Cutaway PVC Half-Pipe Mold Parts
To hold the foamboard mold in place, one can screw it down to a workboard (recommended) or the workbench.
For a more permanent mold, sheath both sides (front and back) of the mold with ¼” lauan plywood. Then cut or glue up (4) 1-½”-wide by 2”-tall blocks from hardwood or plywood to make end blocks for holding the sides of the mold together. Finally, cut out screw blocks from hardwood or plywood to be through-screwed into the end blocks to hold the two halves of the mold together. This makes an inexpensive but light-weight and durable guitar mold.
Play Safe, stay healthy,
Come back next month for a discussion of the Box and Bindings.
MAKING A ONE-OFF MOLD:
The Mini that Woody builds has been created somewhat on the fly. The first two wJs Minis were built a couple of years ago, partly because I had some wood that I wanted to use, but mostly just for the fun of it. I had no mold or fixed dimensions in mind but just wanted to build a smaller guitar. So, I settled on the idea a small dreadnought (24.0" scale) and a body of approximately 90% of the standard dreadnought dimensions.
As a result of the unique size and dimensions, the sides had to be bent on a pipe...the old-fashioned way, as described last month. A unique mold also had to be made to hold the sides in shape until they could be glued up with the blocks, lining and top. Building a mold made from high-density Styrofoam (from Home Depot, or such) is explained below.
A TRUE CUTAWAY: Another, and probably the original, method for building a cutaway is to actually cut away the finished guitar body. The first time I did this I was very nervous.
The trick is to keep the band saw cut perpendicular to the soundboard. The assembled guitar body (without the neck), therefore, needs to be laid face down on the band saw table. The cut-line must be transferred from the top to the back of the guitar. This will be the line for the band saw blade to follow. Take into consideration where the braces are on the top and back. On Woody’s dreadnought cutaway, the transverse brace is cut and one brace on the back. One does not want to cut any part of the X-brace. The structural integrity of the body is maintained because the cutaway actually makes the upper bout more rigid.
Install linings (top and back) and a shaped corner block where the cutaway meets the treble side in the upper bout. Smooth and fair the block and linings with a spindle sander as shown in a photo below. Bending the wood to be used as the side of the cutaway has already been discussed. Keep in mind that it’s challenging to bend a short length of wood into a “U”.
Cutting open a finished hand-made guitar body with a band saw is not for the meek of heart. Now, after more than a few successes, Woody believes that cutting the body is easier than building a guitar with three sides in a mold.
A distinguishing feature of a guitar that Woody is currently building is the Florentine cutaway. The cutaway adds to the challenge of bending and assembling the sides of the guitar. The box will now have three sides and three blocks to hold it in shape, instead of two. Bending a short section of wood into the tight radius of a cutaway can be tricky.
The techniques for bending this short section of wood vary depending on the luthier's experience and equipment. Woody’s uses one of two methods depending on the type of wood.
THE HOT PIPE METHOD: As explained previously, bending wood on a hot pipe is challenging enough. Add to this that a Florentine cutaway side is only about 6” in finished length. Short sections of wood are hard to bend.
The first thing to do in using either of the methods explained here, is to thickness the wood down to 0.090” and cut it to a length which is 4” longer than the finished length. Then the bending of this short section proceeds mostly as a standard hot-pipe bend. What is not standard, however, is that the bend is extreme compared to the 6” length of the section, i.e. the wood is bent into a complete 180-degree “U” in a length of 6”.
To make this bend, the wood will need to be on the copper pipe for a more than a few minutes (rocking slowly to distribute the heat, very important). The wood must be kept moist to avoid scorching. Woody recommends the aluminum foil envelope demonstrated in the LMI video on side bending. If the wood seems to stop bending before a complete “U” is formed, open the envelope (gently) and douse the wood with water on both sides. Spritz or dunk the wood into a trough or bucket of water is the quickest and easiest way to wet both sides of the wood with the aluminum foil pulled back. When the wood has been soaked, close the foil envelope and go back to the hot pipe.
Aluminum Foil Envelope Video:
One final caution. Be sure to apply pressure evenly across the width of the cutaway section so as not to twist it, causing it to split. This would not be good. Use pressure cauls across both ends of the cutaway section.
If you spread the heat over the 3-4" center of the bend before applying serious bending pressure with your hands, bending this short section of wood will be much less difficult.
Chapter 06_ The Mini and the Cutaway
THE COLD-PIPE METHOD: The cold-pipe set up is almost the same as the hot-pipe set up…except no Bernzomatic torch. The heat will be supplied by the LMI heat blanket.
Pre-Heat: Spritz or dunk the 10” x 4” x 0.090” section of the cutaway side material to wet both sides thoroughly. Place the wet wood into an aluminum foil envelope (same as above) as shown in the LMI video on side bending.
Lay the heat blanket on the workbench. Position the aluminum-foil envelope closer to the ‘power-cord’ end of the blanket such that the opposite end of the blanket can be gently folded over the envelope, thus heating it from both sides as it lays flat on the workbench. Place a weight (2x8_12” in the photo) on top of this sandwich (blanket/foil/blanket) to increase heat transfer. Using the LMI controller to monitor the temperature, place the thermocouple into the heat-blanket sandwich between the aluminum envelope and the heat blanket. Leave everything in place until you are confident that the wood has reached 300 degrees. It won’t take long. You will need gloves to handle the envelope and the blanket from here on out.
Once the wood is completely pre-heated, drape the heat blanket over the cold-pipe set up. Keep the wood inside the aluminum foil and place it on top of the blanket on the pipe. The wood is now being heated from only the bottom, the same as with the Bernzomatic hot-pipe set up described above. The advantage is that you have digital control over the temperature of the heat source under the foil envelope. A further advantage with using the blanket as the heat source is that the sides of the pipe will be the same temperature as the top of the pipe.
The rest of the bending is similar to hot-pipe bending described previously. Rock the section of wood to evenly spread the heat. With steady pressure on the pressure cauls, the pre-heated cutaway section will bend slowly until the water is completely evaporated away.
If the wood seems to stop bending before a complete “U” is formed, it’s probably dry inside the envelope. Open the envelope (gently) and douse the wood with water on both sides. Dunking into a trough or bucket of water is the quickest and easiest way to wet both sides of the wood with the aluminum foil pulled back. When the wood has been soaked, close the foil envelope and go right back to the blanket-hot pipe.
Continue the rocking motion with the 1x2 pressure cauls in each hand to distribute the bending pressure evenly. Once completely bent and held in the “U” shape for one minute, take the cutaway section off the pipe and place it in a cutaway mold. Pictured below is a Woody-made PVC half-pipe mold.
MINI SIDES and MOLD
MINI BENDING: A technique of bending wood on a pipe has been documented previously (last month). Bending wood free-hand on a pipe is becoming a lost art. I am glad that I learned how to do it many years ago. It is a skill that Woody has used many times over the years, for many different purposes.
MAKING the MINI MOLD: To keep the shape of the Mini guitar symmetrical (or any unique guitar), the blocks and lining should be glued to the sides while it is in a mold. For a one-of-a-kind guitar, a one-of-a-kind mold is required. Woody has used high-density Styrofoam to fashion a mini mold that worked even better than he expected.
For the Mini, Woody started with (4) rectangular pieces of 1” Styrofoam 8” x 22”. Then he glued two pieces together twice, so that you have (2) 2”-thick pieces 8” x 22”. Draw the half-silhouette of the guitar body on only one of the pieces (centerline along one edge). Stack and tape the (2) 2”-thick pieces together so that now the stack is 4” tall. With a bandsaw and a sharp 1/4" blade, cut the silhouette out of the entire 4” stack at the same time. This will make the mold symmetrical. Woody cuts them on a 9” bandsaw with no problems.
The foamboard is very easy to cut. It is easy to stay precisely on the line without the 1/4” blade drifting. Just the same, always use a sharp blade, work scant and work slowly. When complete, both the treble and the bass the side of the mold should be 2” thick and symmetrical. Glued up this way, the foamboard is much stiffer than one might think. For a one-off project, a Styrofoam mold like this is completely adequate.