I think this will round out the topic of steam bending thick kiln dried walnut stock. On that note of the stock being kiln dried. I should say that I don't have a moisture meter and thus I can't say how dry the stock is. I am in Houston currently so my humidity is very high. I would assume that the walnut may be around 10 percent or maybe even close to 12 percent moisture content. The higher the moisture the better the heat transfer into the stock and the more successful the bend from my experience. The only draw back to stock with a hight moisture content is that it will take longer to set to the form. Until the moisture is quite low the bend will spring back. If you have the choice of green, air dried, or kiln dried... choose air dried. You get the best of both worlds for most applications. On to the final part of this discussion.
When making a bend with thick stock, as was noted in part 3, a strap is necessary to restrain the outer face (convex) of the board from stretching while producing more compression on the inner face (concave) of the board. However you reach a point where the inner face will distort and cause compression failure, showing up as wrinkles. You do not want this even if you are carving away some of the surface. It is very difficult to carve since it creates reversing grain as in highly figured wood but worse. Another problem is that you reach a limit to which the wood can be compressed and you may not be able to complete the bend due to lack of enough force that can be applied to continue compression. I use a 2 ton (4,000 lbs) come along and that will not be enough pressure if the right technic is not used in some cases.
So here is what I did with the bend under discussion. I should note that some use an end block that is adjustable that can release pressure as the bend is being made. I have not tried this method since I am a one man show and need to be doing the bending. However when I grow a third arm I will try it and get back to you with results. It also seems to me overly complicated to make such a set up and I am not convinced that it is the most versatile route or very predicable either. So this has been my solution...
The idea of the end block that you can use to release pressure during the bend is to reduce pressure on the inner face thus allow it to lengthen rather than compress to the point of failure. So I came up with a simple solution to do this automatically during the bend. I simply cut a bevel on the end of my board. It makes the outer face in effect longer and the inner face shorter. This is what results from bending wood. So what does this do? When the bend starts only the outer fibers of the wood are touching the end block and thus being restrained. At that part of the board we are concerned with stretching but as the bend progresses more and more of the wood begins to make contact with the end block and as we pass the neutral point we begin to compress the wood fibers. But not all of the wood fibers below the neutral point are making contact thus no compression at this point. We have delayed compression. So what I am doing is allowing the wood to sort of shift over rather than start to compress. Only once the end fibers touch the end block do they begin to compress. As was mentioned in part 3, the further you move toward the inner face from the center neutral point the more compression occurs. Therefore by delaying the point during the bend at which the end block begins to make contact with the wood fibers less compression will occur overall to that layer of the wood. Basically this balances out the amount of compression that occurs as you move away from the neutral point.
The amount you need to make the bevel varies in each case. Do some experimenting. In this bend I use about a 3 to 4 degree bevel.
Hope that all makes sense. Wish you the best in your work.