As with all manufactured tools and equipment, practice and experience are the only way to understand the machine's quirks and limitations. Just be aware... there will be quirks and limitations. For example: One cannot assume that the temperature of the wood in the bending machine will be uniform under the heated blanket. When learning this machine, it is helpful to place the thermocouple between the wood and the heat blanket near the point where the bending is most severe_ i.e., at the waist and upper bout. This might require a third thermocouple or moving one of the thermocouples. Wherever the thermocouples are located, make sure that they are between the wood and the heat blanket.
If one bends the wood too fast, or the wood is too thick, cracks happen. Again, thickness the sides down to 0.100” before bending.
LMI BENDING MACHINE VIDEO (17 MINUTES; recommended)
This presentation of Woody’s experience with the LMI bending machine assumes that one has seen the online video. As an introduction to side-bending machines, the video is an invaluable resource. Bending of the sides will go smoothly if you watch the video a couple of times and then proceed according to LMI instructions. Be sure that you are alert and sensitive to the wood’s resistance. At 0.100” thickness, 270F-300F, and a spritz or dip into water as suggested in the video, there should be no problems.
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 in chapter 8.
First, let me say this: It is much harder to explain bending on a pipe than to actually do it.
BENDING 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 soften and rearrange themselves.
With the wood (say mahogany) 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. Generally, one doesn’t need to stop to take the temperature of the wood while bending. It's either too hot, or not hot enough.
The idea is to bend the wood without scorching the surface that is in contact with the pipe. Keep the pipe-side surface moist by dipping in the trough, and keeping the gas flow steady from the Bernzomatic. The temperature of the pipe will drop when the re-moistened 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.
TO BEGIN WITH…
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.
Many thanks to Charles for his patience. I took copious notes, and I finally 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 variation in temperature is actually not all bad if you 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.
Play Safe, stay healthy,
Come back next month.
As the wood bends, you will need to remove it to check its shape against a template tracing (or a cardboard cutout) of the guitar’s silhouette. Quickly note what bending is needed, give the wood a spritz, and get the wood back to the pipe as quickly as you can.
From here on out it is just a matter of doing more of the same. Once you learn how to localize the area of maximum heat and the feel of the wood as it begins to bend, you will be able to adjust the moisture, the heat, and the bending pressure as needed to get the bend that you want.
As I mentioned in the beginning, it is harder to describe this process with words than it is to actually bend the wood. Hopefully, this explanation will help you understand what it is that you’re looking at, feeling with your hands and fingers, and how (and where) heat is being delivered to the wood.
Wood bending is a basic skill of guitar building and repair, not some ancient tribal ritual. Practice and you will get it. LMI sells orphan sets of guitar side stock (cheap, they call them ‘practice sides’) just for the purpose of learning how to bend wood. Whether bending on a machine or by hand, practice is the only way to learn.
Woody needed a trough to soak guitar sides. So, one afternoon, he made one out of sheet metal, a few screws and silicone. To his surprise, it never leaked. That was 40 years ago. True story.
Woody paid $550 for LMI’s bending fixture in 2015. After bending on a hot pipe for 30+ years, the bending machine is like a new friend.
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.
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. Place the head of the torch into the copper pipe and mount it so that it is secure and stable.
As the copper pipe is heating, there will be a point when a drop of water dripped onto the heated, upper surface of the pipe will flash boil, bead up and bounce around on the pipe momentarily until it finally rolls off the side. That’s when the pipe is hot enough. That temperature should be approximately 350F.
Another thing_ the last 4-6 inches of the pipe are not at a consistent temperature. The Woody-installed aluminum collar on the propane torch head is to keep the flame aimed at the center of the baffle at the other end of the pipe. Still, the temperature of pipe can be 100F hotter near the baffle than the temperature 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 probably scorching on the underside.
Maintain a steady downward pressure on both ends of the mahogany 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 06_ Bending Sides
It's a good idea to turn off shop music and cell phones when learning to bend with a machine. If one proceeds as demonstrated in the LMI video, it will only take 10-15 minutes to bend a side. That’s almost too quick to see problems coming and take steps to avoid them. The LMI video is very good at explaining how to set up and operate this bending fixture to achieve the results that you want.
Before starting the bending process on the LMI machine, Woody likes to thickness the sides down to 0.100" and cut the taper. The taper is not a straight line because the 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.
Be sure to set up the LMI hardware just as depicted in the video. For mahogany, set the target temperature for 270-285F.
After only a few minutes one can hear the wetted wood reaching the boiling point of water (212F). That's a signal to tighten up your focus! The temperature of the wood will soon rise to the bending point depending on the thickness (and species) of the wood. The heat controller will not allow the temperature to rise beyond the set point. Harder woods are more dense and generally less flexible. These woods (Rosewood, Padauk, etc.) will require a set point closer to 290F or 310F. Proceed slowly on these dense woods.
Once the target temperature is reached, the wood needs to be bent very slowly as you mentally monitor its resistance. When you can feel the wood stretching, proceed slowly but deliberately. After the side is completely bent to the shape of the mold, reduce the heat by 25-50F and allow the wood to cool down while still clamped between the heat blankets. Hold the bent side at this reduced temperature for 10-15 minutes. This will help 'set' the shape of the wood. Ideally, the wood should cool to room temperature before it leaves the bending machine. Then it should go directly into your mold.
After the sides have been in the mold overnight it will be completely dry. The blocks and lining can then be glued in. The shape of the guitar is, thereby, fixed. The complete assembly of the guitar body will be the subject of the next chapters.
Before taking the guitar side wood 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.