Spring Joints

B

briyon

Well-known member
I have been hearing lately (although it sounds like it is nothing new) about using spring joints when edge gluing boards together for say a table top.  I saw Tommy Mac do it a couple of times on the Rough Cut show.  Once on his own and then once when Steve Brown from NBSS was a guest.  This method seems counter intuitive to me. I would think that the more inline you can get your two edges the better the glue joint.  I did a Google search and it seems like this is a pretty popular method.  I was hoping to get some of the opinions of some of the experts on the forum about the benefits of using this type of joint.

Thanks,

Brian
 
I don't do it. It is probably ok for gluing two boards together but it won't work at all for a rub joint. It also doesn't work well (in my way of thinking) in a table top where all of the sprung joints become cumulative.
Mike
 
I agree with Mike.  I don't do it either.  It may work if your boards are narrow but if they are wide it will take a lot of clamp pressure to pull them together.  And then the counter-pressure to pull them apart is greater and if the glue weakens the boards will spring back to their original shape leaving a gap in the center.  I just run the boards over the jointer and glue them up.  In 25+ years of doing it this way I never had a cusrtomer complain about the boards separating.

Sometimes people are just trying to invent a better mouse trap.

Dennis Bork
Antiquity Period Designs
 
With a spring joint, the idea is to preload the ends of the boards (compressing the spring).  That way, as the ends dry and shrink (which happens well before the middle) the preload will be relieved.  Only after the preload is gone will the glue (or wood) go into tension.  Sometime after that is when the failures start.

I do it because I feel it's faster when you are preparing joints by hand.  Getting 2 8' long boards perfectly straight isn't easy by hand.  If there are to be gaps between the boards, you really don't want them at the ends. 

Springing a joint is also the natural way we straighten boards by hand, hollowing out the middle, then listening for straight. 

I routinely spring wide boards and have done 6/4 stock with 18" boards (for a 36" table top) easily.  Obviously the gap is predicated on the width/stiffness of the stock.  It's all about preload.

That said, I don't think it was common 200 years ago and I don't think it works particularly well with hide glue.  But then again, their stuff cracked 200 years ago.  They wrote about it then and we see it now. 

My advice is to try it if you are jointing long stock by hand and are using PVA.  For short, narrow pieces, I don't think it makes much difference one way or the other.

Adam
 
Brian- I don't do it because I feel that there shouldn't be any pressure trying to open the joint. I shoot for no gap anywhere so there's no stress on the joint. Preloading the joint............maybe if the wood was green-Al
 
Brian - I do spring joints for wide glue-ups very slightly, but only very slightly.  But there's a caveat - I don't work with narrow boards (12" wide or less) - generally a table top from my shop will not ever have more than 2 boards, and the total "spring" is about 3 - 6 thousandths (about 1-1/2 - 3  thousandths) per edge.  As Adam notes, the intent is to counter-balance the large difference in moisture adsorption/release between the end-grain of a board and the center face-grain.

It's fairly easy to do this, but you do need to have your jointer plane very well tuned so that a 2 thousandths or less shaving can be taken.

To date, I've not had any table tops crack (that I've been informed of).  Then again, when it leaves my shop the finish coverage is 100%, and the piece hasn't sat out in a barn for the last 150 years as have some of the antiques that have cracked.
 
I hope I wasn't to short with my last response. There are so many variables with lumber that blanket statements don't cover the realities of the day. How was your lumber cut out of the tree? How was it dried? How and where was it stacked and stored? Where was it brought from? What time of year are you building this piece? Do you have to bring in your lumber and let it acclimate itself to your climate? Where is this piece going to live? What type of climate control do they have where it will live compared to where it was built. I made a table for a good customer and ordered lumber from a lumber dealer that I had not worked with before. Two of the glue joints split apart an 1/8th" within 8" of the edge. It was a banded pedestal table with satinwood inlay. I ended up replacing the top. Not a true horror story. I ended up putting the top on another table after it was repaired and then sat in our attic for about 8 years. I guess I wasn't thinking hard enough! I do wonder about 1000th of an inch though.
 
Brian,

So you see, the answer depends on several things, with personal preference as much a driver as anything else. Woodworking, like economics, has no one handed practitioners. 

If the wood is very dry, stable and narrow (I call narrow 6" or less - can't afford the nice wide stuff David uses:) and where there are more than two planks in the glue up, I don't think it necessary to apply spring.  But if the planks are wide and their moisture content is relatively high (say green, or you're building in the summer when the humidity is up), you might want to consider planing some very slight spring into one or both of the boards to allow for end grain moisture release as mentioned.

I joint by hand as I don't have a jointer. I think it's easy and no less efficient to plane in spring - if necessary - when jointing by hand. However, if you're using a jointer it might be counter productive to take the time to carefully *drop in* a long wide board on the cutter head and work to get a smooth clear jointline with spring in it.  If I had a jointer I would probably try to avoid putting spring in the boards and would, instead, try to get the wood to the most stable condition possible before working it and gluing up the joint. (Not that I don't try to do that now.)

And as Mike said, if you're using the rub joint gluing process, spring will prevent the total contact necessary to get the glue to bond. Spring joints must be clamped.

Rick Yochim

PS - A moisture meter is a good investment to consider if you don't have one.



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Jeff - In my case, all of my joints are done with an hand plane, so the total gap in the center of the two edges put together is about 6 thousandths.  This is a pretty significant gap - you can very easily see it, and watch it close when the boards are put into compression.  My guess is that much more than this might result in a split down the center as the boards go into tension during a dry winter.

All that said, I primarily do it because that's the way I was taught - I've not done a series of experiments to see what the effect would be with no spring, 4 thousadnths, 8 thousandths, 20 thousandths, etc... (though that would be very interesting!).
 
I spring almost all my edge joints because it makes sense to me intuitively, but I think it's a matter of personal opinion.  From what little I was able to learn, it seems spring joints didn't come into use until advent of central air heating.  A rustic dining table I made many years ago of white ash routinely opens at end facing window (and therefore HVAC register) in winter but the gap is much more than spring joint gap.  Joints were doweled and glued with PVA.  Wide flatsawn boards - I was young and didn't know better.
I would not spring a joint intended to be rubbed only.
 
If spring joints should be considered then what should be each individual woods springness be in conjuction with the secondary woods which are also used. I do see the consideration of such matters, I just wonder about all of the variables. To many to consider. If you are glueing one board to another without any other infuiences then springboarding(for lack of a better word) probably should be considerded. Glue joints are surface to surface contact. No more no less except
"Spring is in the Air".
 
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