I need to bend and laminate some douglas fir boards for a current project. Sadly, details are my business so I seek insights on the below areas. All the information that I have read says I'm on the right track, but I only have one shot and need to do all I can to ensure success.
Here are the particulars:
Chord length: 65".
Rise height 6".
Air dry boards.
Goal is to bend a 4/4 plank.
7 plys to yield a 7" wide beam.
Boards are 5.5" wide (beam to be 5.5x7 when done).
Glue will be West System 207 epoxy.
Question 1: The boards are all quarter sawn. Is this trouble? I think flat sawn is better but I don't really know.
Question 2: Should I have a form to support them? Will a slight dampening help the wood, but hurt the epoxy? Regarding question one - I can make flat sawn stock, but prefer the quarter due to sawing conditions (we are sawing beams up for this).
Question 3: Could we go thicker to 6/4 plys?
From contributor S:
Is it a support beam? If it is, I am not sure you can build it without an engineer’s stamp. I designed and built my house and there are a lot of exposed fir beams in the main living area. When I submitted the drawings to the building permit people the only thing they wanted was an engineer’s approval for those beams.
Reduce the thickness of your stock until it becomes a manageable cold bend. No on the 1/4 sawn boards. They won't bend (or resist bending) along their vertical grain. Yes, either way you will require a very substantial bending form and clamping method. Why not glue-up your solid beams to form your radius pieces? You can simply band-saw them out.
You stated that you only had one shot at getting this right. That actually isn't correct. I would never attempt an extreme bend without doing a dry clamp up first. I believe that when you try to do this without glue you will discover that it isn't going to happen. At best I think you might pull it off with 1/2" thick material, but even this would be a massive undertaking. Using 1/4" thick material would do the job though. And, with that thickness you could use West Systems, although there is certainly nothing wrong with going to Resorcinal.
There is one other issue that has not been brought up by anyone as yet. That is the uniformity of the bend. If you take a 4' long 1/4" dowel and hold it by the ends in front of you, you'll discover that when you bend it, it bends nicely in the center but remains straight at the ends. That is because there is no wood pulling it into a curve beyond your hands. The ends of a stick will always be straight when you bend without a form. So, in answer to your question: yes, you will need a form to make the bend successfully.
Yes it was a 5.5" x 7" beam. Planks were 7' long to yield a 5.5' curve. Yes we used a form to ensure even fiber stress on the convex side.
We did not do a dry bend - slipping the surfaces under such friction without lube (glue) would be a risk. Resorcinol is a tough glue to use because it requires 125 PSI of clamping pressure to make for the best results. My hand clamps cannot produce this with predictable results so epoxy was the adhesive of choice. Yes it took 1.5 tons of force (best guess) to bend the 11 plys in the stack, which a car jack did a fine job with. I recommend this as the control was excellent.