Thread: Have you heard of this spring back equation for laminations ?

I must have it wrong somehow. If you had 3 laminations, 3 squared minus 1 would be 8. Could the springback be 8 times the Radius of the form?

Or, is it the Radius of the form is 8 times the springback?
Sorry I am confused about this.

Interesting subject but reciently I did some laminations and was trying to figure the spring back and decided to do a test run and just see how far. when I took it out of the clamps the next day there was hardly any . So I glued up another and had very little, as well. My experences this time shoots the theory. Wonder if it is influenced by the type of glue? I used 6 ea 1/8" strips of Oak on the first one with Poly glue. The second one I used the same Oak (4) but had also a couple (2) of Ash. and with it I got perhaps 1/4" springback.

I have a problem with the formula too....

The more laminations you use, the smaller the spring back should be! thus I am suggesting the formula should read


r = R/(nsquared - 1) R devided by (n squared - 1)

thus if a three ply lamination is formed around a 20cm former, when the glue has dried and it is released from the clamps you would get 2.5 cm spring back. I assume this would thus give you and effective radius of 22.5 cmm on your finished product.

With 9 laminations the springback would be only 0.25cm or 2.5mm giving a final radius of 20.25 cm.

I must add that I am no expert, have had very limited experience and find it difficult to believe it could be this simple!!!!!!!

I am just trying to make sense of the posted formula. If any one can confirm or deny this formula I for one would be very keen to know.

Regards chipman

From memory I think the springback is one over the number. ie for three laminations bent 100mm the spring back would be, 1 divided by 3 *3 - 1 = 1/8th of 100 = 12.5mm
Mark

wish I had a scanner. cameras the best I've got. I may have not described it properly. New to it myself.

Here's the description on the formula in the pics.....1 - 4.

Theres more in there article about it. The books called....

'Furniture making techniques for the wood craftsman......the beeest from furniture and cabinetmaking magazine. '

Got it at bookworld the other day going cheap.

Originally Posted by apricotripper

wish I had a scanner. cameras the best I've got. I may have not described it properly. New to it myself.

Here's the description on the formula in the pics.....1 - 4.

Theres more in there article about it. The books called....

'Furniture making techniques for the wood craftsman......the best from furniture and cabinetmaking magazine. '

Got it at bookworld the other day going cheap.

AR there is absolutely no question that you have copied the formula correctly as it is right there in the picture, but the formula contradicts what is written in the text...."the more laminations, the LESS springback"

According to their formula a 3ply lamination over a 20cm radius would have a spring back of 20 x (9 - 1) = 160cm on top of the original radius so the new radius is whopping 180 cm or 6 feet in the old language.

Now try 9 laminations and you get 20 X (81 - 1) = 1600 cm plus the original 20 cm which gives 16.2 metres that is almost dead straight!!!!

Compare these numbers to the ones calculated in the early post of mine which is more consistent to MARKER's recollections in his post.

I am just wondering if that book was cheap for a reason....sometimes books get printed with errors in them and when discovered, they "dump" them at a reduced price anywhere around the world.

Cheer's

Chipman

yeh, I see what you all saying now. maybe something like a division character was obmitted between the R and the n squared.

There's a simpler formula described as well. I'll take photos of it and post here latter. May help to work it out.....I have to go right now though. I'm in trouble with the misses.

ok....(not that most will probably use it anyway )thinking when it comes to curves one just do the curves first and make the joints to fit whatever it turned out to be. Spose it depends on how bound you are to working by measurements or not. I'm always drawn to working of what it is rather than what its supposed to be. efficiency doesn't always allow time for that I spose.

anyway, this one seems to make more sense. springback reduces with laminations. (sorry about the previous equation. I didn't feel the need to test it)

. y = x/n(squared)....

where y is the displacement off the former, and x is......checkout the diagram in first pic.

You can have all the formulas in the world but the real world is is often determined by the real outcome... I too was concerned about springback and decided to do a test run to see what happened and planned to adjust the form accordingly. To my surprise when I released the piece from the form there was not 1/8" clearance between the form and the bent piece. where is the formula here? Six 1/8" plys over a 6" radius adjacent to a 9" radius and a straightway each side.... Poly glue was used. As I said earlier the only change in the two different pieces I made was on had all Oak and one had 4 Oak and 2 Ash the combination piece opened up almost 1/4"



So I deduce from my experiences that there is a difference in the species of wood as well as how tight the clamps were held and the weather and the amount of glue or which day of the week. Although I was a teacher for most of my life and reflected Algabretic formulas in most every day's assignment, when it comes to figuring what wood will do, Equations and super formulas Suck at best.

Originally Posted by Hickory

, Equations and super formulas Suck at best.

yep. tell us what you really think !

no, I agree.

There is probably something behind it, but practically to anyone outside that research its worth bugger all. IMO

anyway something to talk about.

If you ask me wood sort of has a mind of its own!

That new formula is least consistent with practise but at best it is only a guide..... it is a theoretical thing for ideal laminations very thin around a circular form but it could never account for what Hickory is doing.

Best Idea is use it as a sort of a guide or starting point but nothing can replace building a trial or two first, especially if you have lots of them to do. If it is a one off why not try and design it so that the exact shape is not critical.

Anyway, it was worth discussing but one lesson....just because it is written in a book or on the internet doesn't make it right!!!!!


Regards,

Chipman

The theoretical spring back is the depth times 1/n squared. However, practice has been known to confound theory before. I've used it and it's pretty close to correct - worked for the rails on a set of cabinets with curved doors & coopered panels.

Originally Posted by Hickory

... Equations and super formulas Suck at best.

Boyoboy, do they ever. I recently had occasion to compute the bending strength of a thin-walled steel pipe functioning as a simple beam. One would think that such ground had been well and truly plowed, would one not? One would be wrong. A not-so-old textbook at least contained the phrase, "Until this discrepancy is resolved, ..."

Joe

"the more laminations, the LESS springback"

I don't think thats true either. I've glued up curves using ordinary PVA and only TWO laminations with no springback whatsoever. Wood is infuriatingly beautiful stuff, What works with one species may not work with another.