Thursday, September 06, 2007


I finally feel I'm getting the to core of the build. Last week it was the soundboard thickness and this week it's the side bending.

It was a process I was looking forward to with both
enthusiasm and anxiety. It was completely new territory to me, as exciting as it was scary. To prepare for this I read up on as many books I could, most notably Cumpiano and Courtnall. I also posted a thread at Luthierforum and got many responses with helpful hints.

It all lead to the fact that, the best way to start bending was to practice on some scrap and so I did. For this I used some flat sawn walnut
I had and a cut-off from the bubinga I'm building with. This gave me a chance to gain some experience with different bending techniques and setups.

I started with the walnut, sanded to 2mm thickness, and went at it haphazardly on purpose to see what mischief I would get myself into and then tried to get myself out of them again. All in all the walnut felt very leathery in it's elasticity and I had many problems controlling it. It was very difficult to manipulate one area while retaining the bend in another. I also ended up with some kinking, making it impossible to lay the bent side down on a flat surface without one of the bouts twisting upwards. I tried bending both dry and sprayed with distilled water and the wet was definitely the more responsive of the two.

Eventually, after lots of unruly bending and unbending, I kind of got it into a shape somewhat close to the template I was measuring it against, and left it as such. However, the next evening I noticed that it had curled up quite a bit, meaning an overbend had occurred in both bouts. I should mention that I only sprayed the walnut on the face that contacted the pipe, and my guess is that it could have been a result of moisture still being in wood that had evaporated overnight; Maybe a good argument for soaking or spraying both faces regardless of which way the side is being bent. After some dry touch-up I managed to get it back in shape again.

I also had a quick go at the bubinga. It felt considerably tougher than the walnut. Whether this was because of it's density or the quatersawn grain orientation I don't know but, it took considerable more force to manipulate and once bent, it seemed to hold it's shape much better. I tried bending it wet, both after 10 minutes of soaking and from spraying it on both sides but I didn't notice the wood behaving much differently between the two methods. However, like the walnut, it was considerably less manageable when bent dry. I really tried to push it and did manage to fracture it a little when bending it on the tightest spot on the iron, but only after using considerable force.

I also experimented with the bending iron in different positions - horizontally, vertically, with the flat side facing me, with the round end facing me, etc. I ended up with it clamped vertically to the bench with the tip of the egg shaped pipe to my right. I came to the conclusion that the great advantage with the vertical setup was that my eyes were in line with the pipe as I was looking down upon it, making it easier to check whether I was pressing the wood against the pipe with even force across the side or not. On the down side it is more difficult to see whether you are holding the side perpendicular to the pipe or nor. It also took more muscle to bend the wood compared to a horizontal setup where gravity would be working in your favor.

After these practice sessions I felt a little more confident and eventually took the plunge and started on the 'real' sides
. I started by laying both sides on my bench in a bookmatch. After measuring my template with a sewing tape measure, I marked both sides with a line across the position of the the heel, waist and butt and named these accordingly, leaving about 5cm extra stock at each end. I also added arrows to each line indicating which edge was going to be glued the top. This way I was able to avoid bending both sides the 'same way' and thus ruining the bookmatch. However, when I was done bending both sides I realized that I had bent the sides so that the true bookmatch on the inside and that I should have bent them both the other way. It's not a big deal as the sides are perfectly quartersawn thus having a good match on both faces, but nevertheless a shame I didn't get the better of the two on the outside. We live and learn!

After the marking was done, I wetted one of the sides by spraying it with distilled water on each face, put it to the iron and started bending the waist. I used the flatter side of the iron to heat the wood and slowly fed it across
with my right hand while pulling it towards me with my left. Soon the water started to sizzle and evaporate into steam and I felt the wood giving in under the pressure. Slowly the waist emerged and after a few goes it fit the template. After cooling it a little I went on to bend the upper bout. This was done in a similar way, though because the bend was more gentle, less force and a faster feed was needed. The upper bout was a little more tricky to match as it had to be done in a more free form fashion, whereas the waist fitted the bottom of the pipes oval quite well. But after a few goes and some areas of unbending and rebending, it eventually settled quite nicely. The lower bout was done likewise, though with even less pressure and faster feed.

After the whole shape was done a few problem areas became apparent - some where the bend had gone bumpy, a few overbends and some areas where the bend had gone off the template and back again.
I had to respray the side to correct the more severe ones, while others could be dealt with by bending dry, either locally or by sliding the bend across the pipe applying pressure by wrapping my hand around the wood.

Eventually it was pretty much within the tolerance of
±0.5mm and I left it overnight to cool and dry ready for a final touched up the following evening to get it as close to dead on as I could. More touch-up might be needed once the sides are ready to be glued on.















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