Monday, May 11, 2009

Getting Moldy

Although I learned to build without using a mold (also known as a form), it's worth the time to build a mold if it's a guitar shape that you anticipate you will build again. Since the OM is a popular guitar shape, and cutaways are very popular as well, I can envision building a number of these particular shapes. The last guitar was a dreadnought cutaway, another popular guitar shape, so I built two dread molds, one with and one without a cutaway. I'll do the same for the OM shape.

It takes about 4 hours to make a mold. I start with 3/4" plywood, and trace half of the body shape onto it. Using the band saw, I cut the body shape a little bit oversized, then using a drum sander on my drill press to take it down to the lines. A little more sanding to make the edge surface plenty smooth, and I have template for half the mold.

I trace the template twice onto the plywood, and use the table saw to separate the tracings into two rectangular sections. Then it's back to the band saw to rough cut the shape about 1/8" oversized.

I'm going to glue all these sections together so that the mold is 2 1/4" thick, so the shape must match exactly. Using the drum sander to take it down to the pencil lines will not give me an exact match, however, just an approximate match, and that's not good enough.

Instead, I'm going to use a router and router table. The bit has a bearing that is the exact diameter of the cutters. I screw one of the oversized pieces to the template, making sure that all the edges of the rough cut overhang the template.

The bit is raised high enough to cut the entire edge of the piece to be shaped with the bearing rides against the edge of the template. The result is that the two pieces match perfectly.

A router is a great tool, but it kicks out more sawdust than any other tool I own. My router table has a fixture on its fence that will allow me to connect my shop vac and suck up a lot of the dust, but because of the shape of the guitar I have to remove the fence and the dust collector port, so everything is going to get thrown up into the air, where it can get in my eyes, ears, and lungs. Not healthy at all. The router is also extremely loud, so I where ear plugs, eye protection, and a dust mask. Kinda gives me an alien look.

Once of used the router to shape the other two pieces, I glue all three pieces together using Tite-Bond wood glue. Screws provide the initial clampling power, but four clamps make sure there is a tight bond between all three pieces.

Let it dry overnight, and the next morning I unclamp and unscrew everything, and I am halfway there.
If this weren't a cutaway I would have made five oversized pieces instead of two, and then glued everything up at once. But I have to repeat the process using the cutaway outline.

That was a couple of hours worth of work on Saturday morning. Glue it up and let it set for a few hours.

Monday afternoon after work I unclamped everything, drilled holes for the bolts to hold the two halves together, and wallah! An OM Cutaway mold.

You'll see how it's used later.

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