Tuesday, October 14, 2014

Auto-Regulator, Chapter 4: Cutting the arch, part 1

The arch, the upper side horizontals, and the vertical posts come together in a pair of 3-way miter joints at the top of the case. That's the short version. And from an aesthetic point of view, that's really the version that matters. As long as the joint is cleanly made, the eye will freely run along the lines of the case. But from a construction point of view, things are almost never that simple. If there are any gaps, voids, or other breaks in the surface, the eye stops there, and the mind will take note. Much like a shrieking saxophone or clarinet in an orchestra, it won't matter if the melody is miraculous. It's the shriek that you'll notice, and the reverie will be interrupted. So, to make those clean transitions, understanding what's going on is a huge help... and I didn't properly understand what was going on when I got started on this project. So I'm going to break down this deceptively simple looking joint, before we get into how it was done.

On the side of the clock, the vertical post meets the upper horizontal in a 45 degree miter. That's pretty straightforward. I'm going to refer to this as the side miter.

On the front of the clock, the vertical post meets the arch in another miter joint, that's cut at an angle that I've never bothered to measure in terms of degrees. Those miter lines point from the top corners of the case, directly to the center of the clock face. The inner radius of the arch is concentric with the dial, so the miter line runs radially through that edge. I'll refer to this as the front miter.

The curved top surface of the arch meets the upper surface of the upper horizontal members in a 45 degree miter. And I'll refer to this as the top miter. And this is where things start to get funky in the mechanics of the joint.

The plane of the cut for the side miter is at 90 degrees to the plane of the side of the clock. Or, the table saw blade is at 90 degrees to the table, when those miters are cut on those pieces. The cut for the front miter is also cut at 90 degrees to the plane of the surface. That's pretty straightforward. And in my head, that made everything seem very, very simple. That should have been a clue to me that something was awry, I guess. But because the face miter is cut at a different angle as the side miter, the edge where those two cuts intersect gets skewed to one side. So the three-way miter becomes a three way compound miter.

Each cut defines a planar surface. Geometrically speaking, two planes that intersect will define a line along that intersection. Practically speaking, that line defines the edge that's made where the two cuts come together. And for this joint to work, the edge defined by the two cuts made on the vertical post, the edge defined by the two cuts on the horizontal member, and the edge that's defined by the two cuts on each end of the arch... those three edges must come together cleanly along their length, with all of the mating faces coming together fully.

The test joint actually came together cleanly, but if you zoom in on the picture, and see the different surfaces interacting, you'll start to get an idea of just how many things can go wrong in the joint. Oh, and having one of these come together is hard enough. To cut the arch properly, there are two of these joints to consider, one on each end. Which brings us back to the top miter.

To cut that compound miter, the 45 you see on the surface is defined in relation to the top edge of the horizontal, and the back edge of the arch. The angle of the blade during the cut, which is what makes this a compound miter, is defined in reference to the surface of the parts that will lie flat on the saw table.

But the top is curved. There is no reference surface.

Obviously, to be continued...

Tuesday, October 7, 2014

Auto-regulator, Chapter 3: Making the vertical elements

One of the primary design elements of the case was the use of three way miters. In the pedestal, as I've explained, they're not true three-way miters. The movement case is a different story. But there's more to it than that.

Because the facings of the vertical elements are book-matched pieces of solid stock, there's a miter joint around the outer front edge of the vertical pieces. And, because the inside of the vertical pieces will be visibly sculpted, I needed to have matierial to sculpt away. So, the front vertical pieces are both three part laminations: solid core, with book-matched, mitered solid face pieces.

Mitering an end-grain joint is a fairly straightforward affair. Typically, it's used in case constuction, and the biggest miter joints I've usually seen are on things like blanket chests. That may be up to 18" worth of miter, but it's not too hard. In this case, I had a few more variables to consider. First, the stock was re-sawn out of a larger piece. It was mostly flat, and behaved fairly well. Second, the joint has to come together flatly, and mate with the solid core. So, if there was any concavity, or convexity in the joint, I'm going to have issues, because either the middle or the ends won't mate cleanly with the core. Lastly, these pieces are long. 32" of walnut, plus extra to trim back, and 42" of maple, with extra. So they needed to be long, straight, and perfect.

I trust my table saw to do many things. But a few days after being resawn, the pieces were ever so slightly bowed... and I didn't trust that operation to go smoothly on the saw. So, I rough-sawed the miters on the saw, and built a long-miter shooting board to do the rest.

Lastly, I needed everything to be dead straight. Given the thicknesses of everything involved, I wasn't too worried that things would go awry, but to be sure, I made box beams to provide flat reference surfaces for gluing, and glued everything up.

Thursday, October 2, 2014

Auto-Regulator Chapter 2: Below the Waist

I couldn't help myself when it came to naming this entry. The irony is that in this case, everything below the waist isn't where the real action is.

 A note on process: I started this project with a commission to build 2 cases. One in walnut, one in curly maple. So, you'll see parts here for 2 different cases. As I went through these parts, I decided that I would separate the two cases when I went on to build the movement case. The walnut case would be the prototype, (the one where I made my mistakes) and the maple would be the first official 'production' case. Walnut's a little more forgiving to work with than maple, both in the working of it, and in the fact that minor discrepancies are more easily covered up.

The case is divided up into three basic parts: The movement case, the pedestal, and the base. The movement case is what it sounds like. The waist separates the movement case from the pedestal, and the pedestal sits in the base. The base gets leveled before everything else goes up. The pedestal sits on the base, and houses the power supply for the clock. The waist is part of the pedestal, and all of the cables that connect the power supply to the movement pass through the waist, and up behind the rear panel in the movement case.

The base is nothing more than a mitered box. There are grooves inside, and some plywood parts that fit into those grooves, to reinforce the base from inside. Basically, it's a splining technique, but it also allows me to help with mounting the feet. The top of the base is rabbeted to receive the pedestal.

The pedestal is actually rabbeted around the bottom edge, because I wanted the joint line to be horizontal. This is intended to be a production case, and this joint will not be glued. So, in the event of any gaps between the base and the pedestal, I didn't want those gaps to be visible. So, the pedestal lips slightly over the base, and seats solidly in.

The pedestal is, at heart, basically a mitered box, too. The panels that miter together to form that box are mitered frame and panel pieces, that all come together to give the appearance of a three-way miter on the top front corners. It was important to make sure that the vertical pieces are oriented properly, as the grain is continuous with the pieces that will frame the movement case. It's a subtle detail that will probably be lost on most folks, but it's that kind of supporting detail that really makes the difference, and helps to unite the entire piece. (I find that the supporting details, when properly executed, become invisible. But when they're not there, or when they're not done right, they stand out.) The panels that fill the frames have book-matched or 4-way veneer patterns.

Like the base, there are internal plywood frames to reinforce the structure from inside. The bottom frame is open to allow access to the adjustable feet in the base, if needed. That way, if anything settles, the clock can be leveled without having to take it all apart. (That's the theory, anyway.) The top frame is open to allow cables to pass through, and go up into the movement case. The solid wood, mitered top of the pedestal is glued to this frame, and the waist is glued to it, too.

Because the plywood frame is glued into a groove that cuts across the vertical members of each panel, the miters won't actually be supporting the weight of the case, and the movement. The vertical load of the clock will sit on the waist, which sits on top of the plywood frame, which sits in dados that lock that frame directly into the vertical members. So, while it looks like the miter joints are supporting everything, they're not. The load actually hooks into the structure just south of that top mitered panel.

So, as I said, below the waist, the case structure is fairly straightforward. That's not where the real action is.