In four years using this machine I’ve only broken one side. That was because the wood was too thick. It broke at the curve in the upper bout. That’s the sharpest bend on a dreadnought guitar. A cutaway is, obviously, tighter. I usually bend cutaways by hand on the copper pipe set up described above. More on that next month.
First, let me say this: It is much harder to explain bending on a pipe than to actually do it.
BENCING ON THE HOT PIPE: Start by spritzing (or dunking) the 0.100” thick wood to be bent. The wood should not be water-logged from hours of soaking under water. Water does not make the wood bend. The water is there to carry the heat into the wood fibers and resins. When the wood resins are heated, they soften and allow the wood fibers to slip past each other. Bending the wood can also be thought of as stretching the wood on one side and compacting it on the other. Both require the wood fibers to rearrange themselves.
With the wood laid across the pipe, rock the wood back and forth slowly. This should spread the heat over a 6”-8” section of the wood. Center the pipe at the tightest part of the bend with your hands equidistant from the center of pipe. Pressing on the wood with a 6” piece of 1x2 in each hand will allow you to apply bending pressure evenly across the width of the side. Keep rocking the wood slowly left and right until you see moisture evaporating from the side of the wood that is facing you. This evaporation means that the wood surface that you’re looking at is 212F. The wood touching the pipe is, of course, much hotter.
The boiling point of water, 212F, is not hot enough to bend mahogany, much less rosewood. For mahogany, we need the wood to be heated through its entire thickness to 275F (300F for rosewood).
The temperatures that are stated here are approximate. Don’t stop to take the temperature of the wood while bending.
The idea is to bend the wood without scorching the surface that is in contact with the pipe. Keep the pipe-side surface moist and the gas flow steady from the Bernzomatic. The temperature of the pipe will drop when the moist wood is laid on. That’s not a problem. You have a 3,600F flame at the other end of the pipe that will heat things up again quickly.
Bending wood is one of the skills that differentiates luthiery from other types of woodworking. In this chapter, Woody will show some of the differences between bending wood by hand on a hot pipe, and bending with a fixture designed specifically to bend guitar sides.
Woody learned to bend guitar sides from Charles Fox back in 1976. To be correct, Woody was shown how to bend guitar sides in the Fall of ‘76.
At Charles‘ school in Vermont, I think that it is safe to say that I was the worst in the class at bending wood. I just did NOT get it.
I actually learned how to bend wood during the following winter when I went back to Fairbanks and had nothing else to do but stare out the window at the darkness.
The following is what I learned that winter and have modified over the years.
The variations in temperature is actually not all bad, if you are know where the hotter and cooler spots are located. Sometimes you will want to move the wood being heated to a cooler section of the pipe to keep it from becoming scorched. One can’t assume that the wood is being heated evenly across its width or length. There is a way to deal with that. Turn the wood around. The signals to know when to turn the wood around are discussed below.
LMI Bending Machine Video (17 MINUTES; recommended)
COMPUTER-AIDED BENDING MACHINE: To bend sides with the aid of heat blankets and a heat controller Woody uses the LMI side bending machine pictured here. The machine came with heat blankets, thermocouples and a controller to monitor the temperature of the wood as pressure is manually applied for bending.
The bending machine works quite well but that does not mean it’s automatic. Vigilance is required because one cannot see, or feel the wood as it is being heated. The feel that one gets is through the screw press on top of the machine, or the rollers which press the wood against the shape of the mold. Experience in bending wood by hand, on a heated pipe, is very helpful in learning how much pressure to apply…and when.
Before taking the guitar side to the heat pipe, thickness the wood to 0.100”. The guitar sides should also be cut to the proper taper before bending. The taper is not a straight line because the finished guitar’s waist, and upper and lower bouts, are each different depths. Templates for the cut of the sides are available online. One can also copy the depth dimensions from a manufactured guitar if measured or traced carefully.
THE HOT PIPE: The first thing to learn about bending wood on a pipe is how hot the pipe should be. When heating the pipe with a torch set-up (pictured below) the bright blue flame from the propane torch needs to be only 1½” long. As the copper pipe is heating there will be a point when a drop of water dripped onto the heated surface of the pipe will flash boil, bead up and bounce around on the pipe until it finally rolls off the side. That’s when the pipe is hot enough. That temperature is approximately 350F.
Another thing_ the last 4-6 inches of the pipe are not at a consistent temperature. The Woody-installed collar on the torch head is for aiming the flame at the center of the pipe diameter at the baffle end of the pipe. Still, the pipe can be 100F hotter near the baffle than it is 6” back toward the middle of the pipe. The pipe is also much hotter on the top than it is on the sides.
If you see that the water on the top side of the heated section is evaporating closer to the end of the pipe than toward the middle of the pipe, turn the wood around so that the heat can be distributed more evenly across the width of the wood. Once again, keep the heated side of the wood moist to prevent scorching.
Be careful not to touch the pipe while bending, but don’t be afraid to touch the heated wood for brief seconds. A little bit of touch and feel is good. If the wood is too hot to touch, it’s hot enough to bend.
Maintain a steady downward pressure on both sides of the pipe with your hands 10-12” apart. The area receiving maximum heat will be the section in contact with the top of the pipe. You can slide the wood to the left or right if you want to apply more heat to a different area.
Keep an eye on the wood and be sensitive to the feel in your hands for indications that the wood is bending. When it starts to bend, maintain the same pressure on the wood, but spread the area that is receiving maximum heat by moving the wood a little to the left and then a little to the right. Keep the wood rocking, remembering that the top of the pipe is significantly hotter than the sides. As the heated area widens, continue to keep the wood moist with a spritz or a dunk to avoid scorching.
Chapter 05_ Bending