When renovating old guitars and disassembling them, there are two things that are difficult to glue back. Bottom and neck. To begin with, I did it freely, it was stressful and the results varied. Sometimes it went well, sometimes not. It became especially difficult when a newly glued neck had to be re-glued… To get a consistent and good result without stress, I manufacture jigs for both steps.
The problem with the bottom is that you have to have a fit of at most a few tenths around, especially important is that the bottom either fits perfectly or protrudes from the side. Otherwise you will have to grind on the lacquer of the side and you do not want to.
To glue the bottom, I have a plywood plate, a bunch of side forks, a few adjustable sticks with threads to push out the side from the inside with and a pile of triangular bars. Plus the usual pile of forceps of various kinds. The best are the wooden blades, but they are bulky so smaller metal blades are also needed. To glue the thin edge on the bottom and get time to adjust the fit I use fish glue, skin glue I only use to glue the bottom to the neck and bottom pad. A forced model long may also be needed to temporarily force a stubborn side to where I want.
The guitar body is allowed to stand on the cut of the right thickness and the side forceps are mounted where needed with a screw. Inside, I use a cross of adjustable sticks to express the coffin. The pins consist of a threaded rod with a nut at one end of a stick with a matching hole at the other end. In addition, loose pieces of different length type lego with a wooden pin at one end and a hole at the other.
The side forceps consist of a block in book, a plate angle and a screwed in cheap forceps that are slaughtered on its shaft. The hole in the castings is about 5 cm which allows you to adjust the height of the side forcing to the side. You can choose to have both side and bottom edge forced or just to the side.
The fish glue is applied to the thin edge of the side all around, warm skin glue is only applied to the neck and bottom block after preheating with a heat gun. I fit the bottom and quickly pull forceps that attach the bottom to the bricks. The fish glue gives at least half an hour of open time and you do not need to stress. Incidentally, fish glue has the same fine properties as warm skin glue, it is a bit smoother and is longer open, but pulls together the pieces of wood in the same way as hot skin glue when it dries. I've read anecdotes where fish glue has released in a very humid and hot climate, but it might as well have been a bad glue. Uses good quality fish glue, Lee Valley “High tack fish glue”.
Starting on one side and making sure the fit is as it should, the page must not stick out! If the bottom protrudes, the edge can be scraped away later. Adds a puzzle with the triangular counterparts in different shape to get even pressure over the joint.
One side clear, time for the other.
Everyone forces on the spot. This bottom was nice and had not shrunk, if the bottom has just shrunk a little grand you can push the side at the waist and fit the rest of the bottom. Then scrape the overhang at the waist. The page will lean a bit but it is not something you see if you do not look carefully. If the bottom is significantly shrunk, you have to add wood, either with a centerpiece or put on wood at the very edge.
Since fish glue has a strong contracting force when it dries and sticks well even on varnish, the scrap must be wiped away. All side forks are loosened with an extra-long screwdriver, the body with all forceps is lifted off the plywood board and the side is cleaned with a damp cloth.
Here you can see what was under the body and the boxes to store what is needed for the jig. The plywood board is hung on a couple of hooks on the wall.
Uses screws that can be quickly screwed down into the plywood board.
The bottom must dry overnight as it takes 12 hours for the glue to dry. Succeeded with the pass!
This is how it was after scraping and staining in the open wounds along the edge.
The pins can be easily removed through the sound hole. Once I forgot that it was a f-hole guitar I was working with, but luckily the pins went out through the widest spot in the middle of the f-hole 🙂
Gluing the neck is very difficult as at the same time you have to get it in the right position both laterally towards the string holes on the stable and the height so that the stable leg does not get too high or low. The joint in old guitars is rarely as perfect as you can do it on a new guitar, unless otherwise you almost always have damaged wood in the joint when you loose your neck. Making it fit the way you want usually takes a long time and requires a lot of work, but even if you have done a good job, unexpected things can happen when you tighten the forceps when gluing. Laterally and vertically, a few tenths of the difference in attachment can be the difference between right and wrong.
My jig to glue my neck allows me to keep a close eye on what is happening and give me the chance to adjust the neck side-to-side at the last minute as I glue. I can also check that the height at the stable is the right one in the jig with a forced screw (as in the gluing), but I do this before I glue as the neck must be loose in order to adjust if it does not fit.
The jig is foldable to save space and mostly done in birch from a wall hung table from IKEA 🙂 Hangs on a hook on the wall. The jig is built around a long jack. It can be adjusted for various large guitars, ranging from a little parlor to a huge twelve string guitar. If the guitar has an extremely low edge, you may need to place the jig's feet on some pieces of wood so that the top of the forcing does not receive the underside of the neck, but that is rarely the case.
The guitar is mounted in the jig with forceps.
To make sure that the neck is completely straight side-to-side when gluing, I use a small gadget so that I can clamp strings between the outer string holes and the upper saddle in the correct position. Two metal pieces with screws to clamp the strings with, one metal piece with an angle that allows it to be clamped against the edge of the grip board and is also easy to loosen. At the 12th band I can see if the strings are centered on the grip board. If not (usually a small adjustment is always needed) I can push the neck a few tenths at the top with a wedge against the counter in the jig. When all is well, I lock the neck sideways with a wedge on the other side.
The joint is heated and the skin glue on! A quick check that it sits right side-to-side and then it is time to tighten the neck with the long forcing on the neck foot and a couple forces to clamp the neck in the pocket and the grip board against the lid.
The U-from clamps are perfect for clamping the grip board through the sound hole. No bumps inside the guitar to keep track of!
This bonding went well. Perfect side-to-side and even in height. It usually happens to be with this jig 🙂