Sunday, 5 February 2017

'A Modest Gluing Surface,' or, Attaching a 12 course Ladder Extension

Today I want to talk about my method for fitting and gluing a ladder extension on a 12 course "double headed" lute. I've made three of these lutes now, the oldest one being I think almost five years old; the extensions are still firmly attached to all three, so I'm fairly certain that my method is a good one, and I can confidently share it with the wide world of the lute.

For those of you who don't know what a 12 course lute with a ladder extension looks like, here are a couple of photos of the finished product. These are shots of my latest one, a 12 course bass-baroque lute for Evan Plommer, completed in late 2015.

So what's the big deal with this joint? Well, the main problem is that the extension attaches to a very tiny corner of the bent back peg box, and that's almost all the support it gets. The extension, as you can see, carries a fair load of tension from the four bass courses strung to it, which pull it both forward and to the bass side. So the extension has to be well made, and the joint has to be well fitted and well glued; there is very little room for error here.

Let's have a look at the neck-pegbox area before the joint is fitted to see what the issues are.

Here's what the area looks like, once the bent-back peg box has been fitted and glued into place. See that little pink corner of the peg box? That's it.

As you can see, a small corner of the neck and fingerboard have been cut away on the bass side to accommodate the "root" of the extension. Just a quick word about layout: the neck is designed with the width of a 10 course lute, and the edge of the cutout is just a couple of millimeters to the bass side of the fundamental of the eight course. (By the way, the width of the bent-back peg box is determined by the position of the 8th course fundamental too, since that string must thread inside the peg box. Therefore, the edge of the cutout, and the inside edge of the peg box cheek, are virtually aligned.)

Confused yet? Try building one of these things. I remember making the first one of them and being just barely able to comprehend how all of this would align and fit together. I've found that the only solution when you feel this way is make accurate drawings, keep careful notes, and proceed slowly, one step at a time.

Here is a photo that shows some of the layout work I did on the fingerboard before making the cutout.

 And here's a shot of me holding my breath, and plunging in, saw-first.

This was the result--an accurate cutout on the corner of the neck and fingerboard. The back of the cutaway follows the plane of the back of the rebate for the bent-back peg box.

Once that was done, I fit and glued the bent-back peg box--and there I was, looking at that tiny pink pegbox corner, wondering how on earth I could get an extension to stick on that.

Last year, when Evan's bass lute was just finished, I posted some of these pictures to Facebook. One of our eminent lute makers of an earlier generation commented, upon looking at this photo: "A modest gluing surface." He then went on to tell a rather gut-wrenching tale (no pun intended) of having one of these extensions shear off, just as he was delivering it to his client.

I had visions of that very thing happening to me when I was making and fitting this extension for the first time (really, it haunted me in my sleep), and for a long time I racked my brain to find a solution, some way to lock the thing in place. I could just fit the extension and glue it and then... what? reinforce it with a wood screw? drill a hole and insert a dowel? wrap it in duct tape? None of these ideas seemed effective or plausible, let alone elegant. I was stumped.

So I decided to do what I normally do in situations like these, which is just go ahead and work one step at a time, and see what inspiration might come along during the process.

The first step seemed to be just to fit the extension--cut a matching notch in the end of it to fit onto the tiny corner of the pegbox.

There it is marked out. I cut the notch with chisels, and checked the goodness of the fit often. I checked the orientation of the extension by holding it in place with clamps and stretching fishing line from the bridge out to the end of the extension. I had worked out (on my layout drawing) the spaces between all the strings, going both to the main nut and to the extension: this gave me the horizontal orientation of the extension. I also had a clear idea how far forward I wanted the extension to tilt, and I used the same fishing lines to orient it in that axis too.

Here's a look at my clamping setup--I used it to check the fit and orientation of the extension, and later on, to glue the extension in place. This photo shows how I achieved clamping pressure across (note the shaped caul I've placed on the treble side of the neck and peg box).

And this photo shows how I achieved clamping pressure downward. Since there is only a small surface on top of the pegbox to apply pressure, I devised a kind of screw clamp (made with an old piece of threaded rod), with a very small foot below it, to get pressure on that very specific point. This whole rig needed to be clamped securely to the top of the fingerboard--that's why you see the wooden cam clamps in the background. It may look complicated, but in fact it's quite a simple and effective jig for holding and gluing the extension.

So the extension was fitted--now what?

Well, it seemed to me that there were still two problems with this joint. First, there just wasn't a very big gluing surface. Second, there was no real integration--no real link--between the extension and the peg box. I felt that, once string tension was applied, there was too great a chance that the extension would just shear off, and I'd be back at square one.

So, here's the solution I came up with: an internal tenon.

I think that's what it's called, anyway. I cut a small mortise in the inside of the rebate, parallel with its top surface; I fitted and glued a small tenon of pear wood into that mortise; and then I trimmed the tenon so that about 3mm or so of it stuck out.

Then I cut a corresponding slot for this tenon in the side of the peg box.

Once the slot was cut, I could slip the extension into place easily.

After clamping and checking the alignment one more time, I went ahead and glued the extension.

And that was it. Next morning, when the glue was dry, I unclamped and had a look. The fit was tight everywhere, and the alignment still looked good; but more important than that, the joint felt strong to me. I took the extension in hand and carefully flexed it, testing it for strength, listening with my ears and my hands for any tiny fractures, any weaknesses, and I detected none. I was pretty sure I had found a solution to the problems with the joint--the internal tenon gave an increased gluing surface, but more importantly it linked and locked the extension to the peg box.

So, I carried on and finished the instrument, strung it up, brought the strings to tension, and... the extension held. Five years later, it's still holding strong. And as I said, I've used it twice since then, and those lutes are holding strong too--so I'm satisfied that I've found a reliable solution to the problem of attaching a ladder extension to the teeny corner of the peg box on a 12 course lute.

Of course, the joint is still very vulnerable to damage--not from string tension, I think, but from impact. (I would be very careful when walking through doorways carrying a 12 course lute!) But I feel that it would take quite a substantial impact to separate the extension from the peg box. And if that did happen, the extension would not simply shear off--instead, the extension might break, or the corner of the peg box would. That's how well integrated the two pieces are on these lutes.

So that's it for this time out. I must confess that I debated for a long time about whether I should share this little insight I had into how to join this extension. One part of me felt like it was a trade secret, and if I gave it away freely, I might be doing myself or my livelihood some harm. However, I've come to the conclusion that it's best to share these insights when one can. After all, everything I know about lute making came from many people who have shared, and to this day still share, their knowledge generously with me.

And what's the worst that could happen? That there's a sudden rash of 12 course lutes being made with secure extensions? That a lute maker somewhere sees my solution to this problem and uses the concept in some new way to tackle a whole other problem? Or that I see some other maker's solution to a different problem, and apply it in a new way in my work? I'd be pleased with any of those outcomes. Happy lute making--and happy sharing, everyone.