This is my first experience with bent lamination. I'm doing a set of French doors with a radius at the top of 167 1/4" (I know, kinda odd). The doors are not a problem, but I need to bend the top jamb. Everything is going to be soft maple since it's getting painted. The doors are only 5' wide, and it has matching side lights that are 2' wide on each side with curved top frames as well. Question is, how much springback should I account for when I build my jig for the upper jamb glue up? The jamb will be standard 4 9/16" wide finished (3/4" thick) and around 6' long finished. Should I make the entire radius a little smaller, or just curl the ends in a little more on my jig? It's not a real tight radius, so I would expect less springback than if it were say 100".
From contributor F:
I would make the radius exact. Your curve is slight and jambs can be sprung and fastened with shims.
For accurate parts like this I would definitely use this approach again, but I think the more gentle radius you are dealing with plus the fact that you can spring the jamb might make it less of an issue for your project. But you won't really know until you try.
Your project is a very easy bend, so using slightly thicker than 1/8" laminations, regular Titebond glue and either a male/female mold or a vacuum bag should work fine. If bagging, wrap the laminations in plastic sheeting held in place with masking tape, insert into your vacuum bag, clamp at the midpoint to the form, pull the ends down tight to the form seal bag with netting under the vacuum ports and turn on the pump. Cook until well done - two hours? Leave clamped to form overnight, but you don't need the pump on past two hours. Remove from bag and again clamp over form for a day or more of further drying (a lot of moisture gets added by the glue). Make the glue-up 1/2" wider than needed and push it through the table saw to trim both edges. Two people helps. If you made a male/female mold: the inside form should be very rigid and the outside form somewhat flexible so it will give and conform the required shape evenly. We use a Taylor clamp rack when using the M/F mold method. It allows plenty of force and then we use pipe or bar clamps across the ends to pull them in tight. On large glue-ups it helps to have two people to work fast. Use a 4" glue roller (not paint roller) to get an even spread.
2) For bent lamination, use bending luan. As thick as you can get away with. Thicker jamb? Then veneer for paint. Minimal springback. Allow almost none. Has very low springback on panel laminations also. Use hard glue line. Plastic resin works great.
3) Solid bent lamination. Last choice. Use same glue for hard glueline. Thinnest laminate that you can endure.