Steps making a Double Top balsa core soundboard.
During the course of my career, I like others have dabbled in making DT guitars. In the past I made DT's with a Nomex core. A few of these guitars I thought were quite nice, however, I seemed to always prefer my traditional guitars both for volume and tonal characteristics. I was getting ready to phase out making DT's when a customer of mine ordered one. I agreed to make him one and thought this would be my last DT guitar. I decided to try something different and Balsa core instead of Nomex. Previously I experimented with balsa core but it was a disaster, but this time I had something else in mind.
Long story short I finished this "last" DT, but the only problem was, I loved it, it really blew me away. I called Matt Palmer..... Matt wasn't crazy about DT guitars that he had played, and preferred a traditional sounding guitar. I told him you got to try this guitar! I was so excited I told him I'd make him one to try. I finished it in time for Matt's video and recordings here in Santa Fe. Long short short again Matt Loved it and it is now the guitar he uses for both concerts and recordings.
I heard some thing different in this balsa core DT that was quite different for the Nome core DT that I was disillusioned with.
People often ask what are the advantages of a DT. The answer is I don't know! some people don't like them at all and some people love them. I can sum it up this way. DT's generally are very responsive, and produce lots of volume. Traditional guitars have perhaps a sweeter tonal palette. I like both concepts.
Ive posted some Photos on my Face book page and a number of people wanted to know what this was all about and how I did it etc. So, I hope this short blog will clarify what I do. Perhaps others do it differently..... but this is how I do it, it may not be the best way, I don't know.
First of all, I used Hot Hide Glue to glue the balsa slats to the top. The reason being is I want the guitar to sound as "organic" as possible, the opposite of the Nomex plastic sound. HHG crystalizes and I hear a different with this glue, some would call me nuts..... maybe I am, but haven't noticed it yet. Whatever the case HHG is what I use.
I use a syringe and leave it soaking in my glue pot. I also have a couple metal bars. One bar I heat up in my glue pot, and the other bar I leave cold. I apply the Glue to the balsa slat which are 2.5 mm wide, using the syringe. I hold the slat by dabbing a razor blade into it and hold it with that while I apply the glue.
After applying the glue. I press the slat onto the top. The HHG will gel almost immediately creating an uneven surface between the balsa and the top which if left alone will produce an uneven surface on the top. So, after I apply 3 or four balsa slats, I take the metal bar that has been soaking in my glue pot at 140 degrees and put it on top of the balsa slats acting as a weight as well as warming up the HHG that has gelled and re melting the glue so it sits perfectly flat.
While this is being done I cut to length and apply 3 or 4 slats to the other side, then take the hot metal bar put it on top of these and put the cool bar on the first set of slats I did allowing the glue to cool and set. Its important to understand what is going on in regards to the concept of a DT. Two things in my mind: First to reduce the weight, second is to add stiffness across the grain. if you take out the wood add a lighter wood such as balsa, but then replace all the weight with glue then you are defeating the purpose. A high solids glue will add more weight than HHG which is mostly water. I make a small wooden glue stick to clean the glue out from in-between the balsa slats. Between gluing the slat in and cleaning the glue between them its a very intense and monotonous process. Put on a good movie or two while doing this.... ha!
I do this whole process on a Granite surface plate, which for all practical purposes is completely flat, and remains flat regardless of the humidity and temperature changes, un-like wood work benches or surfaces that fluctuate with humidity and temperature. The granite (which weighs 140 lbs) requires a very solid stand. Some friends have told me they use glass for this purpose as well. Whatever floats your boat. However I love this granite block I can plane tops on it, put sand paper on it for quick truing up of nuts and saddles and many different wood components, as well as sharpen my chisels and plane blades by sanding them down with 2000 grit paper on the granite then finished polishing the blades on my water stones. Not to mention you can clean the granite with a glass cleaner then wipe your hand across the surface and feel the slightest spec of dust and be very confident that when you set your cedar top down there is nothing on the surface to dent the soft cedar top. In short countless uses for the granite slab. However, I digress.
After most of the day has past. You might be finished with the tedious task. after letting the top dry over night then you cant plane down the balsa slats level the the rest of the top. At this point it doesn't matter hoe thick the top is because you are going to plane the entire top down to a very thin tolerance then glue a very thin 0.60mm either spruce or cedar veneer over it.
I would not suggest sanding the balsa as it quickly disappears, and the sand paper cuts into the balsa to much.
Some thoughts..... this is basically my routing set up same as probably most people I would imagine, again the perfectly flat surface of the granite adds a new dimension to the tolerances of the final result. This template has served me well, however, adding lines to the template that don't follow the grain of the top is better. As in this new shape. It's important to add wood under the bridge for solidity. I've made the mistake in the past making Nomex DT's and not adding enough wood under the bridge.
The top thickness in the routed areas should be around 0.6 mm finished thickness. The inner veneer should be the same, leaving the thickness of the balsa core up to you. If you wanted the total top thickness to be 2.3 or 2.2 then one must subtract the combined thickness of the top from 0.6 mm to be 1.7 or 1.6 mm.
The top will have noticable waves in it much in the same way the Nomex honeycomb shows up in the right light as you look across the top. this is a trait one must accept in DT guitars due to the thin veneer of the top. Cedar since its dark will do better in not allowing the slats to be seen. However spruce is OK. The solution for spruce is to put a cedar veneer inside thus not allowing any light to pass through.
Hope this helps anyone wanting to know how I do it. Probably better ways that I haven't thought of and I'm learning all the time. Signing off for now.