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A couple of questions about gluing fan struts using LMI white glue, clamps and a dished solera:

1) How much open time can I plan for with LMI white? My shop is a constant 74 deg. F and 60% RH.

2) How long until the glue is fully cured? Since the fan struts are stressed (press-fit) along the glue joint, unclamping will in effect, fully load the joint, and I don’t want to loose any of the dome shape to glue creep by unclamping too soon.

Thanks for any feedback on this.

Joe

Personally, I use hot hide glue, but clamping overnight would be a good idea for Titebond. They probably say you can unclamp a joing after about 45 minutes, but I wouldn't for a stressed joint like that. You want to make sure all the moisture is gone from the joint. I don't mean to be a stick in the mud, but 60% RH is high for building a top. If it doesn't stay there, you are going to have problems!

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Waddy

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First of all, are you sure you want to glue at 60% RH? Unless the guitar is going to spend it's entire life at relatively high humidity, it's a good idea to glue between 40 - 50%.

I don't know off the top of my head if they have specs for open time but it should be at least a few minutes, probably more. Way more than enough time to glue braces.

As for clamping time for spruce braces to spruce tops, an hour is plenty. It won't reach full cure strength but it won't creep under that kind of tension. For larger pieces, particularly bridge plates and bridges, you may want to clamp longer.

But don't trust me on clamp time. Take some scraps about fan brace size and glue them together in your dished form. Clamp one for 15 minutes, one for 30, one for 1 hr, one for 2. Mark them and compare creep the next day.

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Thanks guys. The average RH in my part of the country ranges from a low of 66% in summer months to 75% in winter. So 60% is near the low range that the guitar will likely see around here. However, I don’t know where the guitar may travel to in its lifetime. I can get the RH down to around 50% in my shop which is what I will do.

As an aside note, and I’m not sure if it’s pertinent but according to BBC’s online climate data for Seville and Almeria, Spain, where Torres built many of his guitars, the percent RH historically ranges from a low in the mid 60’s to the upper 70’s and upper 80’s.

As far as clamping time, I guess I’m not in that much of a hurry so leaving things clamped for a day or so as added insurance won’t bother me too much. Although I think I will do some testing on clamp times as suggested.

I’m a little more interested in open time for the LMI glue. My tentative method would be to apply glue, set each fan into position, and then clamp all seven fans at once using cauls that span across all seven fans. The struts will not take on the shape of the dish until clamped so I want to be sure that the glue hasn’t set too much on the first fans before I press them into shape. In a test run it took about 5 minutes between placing the first strut and clamping. I suspect this might be too much open time for the thin layer of glue on the narrow strut and I may have to rethink my overall method. Comments?

Joe

5 minutes is likely fine but again, you could try a similar test to see.

Why all bars at once? Are you vacuuming? Go bars are certainly easy and fast.

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I don't have a go-bar station. I thought I'd try to use clamps since that's what I already have with the additon of a few homemade cauls that are shaped to match the dish of the solera.

Joe

Are you intending to run cauls across all the fan braces to glue them all at once? this would make it difficult to get adequate pressure on the centre fan braces.

Yes, I was intending to use cauls that extend across all fans at the same time. I set up a test using strips of paper placed at the fan locations to verify even pressure. The caul is split with the ends pinned allowing some pressure contouring with the screws. It seems to work fine.

Joe

Open time on LMI White is a little less than Titebond. If you are going to do what you say, you aren't going to have much pressure on the ends of the braces. That could be a problem. I know they are sprung in, but there isn't much pressure there. When using PVA glues, it's better to have consistent pressure along the glue line. You could create weakened joints at the end. Also, if you are going to set them all at once, I'd practice a few times to make sure you are getting everything done within the needed time.

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Waddy

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The other major thing to consider when gluing these is doing it in a sequence which will alow you to carve and shape the braces as desired without runnng into another brace. Depending on the pattern that you are making, this will dictate the gluing sequence.
I use a go-bar deck and a shaped solera with fish glue and leave everything for at least 12 hours.

Waddy, sorry but my picture showing only a single caul was a bit misleading. Here are a couple pictures of a full dry run showing all the clamps and cauls for the fans, including foam-backed cauls for the strut ends. Practicing a few dry runs to get the timing down sounds like a good idea.




David, thanks for the picture – it looks like you did something similar with go-bars and short cauls spanning two struts, and yes, I see where it would be difficult to shape some braces in close proximity to others. I will try to plan the sequence carefully.

Joe

That's much better from a clamping standpoint, but time will be your enemy, getting all that in place within a few minutes. Practice makes perfect. You might think of different ways to get the glue on the braces. I feel that will be a slow point in the process - applying and placing. Make sure you have good marks for placing. You might consider using little tape dams at the ends and on each side of each brace at the ends and in the center. I use that method to glue bridges down. Using PVA, you are going to have slipping problems when placing and clamping. The braces are going to slip off location. The tape dams will help that not happen, and they also give you a positive location once glue is applied. Tape dams are made by cutting through a number layers of tape, on the roll, You can make them 1/4" by the width of the roll. Use a razor knife and cut through the layers. I like the green painters tape as it releases nicely. I have a roll that I devote to tape dams.

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Waddy

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Sound Clips of most of my guitars

Update: I lowered the RH in my shop to just under 50%, let the wood acclimatize for a few days; did some testing of open times and clamp times for the glue as suggested, a couple dry runs of gluing/clamping to make sure the timing would work, then went ahead with bracing. It seemed to go without any major hiccups.



So far there has been no measurable spring back (dome precisely matches the solera dish) which I guess is a good sign. Thanks to everyone for the helpful advice.

Joe