Bent Lamination Springback

      Advice on limiting springback in a laminated piece with a fairly gentle bend. July 13, 2010

Question
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".

Forum Responses
(Cabinetmaking Forum)
From contributor F:
I would make the radius exact. Your curve is slight and jambs can be sprung and fastened with shims.



From contributor E:
I did my first bent laminations late last year and had all types of problems with springback. I ended up making my parts (drawer faces with 24" radius) out of 23 pieces of white oak veneer - no springback!

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.



From contributor L:
We do lots of radius work and from my experience springback comes mainly from choosing too thick of a lamination and taking it off the form too soon. Some glues don't work as well because they are rubbery even when dry. The give in the lamination glue lines gets worse when there hasn't been enough pressure or there's been uneven pressure. We've had too many laminations on occasion and the part will pull in to a tighter radius as it fully dries after it is off the form.

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.



From contributor Y:
From experience:
1) I would try to use solid material, joined and radius cut. Good for paint grade, also.

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.



From contributor L:
My problems with bending luann, wiggle board and the like are that: 1. It never bends evenly, usually facets, and the facets show through the veneer faces, 2. It is so rough, both from the way it bends and from the rough outer veneer, that it is not the best for gluing veneer to, and 3. How do you deal with the coarse end grain that shows on the edges of your jamb? The exposed edges on a jamb are greater than the veneer in thickness. For tighter bends we use hard setting glue, but for an easy bend itís not worth the mess. Hard glues (urea formaldehyde) don't have gap filling ability. When they fill a gap, the resulting glue line is very brittle. What method you use and what is acceptable will depend on your available technology and skill set.


From contributor Y:
I always try to use bending luan or bending birch as a first choice for laminating. I make the radius forms with a bandsaw or router and guide system. They are a smooth radius so the problem of facets from laminating hasn't been a problem. I don't remember having had a problem with the roughness of the luan and veneer. I probably have touch sanded with coarse paper if there was. I would laminate, joint and then saw the opposite side of the jamb. The edges should be smooth enough to glue the veneer.

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