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How Much Springback for Thin Laminates?

bmyyou's picture

I'm making some bent chair legs with a 2.25 inch bending radius out of thin walnut veneer strips (1/45 inch thick).  Do I need to make the form's radius (thus the bending angle of the form) to account for springback of the laminated leg after the glue up?  I would like to use the forms that I made for steam bending thicker stock that had a smaller radius rather than make new ones to exact desired dimensions/angles, but I will if needed.

Thanks for the feedback,

Brian

mrossk's picture

did you really mean a 2.25" (post #170782, reply #1 of 5)

did you really mean a 2.25" radius?  That is a super tight bend.  You'd have to have really thin laminations, if it were even possible.  More generally though, less spring back is usually accomplished with more, thinner laminates, rather than fewer thicker ones. Also, a rigid when dry glue, like UniBond or something similar is superior to a PVA glue which stays slightly flexible.

bmyyou's picture

Here is a picture of the (post #170782, reply #2 of 5)

Here is a picture of the chair legs that I'm making.  The finished leg is about 1 inch thick so I'll need about 46 strips of veneer to make up one leg.  Not sure if the radius is exact as I mentioned but it is small.  So looking at this picture will there be springback with this bend from the laminate or should I make the form to exactly fit the angle of the finished leg?

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mrossk's picture

I'm sorry that I can't say (post #170782, reply #3 of 5)

I'm sorry that I can't say with any authority that "yes, you will definitely have 'x' amount of springback", though my intuition says yes, you will have some.  The best I can offer is that before you try it for real, make a simple form of the approximate radius you think you have to bend and do a test. In addition to the springback, you also have to consider milling the pieces that thin so they don't blow up in your planer- I think you'd need a decent thickness sander to get it done. And as I mentioned, some sort of two-part rigid glue is a must I think.  I'm curious to see how you get on, please update as you can.  Best,

Michael

Westchester's picture

Chairs (post #170782, reply #4 of 5)

Could you make the legs from plywood and veneer all 4 sides ?

SA

ThomasWS's picture

Here is the equation... (post #170782, reply #5 of 5)

Not sure if you've already finished this work, but, for future reference, here is what is termed a generally accepted equation for percentage of spring-back based on the number of laminations:

S = 100 / ((n**2)-1) (See citation below)

Note that the percentage is reduced by the square of the number of laminations so, with more than 20 laminations, the spring-back is virtually nil (and probably far less than the accuracy of the bend in the first place).  Note, also, that this equation doesn't factor in stiffness of material and does not account for the amount of dimensional change due to glue shrinkage during cure - something that is a much bigger factor with an increase in the number of laminations.  

Stiffness tends to cancel out IFF the glue doesn't shear AND assumes that you don't actually 'break' the wood cells, but, in fact, do uniformily compress and stretch the cells (ie, if you can bend it, it will stay...) As pointed out earlier, using a hard glue like Unibond, is highly recommended, especially with such a small bending radius.

This equation comes from "Wood Bending Handbook", W C Stevens & N Turner, Forest Products Research Laboratory, Buckinhamshire, UK, 1970, p. 68, reprinted by Woodcraft Supply, 1978.  I see that there is a new, "American" version of this book published in 2007.

Tom,

Making sawdust in the heart of NC!