Saturday, 14 December 2019

Building a 13 Course Lute, Start to Finish, 11: Gluing the Bridge to the Belly

Welcome back. In today's instalment of the series, I'll be gluing the newly-made bridge to the belly of the 13 course lute that's under construction in my workshop. I do this job first thing in the morning, in a warm workshop that has a relative humidity of around 42-43%. I use fresh hide glue, which I've begun soaking the day before. I'm well-rested, alert, and ready to do my best work; and to ensure the best chance of creating a strong glued joint, I should have the work finished before lunch time. Here we go.

If you've been following the series so far, you will know that I've previously fitted the braced belly into the body, and located on the belly the exact position of the bridge. You'll recall, too, that I marked that position with two pin pricks in the belly (and blackened them lightly with the sharp tip of a pencil), one at each end of the bridge's string band.

All that's been wanting is the bridge, and now that that's ready to go, I can prep it, and the belly, and and stick the two together. I have a couple of things to do to prepare the belly.

First, with a couple of small pieces of double-sided tape I stick a caul to the back side of the belly, exactly opposite the bridge. As you can see, it covers a fairly large area, and I've cut recesses in it so it fits over the tips of the treble bars (that's where the treble end of the bridge will end up.)

The caul is the same one I used to glue the ebony cap to the bridge in my last episode. As I described there, the caul has a slight curve planed into it--about 1mm across its entire length--which meant that I was able to build that amount of curve into the bridge itself. Now, I'll use the same caul to ensure that the curvature is maintained when I stick the bridge on the belly.

If you go back even further in this series, to the episode where I glued the braces on the belly, you'll see that I also used shaped cauls under the belly to glue the first couple of bars above the bridge with a similar amount of curvature. The purpose of all of this curviness--this very slight doming of the belly at the bridge and above it--will, I hope, become clear in the next episode (so please stay tuned!)

The second thing I do to prep the belly is give it a light scraping with a very sharp scraper in the area where the bridge will be glued, not to remove much material, but simply to remove the oxidized and dusty layer and refresh the gluing surface. (The pin location marks I made on the belly will still be visible after this scraping.)

Now I can go on to prepping the bridge. First, I shorten the ends a little. The original of this bridge, on the Paris Schelle, has ends that are cut short, and perhaps at one point had separate tips made from another material. 

Mine will have ebony tips that I'll add after the bridge has been glued.  

Then, as I did with the belly, I refresh the gluing surface of the bridge. I've handled the bridge a fair bit throughout the process of carving it, and inevitably some oil and grime from my hands has transferred to the underside. In addition, though I was quite careful not to get any finishing oil on the bottom surface, I want to make sure that any residual bits are gone from the edges of the gluing surface. Just as with the belly, I need to remove a thin layer of material on the bottom of the bridge to expose fresh wood.

Here's how it's done: by running the bottom of the bridge across the #5 bench plane, upturned in the vice. The blade is freshly sharpened and very closely set, and I move the bridge over it slowly and steadily.  

In the previous post, I mentioned that I wanted to keep track of the best direction for planing the bottom surface of the bridge (I actually drew an arrow in pencil on it to remind me.) This is why.

If I've done my work well, I should be able to pick up a single curled shaving from the workshop floor, unfurl it, and see that I've removed a thin, even layer of material from the bottom of the bridge. Both gluing surfaces, the belly and the bridge, are now prepared, and I can begin the process of gluing up.

I lay out a double layer of masking tape, and cut it into triangle shapes.

I lift them and fold one corner over. I'll use these to locate the bridge for gluing.

I position the bridge very carefully over my pin marks. I can't possibly show in a photo what my eye sees when I'm doing this: front edge of the bridge is set exactly upon the pin mark, and the pin marks are directly under the single first course and bass-side 13th course string holes. 

Holding the bridge in place, I can then bring in my locator tabs. The double-thickness of masking tape will provide a positive stop when I want to get the glued bridge into position.

I can't really stop to photograph the stages of actual gluing, because it goes very quickly once I am ready. Instead, I'll show some shots of my dry-runs, which should give a pretty clear idea. Have I mentioned dry runs before? They're the best thing ever. For a process like the one I'm about to do, which has a number of discrete steps (none of which I want to omit), it's a great way to get the process out of my mind and into my hands. I might practice this gluing-up process a half-dozen times, making sure I know exactly how many clamps I'll need, where I can set them so that they're close at hand, where my glue pot should be, et cetera. All the while, I'm keeping an eye on my glue, which I am now bringing up to the optimum temperature and consistency. In a while, all of these different strands of the work converge, and my brain gives the order to go--and what has been a up to now a dry run, suddenly becomes the real thing.

I warm the belly and the bridge with a hair dryer.

I brush hot glue on the bottom of the bridge.

I stick the bridge down, and squadge it about slightly to ensure good contact and to bring it into position against the stops. Then I put cam clamps at each end of the central part of the bridge (i.e., not on the finials), and partially tighten them. (To tighten them fully at this point might risk pulling the bridge out of position.)

I move the rest of my cam clamps into position, tightening each one partially. All these partly-tightened clamps give enough pressure so that the bridge will not move, so now it is safe for me to go ahead and tighten them all up.

And there it is. I can't imagine that the whole gluing process takes much more than a minute--maybe closer to 30 seconds. If I'm well-prepared, it happens very quickly.

Now I can set this aside, and go on to other work for the rest of my day.

Next morning, I remove the clamps, and remove the locator tapes and glue squeeze-out.

As I do with other hide-glue cleanup jobs, I wet the glue with a brush, and then apply wetted paper towel on top of the glue. I like this method better than continually brushing on water: the towel holds the water in contact with the glue, and keeps it from evaporating or soaking away. 

In about 10 or 15 minutes, the glue is softened enough for me to remove it with a sharpened stick.

I'll let this dry for another day, before fitting and gluing the ebony tips.

And here's how I do that: put a locator tape to the side, and shape the end of an ebony stick to fit and match the profile.

Like this. (I'm not quite there yet.) When the piece is shaped, I cut it to length, and stick it on with hide glue.

Moderate pressure from a cam clamp will keep them in place. They are not structural in any way, only decorative, so the routine for gluing them in place is a bit less... regimented than for the bridge itself.

And that is all for this job, and for this week on the blog. Next time out, I'll talk about gluing and shaping the particular piece of sitka spruce that's responsible for all of these curves and arches. Here's a sneak peek:

Until we meet again!

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