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Bent wood laminations1/2
I'm a carpenter by trade and have a full-time woodworking shop that I do for my projects. Currently making a 36-in exterior door that has an 18-in radius top and I made a laminated door jamb out of four plies of poplar. Skipping ahead through the basics of a proper bending form I got approximately 5/8 of an inch spring back. It's approximately 70 in my shop I use tight bond three and placed my calls and clamps about every 5 in around the perimeter of my lamination. Any thoughts on how I can limit the spring back being as it is a door jamb the reveals have to be somewhat perfect any suggestions are welcome thank you.
Also to describe my form I did a full 18 in radius from side the side and to extend the jamb legs down I've got two 5inch
You will get better responses if you post this in the Architectural Woodworking Forum.
One thing you can be sure of, anything made of bent laminations, coming off a form, will not be what you planned for. Or needed.
There are formulas and rules of thumb, and other methods (dart board) - none of which are accurate except by chance. Generally, the more laminations, the closer the laminations are to what the form is. Four layers of Poplar will not hold much of a curve. You also need to use a rigid setting glue. TB III is the opposite of what type of glue you need. TB I, epoxy, resorcinol, urethane, plastic resin, will all work well.
However, I seek to change your ways. I suggest a 'brick laid core with a face". This gives you a sawn curve that is accurate and predictable. Saw out blanks to the inside radius, plus 3/32". Glue them together, to get your 6-5/8" or what is needed. Butt the ends of your blanks by cutting to a tangent. Once out of the clamps, clean up the inside face and add a 3/32" thick piece of Poplar or whatever to present a clean face.
If you are making a rabbeted solid jamb, then you will need to make two assemblies, one with a 1/2" longer radius than the other, to create the rabbet.
The sawn core is rigid and predictable. The thin face is pliable and glues easily, and presents a good look. No form needs to be built. Looks as good as bent laminations, but performs better.
David I am not sure I am following what you are saying.
Do you have a drawing or photo that will help me visualize it?
Bill - Let's say you need a 36" inside radius head - ISR. And being a door jamb, you need to end up at 4-5/8" wide, and 180 degrees.
Make a template - layout one curve at 36-1/8" and another, concentric at 37". Cut out and reproduce onto your wood - surfaced, flat and true. Let's say your wood is 1" thick. Lay your template and trace onto the wood so as to get as many parts out of the piece of wood as practical. Saw them out. Cut the ends so they can butt to each other when placed into the arch.
Draw your two radii onto the bench or plywood or where you plan to assemble. Lay your parts onto the drawn curve and build it up staggering the joints - bricklaid.
Take it down, spread your glue and rebuild. You can nail as you go, or clamp a couple of pieces, then wait and add a couple more.... Then you will have a curved wall, greater than 180 degrees, and 5" tall. Clean up with a sander and or scraper. Add a 1/8" solid ply to the face, using several bending wood cauls, and clamping every few inches. Use a rigid glue line glue for this last step. TBIII or equal will allow that skin to creep.
When completely dry, pass it under the planer, over the jointer, or thru the saw to get it to 4-5/8"
2nd post on here where poplar and exterior doors have come up. Even for a jamb, poplar is horrible. You will get minimum spring back if you use thinner laminations and a glue that is rigid when cured. I can't say how much thinner of stock since we don't know what you used.
The springback is greatly reduced if you use plastic resin glue.
What we do is make a band saw swing table at 19R. Poplar is fine for this. Cut 18-24" long boards ( 4 or 5/4 work best) and band saw them each to get pieces not less than 1" at the ends. This gives you a stack or arcs, all with a 19R on both sides. Take the top offall from one board and clamp it to the bandsaw table square to the blade leaving 7/8 between the top of the blade and the inside of the arc. Slide all the arcs through the saw. you now have a stack of arcs 19 OSR ,18.125 ISR. We put a sled on the table saw and cut all the ends, again using the top of the first bandsaw cut as our cradle. You now stack these, glueing and stapleing as you go until you have built a jamb head. You either build to size or cut the ends when you're done. I like to cut at the end because it gives a perfect edge. Beltsand the interior and wrap it with 1/8 solid skin. You have a head that is 18ISR 19OSR. We do a 1" leg and shoot a scab on the outside.