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Step #7–Joining the Book-matched Plates

I don’t have many photos of this, as I forgot about the camera when I was joining the plates for the cello. Not all instruments have a glue-seam down the centers of the plates, but most do, and especially most cello-sized instruments. It is simply too difficult to find good tonewood that is big enough for one-piece backs. One-piece tops are available (especially if you use Sitka spruce, as the trees are huge), but nearly everyone uses two-piece tops. One reason is the idea that since we usually “book-match” the plates, (meaning that a pie-shaped wedge of a log is split lengthwise to make two very narrow pie-shapes, then the wide edges are planed and joined in a hide-glue seam, to make a cross-section similar to that of a low, double-pitched roof) the grain begins tight, both sides of the glue-seam, then broadens fairly symmetrically, as it extends toward the outer edges. I don’t know whether this really makes any difference, acoustically, but it looks nice.

Planing the Center Seam

I used to do all my center joints with a #8 Stanley Jointer Plane (hand-jointer). That was a lot of work, and difficult to get it really flat. Now, on large instruments, I run the edges through a powered jointer first, to get the glue-seam as close as I can to being perpendicular to the inside plate surface. The jointer is a fast, reliable way to get close to a good joint, but I have never gotten a good enough joint JUST using the jointer that I could go ahead and glue the pieces together. I always end up using a good straightedge and a small, very sharp hand plane to fine-tune the joint faces until the fit is perfect– airtight. I check the full length of the face, at each edge and the center, then diagonally across the face, in both directions. If I have to, I use a scraper to remove very small humps. Once I have one plate perfect, I put the second one in a vise, and use the first one as a “try” surface–I run through the same routine with the straightedge, but then put the first glue surface on top of the second, and rotate it, to see if there are “pivot points” which indicate high spots. I use a scraper to bring those down until there seems to be an even friction all over, and an absolutely airtight, light-tight joint everywhere the two plates touch. Some luthiers use a “shooting board” to get the same results. This is essentially a plane affixed at a 90 degree angle to a flat board, along which the instrument plate is pushed; the plane stays still, and the board moves.

Preparing to clamp the book-matched plates

Then I saw out notches at the ends and (usually) in the center, near where the C-bouts will be, so that I can get clamps in three places. I try the clamping first (quite tightly) while the plates are dry– if the notches where the clamps sit are at a bad angle, they may force the plates apart rather than hold them squarely together.

Ready to Glue

When I am ready to glue, and the hide glue is hot, a brush is ready to use, etc., I heat the gluing surfaces with a clothes-iron, until they are hot to the touch. Be careful on this step, as it is easy to scorch the plates–especially the spruce. Another way to do it is with a heat-gun or hair-dryer. I have one plate secured in a vise, and the other ready to match against it.

With the plates hot, and the glue hot, (and pretty runny– not water-thin, but not viscous like honey– maybe more like thin syrup) I brush the glue liberally on both plates, then quickly match up the plates, rubbing the top one hard against the bottom, so that glue seeps out all around the edges. I want it to squeeze out everywhere. I only have a few seconds to get the joint tight, and lined up exactly right, because the glue quickly begins to gel, and then it will lock up. I line up the corners, then apply clamps at both ends.  I take the plates out of the vise, and apply a bar-clamp to the center notch. Set it aside and allow it to dry overnight. It will actually be strong in much less time, but I REALLY don’t like to have center-joints fail, so I allow them to dry overnight.

Completed Plates

You will notice that in the two photos below, I did not center-notch the maple plates. I don’t recall why…sorry.

Sitka Spruce, from the Wood Well (tonewood dealers near Port Townsend, WA).

Big Leaf Maple, from the same source. On the Spruce plate, you can see the remnants of the glue that squeezed out during clamping. They are not visible on the maple, probably because they are camouflaged by the curl (flame) of the maple. (nice wood, huh?)

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Step #6–Leveling the Garland

Stanley low angle plane, Sanding board

I began leveling the edge of the garland by checking the garland against a flat surface (plywood with formica– probably not dead-flat, but close), marking the most obvious high spots, then going around and removing them with a sharp small hand-plane (Stanley Low-angle).

Sanding Board

For final flattening I used a “sanding board” I made from a wide sanding belt (salvaged from a cabinet shop, where they are used and thrown away) glued to a doubled layer of plywood. (The plywood was 3/4″, which is not stiff enough to stay flat. I doubled it, using glue and screws, so it is 1-1/2″ thick, and about 24″ x 48″ flat dimensions.) I used a spray contact adhesive to affix the abrasive fabric to the board. I think the grit is about 220, and pretty worn. A little coarser would have made the job go faster, but this does really smooth work.

The board looks like this, after it has been used a while:

Using the sanding board

The way it is used is very simple: press down firmly and rub back and forth vigorously. There was probably between a half-hour and an hour of hard work, rubbing the two sides completely flat. One thing I do to ascertain that it is coming out flat, is to mark up the surfaces of the edges and ends of the blocks with a pencil. When all the marks wear off, it is flat. If you don’t use the pencil marks, it is hard to see the last few low spots.

Good exercise, I suppose. Tires me out, anyway.

The next step is to trace the shape of the plates, but we haven’t talked about joining the plates, yet, so that (joining the plates) is next on the list.

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Step #5—Installing Linings on the Cello

Strengthening the joint between the ribs and plates. Willow or Spruce.

After the ribs are exactly the way you want them, you will still need to add linings. The linings are strips of wood (usually either spruce or willow) that add thickness to the edges of the ribs, to triple the gluing surface and strengthen the joints between rib garland and plates.

In a violin, the linings are 7-9 mm wide, and 2mm thick; in the cello they are 12-15mm wide and 4mm thick, or so. I happen to prefer willow for blocks and linings, but many well-known makers prefer spruce. I just happen to like the way that willow behaves under the knife, as well as the way it retains a bend, once formed to the shape of the rib.

Cutting the Linings

One very effective way to make linings is to use a bandsaw or table saw to cut “boards” a little thicker than your intended goal for thickness, but several inches wide, and as long as you can manage. Then plane or in some other way re-thickness the “board” to exactly the thickness you want for the linings, and use a wheel-type marking gauge (http://www.rockler.com/product.cfm?page=21921) to score the edge from both sides, deeply enough that you can simply break off each strip. The result is linings that are exactly the same, every time, and with minimum effort.

Bending the Linings

You can use the bending iron to pre-form the linings to approximately the same shape as the bouts to which they will be affixed, but if you use willow, as I do, they are easily formed by hand, especially with some dampening with water.

Fitting the Linings

I inlet the linings into the blocks at either end of the rib, and carefully fit the lining so that it will fit tightly when installed. Once I have them all cut, I go through and glue each one: I remove the lining, brush it thickly with fresh, hot, hide glue, and re-install it, clamping continuously, with the small spring-clamps. I try to keep the edge of the lining just proud of the edge of the rib. I want to remove lining wood, not rib wood, if possible, when leveling the garland.

Gluing and Clamping the Linings

As I recall, it required about 108 or so of the small clamps to clamp all the way around one side of the cello garland. It is a good idea to have a few tiny c-clamps, as well, for stubborn spots where the spring clamps just aren’t providing enough pressure. The small f-clamps work well, too.

Using the Clamps; Warming the glue-joint.

The way I use the spring clamps: I pinch the lining to the rib with my left hand, tightly enough to see the glue squeezing out, then, while the joint is still under pressure between my thumb and forefinger, I apply the green clamp. Then I move over one space, and repeat for the next clamp. If I can’t get the joint tight that way, I use a tiny C-clamp, or one of the f-clamps in the photo. Also, I don’t hesitate to warm the ribs with a heat-gun, or my clothes-iron, to re-liquify the glue.

After both sides are complete and dry, I begin leveling the garland (next step).

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Step #4–Bending and Installing cello Ribs

Bending iron, bending strap, clamping caul, International Violin Co.

Proper tools

The best tool for this job is the traditional bending iron. You could make one that would be quite functional, for very little cash outlay, but the nice thing about the “traditional” ones, available at International Violin Co. and many other sources, is the variation of curves available. The Iron (by the way, they are no longer made of iron, but aluminum…the name has stuck, though) is “egg-shaped”, and on the small end has a tight enough curve to form the tight bends at the corner blocks, but at the center, it is flat enough to gently curve the portions of the ribs where they span the lower bouts of a large instrument such as a cello. 

The Bending Iron MUST be hot!

The key to successful rib-bending is that you MUST get the wood hot all the way through before commencing the bend. If you don’t do this, you can almost guarantee you will break the rib. How hot? Good question.

I used to use a clothes-iron, and wooden molds, around which I would bend my ribs. The clothes iron was rigidly fixed in position so that the hot side was up, and the thermostat control was set on “linen”. At that temperature, if you are not careful, you can scorch the wood. At that temperature, also, if you dribble water on the iron, it dances, and bounces as it boils away to nothing. It can’t just sit there and steam. So that is my test on the bending iron as well. I want the water to dance and boil instantly, not just sit there and simmer. On the commercial bending iron, it takes 20 minutes or so to get hot enough…maybe longer. Be patient: this is critical. The iron has to be hot.

Bending Strap

The bending strap can be bought, but, if you can secure a thin strip of aluminum (aluminum roll roof-flashing is almost too thick, but it will work) you can affix handles of wood, and it will be fine. For violins, I cut up one of those large beverage cans, in a spiral (as a cardboard tube is made) , to produce a strip about 40mm wide, and long enough for a bending strap.

The bending strap accomplishes several things. The most important, in my mind, is that it supports the outside of the curve and prevents the wood from “stress-relieving” through fracture. Especially in heavily figured woods, it is very common for an unsupported rib to break along the curl. (Rats! Your beautiful rib just became beautiful firewood!) The second thing is that it allows you to put pressure on the whole part of the rib being bent, as it gets hot enough to really hurt you if you touch it, without endangering your fingers. It is a good idea to put handles on the bending strap. (By the way, some people use cloth, instead of metal, for their bending straps. Brass or aluminum are the most common metals used, though.) Metal transfers heat pretty rapidly, so wooden handles are very helpful. Thin gloves aren’t a bad idea either, but I usually think of that after I am well into the bending process and it is too late to stop and put them on.

How Much Water?

Some folks bend their wood dry. Some soak their wood. I use just enough water to get the wood wet on the outside (spray it on; no soaking) and enough to produce a little steam while I am heating the wood. I want it completely dry when I remove it from the heat.

Bending the Ribs

Method

Press the wood against the flatter porton of the iron while it is heating (count off at least ten seconds), then slide it toward the end with the curve that most closely approximates the bend you need, and slowly, firmly press the wood into shape with the bending strap against the bending iron. Hold it another ten seconds or so, and then let off a little, to see if it will hold that curve. If it springs back, pull it tight again, and hold it some more. Pay attention to smells, at this point. It will smell like something baking in an oven. You do not want to scorch the wood. (Try this with wood you can afford to lose, a few times, before using the nice expensive rib-stock you bought.) I broke and/or burnt a number of ribs, trying to learn this skill. But I know a guy who uses an old piston from a diesel engine for his bending iron, and heats it with a propane torch. Others have used water pipes, curling irons, charcoal briquette lighters and the like. I finally got a “real” bending iron after I had made 15 instruments.

Specific to cellos: I made my thickness 2mm, and used a large bending strap, as shown. While you can’t see the mold in the photos (I forgot about the camera until the real cello ribs were done and installed) what I did was to set the cello mold nearby, and bend a rib, then check the bend against the actual mold, to see if it was close to correct. Then, back to the bending iron, to correct it. When a rib was satisfactory, I immediately clamped it in the mold, to hold that curve while it cooled. Once it was cool, it would hold that curve permanently.

Sequence

The reason we do the inner c-bout ribs first is that there is frequently a fair amount of pressure put on the ribs during the gluing/clamping process, and if the corner blocks were cut to their final shape, they could flex, and permanently change the shape of our corners.

After the c-bout ribs are glued with hot hide glue, and thoroughly dried, I shape the outer sides of the corner blocks. The black lines you can see below are ink, where I accidentally allowed the template to move as I was marking the corner blocks. Those marks were removed when I leveled the garland, which we will talk about in a later article. The curve of the outer side of the blocks is extended through the C-bout rib, so that the upper and lower ribs overlap the C-bout ribs.

Clamping and gluing

I cut clamping “cauls” (curved wooden blocks) to help hold the ribs tightly to the blocks while gluing. The holes in the mold are positioned in such a way that the clamps will be pointed the right direction to achieve maximum hold while gluing. Do four or five “dry-runs” before actually applying the hot glue. In fact, it is a really good idea to have things clamped in place, and just remove the clamps on one end long enough to apply the hot hide glue and re-clamp immediately. Clean up around the joint, immediately, with hot water and a rag or brush. Let the glue dry overnight. If you have to re-work a joint, a teakettle’s steam, as close as possible to the spout, will unglue a joint in about 20 seconds. Watch the fingers. Steam is hot. (Close to the spout, where the steam is invisible, is where you want to be.) You can get a serious burn if you are careless.

I use a $3 “potpourri heater” from the Goodwill store for my glue heater. I put an old yeast jar in it, and fill with water around the jar. I keep the lid on the jar when I am not actively using the glue, to keep it from drying out. It seems to work pretty well. I am told that 145 degrees F. is the correct temperature for hide glue. You can get good glue from a variety of places. I’ll talk more about glue, and the glue-pot, in a later article.

After the glue is completely dry, I cut off the excess upper and lower ribs at a right angle, exactly at their intersection with the C-bout ribs, so that the glue lines follow the inside corners of the upper and lower rib ends, and are essentially invisible.  (I do NOT “miter” these corners–I hide the joints by putting them right on the corners of the squared-off ribs.)

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Step #3 Making the Mold:

Interior mold

Cheap, plywood, collapsible mold

The mold is a structure to hold the shape of the ribs until they are glued to the front and back plates, as well as to hold the blocks rigidly until the corpus is assembled. Since the plates actually derive their respective shapes from that of the mold and ribs, it is very important to get the mold as accurate as possible.

Planning Block attachment–and removal

In small molds (violin, viola, etc.) the end and corner blocks are simply glued into the mold, and the glue line is broken later, in order to remove the mold from the body of the violin or viola. (Remember that the blocks are a permanent part of the instrument– the mold is temporary. It has to be right, but it can be “cheaply right”, or “expensively right”. I went with “cheaply right”.)  In larger instruments it is possible to secure the blocks using screws from the inside of the mold, so that it is not necessary to break glued joints in order to remove the mold.

Collapsible Molds

Furthermore, it is desirable to be able to remove the mold piecemeal, so as to not stress the rib structure trying to pry the mold out. (I have broken more than one rib, trying just a little too vigorously to remove the interior mold.)

As you can see, above, the main body shape was simply built up with layers of plywood. I had a bunch of scrap plywood in my shop, so I traced out individual sections, from the mold template (from step #2), cut them all out, stacked them, and secured them with glue and drywall screws.

What you may not be able to tell is that the left and right sides of the main part of the mold can be released from the center blocks on each end, to facilitate mold removal. They are not glued there– just held with screws. The front and back sections are one-piece each, but are easily removed before the linings are installed. (The linings are what makes mold-removal tricky. We will talk about linings later on.)

So, essentially, the whole mold is in six pieces: left and right, top and bottom end blocks, and front and back sections. After they were all assembled, with extra room left around all edges, I checked one last time, using the mold template, and cut the mold out on a bandsaw, then smoothed the sides on an oscillating spindle sander (like a drum-sander). The sander keeps the edges at a right angle to the front and back. It is really a handy tool, which has paid for itself many times over.

Above, you can see that the front and back sections of the mold are secured only with screws, so they can be easily removed, after the ribs are in place, so the linings can be installed. I used about eight screws each, front and back.

Here you can see how the deck screws pass through the end block of the mold, to secure the end block of the cello. You must be sure to use short enough screws that they will not protrude all the way through the block. You only want the screws to penetrate the blocks by 3/4″ or so, and right along the center line. If you ever run into one of those screws with a cutting tool, you will be shocked at the damage it will do to your expensive tool. Notice that in this photo, the blocks are already shaped. In reality, they are installed in rectangular form, and shaped afterward. See the next picture.

Tracing the Template shape onto the Blocks

This is how the mold template is used to trace the shape of all six blocks. You trace one half, then flip the template over to trace the other half. Be very careful to do this step accurately, as it will determine the final shape of the cello.

Shaping the Blocks

I cut the inner curve of the center corner blocks close to the line with a bandsaw, then used the spindle sander again to get exactly to my line. I wait until after the C-bout ribs are installed to trim the outer part of the center blocks. I’ve never had a block fail, but I have been told that if you cut them too soon they can be fragile and flexible, and you may end up either bending or breaking them.

Next we’ll talk about bending ribs.

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Step #2–Making Templates

Mold template, scroll template, arching template

How do I transfer those lines?

Having chosen to use the 1712 Davidov Stradivarius cello as my pattern, I went to a copy shop, and had them make a photocopy of the drawing on the back of the poster. They made three copies, as I intended to cut the copies up in making the templates. I pasted the cut up photocopies to thin plywood (“door-skin”), and then cut out the shapes seen in the photos below. An even better material for templates, produced in exactly the same way, is 1.5mm Aluminum.

Still another way, which I use on smaller instruments, is to use clear, 3mm plastic (lucite, plexiglas, whatever you call it) and trace with a scribe, directly over the original drawing. The resulting template is accurate, transparent, and does not warp with a change in humidity. Your choice…

How Templates are used:

The idea behind the templates is that I should have a flat pattern to trace around when determining the shape of the mold and a curve to match the shape of the archings as I carve the topography of the plates. Photos below: Continue reading “Building a Cello–Step #2: Making cello templates for molds, scroll, and arching”

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Step #1– Choosing a Model

Ruggieri, Montagnana, Stradivari, Amati, Maggini, Guarneri, Gofriller, Gasparo da Salo, Strobel, etc.

What Cello should I build?

There are almost as many different models of cello as there are of violins. Stradivari made them, as, apparently, did Giuseppe Guarneri, as well as those before and after them. Both Andreas Amati and his illustrious grandson, Nicolo Amati, made celli. Franscesco Ruggieri, Andrea Guarneri, Gasparo da Salo, Giovanni Maggini, Matteo Gofriller, and Domenico Montagnana are other makers famous for celli. But I really was going to choose between only three possibilities:

1. Someone had given me a plan for the Montagnana “Sleeping Beauty”, which has a good reputation for a big sound.

2. There are two complete sets of plans in Henry Strobel’s book, Cello-Making, Step-by-Step–one a full-size model, and the other a 7/8 size.

3. I had bought a poster from The Strad Magazine, with a detailed drawing of the 1712 Stradivarius known as  The Davidov.

The Davidov was being played by Yo Yo Ma, a world-class cellist. I really liked the looks of it (the Sleeping beauty is kinda chunky), and it is a known entity (those in Henry Strobel’s book are lesser known). So, with all that (and the fact that Paul Schuback assured me it was a good choice) I settled on the Davidov. Nothing arcane about it– I just grabbed the one I liked, really.

One thing that had been a vote in favor of Henry Strobel’s plans is that the plans were already drawn, including the lines for the mold. The poster gave all the information for the original instrument, but I had to come up with the mold template myself. That was OK, but I ended up a few millimeters oversize. My fault.

Photo-Essay of my first cello-build

In case anyone wants to see this whole series at once, here is a link to the original blow-by-blow photo-essay on Maestronet. It took 13 weeks of spare time work from start to finish–between 250 and 300 hours, I would guess.

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