Scrap Yard Challenge
I've been toying with this idea for quite a few years but only now I finally got the main ingredient - stainless spring steel shim stock.
Some theory:
WHen you bend wood across heated iron the inside of the wood surface gets compressed and outside stretched. Since inside gets hotter sooner than outside the compression dominates and wood can be bent. But once you bend wood with funky figure which makes the outside surface less resistant to stretching or bend to tight radii then the outside will start tearing apart (lifts) or you will get kinks. Thin steel strap helps keep the wood pressed against the iron and keep the bending forces more even with less kinks (but one needs to pull VERY tightly the strap) but it will not prevent the tearing completely with highly figured woods. Lots of experience and patience is required to get some odd pieces bent into tight radii. Sometimes the only help is some extra thinnng of the rib stock so it can bend easier.
For bending thick wood beams workers use steam bending. Wood is first steamed in steam chambers and then bent against forms using bending strap that is RIGIDLY FIXED to the wood at both ends. This takes all the stretching forces from wood to the strap and wood is subjected to compression only - which it can take a lot in the freshly steamed state.
Some history:
When I was building my first mandolin almost 20 years ago, I was bending curly maple binding in 2mm thickness (which is exactly what I use for ribs) and I needed to bend it to really severe radii - the smaller headstock scroll binding piece had diameter of roughly 6mm or 1/4". Most makers just carve the binding piece out of solid piece of wood or use several thin layers of wood. I didn't want to do any of that so I did it just like the steam benders do. I took piece of steel sheet strip (galvanized roof flashing) and bent one end to a tiny squared hook to fit around end of the wood strip. This assured the one fixed end, the other fixed end was provided by holding the wood strip against the metal strap with flat pliers (strong enough but without crushing the wood). I wetted the wood and held it against my hot iron till all the water turned into steam and grabbed the hooked end with round nose pliers and bent it just like I would bend wire. Worked good enough and after several attempts I was able to bend the binding strips without breaks or kinks or crushing it unintentionally.
Few days ago:
I got my SS spring steel shim few days ago and immediately started planning how to bend the ribs in similar manner. It was clear that I can make the bending strap with a "hook" for one end of rib, but the other end was problematic. I used simple clamp in the past to hold wood against the strap but it would stand in the way of bending scroll piece almost all the way around my 2" diameter bending iron. I needed to clamp the strap/wood sandwich directly to the iron without obscuring the area too much so I can bend the end of scroll piece all around.
Now the scrap yard challenge....
Like with most of my other fixtures I strated by sweepeing the floor of my workshop and found some old plumbing pipe hanger some bolts and nuts an old clamp, some sheet metal and other metal trash.
I cut some length of the strap material (stainless spring steel 0.3mm thick, 1.430 steel if you want to know) and bent rectangular handles from pieces of thick steel wire and took pieces of sheet to attach them to ends of the strap. On one end I added 2mm thick spacer that created the "hook" to tuck end of wood against. (see pictures)
I had to shorten the pipe hanger as it was one size too big diameter for my iron, added cut piece of bolt and attached the half clamp. I had to file off sides of the bolt (8mm metric) to fit into the 6mm thick slot in the clamp. I replaced the clamp pad with cut off piece of steel I found laying around (I guess it was holder of my kids' bike helping wheels). That would clamp the strap/wood onto the bendingiron securely (less wide profile would be better).
First test:
I turned on my iron and took piece of random rib stock offcut. I didn't even smooth it out perfectly and intentionally left one end with saw marks across and some rough surfaces that would cause the piece break pretty much immediately when bent in conventional way.
The rest of the first test is in the video linked below and in pics.
http://gjgt.sk/~minarovic/bending.avi
Summary:
The bending went really well. I got one lift when I applied the clamp too aggressively in the start without waiting for the wood to heat enough. After that I could bend the wood with very little force needed - three fingers. The bend was perfectly smooth with no kinks and even the rough end of the wood could be bent without breaks or kinks (that was my second attempt not on the video)
I found out the pipe hanger is too flexible and weak for the required clamping force and the clamping pad turned around a bit but it worked OK for this prototype. I've got better design of clamping method on paper but it would require some welding etc.
Lesson:
Don't attempt to drill holes in hard spring steel shim stock without drillpress and proper holding vice or fixture. The drill broke while holding the pieces with my bare hand near it and I drilled/punched hole right through my left hand thumbnail and all through the thumb. Luckily it was just 2mm drill and close to edge of fingernail and it healed well in just few days.
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Originally Posted by CarlM
methods for a lot of my tight bends. It's amazing what a difference it makes to have the wood entirely under compression.
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