Top thickeness sanding is one of the most important steps in the entire guitar building process. The tone and the structural integrity of your guitar is directly related to how thick the top is. A sharp hand plane is a fine way to thickness a top. It allows you to get a feel for the wood and determine things like grain run-out. However for this video, I will be using a drum sander. You can also use a combination of both tools. If I only use a hand plane, then I complete the process by sanding the inside of the top up to 180 or even 220 grit.
Let me start by saying that I’m not necessarily about a specific number for my top thickness but rather about the stiffness and musicality of the top. I am working with certain numbers in mind but I don’t let those determine how thick or thin to make each individual top. I only use these numbers as guidelines and then I let each top tell me how thick it wants to be. After thickness sanding the top, I check it calipers to make sure it’s in my guideline parameters. Before thickness sanding the top, the rosette must be installed, leveled, and the entire outside of the top sanded to about 180 grit. You can start sanding with 80 grit paper on your sanding drum. However ideally, your drum sander should have about 150 or even 180 grit paper on it as you get closer to your guideline numbers. Most tops when purchased already come thickness somewhere between 4 and 4.5mm or .157”-.177”. As I thickness them, I noticed for my purposes they become musical somewhere around 3.5mm or about .140”. This is the point where I really start paying attention to the stiffness of the piece of wood. Notice how I hold the wood between my fingers and lightly flop it. This gives me a feel for the stiffness and also allows me to hear what it sounds like. What I listen for is what I describe as a sheet metal sound. This sound is the musicality I mentioned earlier. You should be hearing the sheet metal sound around 3.5mm or .140”. The more you thickness the top pass this measurement, the more musical that sheet metal sound will become. There will come a point where this sound will peak and if thickness more it will gradually decrease until it just sounds like a piece of cardboard.
The goal is to stop thickness sanding at this peak. Now I start paying attention to numbers by checking the thicknesses with my calipers. Trust your judgement. As you gain experience, your senses will tell you when a top is too thick or too thin or too stiff or too floppy. The calipers only verify this. This peaking of the sheet metal sound that I spoke of is only for my steel string guitar tops. When thickness sanding my classical tops, I have to shoot more for a target number because I take the top so thin that before bracing, it really only sounds like a piece of cardboard. Just as with the steel string guitar though, this target number is only a guideline. The tops are really beginning to loosen up now as I get closer to my final thickness. Watch as I flop each top and listen to it, Occasionally, I also check it with the calipers. A judgement call needs to be made of whether or not to send it through the sander again or stop and work with this final thickness. As I work, let me tell you about the pieces of wood used in this video. They were Adirondack, a piece of bearclaw spruce, Port Orford cedar, and several pieces of Sitka spruce. I’m also going to give you some generic guideline or parameters for you to work with. For spruce, a good set of parameters to work with are between .105”-.115”or 2.6mm-2.9mm.For cedar, I work with between .110”-.112” or 2.8mm-3.1mm.
Finally one of the tops has arrived at what I considered to be optimum thickness. It just happens to be the highly figured bearclaw spruce and it came in at around 2.9mm or .114”. The next piece to arrive at final thickness was the piece of Port Orford cedar. This was a really stiff piece of cedar and it came in at 2.8mm or around .110”. The Adirondack decided it wasn’t done until 2.75mm or .108”. All of the Sitka tops had to go through again but eventually all of them ended up in around 2.6mm or around .105”. Remember the specs I have given in this video are only guidelines but I highly recommend staying within these parameters until you have gained enough control over your tone to do otherwise.