Friday, May 29, 2009

Feelin' Like Frettin'

With lots of things to glue but humidity in the 70% range, I move on to other jobs that I was planning to do later. One of those is cutting the fret slots in the fretboard.

There is actually both a geometrical and a mathematical way of figuring the distance between each fret. I can't show you the geometrical way, and I'm not sure you want me to try to explain the mathematical way, but for those with pocket protectors, here it is: take the scale length (the distance from the front of the nut to the front of the saddle) and divide it by 17.817, and that gives you the distance from the nut to the first fret. Subtract that number from the scale length, and divide by 17.817, and you get the distance from the nut to the second fret. Keep doing this for as many frets as you plan to have.

With a calculator it's not that hard, but unless you are using an odd scale length there's no need because someone's already done the math for the most common scale lengths. I'm using the most common scale length for both dreadnought and OM guitars, 25.4".

But here's what you get: the distance from the nut to the first fret is 1.426", from the nut to the 2nd fret is 2.771", 3rd fret 4.041" get the picture.

How exactly do you measure 2.771"? Even if you round to the nearest 1/64", you still get 1 27/64", 2 49/64", and 4 3/64". When you're measuring in 1/64ths of an inch, it's easy to be off when measuring and marking. A less-than-sharp pencil can mess you up.

And fret spacing is pretty important if you like things like playing in tune.

For guitar #002 I just ordered a pre-slotted fretboard from LMI, but over the winter I was able to get the tools necessary to do it myself. This is the same system that LMI uses and that we used at Vermont Instruments when I built #001.

I spent a good part of yesterday and today building the table saw sled for the slotting system. Here are a couple of pictures of the sled seated on the table saw.

It's made of 3/4" MDF (medium density fiberboard--think tightly compressed cardboard) which is nasty stuff to work with but great when you need something that is absolutely flat and square and won't warp. In this regard it's better than plywood.

Underneath I attached two metal runners that fit in the two miter slots of the table saw.

The regular blade is replaced by a special fretting blade, which is smaller and thinner. Because it is so thin it will distort when encountering the hardwoods like ebony and rosewoods that are used for fretboards, so it is sandwiched between two thick metal plates.

At the heart of the system is an acrylic template which has notches along both edges corresponding to the fret positions of two popular scale lengths, 24.9" and 25.4". I mark the center line on my ebony fretboard and, using double-stick tape, tape it to the template so that it matches the centerline of the template.

Installed in the fence is an indexing pin which fits into the notches.

By holding the template against the fence and pushing the sled over the saw blade, I get a kerf cut into the fretboard that is the perfect width for the fret wire.

Before I can do that I have to know what depth to cut the slot. This is complicated by the fact that the fretboard is going to be radiused i.e. it will be slightly domed across it's width. If I just cut to the depth of the fretwire tang (the part under the fret that gets hammered into the fret slot), then when I radius the fretboard the slots will be too shallow at the edges. It's better to be a little deep in the middle than shallow at the edges. But how to figure this out?

Luckily I have a pre-slotted and radiused fretboard for a 24.9" scale length, and I find that a piece of posterboard fits perfectly in the slots. I cut a small piece of posterboard and use a straightedge to make sure one side is perfectly straight. I insert this edge into one of the slot and, with a very sharp pencil trace the outline of the fretboard. I use a razor knife to cut along this outline, and now I have a guide for how high to raise the saw blade. I just hold it over the blade with the two outside edges resting on the table, and raise the blade until it touches the highest point on the gauge.

Boom! Now I'm ready to slot the fretboard. The first notch cuts a slot which will be the front of the nut. Then I move the template to the next notch, run it over the blade, and repeat for twenty frets.

When I'm done, I have a slotted fretboard.

Took me several hours from start to finish, but for every guitar after this it will be a thirty minute job including switching the blades out and cleaning up.

And I can now say that I did everything on the guitar myself.

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