Questions Answered - Spring Edge Edge-jointing Boards

Question:

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Paul (and Team)

I was curious if you could give us your thoughts on spring joints. I’ve seen them mentioned in various places when discussing panel glue ups. I think I understand them in principle. By creating a slight concavity in the middle of the two boards to be joined, the ends are automatically pulled tight during glue up. My questions have to do with the purpose of such a joint and if they are practical for everyday use. Does this make for a tighter joint? Does this help to reduce the number of clamps needed for a glue up? Is there any benefit in strength of the joint compared to joining two parallel edges or even match planed edges?

Thanks for all you and your team do!

Adam

Answer:

This technique was introduced in an article from a well-known US woodworking magazine in the early 90s, but its origins trace back to when woodworkers had to rely on less sophisticated equipment. The strategy entails forming a slight convex shape along the edges of two long boards, allowing them to bond effectively during the glue-up process.

Utilizing this method permits the use of only a couple of clamps, or alternatively, you could deploy two timber dogs, also known as nail dogs, to tightly draw the ends together. This automatically compresses the joint seamlessly across its entire length. While the amount of convexity involved should be minimal to avoid creating hollows, some curve is necessary to ensure pressure at the ends to effectively close any gaps. A slight arc is all that's necessary for this method to work efficiently, as too much pressure might lead to undesirable results.

The main advantage of employing this technique is that it minimizes the number of clamps required to secure a panel. In fact, just two clamps can suffice, which is particularly beneficial for smaller workshops with limited clamp inventory.

Traditionally, dogs were commonly used in glue-ups in the past due to their effectiveness, especially in situations where clamps might not suffice. I personally have several pairs of dogs that I find immensely handy in certain setups.

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The design of the dogs allows for pressure to be exerted at the very ends of the boards, thereby facilitating a tight bond across the entire joint.

It’s also worth noting that screw-threaded clamps weren't readily available to woodworkers in earlier times, necessitating the use of alternative methods like wedges and dogs for securing glue joints. In those days, the only adhesives available were natural glues like hide or fish glue, which performed quite differently from modern options such as PVA or epoxies.

The most traditional method used for edge joining was rub-jointing, which involved rubbing the two glued edges back and forth against each other until the glue was evenly spread. Once a certain thickness of glue was reached, it would create a bond strong enough to hold the pieces together without further movement.

To ensure proper alignment, glued boards would often be rested against supports in a vertical position, allowing the glue to cure without clamps until the next day. This method necessitated that the edges of the boards were pre-planed to perfection, avoiding any gaps or unevenness. Historically, the practice of convexing board edges was likely a response to the limitations of tools and equipment available at the time, as screws and clamps were less common in the woodworking arsenal.

Consequently, while this method allows for efficient assembly, it’s important to acknowledge that higher standards of craftsmanship can typically be achieved with more meticulous clamping techniques. The traditional use of dogs might leave visible square holes in the end grain of panels, which is acceptable depending on the project.

The use of clamps or dogs must be managed carefully, as they can cause the boards to bow apart due to the compression of the wood fibers. Thus, it’s wise to ensure clamps are applied early in the process to prevent excessive gaps from forming.

In conclusion, the effectiveness of convexing boards versus using clamps remains debatable. In my experience, striving for meticulous practices often yields better results by eliminating any potential discrepancies during the assembly. Indeed, the finesse involved in woodwork allows for pieces to withstand significant everyday stresses, from chairs to musical instruments, as their integrity relies on the precision of construction.

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