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Sylvan Wells Guitars
(781) 361-1471
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Vacuum and the Guitarmaker What is vacuum? Vacuum is a space that contains air or a gas at less than atmospheric pressure. 1 sq. in. column of air from sea level to outer space exerts 14.7 pounds of downward force. Application of this force to a larger area like 1 sq. ft.(144 sq. inches), generates more than 1 ton of holding force (14.7 x 144 = 2,116.80 lbs.). This is very useful in guitarmaking. At a recent ASIA Sympsoium I demonstrated a method of a holding a guitar with vacuum on a workbench. I drilled a hole in a piece of solid wood about 5" by 6" with a vacuum line connected out the side. When I placed the template on the bench covering the vacuum hole, and put the work piece on the template, I created a vacuum cavity. Turning on the vacuum pump sucks all the air out from under the piece. The vacuum holds the template and piece to the table with about 300 pounds of force. Making this vacuum template is quick and simple. I have used a template made from wood. It's cut and sanded to shape and then a hole is drilled through the center. This allows vacuum to be applied to both sides of the piece. Next, the piece is outlined with a closed cell foam tape (this tape is household insulation available from any home improvement center but it must be CLOSED CELL not open cell-they sell both types) on both sides (top and bottom). The foam provides a seal between the template and the work piece on top and the table on the bottom. Calculating the holding force is just very simple math. First, determine the area under the vacuum. For this example, assume about a 5" X 6" area or 30 sq. in. With the pump on, I reached 20 inches of mercury ("Hg); about the minimum needed to hold securely. For every 2" Hg, we get about 1 pound/sq. in. (psi) of holding force; a 2:1 ratio. Therefore, 20" Hg equals 10 psi. The 10-psi is multiplied by the 30 sq. in. of vacuum surface area. This equals 300 pounds of force holding the template to the tabletop and work piece to the template. Why use vacuum for guitarmaking? It is fast but so are clamps. It is strong but so are clamps. There are many reasons. Here are a few. Vacuum applies even and uniform pressure over the entire area where vacuum is applied. A mechanical clamp applying 300 pounds, applies it to a point which receives the "hot spot" of the clamp. Everything else gets something less. Not so with vacuum - even pressure everywhere. Additionally when you use a membrane to act as a caul there is no movement of the pieces being glued. All pressure is straight down. This is, for me, the crucial reason. This is the perfect scenario for gluing back and top braces. If you are gluing up backs and tops for an acoustic guitar vacuum clamping allows you to basically pre-shape the fragile braces before they are glued on. Even though some of the braces are quite small and fragile, vacuum clamping, without any torsional forces, allows the easy gluing of such pieces since they will not move around. Another important use for vacuum in guitarmaking is the ability to "clamp" an entire surface by putting the work in a "bag". This is the most efficient way to glue a top on a solid body guitar. In fact, it is the only way I know to achieve good glue bond on a drop top stratocaster type guitar. There are many kinds of vacuum pumps used in industry. For guitarmaking most of our needs are relatively modest. Most of the guitar supply houses sell air powered vacuum pumps for about $130.00. They require an air compressor of at least 3/4 hp. to work. As a practical matter, you need to be able to maintain 100 lbs of air pressure throughout the cutting or gluing operation. And, unless your air compressor is outside your main work room, the process can become quite noisy. The noise is the main reason I do not recommend air powered vacuum pumps. Some of the supply houses also sell stand alone electric vacuum pumps. These can vary in price from $2-300.00 to thousands of dollars. I used to use two different air powered systems in my shop (noisy!) but I have now switched to a small GAST diaphragm pump which I purchased on eBay for under $100.00. Ebay is a great place to buy a pump. You can get a good diaphragm pump for under a $100.00 if you take your time and study what is available. Do a search for GAST vacuum pumps and you will be pleasantly surprised at what you will find. Just make sure that the pump will pull at least 20 lbs. of mercury and it will work great for guitar work. Sources of Vacuum SuppliesLuthier’s Mercantile International Vacuum Fixtures and Vacuum Pumps Hold Downs, jigs, bags, air compressor vacuum Stockwell Rubber Company 1/16” COHR 9235 36”W
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