My first air hose form was made out of laminated plywood, but had a couple of problems. The biggest was that the bolt holes for the tie straps would deform under load (1.5" x 72" x 80 psi = 8600 lbs). Which made it tough to keep the form nicely aligned. Second, it took a lot of wood to make the form and then that form took up a lot of space. So messing with multiple forms became problematic.
Being an engineer with ADD who sits in a lot of boring meetings gave me time to overthink this. What I came up with was a single form frame out of metal that could have various wood inserts swapped in and out.
All of the load is assumed to be carried through the metal components, the wood is just there for shape. Since the air in my hose expands in the oven and my pressure often goes over 90 psi while cooking, I ran the calcs for minimal deflection at 10,000 lbs of load. I decided to go with 1530 extrusions from 80/20. (1.5" x 3.0") 80/20 extrusions are anodized aluminum and very straight. Steel tube would have worked as well and been a bit cheaper. However, 80/20 also lets you add moveable feet to stand it up and clips for attaching the wood inserts.
The tie-straps are also aluminum (6061-T6) and sized to carry the load. (Note they actually carry 20,000 lbs combined.) I think the cross sections are 3/4 x 1/2. They bow out from the form in order to allow room for the air hose or wood inserts wider than 1.5". These straps are through bolted to the extrusions at four fixed locations with 5/16" grade 8 bolts. These through holes were drilled carefully and precisely to make sure everything stays square and true. If not all drilled the same, you could end up with some of the straps not carrying their share of the load. If not drilled true and square, the top of your form will cock over slightly relative to the bottom.
The straps in the picture are sized for 6" wood inserts which works well for longbow forms. I also made a set of tie straps that allow for 12" wood inserts when I want to make bows with more curvature in the limbs.
I've found the form bolts together very quickly and easily. It weighs about 60# with a bow in it going into the oven, which isn't that much heavier than my all plywood form. My glue-ups have all come out of it nice and straight and trouble free. I think it was definitely worth the investment in materials and construction.
If y'all have more specific questions I can try to answer them. I can probably also take some more pictures if you have certain areas you want to see in more detail.
Topic: What did you do today, 2015.... (Read 22867 times)
