical basis as well as practical results, I can predict that the joint will be as strong as the wood for scarf angles finer than sqrt (1/0.33) = 1.732. This would be difficult to clamp of course, and absolutely intolerant of scarf cutting errors, so it is impractical.

If a finer angle is used, any slight curve along the length of the cut gets divided by the slope. For example, if my scarf cut has a 1 mm curve error along its length between the 2 joined pieces, that would likely result in a weak spot where the glue, even if it filled the gap, would be brittle. In contrast, in a 1:8 scarf with the same error the clamping would close the joint leaving a 0.125 mm mismatch in the thickness of the face veneer at the joint, which could be sanded off without too much impact on joint strength.

Nonetheless, for a joint that can easily be perfectly cut, such as a thin batten, the angle can be coarser than current best practice indicates. I have a jig that enables me cut accurately and a clamping jig which allows me to use very coarse angles if I wish to, as it prevents the pieces sliding apart. Although I routinely use 8:1 I have on occasion used 4:1 to save on material and the joints were fine, but I took care to ensure the end grain is saturated with glue before clamping.

As Par and others noted much earlier in the thread, finer angles are advisable if the plank is to be bent sharply, and particularly if it is to be twisted. I have not done experiments twisting scarfed joints.

[Boston]

I really need to draw up that old router guide we used to have
all you do is shove a board in the end clamp it down and plow it out
thing weighed a ton but you could do perfect scarfs as fast as you could run a router

cheers
B