Official Luthiers Forum!

So, in the middle of a couple of other builds I recently decided, kinda on a whim, to build a parlor guitar with some of the freebies I've got laying around the shop (wood that was included as packing or extras with bigger orders).

One of the first things I did was join the plates. I didn't have my glue pot set up that day (I usually use HHG for this) so I used TB1 (fresh). The joints were completely invisible, looked great. Yesterday I took the back down to close to final thickness and cut out the profile. After doing so I snapped the scrap in half and was surprised to notice that it broke relatively cleanly along the joint, taking only a few fibers with it. It's .09" walnut and I did have to bend it pretty far before it gave way-about to the point where I'd expect it to break anyway. But I did find the break along the joint a little distressing.

Two possible causes came to mind, first being that I starved the joint. I do probably use a little more pressure than necessary for such a thin joint but I've never had a weak center seam using the same method and HHG. The second thing that came to mind is that my shop has been between 90-95F for about the last week (I just installed an AC yesterday) and it was about 93F when I snapped the piece. Wondering if that's hot enough to weaken a TB1 joint.

Now I'm wondering if I should proceed as normal (again, it did take a fair amount of pressure to snap it and it will be reinforced by the cross-grain graft) or if I should cut it in half and rejoin which would be slightly tricky since I don't want to loose much thickness.

If this was a commission or one of my other guitars that I plan to sell I'd definitely re-join. But this is just kind of an experiment I'm doing on the cheap that I'm working on in between other projects. When I'm done with it I'll probably give it to my friend's kid.
At the same time, though, I don't really want to put in the time just to have the center seam on the back open up down the road.

What do you guys think?

Edit: When I did the same thing with the scrap from the cedar top last week (joined at the same time) it did not break along the seam but, being cedar, it did take less pressure to break.

All the tests I've ever done with TB1 have failed elsewhere than the joint.
I've even done a few with purposefully less-than-perfect joints, and even some of those held up.
So I dunno.

_________________
Old growth, shmold growth!

Yeah, I'll probably re-join it...guess I was just looking for some excuse not to-really wasn't planning on spending much time on this one.

One question, though: if the joint was weakened by the heat would it be structurally sound after it cools to normal temps or is a cured TB1 joint that's overheated going to remain weak after cooled?
I didn't trim right up to the line so I may try trimming another little piece and snapping it next time I make it out to my shop (now that it has AC).
...I'll probably re-join regardless but it'd be nice to know the cause.

Put in a backstrip and a back joint reinforcement. After that the original glue joint won't be doing much work anyway.

_________________
Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

If you rejoin you really should re-plane. That's one advantage hide has over TB1, if you deliberately snap your back in half you don't have to clean the old glue off to rejoin it.

_________________
Old growth, shmold growth!

90-95 degrees is not overheating for titebond. I regularly keep my shop at 90 degrees in the summer when I am not working in there. With the heat here in AZ, it is just too costly to keep the a/c any lower when I am not there, and there is no harm at that temp. If your shop was over 105 for a period of time, then I would worry about your glue joints.

To clarify, I am referring to already cured joints. I don't know how 90 degrees would affect open joint time since I cool my shop down to work. I should also mention that I store my glues in the house where it is cooler. Was your glue stored in the heat?

So, when you look along the inside edges of the broken joint, you see a little pulled out wood in one area but the rest just looks like dried glue on both halves?

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

-a dirty joint?
-old glue?
-not enough glue?

I thought the same as Todd, glue surface skinned over. The only other time I've seen tb fail like that is when my glue surface is too smooth, like maple edge grain fresh out of a planer.

TB should glue walnut with no problems. But.... I have also seen thin plates that were hard to get a good joint. The only thing I do different than some is I put a heavy coat on both surfaces and close it soon, but not clamped tight. I wait about three or so minutes for the glue to soak in as much as it wants to. (keeping in mind that the joint is closed, just not clamped tight.) Then I clamp as tight as I dare.

TB usually is impossible to starve but I'm thinking that with a thin plate the surfaces are very close so the glue can squeeze out leaving a thin joint starved. It is only .06" or so to the surface of the plates.

_________________
Joe Beaver
Maker of Sawdust

I run a bead along one plate and rub them together laying flat on the newspaper. Then I wedge clamp them. I put little weights on each side of the joint to keep them flush and level with each other. I've joined plates that I pre-thicknessed that way and only had to scrape the glue off, but lately I like to leave them a little over thick, and do the final thicknessing after the rosette is installed.

I clamp them as soon as possible since it's impossible to starve a titebond joint if both surfaces are coated and the joinery is tight.

_________________
Old growth, shmold growth!

Go ahead and add the back graft, then rout a center strip into the back. That might be easier than redoing the joint. Even if it is a bad glue joint it would be locked in on both sides.

I agree that the most likely scenario is that it started to skin over before joining because of the warm weather. So another +1 to that. Applying glue to both pieces reduces this issue, IME. Cooler temperatures probably help too.

I don't think it's possible to starve a joint with TB.

if you haven't tried titebond extend ...do so ...after the discussion here awhile back by Hugh Evans I got a bottle and love the stuff...a little longer open time than regular titebond and, seems to me, that the viscosity is a little different making it is very easy to work with...I can't imagine using anything else in a application requiring wood glue for building guitars..

_________________
MK5acoustics.com

work quickly? fwiw, i can get a plate joint locked up in under 2 mins.

I can see how more open time could come in handy for certain jobs. Meanwhile the increased strength and creep resistance with Titebond Extend is useful all around otherwise.

_________________
Old growth, shmold growth!

I have noticed that when the shop is that hot that the open time for TB1 has been considerably shorter (RH is kept @40-45%) so I guess skinning over is a definite possibility but I do get that particular joint closed pretty quickly. I use a tape hinge, run a bead, spread it with my finger, close the hinge, pull a few pre-cut pieces of tape across the opposite side (at this point the joint is closed) then lay it on a board that has 3 stops on either side of the plates, clamp down 3 cauls width-wise to keep things flat, and drive in 3 wedges between the plate and the stops on one side (the plate butts up against the stops on the other side). The joint is closed in 30 seconds (though I do spread the glue relatively thin to keep the squeeze-out nice and neat which probably doesn't help in hot situations) and the whole procedure takes maybe a minute and a half.
I'm going back out to my shop later this afternoon. I'll take another look at the piece I snapped to see if there are any more clues.

After reading through this fred, this thought just came to me: What about just inlaying the back reinforcement strip? Making it out of cross grain, seems like the hard part would be getting a consistent width to within about .003" to look clean. Thoughts?

_________________
Pat

That's what I've used on all of my other instruments up to this point (except my first one in 2004 which was done with LMI white).
The only reason I used the TB1 in the first place was that I didn't have any HHG at the shop that day and literally started this build on a whim. I usually mix HHG up and store it at home since I don't have a fridge at my shop (it's a 40 minute train ride or 20-25 min bike ride from my place) and take it out to the shop with me on days I plan to join some plates or brace a top, etc.

I'll take some to the shop next time I head out there (was feeling a little out of it today after a concussion at the end of a 16hr workday, followed by a couple of hrs at the ER late last night and decided I might do more harm than good at the shop so stayed home).