Springback formula for lamination

[Mel Fulks]

Mike and Sam have been brave enough to mention it so I'll follow up .Years ago a guy in England (I think)
sent a formula to Fine Woodworking . A most contentious and even acrimonious debate followed. I vused the formula for years with good luck. Has any one else tried it or remember it?

[Sam Murdoch]

Maybe this page has the answer you are looking for Mel - http://americanwoodworker.com/blogs/techniques/archive/2009/10/07/bent-wood-lamination.aspx Hope Derek will see this before he starts his glue up.

[Peter Quinn]

We use a simple formula..... Make the lamination first, use a rigid glue to do so, fit everything else to that. Have done curved jamb heads, curved panel jambs, etc. Never knew there was a sciency formula to follow, love to get a gander at that.

[Mel Fulks]

I don't think that's it . Not being cagey ,my math ability is pretty limited and I kinda used geometrical construction to draw it and change to quick ratio type thing. My preference would be for one of you more computer literate to search FW archives and post link. It was 15 to 20 years ago ,not sure if it was in the letter thing or tip thing .I have photo copy here somewhere as well as my ratios. Species does not matter in the formula in any direct way,only that plys will bend without
breaking and not so thin that they won't spring back. I won't debate it ,but certainly glad to pass on written material if
you do not turn it up. It was pretty contentious and its contributor answered criticism from doubters at least twice wwho were too lazy to try it. Writer was an engineer and said formula was based on one for beam deflection.

[Chris Fournier]

There would be some pretty heavy engineering and math required in any formula that you be worth it's salt and it would have to take into account the modulus of elasticity of the wood species and the grain orientation of the plys, forces involved in various radii and arcs etc. I doubt that anyone has put the effort into creating a scientifically robust formula that has real world validity in the shop. I have found that suceesful bent laminating invloves a lot of intuition and "feel".

[Ken Fitzgerald]

Testing the system.

It seems to be working.

[Mike Henderson]

I think I saw that tip and used it in an article I wrote. If it's the one you're thinking about, it's:

y = x/n^2

Where

y = springback
x = original deflection
n = number of laminates

Mike

[bill tindall]

This topic was recently discussed on the WoodCentral forum. A teacher from England provided several formula for calculating spring back that are similar and yield similar results. I find the formula are useful for prediction.

[bill tindall]

got it FWW Vol 119 page 10

[Sam Murdoch]

I don't think that's it . Not being cagey ,my math ability is pretty limited and I kinda used geometrical construction to draw it and change to quick ratio type thing. My preference would be for one of you more computer literate to search FW archives and post link. It was 15 to 20 years ago ,not sure if it was in the letter thing or tip thing .I have photo copy here somewhere as well as my ratios. Species does not matter in the formula in any direct way,only that plys will bend without
breaking and not so thin that they won't spring back. I won't debate it ,but certainly glad to pass on written material if
you do not turn it up. It was pretty contentious and its contributor answered criticism from doubters at least twice wwho were too lazy to try it. Writer was an engineer and said formula was based on one for beam deflection.


Maybe this one Mel? - http://www.richardjonesfurniture.com/PDFs/LaminateSpringback.pdf Which is the same formula as Mike Henderson noted above but with more explanation.

Found that link on this link from Fine Woodworking - http://forums.finewoodworking.com/fine-woodworking-knots/general-discussion/laminate-springback-calcs-article

[Mel Fulks]

That's it! Probably not suitable for accurate free standing stuff,without adjusting the form after initial test. For base ,jambs
etc. It always works. We keep most of the forms,I mark them with radius of form itself and and radius it produces and
number of plys. Sometimes when needing something like base trim that can take nails I use an existing form made for uuse with 3 or 4 plys with only 2. Greatest part of my work is done with 2,3 ,or 4 layers.

[Mel Fulks]

P.S. Using glue with or thinned with water ,the work will will close up some over time. Only an issue with something
freestanding.

[jack forsberg]

interestingly Glue is not a factor in the study. my experience is other wise. I think for critical curves that are not held in frames during drying time spring back can be a big deal. On the other hand like steam bending drying forms help greatly reduce spring back while moisture is reestablished in the wood.

[Mel Fulks]

I use Italian bending ply for ellipses,and on some door frames still prefer the wood bricks. I'v had a couple of employers
tell me the formula would not work....then ask me to give it to them!. In all but one case I have refused. One boss would
occasionaly ,right in the middle of a conversation on some other subject ,would blurt out " I want that formula!" Like some
moustache twirling villain .On the way to work one day I found a a big thin sheet metal toy pirate chest.Took it to work
And waited for him to take the bait. Didn't have to wait long :"Melvin,what's that chest for?" I answered "Aargh! Thats
where me keeps me secret formulas!" .....in a very good pirate voice.

[peter gagliardi]

I always use a rigid set powdered resin glue like Spectrum Adhesives #0502 . My laminations are left in form for 2-3 days, sometimes longer- no appreciable spring back almost ever. However, I have had the occurrence when doing spiral stair hand railing that the handrail actually springs inward a small amount!!
I do a large amount of curved work in the shop, and have found that if radius is tight, you can laminate a couple of the plys together one day, then come back and laminate the balance of the plys one or two days later, and it will NOT spring at all, it will match your drawing perfectly when set upon it.
The secret for me at least is don't be in a hurry!