Experienced woodworkers know that achieving a sturdy joint hinges on understanding grain orientation and wood movement. This is especially true in the round joints commonly used in chairmaking. By understanding grain orientation and how it affects wood movement, all woodworkers can reduce the risk of joint failure. This fundamental principle underpins the construction of different woodworking joints, guaranteeing long-lasting durability.
Transcript
Okay, so the second component of making a good joint…
First component is straight long grain, second component is grain orientation and wood movement. Or if not grain orientation and wood movement, it's how grain orientation affects the movement of the wood within the part. And this is a general principle for all wood.
00:24 Grain Orientation: Radial vs Tangential
So if you look at this, this is a white oak board, a white oak leg blank. And you can see that the growth rings are going roughly parallel to these two edges. So the growth rings will be called tangential plane because you're talking about being tangent. If you put a straight line up to the growth ring, it's tangent to the growth ring. The growth rings are always slightly arced as they go around the tree, but tangent to the growth rings is called the tangential plane. And the faces that are parallel to the growth rings, this one and this one, called the tangential face.
The other grain in the wood is called the radial grain. And in oak, in this white oak, you can actually see it. You can see the radial grain in this shot of end grain. You can see it. The radial grain goes from the center of the tree to the outside. Where you can think of it as radiating from the center to the outside or heart to bark. However, you can think of it. I like radiating because it's radial grain.
So this is the radial gran. The tangential grain is that way. Radial grain is always perpendicular to the tangential grain. So these would be the radial faces parallel to the radial grain. Radial faces, tangential faces. Tangential grain, radial grain.
02:20 Grain Orientation and Wood Movement
In general (and all we need to know is the general principle, we don't need to know exactly how far wood moves. We just need to know the general principle), wood will move twice as much along the tangential plane this way (twice as much this way) as it does along the radial plane.
So we also have the long grain of the wood. And for woodworking purposes, there is a very, very tiny amount of wood movement along the long grain. But in terms of woodworking and the length of woodworking joints, we can say there's no wood movement along the length of the part along the long grain.
So we need to take all those factors into consideration when we're orienting (the parts in the joint). So this would be a front leg. When we're orienting, let's say, a rung going into a front leg, we want to orient it in a way that will yield the best (match). You want to make the mortise and the tenon, mortise in the leg and the tenon on the rung as compatible as possible when it comes to wood movement.
So there are two sets of relationships.
04:00 End Grain Of The Leg Relative To The Long Grain Of The Rung
The first relationship is the end grain of the leg relative to the long grain of the rung. So we're going to assume this long grain is nice and straight and we have no wood movement along the long grain. In the end grain of the leg we have more wood movement this way (along the rings) and less movement this way (across the rings). So the best way to orient the long grain of the rung relative to the end grain of the leg would be like (axis of rung perpendicular to the growth rings) this because we have no wood movement here, smaller amount of wood movement here. And if we went in this way (axis of the rung parallel to the growth rings), no wood movement here, but more wood movement this way. So we want to make these two parts of the joint as compatible as possible.
The problem with this is that we also have a rung coming in from this direction (rotated 90 degrees from the first rung). So if we did nothing, this (original) rung would have a really good joint, the best possible joint. And this rung (the new rung) here, coming in parallel to the tangential plane or parallel to the growth rings, would be the worst possible joint in terms of wood movement.
So in a chair what we do is split the difference. So we'll just, this part's going to be turned round so we can rotate it to any orientation we want and, it would come in like this. So if you look at a post-and-rung chair that's done correctly and look at the grain at the tops of the front legs, you'll see that they're rotated 45 degrees. And that's the reason for that. So we're making each rung equally unhappy or equally happy one way or the other. So the growth rings here are going like that in the leg. So that's a compromise. In the chair you need to compromise. And basically we're splitting the difference.
06:32 End Grain Of The Rung Relative To The Long Grain Of The Leg
But we also have a relationship of the end grain of the rung relative to the long grain of the leg. So the rung is small, we've marked the growth rings on here. So again, same thing, tangential plane this way (parallel to the growth rings), radial plane this way (tangential to the growth rings), more movement this way (along rings), less movement would move this way (across rings), and along the length of the part (the leg), no wood movement.
So the very best way to orient the end grain of the rung relative to the long grain of the leg is to have the growth rings parallel to the floor and that puts the radial plane parallel to the long grain of the leg. And in a rung we can do that. We don't have to compromise. That's what we do.
And that brings up a rule of thumb. The rule of thumb is:
The radial grain in one part should run parallel to the long grain in the mating part.
That occurs in every joint in the chair. We do that. Even in joints that are not (round). We do that in slats, we do that in runners, and we especially do that in rungs. So, rule of thumb, radial grain parallel to the long grain of the leg.
In practice, what we do is, this is a rung that's been turned and you often cannot see the end grain on the part. We've marked it on here. So the end grain is this way. So that's the tangential plane. The radial plane won't be like this. But the way we look at this, the way we look at a turned part, is you can see these cathedrals in the surface of the turned rung. Particularly if it's tapered. So the rung is tapered, and what we're doing is revealing one growth ring, another growth ring, another growth ring. And they create these shapes in the surface of the wood that I call cathedrals. Some people call them chevrons.
So what we do before prior to assembly is we look at every rung and we know we want this surface, parallel to the floor, and facing up. So we'll just look at it and determine which surface is the tangential face, which is this one (with cathedrals). And there's always a second one on the bottom. So either one can be facing up. We pick one. So I would pick this one, put a piece of blue tape on that. So then I would mark, I'm just gonna mark it and then show it to you with the tangential face parallel to the bench. I know I can draw a line perpendicular to the tangential face, which would be the radial face. So that line represents the radial plane.
And when we put that part in the joint, if this were a leg, and we were putting this part in the joint, we would want-- if we put blue tape here, this was the top of the leg. We would put the blue tape up. But we don't always get the radial plane exactly in line. That's what this is for. We rotate this until this is parallel to the length of the part.
And that gets us essentially perfect matching grain orientation between the end grain of the leg of the rung, long grain of the leg. And we do that every time. So we're maximizing the compatibility wherever possible of the wood movement of the two parts and getting them to be as compatible as possible.
The other places that we have, where we consider that, this is a slat, it's quarter-sawn. You can see that the, you can see the quarter-sawn grain marked off here. So the growth rings are going this way (across the thickness). Which means the radial plane is straight up and down the part. And when that slat goes into a leg, it's going in like this. Rule of thumb, radial plane in one part runs parallel to the long grain of the mating part.
And the same thing for rocker runners. (They) are quarter-sawn for the same reason. If this were a runner and this were the bottom of the leg, radial quarter-sawn this way, radial plane is this way runs parallel to the long brain of the leg.
And this makes very beautiful slats, I think. Especially in oak… you get all those rays.
So that's the second component of a good joint.
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