Cutaway PVC Half-Pipe Mold Parts
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.
Store the cutaway in the mold to keep the shape until ready to install.
The rest of the bending is similar to hot-pipe bending described in an earlier chapter. 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.
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 lightweight and durable guitar mold.
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 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 is 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.
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 (as pictured below).
Once the wood is completely pre-heated, drape the heat blanket over the cold-pipe setup. 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 setup 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 [A picture containing text, indoor, person Description automatically generated] the top of the pipe.
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.
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.
The cutaway is first laid out and drawn on the face of the guitar, then it must be transferred to the back of the guitar. The fully assembled guitar body is then laid face down on the band saw table. The line on the back of the guitar will be the guide 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.
Once the cutaway “C” has been cut into the upper bout of the guitar, install the shaped corner block where the cutaway meets the treble side in the upper bout (the Florentine point). Linings are then installed in the top and back of the cutaway. Smooth and fair the block and linings with a spindle sander as shown in a photo below. The cutaway side wood can now be installed.
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 woody 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 in a previous chapter. 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.
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.
To build the Mini Mold, 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.
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.
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.