The molds are complete -- glued up, assembled, and a few coats of lacquer. The lacquer is just to protect the MDF from moisture. Moisture makes MDF swell a lot and ultimately crumble apart. These molds will see their fair share of moisture, as you'll see later in this post. Here's what one of the finished molds looks like, with the spreaders in place. The spreaders will squish the sides against the mold while I'm working on them. I made two molds and sets of spreaders, so Josh and I can be working on the guitars at the same time.
Next, I made a 40' radius dish. A radius dish is a disc (this one is about 1-1/2" thick, 23" in diameter) with an inverted dome cut into the top. The dome has a 40' radius. Imagine an enormous sphere that is 40' in diameter, lowered into my radius dish, and that is the shape I'm going for.
The purpose of the radius dish is to help me put a 40' radiused dome on the top of the guitar. Even though these will be traditional "flat top" guitars, flat top guitars aren't really flat on the top. They are mostly flat. If they were all the way flat, they would break. Whenever the humidity decreases, the wood on the top would try to shrink, and since it is glued to the sides, there is no way for a flat top to shrink without splitting. A slight dome gives the top room to shrink (and, in the process, flatten a bit) without cracking when the humidity decreases. It is also quite a bit stronger than a flat top. I've always used a 25' radius for my guitar soundboards in the past, but Martin uses a 40' radius, and since Josh wanted a very Martin-esque guitar, I'm making a new radius dish.
I've made radius dishes in the past using a router with a set of arched guides and a lazy suzan to spin the dish while the router goes to all the right places. It's too complicated to explain, and very messy. But today, I have a CNC machine, so it's a pretty simple process. Here's a video of my CNC router cutting the dish, which I think is super cool:
And here's as picture of the finished dish. The 40' radius is very slight, so the dish looks flat until you put a straightedge on it:
Well, that's enough mold-and-jig building. Time for the real fun to begin.
I build my guitars from scratch. Quite literally, I buy nearly all of my material as rough-sawn lumber from a local lumber yard. You have to be very careful to pick out lumber with the right grain orientation, which is important for strength and stability. But if you know what to look for, there's plenty out there. Alternatively, you can buy specialty tonewood from some of the luthier supply houses. "Tonewood" is wood that you buy from a luthier supply house that costs 5-6 times what you would pay for "normal wood" at a lumber yard. There really isn't any other difference.
These guitars were inspired by an all-mahogany Martin that Josh and I saw at a local music store, so Josh wanted to make these guitars from African Mahogany. We'll use that for the soundboard, back, sides, and neck. (Most everything.) Here's the raw lumber:
All of this gets resawn on my bandsaw to very thin slices. Starting with the center board in the picture above, which will be for the sides, I begin by jointing one side perfectly flat on my jointer:
We plane the other side perfectly flat and parallel to the first side, using the planer:
Then I slice it up on the bandsaw:
Now, each slice is about .190" thick. The sides need to be thinned to roughly .080" for them to bend without cracking. I thin them on the wide belt sander:
Next, I stack the super-thin sides with popsicle sticks in between them, and then put a heavy rosewood board on top. The purpose is to let the sides acclimate to the humidity of my shop now that they have been resawn, and the popsicle sticks create a bit of space between the sides so that the air can flow between them readily. The heavy rosewood keeps the sides from warping as their moisture content adjusts.
Then I go through the same process with the big piece of mahogany for the backs and soundboards. I upgrade the popsicle sticks to some bigger sticks to ensure I have good airflow on these larger pieces.
And now, for the part you've been waiting for. How to bend the sides. Having thinned the sides to .080" thick helps, but it will still break if you try to bend it. So, I spritz it with some water, push the wood against a hot pipe, and let the steam get into the wood fibers. I keep repeating until I can bend the fibers a bit. When the fibers are hot and wet, they will bend. The concept isn't complicated. But it is finicky. If you push too hard, it will crack. If you don't push hard enough, it won't bend. If you don't wait long enough for it to get hot before bending, it will crack. If you let it get too hot for too long, it will "case harden" and become impossible to bend. If you get it too wet, it will ripple in a nasty way. If you don't get it wet, it may case harden before it bends. There's lots that can go wrong, but with some practice, it just bends.
And so, here are the videos. The first video shows me just beginning to bend.
This next one is a bit longer. Bending sides takes some time. I have to bend it, compare the shape to the mold, bend some more, and keep going until it is close enough that I can clamp it into the mold. Some people are faster than me, but my sides come out pretty nice. I think this side took me about 20 minutes overall to bend until I had it ready to clamp into the form. (The video doesn't show the whole thing.) Don't feel like you have to watch the whole thing, but it is kinda therapeutic.
Here are some pictures of the bent side in the mold. Those spreaders I made press the side firmly against the mold. The sides will sit there for a day or two to dry and then get used to their new shape. (You can see in the first and third pictures below that some areas are darker because they are still wet, while others are lighter because they are dry.) I made the outer edge of the spreaders .09" smaller than the inner edge of the mold, so the spreaders match the inside edge of the sides when the sides are pressed firmly against the mold.
And that's how you get wood into the shape of a guitar.
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