If you were to mention a woodworking joint, to a non woodworker, they would probably know of a joint/have a picture in there head, and think nothing more than the joint, joins two sections of wood together. Of course this is true however, there is a lot more to a joint, then just joining two sections of wood together. This often gets over looked by people, and they do not understand the mechanics of the joint. Which seems a shame, considering that understanding joints, will help you better choose a good joint, on a structure you may be designing, which will ultimately strengthen that structure.
I think a lot of people choose a joint, because they know it to be strong, because someone said so, and without question, not fully understanding why they are using it. Take the good old dovetail in drawer construction. The pin portion will always be on the front of the drawer, because of the nature of a dovetail joint. The pins are like a group of wedges, fitting into wedge like grooves/holes. The pins board can only enter the tails board left or right, and because of the wedge/tapered shape of the pins, it can not exit forward. Any forward motion is prohibited, by the interlocking properties of the dovetails.
Once you understand this, you understand the concept of a drawer, why it is made this way, why there are examples still in use after a hundred years, and it makes perfect sense for the drawer to be construed this way, when someone is going to be pulling the front of the drawer, for the next hundred years.
Have you ever used a mortice an tenon joint in the past, and thought (before using it), I'll use a mortice and tenon joint because its strong, that sucker will do the job? I know I have, without even considering what the joint is doing. If we take for instance, a chair, specificaly the two back legs, that extend from the floor to the shoulders, comprised with mortice and tenons. The two legs are joined left to right with back rest rails, a seat rail, and sometimes a undercarriage leg rail. So when we look at what is happening, there are a few things going on. So obviously, the joints are keeping the leg together. When we sit, the force pushes the end of the tenon on the seat rail, onto the edge of the mortice, which transfers to the leg, then to the ground. When we lean back the force is being excerpted onto the face portion of the joints. All of the joint(s) parts work in unity, as do all joints.
Providing the joint has been cut well, we can expect the following - The tenon fits into the mortice, all faces of the tenon are making good contact, with the inside of the mortice, thus causing a tight joint. The shoulders of the tenon, press against the surface of the leg(s). Because the tenon is a tight fit, it adds stability to the shoulders, and because the shoulders are in contact with the leg face, they help keep the tenon from moving around, so all components are serving a purpose, and are in unity with each other. Once you understand this, you can understand why a chair will resist racking, and why, when someone balances on two legs, of a well made chair, the chair doesn't break!
Obviously this can be expanded upon, using wider tenons, and cutting double tenons (castle like shapes), which would resist movement more, then a standard tenon. There are lots of variations that could be noted but, once one understands the basics, the reasons behind variations become clear.
Understanding joints is also understanding the structure of wood, and vise versa. I think once you understand one, the other becomes apparent. Many of the joints we know today are very old, with the earliest known dovetails being recorded from ancient Egypt (3000BC). The fact that they are so old makes sense, because ancient people(s) had more connection with the natural world, understanding and using natural materials. Unfortunately, a lot of the knowledge has been lost, but there understanding of wood, is evident in the old woodworking joints.
Looking at a Japanese style scarf joint we can see this, there are 4 sections of endgrain butting against each other, with the joint intertwined together, and wedged in the middle, against more endgrain, forcing the four contact endgrain pieces together, and keeping them there. When you think about it, the joint is pretty ingenious. You have all that endgrain butting against each other, and endgrain is the hardest part of wood, and less susceptible to compression. If downward force is applied around the joint, and the joint is to fail, then the endgrain must compress, in order for the joint to fail. If the joint endgrain sections are square, and indeed the joint it's self is cut well, when assembled, the joint becomes very strong. It would be interesting to stress test a scarf joint to see how it would hold up. Although I'm not a fan of cutting a joint and not getting a practical use out of it, I think my curiosity would allow for an exception, on that particular occasion. Keep an eye out for that post, It should be interesting if I get around to it!
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