Saturday, 26 October 2019

Building a 13 Course Lute, Start to Finish, 5: Thicknessing the Belly

Welcome back to the series, in which I document all the individual, discrete steps I do in building a 13 course baroque lute. So far, I've made the back; fitted, shaped and veneered the neck; and glued the neck and body together. Now I can set that assembly aside to chill for a while--only a figure of speech, mind, since my shop is actually nice and cozy-warm as rainy autumn descends on Vancouver. Seems like a good time to be inside, getting to work on a new and crucial piece: the belly.

Here are all three bellies, in their final thicknessed form. From left, 8 course, 13 course, and 7 course.

This is where the process really starts--with soundboard sets (on the shelf above) stickered and left to season for some years before I prepare them for use on a lute. (The plastic sheeting on top is to protect against a sometimes-leaky roof.) What you see in the rack below are glued-up soundboards arranged by size. From right to left, they are large, medium, small, and--on the far left--"other," a category that includes bellies of odd sizes, odd grain patterns, and other unique features.

I generally set aside two days once every couple of years to glue up a dozen or so bellies. It makes much more sense to set up the workshop to do a bunch at once, rather than one at a time for each new lute. For one thing, it saves a lot of time; for another, having a lot of joined bellies on hand makes choosing one for a particular lute that much easier--I can assess things like size, grain, stiffness, density, tap tone, and so forth, and select the belly that I think is most perfectly suited to an instrument.

(I had my two-day belly joining fiesta about this time last year, but I took no pics of the work. I think that process would make an excellent blog post on its own, however, so next year when I set up and do it again, I'll take lots of pics and report on my process.)

Here's the belly I chose for this 13 course lute. You can see some notes I wrote on the board concerning grade and tap tone; these comments are left over from when I received the boards from the sawyer, and from when I joined the belly last year.


You can see my belly template here. I figure out the best position for the belly on the piece, mark it out, and then cut it on the bandsaw leaving about one cm extra around the perimeter.
The next thing I do is select which side of the belly is going to be the 'good' side, the one facing out, and I smooth it as best I can, first with hand scrapers, and then--as you see here--with a piece of shave grass, rubbed across the grain. 

I chamfer the edges of the belly on both surfaces, so that in the thicknessing process to come, I don't accidentally snag a piece of grain in the wood, and tear out a chunk with my plane. 

I then get down to thicknessing, using a caliper for reference and writing thicknesses on the belly. At this point, the numbers are written large and cover rather large areas; they will become smaller, and cover smaller areas, as the process goes on. 
The quickest, surest, and most accurate way to remove material is by using a very finely-set low angle block plane, and planing exactly perpendicularly to the grain. I'll remove material this way until I get within about 0.5mm of my final destination.

But what is my final destination? I can't exactly say for sure until I start working with the wood. I start out with an idea of a pattern of thickness I want for the belly, which I've developed through observing belly thickness patterns in old (and some fine modern) 13 course lutes. However, what the actual final thickness numbers will be depends on my assessment of the board as I get up-close and personal by thickness-planing and finally scraping the belly.

As I work, I find myself naturally checking thicknesses in a kind of grid pattern (and the grid gets more detailed the thinner I go.) 

I also draw in the locations of the top block, rose and bridge as I work, since these areas are crucial to the kind of overall thicknessing pattern I'm using.

When I get very close to the final thickness, I write large numbers indicating how many tenths of a millimetre further I need to go.

At this point, I use a very sharp hand scraper to take the belly to its final thickness. I try to blend together the various thicknesses in the pattern, making sure there are no abrupt transitions.


When I'm working at this stage--when the belly is very thin, less than 2mm overall (and in some places much less than that)--tapping for a tone to listen to is pretty fruitless. Instead, it seems to work much better to flex the belly to test its strength and resilience, both across the grain and--as I'm doing here--along the grain as well.

When I've gone as far as I think I want to go--when I've flexed, fretted, and tried my best to commune with the living material in my hands--I put my scraper down. My last step is to erase all the numbers I've written on the back of the belly.
I then hang my thicknessed bellies for approximately 24 hours in Grant Tomlinson's light box (in his workshop, right next door to mine.) This is a closet lined with aluminum foil and fitted with ultraviolet tubes, which we use to cure the varnish on lutes we are finishing. In this instance, however, I don't turn on the UV tubes, but turn on only the incandescent bulbs on the floor of the box.

The heat generated by these two 200-watt bulbs will be enough to lower the relative humidity in the box to around 20%.
After the 24 hour low-humidity period is up, I'll let the bellies return gently to the ambient shop humidity (of between 40 and 45%) over a couple of days. Then I'll prepare them for gluing rose patterns, and then, shortly after that, carve the roses. That will be the subject of my next instalment.

The name for this practice of taking soundboards from relatively high, to low, and then back to  higher humidity, is humidity cycling. Grant Tomlinson introduced me to the practice, and I've used it for years, as he has. The theory behind exposing the wood in a controlled way to higher and lower humidities is that it will eventually become (slightly) less prone to extreme fluctuations of expansion and contraction, and will make for a more stable instrument. That's the theory, anyway. I've read conflicting reports and opinions on the practice; some swear by it, while some think it's hogwash. (Same as it ever was, in the world of lutherie.) I'll continue to do it, since at the very least it causes no harm, and may, in fact, bring a small benefit. I see no reason to stop.

That is my report for this week. It's been a while since I posted, and if anyone has been waiting impatiently, I do apologize. My only excuse is that I've been busy building lutes!




3 comments:

Termokurt said...

Good job, Travis, Most piano builders have a climate chamber where they store their soundboards at around 20% relative humidity for a longer period of time: https://www.youtube.com/watch?v=3pn_YuoVbUA

James Louder said...
This comment has been removed by the author.
James Louder said...

The late Frank Hubbard, in the builder's manual for his famous harpsichord kits, also recommended thermal cycling for the sound board. If I recall the drill correctly, you were supposed to stand the soundboard in a closet for a number of days, where it could be humidified (did he suggest bowls of water?); then take it out and let it slowly re-addjsut to the ambient humidity. Or did he suggest first replacing the water bowls with a heater/d-humidifier, dry it completely; then bring it to ambient? Damn...ol' dog losin' his mem'ry!

Anyway,the point is, if you don't want things cracking all to hell, you have to discover what your maximum and minimum dimensions are. Still more, you want to relieve any residual stress that the wood may have picked up when you glued the two halves of the soundboard together; and to get the thing working as a whole for the first time. And if, for your sins, the board has some hidden potential to warp really badly, this is when you're going to find out--nope, this one's for your buddy who makes dollhouse furniture. Such nasty surprises aren't confined to lutherie. I've put keyboard blanks through a similar process and been mighty glad that I did.

Thanks again for sharing this account of your work and how you do it. I'm really looking forward to the next installment!