"Spring" joints?

Re: "Spring" joints?

Bill Tindall, E.Tn.

Furniture factories don't use spring joints. They do however, ensure that the edges to be glued mate perfectly with no gaps at the end. I believe the only benefit of a sprung joint is to ensure the ends meet(if there is a gap in the middle the ends must have met). If you can ensure ends mating by good edge preparation the spring joint does nothing beneficial and there is always the risk of screwing up the edge preparation by making the hollow.

I believe it a myth that the sprung joint adds any significant holding power to the ends of a joint. A well prepared joint is stuck with something like 3000 psi of holding power. In the sprung joints you have made in the past how much force did it take to close the gap? Much less than 3000 psi?

I only shoot for a gap of a few thousandths

Dick Coers

And that is very easy to remove with clamping.

Are you using KD wood?

Keith Newton

I wouldn't bother, unless you were using AIR Dried, which might loose some moisture faster on the end-grain than across the grai out in the middle. Otherwise, there shouldn't be any more tension on the ends than the rest. Wood movement is all about changes in moisture content.

I never really understood "spring joints".

Yonak

My woodworking professor in college pushed them and I always argued with him about their efficacy. I say, why put any tension on a joint at all ? As Bill says, if the ends are tight, all is good.

Re: I never really understood "spring joints".

Steve jenkins

If I have any gap on the ends I will rejoint . Any tiny gap in the middle I don't worry about it if it pulls tight easily

Re: "Spring" joints?

Don Stephan

Why can't spring joints be like dominoes or mortise and tenon, spread glue on one side of a joint or both sides, yellow glue or hide glue, flatten and straighten before gluing or force alignment with clamps and go sticks, pins first or tails first, . . . a matter of personal choice?

I believe the glue line is less noticeable when I spring long joints, and I haven't had glue failures since I started springing long joints, so I plan to continue springing long joints. But I'm not going to argue with someone who doesn't want to use spring joints.

Re: "Spring" joints?

H.C. Sakman (Chico)

Spring joints are used where there are opposing two bowed boards and once glued they cancel each-other out and remain flat/straight. Mind you, both edges of the boards should be bowed, not only the glued edges. Otherwise, IMO, intentional spring joint would add nothing but tension to a glue-up such as a table top and eventually it will fail/crack somewhere due to that tension caused by the intentional spring joint.

Chico

I must add...

H.C. Sakman (Chico)

...that minute gaps are not an issue to bring together by the clamps. Some boards will have that no matter how well and carefully you joint them.

Chico

AGREE

The easiest way of explaining it...

John in NM

Is that springing the joint is a way of ensuring that the ends are tight. Really, the practice is nothing more than that.

I usually do it because it's pretty easy - couple swipes with a plane and you're done. One shop I worked for had an old Oliver that I could tune to perfection - but my wedge bed jointer is not so easy to get right. I prefer to use it as a kinda "fine roughing" machine and finish the long joints with a plane.

The method started with ends shrinking more

Dick Coers

It was an early process with the knowledge that the ends of boards would loose moisture out the end grain faster than the center of the board. This would stress the joint, just near the ends. So by preloading that section with a sprung joint, it would hold the glue line. Can't say one technique is better than the other, but it just takes a few seconds to do so I feel it's time well spent.

i thought they

Joseph Piotrowski

were just to reduce the amount of clamps. so much to lean...

Historical

Bill Tindall, E.Tn.

In the beginning I suspect you are right about the clamps. I have a book that lists shop inventories of shops in Chester Co. PA in the 1700 and 1800. Few clamps are mentioned. I expect the practice, like most of the practices of a past time, were optimized for what was available. In this case clamps were expensive, wood was not dried uniformly and hot hide glue worked in this situation. In modern times clamps are cheap and machines can (not necessarily will) prepare a perfectly straight edge. The practice has disappeared from commercial production. None of the shops we sold to ever used a spring joint. I doubt any had a hand plane in good working order.

Clearly it is not necessary. On the other hand it is insurance for a shop that can plane an edge. ....or at least that is how I see it.

What I question is the preloading

Bill Tindall, E.Tn.

I understand the argument. However, compared to glue strength I *think* the force added is diddly. When I have added this feature to an edge glue of 6" boards the force needed to pull out the spring was a small part of the force I normally add to effect the glue bonding. Hence the diddly force added assumption.

Where is a mechanical engineer when one is needed ??

Re: "Spring" joints?

William Duffield

A crack can start with much less tension at the end of a board than in the middle. The spring joint puts tension across the middle of the board in order to keep the ends of the board in compression as the humidity drops every fall and the ends dry faster than the middle. The failure doesn't occur at the glue line. We all know the glue is stronger than the wood. The failure usually starts at an invisible end check that developed as the log was drying, but then closed up and visually disappeared as the center of the board shrank with drying to equilibrate with the ends of the board. With a table saw, chop saw or radial arm saw, it is easy enough to cut an eighth of an inch at a time off the ends of the boards to discover how far the end checks have progressed and eliminate them, but if you were cutting them to length with a hand saw, that would have been a luxury, so sprung joints were perhaps more important (and much easier to implement) in a pre-industrial shop than in one with forked-tailed apprentices. If you have a barn full of 16' flitches, you could take all the stock for your table and bureau tops from the centers of the boards, but many of us would consider our time to ferret out checks not worth that much.

You can't show off a spring joint like you can an inlaid butterfly key.

Re: "Spring" joints?

Brian Holcombe

I agree with William. Spring joints work very well when used appropriately. Like most techniques it is important to deploy them with consideration. A really wide thin panel....don't spring. A moderately thick table top, spring.

Re: "Spring" joints?

Barry Irby

I made a head board for my son's bed. A walnut panel about 7' by 4' Air dried walnut about 3/4" thick. Can't remember but some of the joints may have been sprung a bit. I cut a slice of the end 3/8" by 3/4" by 48". So its a "board" 3/8" long and 48" wide. Its been standing up in a corner about 8 years and holding together just fine. I intended to measure it once in a while to see what the effects of humidity were, but never think to measure it.

It seems if the joints were prone to fail at the ends this would have fallen apart.

Re: "Spring" joints?

William Duffield

It is not necessarily the joints in a wide panel that would fail.

Your test strip would have been much more likely to have failed as it was coming off the saw than at any later time, unless you later accidentally knocked it over onto the floor.

A "board" that short will not have the internal stresses caused by differential expansion and contraction that a much longer board would develop over the course of a year of changing humidity.

Not and engineer, but this,

Keith Newton

As I mentioned above, the starting MC for air-dried wood, would be the best reason to do this.

Lots of us home-grown woodworkers are using AIR-DRIED wood which will come indoors to conditioned air, which will eventually bring the MC down from 12% to 6%. Using the shrinkage calculator for flat sawn Walnut, on a 40" wide table top will shrink 0.6686 inches, or 21/32 inches, or just over 5/8" if your challenged by fractions.

Now, it's not that a properly prepared glue-line is where it will fail, if anything does fail, but wood isn't a homogenous material, and finishes aren't all that great a barrier to moisture changes in wood, and we all know that it can loose it faster from the engrain than across it, some species more than others. So trying to get a little compression where your expecting there to soon be tension seems like a pretty good idea. Beyond that slowing that final drying especially on the ends is too.

Re: Just wondering about Humidity changes?

Larry Clinton At Frankfort, (Central) Indiana

Today, just about every home I have been in has central air conditioning and heating systems. They are also pretty well insulated, sealed with many having automatic humidifiers in the furnace system.

I really wonder how much humidity changes occur with these systems in use. I have never recorded the humidity in my home. Wonder if any here has taken the time to do this?

Granted there will be some changes, but I really doubt they are any where close to the changes that occurred in the days of wood heat and no air conditioning.

My shop is equipped with central air conditioned and heat and 99% of my projects are constructed with wood that has been stored for at least 2 to 5 years. So I don't worry about the abrupt changes from "new" lumber prepared in unheated / air conditioned shops which then are moved to a controlled home.

I do

Bill Tindall, E.Tn.

House humidity varies from as low as 35% winter to 80% summer. Shop gets dehumidified when it gets to 60%. Shop varies 45-60%. However, wood that is finished doesn't change moisture content or size near as much as one would predict from these data.

I spite of the nostalgia lavished on old time woodworkers I believe they often were using wood not thoroughly dry and furniture, especially chairs, got set near fireplaces in the winter where the humidity was very low. With modern glue, modern lumber and modern central heating/cooling I don't believe we face anything close the challenges with dimension change that was common in earlier times.

Re: I do too

Mark Mandell - Gone Round In New Jersey

While we have no qualms about opening the windows to let those gentle breezes freshen the house, we also condition and humidity-control our home. I have 2 70pt. dehumidifiers running in my basement shop/office from March through October. Ambient RH ranges between 40% and 70% seasonally. I have measured the MC at various points in the house (furniture, trim, exposed framing) and find that it may seasonally fluctuate between 9 and 11%.

In that context, I have two "breadboard" coffee tables, 1 all ash, and one cherry with walnut breadboards. They are 1" thick and both are fully finished with my oil/shellac/waterborne routine. Both of the table tops will expand and contract with seasonal changes relative to the end-boards. Both tables are 28" wide. The total range (dry January vs. wet August) of dimension change across both of them is about 1/16"; just enough to let you "feel" the joint.

Cherry and walnut were air-dried; the ash was KD.

As a beginner who transitioned from plywood to solid hardwoods, I made a few mistakes in both purchasing wood and planning on movement. Fortunately, nobody got hurt when the joints let go.

Re: Just wondering about Humidity changes?

John in NM

I've never owned a house with such modern systems, even in Tucson where our last house had A/C (lived in two others there with just evap. cooling).

Our current house does not have cooling as the nighttime breezes are generally enough to keep it cool.

I'll occasionally notice a cabinet door or drawer rubbing in the summer, but not too often. I had a table I made where the drawer was stuck closed one summer until I adjusted the side dimensions a bit. I see more movement than Mark notes, but not a huge amount more, leading me to believe there may be something in what Bill says about kiln drying.

Movement

Mark Mandell - Gone Round In New Jersey

That's less than a 1/16 total expansion across a 28" width.

While I allow and plan for it, that's no enough to break a well made joint with modern glue with a cross section of 2-3 inches. On percentage basis it constitutes movement of 1/448th or .002232 or about 1/5th of 1 %.