Spring Joints still a good idea and with how many boards?
jerrytc
| Posted in General Discussion on
Good Morning.
While listening to Gary’s podcast he frequently talks about spring joints. Heading to YouTube I see little discussion on how to execute this joint from any recent presenters.
Local wood shop store expressed concern that a spring joint would starve the ends of the joint of glue and also stated that today’s advance glue types greatly reduce the risk of boards parting at the ends, as in nearly never.
I am assuming spring joints are used when gluing only two boards?
Are spring joints a good practice with longer boards as there would be little spring in a shorter board?
As you can plainly see I fit into the newer wood worker category.
Thanks
Jerry
Replies
l would always spring my joints. I think it’s a good safety measure in making furniture because ends dry out and shrink. If you don’t do it you may have joint failure issues.
As far as starving the joint, that may have more to do with too much clamping pressure than anything else. I don’t think it is glue related either as hide glue, which is old technology, works amazing and is extremely strong.
There is an article on Fine Woodworking.com by Bob Van Dyke that explains the how and why of springing a joint (issue 208 Nov/Dec 2009). Cheers!
The purpose of spring jointing boards together into large areas like tabletops is to ensure the ends don't un-spring when they dry out faster than the timber further in-board from the board-ends. This is a bigger risk where humidity varies a lot and/or where the end grain is fully exposed to the atmosphere.
I have a dining table made by Albert Jeffreys (an apprentice of Robert Thompson, the Mouseman) that has a 2 inch thick oak top 3ft by 6 ft. It was made in an unheated workshop where the humidity was probably somewhat higher than is the case in a typical modern house with central heating. The end grain of the top is fully exposed and all of the boards used in the table top have very slight gaps between them for about 6 - 9 inches at each end of the table top.
So even professionals get it wrong. :-)
My own similar table tops have never split at the ends. I always spring-joint them, having seen the Albert table top and learned the lesson before beginning wood working myself. It was a FWW article in the magazine, decades ago, that explained this end-gapping and how to avoid it.
*******
As a contrast, consider this video by Paul Sellars:
https://www.youtube.com/watch?v=ehiSOFuBlRo
He shows the use of "timber dogs" (double-pointed metal mini-bridges) to pull glued-edge boards together as an alternative to clamps. For this technique to work, the board edges have to be planed in the opposite fashion to that for a spring-joint. The centre of the edge must have a slight convexity rather than a slight concavity.
When the metal timber dogs are hammered into the board ends, bridging two boards, they pull the boards together at their ends, due to the shape of the timber dogs. This makes the convex parts of the board edges come together first. It's like a spring-joint but in reverse.
This used to be a common way to join glued-up boards, in them olden days, 'cos clamps were expensive compared to timber dogs. Perhaps this is why so many of them olde table tops have split ends? :-)
****
Another way to avoid split board-ends due to humidity changes and differential drying-out rates, is to use breadboard ends. These don't stop the board ends expanding and contracting across their widths (as long as they're made and fitted properly) but they do slow down the rate of change of moisture content there to better match the rate of change elsewhere in the boards, thus avoiding the forces that cause the ends to split.
The breadboard ends cover the end grain so the atmosphere has to give or take moisture from the ends of the boards in the same way that it does from the middle of the boards.
Lataxe
As a hobbyist with few clamps, I use spring joints because it simplifies clamping and forces me to focus on making a decent joint rather than relying on a bunch of clamps to close it. It's also very easy to check for flat and adjust as necessary during the incredibly-stressful glue-up... IF you've checked that it'll go together flat in the first place.
On a table top (like something >30 inches), I put in slightly less spring in the middle boards. My test is if I can push them together w/ my hands, it's enough spring. Remember you're adding the stresses each time you put another spring in there. The number of boards really depends on the wood you're working with--species, dimension. You'll probably be fine regardless, but in general I try to minimize the amount of stress I'm inducing in order to get things in their final resting place after glue-up.
IMO there is no real reason to use a spring joint other than if you're concerned the ends are going to shrink or check, thereby opening the joint.
I think it developed as a technique simply to save clamps and speed up the process.
This forum post is now archived. Commenting has been disabled