Building a 13 Course Lute, Start to Finish, 12: The Fingerboard Spacer; and, Preparing to Glue in the Belly

Hello, friends--I hope you're enjoying a fine holiday. I'm in the midst of travels these days, but I thought I'd put together a little treat to celebrate the new year.

As the title of this week's post indicates, I'll be dealing with two themes today--gluing and shaping the fingerboard spacer, and preparing to glue in the belly. They are very closely related, and really need to be done in tandem (and I think they're best described in tandem), since the shape of one essentially dictates the shape of the other. 

At the outset of this series, a reader posed a fundamental question that I've been coming back to ever since: how different is building the baroque lute from building the renaissance? My answer: very different, and though the differences are many they all find their source in the shape of the fingerboard. If you've been following the series, you'll know that at practically every step of the way I've been making allowances for and anticipating its arrival--I've taken account of it in checking the neck angle and developing action; I've arched the belly in subtle ways by gluing curved braces across it; and I've curved the bridge slightly, during both the making and gluing of the bridge, to match that belly arching. Now it's time to see how all that preparatory work pays off.

For those who may not know, one of the distinguishing characteristics of a baroque lute is its sometimes very highly curved fingerboard. The amount can vary, but I've designed this lute to have a fingerboard arch of around 4.5mm at the nut end, and around 2.5mm at the body-neck joint. Compare that to a renaissance lute, which, depending on how many courses it has and consequently how wide the neck is, might have an arch of around 1mm or less at the nut end, and about 0.5mm or less at the body-neck joint. On a renaissance lute, this curvature is achieved by gluing down a thick piece of ebony for a fingerboard--between 3 and 4mm thick--and planing the curve directly into it. However, this option is not available on a baroque lute, since for the curves I'm looking at, the piece of ebony would need to be somewhere around 7mm thick. The result would be a) a very heavy neck; b) a ton of work to shape the piece; and c) a terrible waste of a precious material.

The solution to the problem is to glue and shape a piece of strong, stable, light material on the top of the neck--a fingerboard spacer--and then, ultimately, cover that with relatively thin piece of ebony (as we'll see in a later episode, around 2mm thick--barely more than a rather thick veneer.) The material I use for this fb spacer is sitka spruce--which is strong, stable, light, readily available locally, and easy to work. Here are some process photos to show how I glue it on.

On the left is a caul that matches the twist I've planed into the neck's top surface (see post #8 in this series for an explanation of that.) The fingerboard spacer is on the right, and will be glued onto both the neck and the top block. It's a bit oversize in length as well as width, so I've masked off the upper portion of the body to keep glue from dripping down inside.

After lots of dry runs (where I figure out how many clamps to use, what sequence to use them, where to set them, etc.), I'll warm the parts up with a hair dryer, paint glue on both surfaces, and then put them together and clamp. With such a big gluing surface things will tend to swim around a bit during clamping, so I need to check frequently as I tighten to make sure all the pieces end up in the correct position.

I paint extra glue on the outside of the joints--as I've mentioned before, the glue shrinks as it dries, and will help to pull together the edges of the two pieces.

Careful planning--and lots of dry-run rehearsals--help me to achieve a nice orderly result.

Next day, I clear away excess glue on all edges, and then trim the piece. I begin by cutting off excess length at the top block and the nut.

Then I trim the sides--first with a knife...

...and then with my trusty low-angle block plane (and a lot of wax on the sole.)

I leave a small bit of excess along the sides, but trim the ends very closely.

Now with templates I can lay out the fingerboard profiles at the body end... 

...and at the nut end.

And then I can get to work putting a good shape on the thing.

It can be handy, when doing so, to chamfer down to the line with a sharp chisel.

As I near the shape, I start working with rasps, then files, then finally a sanding block.

Although the surface is curved in cross-section, it must be kept very close to flat in long section. As I'm working, I draw (and re-draw) lines that follow the layout of the courses in order to keep track of the flatness. 

The straightedge held on each line, with strong light behind, shows the developing profile clearly.

Here's what the edge of the shaped fingerboard spacer looks like (I put a piece of paper behind to show it more clearly.) The spruce is feathered out pretty precisely to meet the edge of the neck veneer.

There's the nut end profile (sorry for the lack of focus!)

And here is the profile at the edge of the top block. You will notice that at each side of the fingerboard spacer, the ends of the curved profile meet up, rather bluntly, with the flat top block surface. I'll need to ease the corners of the top block a little, to continue and blend in the curve gracefully.

In practice, that means I also need to ease (slightly) the edge of the outside rib. I use a sanding stick for this little job--just a strip of particle board with a piece of sandpaper stuck to the end.

Here's what it looks like without me in the way. It's long enough to use lengthwise as well as across the body, and it keeps the top surface of the edge rib pretty flat while I'm shaping the contour. 

This is the kind of shape I'm looking for--the curve of the fingerboard spacer extends to the outer edges of the top block, and that curve blends smoothly into the forward part of the edge rib.

It's time to see how the belly looks inside the body. You can see here how the arched braces I glued to the upper part of the belly  match the arching of the fingerboard spacer. So far, things look good.

Two other things I'll be paying close attention to are, first, how the belly fits into the curves that I've just shaped into the top block/ edge rib area, and second, what the action looks like. This is the very first time during construction that I will be able put on an actual string and stretch it from bridge to nut to see what the true string height above the fingerboard will be. It's a bit of an exciting moment!

Here's what it looks like. I've marked the string spacings at the nut, so I know exactly where to stretch my piece of nylon fishing line. I've also marked the positions of the first and eighth fret, which is where I want to put down wooden spacers to get a representation of the action of the lute as it is now (and how much I need to adjust it before I can glue in the belly.)

Another view. Notice two things in this photo: my collection of very accurately made wooden spacers (I've made them in sets of ten, graduated in tenths of a millimetre); and the leather collar on the tip of my index finger, around which I can wind and pull tight the nylon line without it cutting into my finger.

A quick word about the method used here to check the action. The spacer I use at the first fret represents the theoretical height of the string above the finished fingerboard, so when I stretch a nylon line between it and the bridge, I can slide a spacer under the string at the eighth fret to derive the exact string height. I measure the action this way for the first eight courses only. What I'm looking for--what I'm hoping for, actually, when I do the preliminary action check at this point--is an action that does two things: first, rises very gradually by a small amount from one course to the next; and, second, is overall very slightly higher than the action I am hoping for, before I glue in the belly.

My initial assessment of the action is that, as I hoped, overall the action is slightly too high. However, I've also noted that the action of the middle courses--roughly courses 2-7--is a little bit low, relative to the outer courses. The arch of the fingerboard spacer is a bit too high down at the lower end, so I'll need to remove the belly and do a bit of reshaping.

During the re-shaping process, it's handy to use a contour gauge to get a clear look at how sweet the fingerboard curve is. (I also use this gauge to 'take notes' on the developing arch, by tracing the shape into my notebook.)

I write notes and squiggles as I work, to indicate where I need to remove (or not remove) material.

Here's how the action ended up looking after I had finished up re-shaping the spacer. As you can see, the action rises gradually--by 0.1mm from one course to the next. This looks very good to me. 

The action across the string band looks good, but as I said above, it's all a little bit high--about 0.2mm. This is a good thing--I've been planning for and anticipating this all through the building process. It means that I'll be able to lower the action, in a very controlled way, to get it to the exact place I want before gluing in the soundboard.

In order to lower the action at the 8th fret by 0.2mm, I'll need to lower the edge rib by about 3 times that amount--somewhere around 0.5 or 0.6mm. It's hard to say exactly; all a person can do is start shaving it down, lowering it a little, then taping the belly back in, and checking the action.

To give a little visual guidance, I use a marking gauge with a sharpened pencil lead to mark a line about 1mm away from the edge, all the way around the bowl.

Then I get out my low-angle block plane and get to work. Most of the material I remove will come from around the location of the bridge--say, the bottom third of the body.

As I come up around the sides, I'm looking to blend in with the curve already planed in the edge rib (the 'belly scoop'--see episode 2), including the curves that I've just made on each side of the top block.

I use the sanding stick to blend in the work I've done with the block plane, and make nice, even transitions all the way around.

The only way to get a clear look at my progress is to tape the belly in place, and check the action. I will do this two or three times--removing some material from the edge rib, taping in the belly, and checking--before I get the action to the place I want. 

I want to make sure the action has come down equally on both sides, there is still a good belly scoop, and the curves and contours I've shaped into the edge rib are smoothly integrated. It's tough to show this in a photo; here's my best effort.

I think this is a good point to leave things for today. There are still about a dozen little jobs to do before I can glue in the belly; I'll show them all next time, and I'll show how I glue the belly in as well. This episode has not been the most spectacular or dramatic, perhaps, but I trust it's shown the kind of detail and concentration needed to bring together a lot of different strands of the building process. What I've shown in this post is essential work in creating a playable instrument.

Happy new year, everyone!