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Week 7: April 23 - 29

This week, the sides must be bent. I've done some bending before, making a couple of small Shaker-style oval boxes of white oak and cherry. That was done by boiling strips of wood in water and strapping them around a form. The free-standing method of instrument building involves shaping the ribs against a hot tube by hand and eye alone. I look forward to the challenge.

First some preliminaries must be taken care of.

The last item for the soundboard is to make and attach the tailblock. The making is straightforward; it's just a block three by four inches and 3/4" thick.  The grain runs parallel to the grain of the sides, so that the surface glued to the soundboard and the one glued to the back is long grain rather than end grain.

Because the four-inch dimension is to be across the grain, there's no way to cut it out of the neck blank log, since the widest dimension is just shy of four inches. Presumably LMI intended the tailpiece to be made with the grain going the other way. While recommending the horizontal grain, Cumpiano and Natelson  acknowledge that folks disagree about it. Since I have a good, straight-grained piece of mahogany knocking around the shop, I decide to use that and follow the C&N program.

It's critical that the end that attaches to the soundboard be perpendicular to the face that's going to be glued to the sides at the butt of the guitar. I use the fore plane to level the edge of the board before cutting off the block, checking with and engineer's square. After cutting out the piece, I final-level it with the block plane.

This is the purpose of the little tab at the end of the workboard. While clamping the block to the soundboard, the tab provides a ledge to set a square. The excess wood at the bottom of the soundboard is trimmed away to allow the square to reach. Because the block doesn't come all the way to the edge of the soundboard - it's set back by an amount equal to the thickness of the side material - the small combination square shown is the best way to get a reading: you can slide the ruler up an eight of an inch to clear the last little bit of the soundboard material and come into contact with the tailblock. I did check this arrangement against an engineer's square to make sure that sliding up the ruler didn't introduce a small angle into the square. This square, by the way, may be the classiest tool I own. It's a Starret four-inch combination square that my Dad recently gave me.

Here's the soundboard complete. I've left the braces rather heavy according to most descriptions I have seen. They pretty exactly match the plan that I'm following. Tapping the soundboard yields a crisp, high-pitched sound. I'm doing my best to remember exactly what I'm hearing and to record the sizes and shapes of all the braces, so that when the guitar is finished and I analyze the sound, I will know as nearly as possible what led to it. 

Something I left over for later back when I made the workboard was to make a cork rim to support the edges of the soundboard after the arched braces have been applied. Operations up to now, like attaching the braces and the head and tail blocks, have not required it, but it will be needed when the sides and the back are attached. I had a hell of a time finding the sheet cork. Not at Frank's (crafts and garden supplies), not at Woodworker's Club (Woodcraft), not at Home Despot, though I can't stand being in there long enough to make a thorough search. Not at the couple of local hardware stores I tried. The guy (I should know his name - he knows mine) at Woodworker's Club in Norwalk, Connecticut is always super helpful and knowledgeable in suggesting other places to check for things he doesn't have, and he suggested Michael's craft supplies (among other places), which is where I finally found it. The cork is 1/8" thick and attached to a sheet of stiff paper.

The last thing I can think of to do to avoid bending sides is to make the workboard cleat. A bolt through the workboard at the location of the center of the soundhole holds a block of wood that contacts the upper transverse brace and the juncture of the X braces to hold the soundboard assembly firmly down on the workboard. Because this is in place when the back is attached, it's important to remember to put the wing nut on the underside of the workboard at that time. It's a little more trouble, but because I know that I will forget it if I have a chance when the time comes, I decided to always do it that way.

Bending the Sides

The first thing I learned about side bending on a bending iron is that you need a lot of space! You've got about 35 inches of board flapping around and you are going to be applying it to the iron at many different angles and from both sides. Make sure you have clearance above and below as well as on both sides. You also need space around your template for when you make the frequent checks against it for the shape of the bent sides. The setup I have here is barely workable. The handle of the vise was sometimes in the way, though I pushed the iron out to where I could just clamp it. The way it is here, I could only clamp it on one side, and sometimes it would turn in place. I will have to arrange a better setup for next time.

Yes, there will be a next time. I will drop the clue that the process was successful to a good degree, and quite enjoyable. I've never had the opportunity to see this being done, so I relied on the thin chapter in Cumpiano and Natelson  and a couple of other sources. I had wondered about the brevity of the information I was able to find on this, but now, trying to describe the process myself, I no longer wonder. It's pretty hard to describe, but here goes. I'll try to point out things that I think are helpful; things I didn't know from reading, things that surprised me about it.

First off, I don't have pictures of the process. Maybe another time I will recruit my wife to take some, but the procedure is so intense and hands-on (and damp!) that I just couldn't set up even a time-delay shot or two. I don't think still pictures would convey a lot of information about how to do it, anyway.

Preparations

The sides need to be matched and jointed. Place the sides together so the figure matches. Usually, the pieces will have come from a single board and matching involves placing them back together in the orientation they had in the board and then opening them like the pages of a book so that two edges are together. There are two ways to do this, so pick the one that looks best and choose the side you want to have showing on the outside of the guitar. On my materials, there was really no difference inside and out so I could pick the figure orientation without considering this. At one end of the pieces, the figure took a slight curve, and I decided to put this end at the tail of the guitar. I put the two pieces together face to face and, using the fore plane, jointed the edge that would be contacting the soundboard so that it was straight and smooth. 

Both pieces had begun to split at the tail end, probably due to bringing them into my house in mid winter when the inside humidity went to almost nothing. I had to plan on not using the last inch or so. Fortunately, this is a small guitar!

I rolled a small roll of electric tape along the outline of the template, and marked the location of each major turning point or focus of a bend in terms of the number of rotations the tape made. I then transferred these measurements to the side material so I would have reference points to work from. I made each mark on the convex side of the bend, so I could see it while holding the concave side against the bending iron. This turned out to be only a little bit helpful, as I checked the bend against the template so frequently I always knew where I was without referring to the marks on the wood.

Soaking the side material

The first step is to soak the side material in water. I looked around for an appropriate container, and came up with a 36" window-box liner from Frank's garden and crafts store. It wasn't a good choice, as it turns out, but it was better than anything else I found. 36" turns out to have been the outside dimension. The sides and ends of the container narrow down to a little shorter than the side wood and a little narrower than the side wood, which is 35" long. Of course. I was able to sort of wedge it in there. Brosnac's book shows a tray made of sheet metal that he uses for boiling the sides in. I may go that route if I can't find anything ready-made of plastic. On one side I soaked the neck end first, bent the shoulder on the pipe, and then was able to fit the whole piece in. On the other piece, I used more water and soaked all but six inches or so of the butt end. When I stepped back to look at it, the end sticking out of the water had warped almost into a "C" (as you look at the end grain). I was concerned that this stress might increase the split. I turned it around and got that end under water and it unbent pretty quickly. At first I intended to use water from a teakettle that had been brought to the boil, but it barely covered the bottom of the trough. I used hot tap water instead. I soaked for about 20 minutes.

Bending

The LMI Professional Bending Iron takes a long while to heat up. So long, in fact, that when I first tried it out I was afraid that it was defective, because after several minutes on "HI" (Hello!) it was barely warm. It gets there eventually. I have the blisters to prove it. I let it heat up at the "HI" setting, and then backed it off to 6 (it's marked "LO", then 1 through 6 and "HI", so 6 is  the second highest setting). I set up the iron on its side so that the wide parts were on the two sides and the sharper curve was down. In section, the iron is egg-shaped, and the pointy end of the egg was toward the floor. I ended up doing almost all the bending against the blunt end of the egg.

I began bending the first side at the pencil line I had marked for the narrowest point on the waist. I laid the wood against the iron at this point and rocked it back and forth to get the heat into a band of wood about 3/4" wide. The water in the wood hissed and popped, and I could see it surging up through the wood, or perhaps just the water on the near side of the wood was boiling. I began to apply pressure on both left and right while continuing slowly to rock. The wood began to bend, almost imperceptibly -- until I held it up and looked at it, I wasn't sure there was a bend at all. After a minute or two, the wood seemed to "let go" and I could easily push a bend into it. I held the wood in its new shape and checked against the template -- still had a way to go. 

By this time the wood was dry on the surface on both sides, so I dipped it in the water for a few seconds and went back to work. I thought I was focusing too much on the one point, so I went a little to one side and the other of the pencil mark and rocked and pushed until I felt a bend starting. This was a mistake. The waist on this guitar is a pretty sharp curve, and I needed to get most of the bend focused at that pencil line. I found this out later when I tried to get the lower bout to come down to the line. 

It takes some pressure to get a bend into the wood, and you have to not worry too much about tearing or cracking the piece. Otherwise, you are going to rock or slide it back and forth until it scorches without any bend getting into the piece at all. In the first photo of the bent sides you can see scorching on the piece above and to the right, and almost none on the other piece, which I did second. You can learn by doing! The scorching is there on the inside of the waist bend as well. I hope the scorch marks will scrape out. If not, they will be a little mark of history. When bending the waist on the second side, I found that the pressure can be applied sooner and more strongly. The bend will still start gradually and then almost suddenly the wood will be supple. I can only speak for excellently quartered and straight-grained rosewood at this time, but by the time I was finished with the second side I was applying considerable force at each point and never heard a tear or crack. It's a matter of getting a feel for it, which, for me anyway, didn't take very long. 

While the waist bend needed to be done almost entirely in one place against the iron, the upper bout bend is less pronounced, and needs to be introduced a little at a time as you slide the wood along the iron. I found that it worked best to apply pressure while rocking back and forth for just a few seconds, then move over maybe an eighth of an inch and repeat. Do this only three or four times and check against the template. If you get ahead, you may over bend further along the curve. 

The long, changing curve on the lower bout  was almost more difficult to bend than the tighter curves. The best method I found was to be very careful not to get ahead, but to work one short section (about 1/2" at a time) until it would lie right against the line in the template. Any time I tried to work a longer section I found that I had overbent out along the curve. 

The wood dries visibly as the process goes along, though internally it remains quite damp, as you can tell as soon as it cools a little. To prevent scorching, I tried to dip from time to time whenever I worked a section long enough so that it looked dry.

That's about all I learned from this experience. It took about four hours to bend both sides, much of which was pondering just how to do it or correcting overbent sections. 

Here are the sides an hour or so after the bending was done. As you can see, there was some "springback" evident. The wood was still quite damp at this point. Cumpiano and Natelson advise letting it go overnight. They say they leave the sides free during this period, while others clamp them in a form. I decided to clamp them to the headblock and tailblock in their assembled configuration. It probably migrated some moisture to these parts, as I realized later, but I hope the amount was insignificant. Probably was. Anyway, it worked very well. The springback you see here was reduced considerably by the next day and the sides seemed to have settled into their shape.

Here one side is clamped and glued at the tailblock and headblock. The cam clamps are just lightly holding the side down to the line. The side was first clamped like this and the two ends marked -- the headblock end was marked just along the inside edge of the mortise on the headblock, and the tailblock end was marked about a sixteenth of an inch shy of the center line. I trimmed the ends off square with the dozuki saw.

I've decided to use a modified "tentellone" method to attach the sides to the soundboard. Tentellones are small individual gluing blocks that provide more glue surface than the thin edge of the side material. With the sides lightly clamped to the soundboard as in the photo above, the blocks are glued one by one to the top and sides. Because they are small and light, the surface tension of the glue is enough to hold them in place until the glue sets. While Cumpiano and Natelson  describe this method, they prefer using continuous kerfed linings that are glued to the sides; then the assembly is glued to the soundboard. This involves locating the ends of the braces that reach all the way to the sides and chiseling away the already-glued lining at those points; later, when the sides are attached, individual blocks are placed on the sides over the braces. I have a couple of reservations about this. First, gluing something on only to chisel it off later goes against the grain. Second, the operation of gluing the entire side at one stroke is an invitation to disaster and an awkward setup, requiring a lot more clamps than I like to use (or than I even have). By clamping the side down dry, I can fiddle and adjust until it lies just right along the line without smearing glue everywhere.

I am using a modified tentellone method, because I am using a piece of continuous kerfed lining that came with the boxed materials and gluing in a short strip at a time, containing from one to six blocks. This will be quicker than individual blocks, while allowing the assurance of positive gluing contact for every part of the strip. I can glue blocks over the braces as I go along. I found that at the incurve of the waist, I needed to place the blocks one at a time. On the long straight part of the lower bout, five or six blocks was the limit. Any more than that and the blocks toward the middle of the section would want to pull away from the side. I ended up having to hold one section down for a good ten minutes until the glue was tacky enough to hold it. One trick that worked well for me was to put down the strip and slide it back and forth along the side a few times. This thinned and spread the glue enough so that it really grabbed and I could let go right away.

Using this method I was able to trim a block at an angle and get it right into the sharp corner by the X brace just above the waist. I really don't know how much of a difference this will make in the long run. I don't like to think of hitting a big void when I try to glue the binding in, though.

I think that overall, this method is superior to the continuous kerfed lining. It allows the blocks to be placed right up to the braces like this without any chiseling away and regluing. It allows leisurely adjustment of the side along the line before any glue is applied. The jointed edges of the material lie flat along the soundboard, and there is no need to plane and sand down the linings after gluing them to the sides.

Here's the second side glued to the blocks and lined.


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Copyright 2001 Stephen Miklos