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PostPosted: Thu Feb 07, 2019 3:55 pm 
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Walnut
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Joined: Fri Dec 28, 2018 2:03 pm
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I created a post here a few weeks back about a laminated neck I wanted to make out of sapele and white oak. I ended up ruining all that lovely wood. I made mistakes cutting the scarf joints, but even more problematic was that the laminations did not seem to really hold together well. I picked up some of the scraps after I had wrecked all the wood screwing up the scarf joints, and I could snap the wood at the glue joints with my bare hands. I mean, I really had to grunt and strain to do it, but they did snap along the glue line.

When I glued up the three pieces of wood, I made a [ Sapele | White Oak | Sapele ] sandwich, and I used Tight Bond 2. I had 3/4" thick white oak boards that I used as clamping cauls to distribute the clamping pressure, and I had 4 bar clamps underneath the 5 boards (2x white oak cauls, plus the S|WO|S sandwich), and I had 6 super heavy duty C-clamps, plus 1 nice F-clamp clamping from the other direction. I even broke out my goofy Kregg jig's vice-style clamp that you screw down and then squeeze hard until it locks clamped.

So with 12 clamps and hardwood cauls, I cannot imagine that my problem was that I didn't have enough clamps. I also absolutely slathered the wood on both sides with glue. I had a plastic sheet down to catch all the drips, so I just went nuts with the glue. I used a paint stirrer to spread it all out evenly and over the edges. There was not a single micron of wood that did not have glue liberally added. When I clamped it all up though... I think I may have over-tightened the clamps. I started with the whole assembly resting on the pipes of the pipe clamps, and slowly tightened the pipe clamps just until each one was touching the wood; no pressure. Then I added the c-clamps from above, aiming for the middle of the boards, and still just screwed them down until they were just holding, and not falling over. Then I went back to the pipe clamps, and gave each one a half turn, starting from the inner-two, and then the outer two. Then I did the same thing with the C-clamps, and just kept doing that until, well... until I couldn't turn the handle anymore. I didn't 100% Hulk-out and really break out the crowbar, but I did turn those clamps until the wood sandwich have compressed the oak cauls so that when I took them off, you could feel a distinct ledge where the oak had been compressed.

So... I've heard before, that you can over-squeeze a glue-up, and end up starving the joint of glue by literally squeezing it all out. But then I've also heard from several sources that it is total B.S. and that PVA glue is so strong, that all you'd need is a 1 micron-thick film of it to create a joint that is stronger than the wood around it. And that even a 10 ton press could not completely squeeze it all out.

So... keeping in mind that these are all just YouTube videos that I've been using for reference, can any of you guys speak to how to dial in the "right amount" of clamping pressure?


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PostPosted: Thu Feb 07, 2019 4:22 pm 
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Brazilian Rosewood
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Put me in the camp of not believing you can starve a joint by clamping all the glue out. I have never experienced anything like this and I have clamped some joints hard. I suspect your problem was a glue line that was too thick and or poorly prepared surfaces.

Several people will chime in soon to tell you not to use titebond II. The general consensus among instrument makers is to use titebond original. I do, however, use titebond III when I make up binding/purfling logs because I want the heat resistance but these are not stressed joints.

Wood glues like titebond do not gap fill and really want perfect wood to wood contact. The parts should fit perfectly together and be clamped well enough to keep a thin glue line. Surface prep is also important. Ideally you will be gluing freshly prepared surfaces. That means planed, scraped or sanded close to the time the glue up is made. If you used the "factory edges" of your boards the surfaces were likely oxidized and/or contaminated with who knows what.

Tell us more about the preparation and fit of the parts before you glued them.

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PostPosted: Thu Feb 07, 2019 4:41 pm 
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Walnut
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First name: Brian
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Just about every time I have off cuts that share a glueline with what I'm working on I also try to break it just to test the glue. It's good practice and gives you good info to have. I quit using epoxy for a lot of applications for that very reason.

Not being familiar with TB II, I went to their website to see what it had to say. when comparing the "features and benefits" of TB II to TB Original the TB II sales pitch did not include the phrase "Bonds Stronger than Wood" but TB Original did.

Maybe there's some truth in advertising -- if TB II's bond is not stronger than wood, it acted exactly as advertised.

As mentioned TB II is not recommended for luthier work very often.

Brian


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PostPosted: Fri Feb 08, 2019 6:39 am 
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Brazilian Rosewood
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You can starve a joint for glue. But it is much more common to apply to much glue.... This requires higher clamp pressure to squeeze all the excess out and can lead to all sorts of issuers from the one you describe to uneven flute lines and weakened joinery. Too much is just as bad as too little, proper application of an appropriate amount of glue was something I always had to pound into new cabinet makers in my shop.....

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PostPosted: Fri Feb 08, 2019 9:22 am 
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Walnut
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I used Titebond II on my first build.
I no longer use it and just stick to Original now.

I never had any issues with TB2 hold strength. It seemed plenty strong enough. Something I did notice was the shorter open time, compared to original. I know its only a minute or so, but sometimes a minute more is all you need. I also noticed that TB2 dries darker. I can see the glue line pretty well on the Engelmann top I used it on. Given, it was my first time, but I did candle the joint and had no gaps.

Maybe the next glue up try wipping away the excess squeeze out as you are clamping. If you continue to apply pressure and there is no squeeze out then you probably should stop.

Just my 2 cents.


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PostPosted: Fri Feb 08, 2019 1:55 pm 
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Koa
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Dolmetscher007 wrote:
.... I ended up ruining all that lovely wood. I made mistakes cutting the scarf joints...


I can't offer any advice on your glue failure - I've never used TB II and I've never had problems with Original.

However I am interested in your scarf joint. I've done a lot of them, again, with only minor issues, which I think we discussed in your other thread. What exactly went wrong?


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PostPosted: Wed Feb 13, 2019 12:08 pm 
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Cocobolo
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In my gluing I include a factor I call "Sit Time" whenever I use my TB Original or epoxy. My "Sit Time" allows the glue to be absorbed into the wood better. Penetrating the surface deeper. More often than not, I also rub the glue into the wood fibers with my finger on smaller glue ups. Then I will let the glue sit for 5 to 10 minutes to be absorbed more before clamping. Some more porous woods actually absorb so much glue during the "Sit Time" that I will reapply additional glue. Also, If I let the glue sit too long, I will again add a little fresh glue.

I acquired this "Sit Time" absorption habit about 15 years ago when I was gluing (with structural epoxy) together a Sitka Spruce framed 2 seater wood aircraft called a KR-2S. The wood frame was epoxy glued and included corner gussets for additional strength.

Here's a picture of part of my aircraft frame during gluing. I sold the aircraft when 60% done to to cash flow issues. :(
Image

I did a lot of joint strength testing prior to construction and found that a joint with "Sit Time" never failed. But some joints with no "Sit Time" did fail with pressure, reveling inadequate penetration into the wood. Also my experience is and it stands to reason that wood joints where the glue has penetrated both surfaces sufficiently are affected less by excessive gluing pressure.

Ever since my testing back then, "Sit Time" whether needed or not has become a habit for me.

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