Firstly, a quick introduction. I'm a beginner builder from Australia currently working on my fourth and fifth instruments. I've lurked this forum for months and I'm looking forward to participating. It seems to be the most helpful and friendly I've encountered so far.
I was hoping some of you might be able to offer some advice and ideas about bending a venetian-style cutaway. I've made one unsatisfactory attempt and hope to gather some ideas to change or improve my process before trying again. First, I will layout the process for my unsuccessful first attempt.
The instrument in question is a very large jumbo (17" lower bout), similar in style to a Martin J-35. I'm building it for a family member who requested a cutaway. The back and sides are padauk.
I'd never attempted a cutaway before, but looking on it as an educational opportunity I dove in.
I was not able to find much information online about how to bend a cutaway. None of the construction books or DVDs I own delve into the topic at all. The only piece of advice I saw offered consistently was to thin the sides more than normal. I also saw several photos that seemed to show a fox-style bender with an extra press ram being used.
I happen to bend using a fox-style machine (purchased from Blues Creek Guitars) so the first step was to build bending forms for the cutaway and non-cutaway sides of the instrument:
That done, I fabricated an additional press-ram for my bender:
I must admit I still didn't feel confident the set-up I'd created would actually work very well; it just didn't seem very controllable; having to manipulate the second press-ram up from the waist and around the cutaway 'horn' and then into the cutaway itself as it was wound down seemed like a clunky process.
However, I embarked on some test bends using scrap:
As you can see (from right to left) over the course of four practice bends my results improved a little. Still not a perfect fit at the point the cutaway scoop transitions into the neck, but close.
It was at this point that impatience got the better of me and I decided to go ahead and bend my sides. I thinned them to 0.070 (1.8mm for my fellow Australians!) and bent at around 300f to 320f. Sandwich was (bottom to top) steel slat, wood (spritzed lightly, wrapped in tinfoil) blanket, steel slat.
Here's the cutaway side in the bender:
I was happy at this point as the steel slats and the wood in between them seemed to be conforming tightly to the cutaway curves of my form. In my previous two instruments I never had much springback with just a single heat cycle. But they weren't cutaways. Fearing springback, I followed some advice I'd seen in several places around the web and did two heat cycles, about four hours apart and then left the side in the bender for an additional 24 hours before releasing it. Here's how it looked:
No cracks, lifting fibers or cupping and the lower bout and waist bent perfectly... but the cutaway portion of the side experienced the same problem I'd previously encountered on my test bends - the 'horn' bend went acceptably, but the recurve portion seems to have sprung back so that the end of the side does not successfully line up with the side of the neck block. This pic shows the problem:
At this point I tried to 'touch-up' the bend on a hot pipe but my skills and setup (heat blanket sandwiched between steel sheets clamped into a curve of the appropriate radius) in this area are poor and I was unable to get the bend much tighter. Eventually fibers began to lift, cracks (parallel to the grain) began to appear and the timber began to scorch beyond what I felt I could sand out. Perhaps I'd overcooked it in the bender and it had no more bend left to give? Perhaps I'm just too inexperienced at pipebending!
The good news is I anticipated this situation and had purchased backup padauk sides which match the back fairly well.
So after that rambling tale of adventure and mystery, can anyone offer advice or tips on where I'm going wrong and how I can move towards getting this bend executed successfully? I've gotten this far on my own but I'm unsure about what parts of my process I should change before trying again.
All advice greatly appreciated!