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Guitar wood bending

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Blog entry by Max11221122 posted 07-07-2020 12:34 AM 484 reads 0 times favorited 3 comments Add to Favorites Watch

This article shows two methods to bend wood for guitars without a steam box. One is to use water and clamps and the other uses a steam blanket and a fox bending machine. This isn’t the hardest part of guitar building. If you go slow and take your time everything will work out. Many people ask: How do you bend wood? Even without the steam box the answer is that we need water and heat to create steam in some form. This is the key to guitar bending. The steam loosens the fibers of the wood and hydrate the cells making the wood pliable.

Background

Let’s think about the lifecycle of the tree and analyze some of the different trends that happen as the tree ages. It starts in the ground and grows large. A tree can be 100 to who knows how many years old. While it’s growing the wood needs to suck up water from the ground. This water works as a coolant for the plant. It dissipates the heat created by the growth of new cells. The moisture content of the wood is at it’s most here. Once the tree is chopped down It stops sucking up water from the ground. The internal moisture content is set at this point and it can only go down. If we take a small branch off the tree, it will bend with no trouble. We can twist it and turn it into a circle and do whatever else. This is because it has so much moisture in it. Once that twig dries out it will snap when bent. Next the wood is sawn into usable boards and then kiln dried, sometimes with vacuum. When they still have a high-water content, they are considered “green”. The drying process makes the wood usable. It is brought down to a moisture content that makes the wood workable. If it is too green than it will compress while in use and shrink. For guitar making we want dry wood it has many useful properties. It’s hard and stiff, among others. Yet it loses that bending ability. To be successful, we must soften the wood for bending.

The wood needs to be convinced to become a guitar. It has spent years and years as a tree and only know one shape. For this reason, going slow will make or break the success of the project.

The important part of knowing how bend wood for guitar sides is applying pressure to the wood. Whether this is a roller or with clamps, it is a necessity. Something needs to convince the sides to take the shape that we want it to. Sometimes it’s necessary to make a tight turn, so it’s important to apply consistent pressure at each point. The good thing is that once the wood has taken the new shape it will keep that shape.

We can think of wood as a thermoplastic. It is made up of a grid of uniform pieces, the fibers of the wood are stacked up and this creates the grains. With water and heat the fibers become lubricated by the water and can slide against each other. Just like if we heat up plastic it will bend, when we heat up wood it will want to bend as well. This is only if it is wet enough. Wood that is dried out will scorch and burn. The temperature to keep the wood at is hot enough to evaporate water but not enough to scorch the wood. The water in the wood cells acts a lubricant and lets them slide next to each other. Thus, creating a pliable piece of wood.

Three caveats to consider:

1. Some pieces will never bend. They are too thick in certain sections. It’s possible to go at a turtle’s pace to make these bends, but it is a challenge. The wood must be less than a quarter inch to give the bend a good change of success.
2. Some bends have too sharp of a corner. A Florentine cutaway guitar that has a sharp edge will never be possible to bend to shape. There is a cutoff radius of curvature for each piece of wood depending on the size, shape and species of wood.
3. Some species of wood are harder to bend than others. Zebra wood is difficult because it wants to split at the tops where the black streaks are. It is important to experiment with different types, sizes and shapes.
There are two methods of wood bending to be shown. How to bend wood with heat and water and how to bend wood without heat and only water. One will be with a fox bending machine and the other will be with clamps and a jig. Both show how to soften wood for bending.

Clamps

The first method that I’ll show uses tepid water, a sink, a jig, oven, and clamps. Quite simple compared to using a steam box, heating pad or other equipment. This process is best used on a jig with a natural radius of curvature. The wood can make some dramatic bends despite this, even without steam.

The strips of wood are soaked in water. The water can be warm but it’s going to cool down over the time that it’s soaking. This step isn’t to add any heat to the wood it’s only to make the water absorb through the wood. It’s left in the sink for about an hour. It can be left in longer but at a certain point the wood has absorbed all the water that it’s able to. The strips are thin enough that this doesn’t take a long time. After the hour is up the strips are tested by giving them a little bend. If it’s pliable then they can be taken out. If they are still tough, then time to soak for longer. The braces only need to bend enough to fit into our forms. The forms have nice gentle curves so it’s not asking the wood to bend far. It’s much less than bending the upper bout of the side, a notorious section.

The four pieces are clamped to one end of the jig. Then a clamp is applied two inches up, then another is clamped two more inches up. This is repeated until the wood is bent around the jig. Before clamping, check to make sure the wood has some give and push it into the form. The clamps are applied closer where the jig curves this is because there is the most force on the wood.
The wood is clamped into the jig and put in the oven. They are layered three of four pieces deep. Doing many at once allows for more guitar braces to be created.

Here’s Jay taking the braces out. It’s hot! Those clamps absorbed a lot of heat. The heat of the oven absorbed into the wood and turned the water into steam, this steam evaporated in the oven leaving only dry wood left. The temperature is set to 350 degrees. This is enough to prevent the wood for burning but hot enough to get rid of the moisture. This temperature is appropriate for the guitar side bending temperature as well.
Steam has been created in this process. But the steam was not critical to make the wood bend. The strips were thin and wet enough on their own to make the change with clamps.

The jigs are left to cool down and hold into the form. It would be a problem to release them from the jig now and have them relax back to a flatter position. If the pieces were wet this would surely happen. They are still malleable and don’t understand what shape we want them to be in. Therefore, we apply heat; it evaporates the water and makes the wood hold its shape. At a certain point this is true because we need to dry out the wood, it isn’t useful to use a wet mess.

The Bending Machine

The second method to guitar side bending is using a bending jig. This is called a fox bending machine with a few adjustments.

The idea is to bend the middle section of the side first by slowly applying pressure with the radial arm. A fixture that matches the curve of the side jig is pressed down into the wood by turning a large bolt then held in place. The piece of wood sites on a jig that has the same shape as the side that we want. To get the outer curves two rollers are used. They are held in place by two large springs fixed to the base.
We need to make this machine usable for many different sizes of guitars. We have many jigs that reflect the different body sizes that we used from a 17” bass down to a 6” super tiny model. The holes in the side allow the roller to be positioned in any place. This makes it easy to find a position to set the roller.
The side wood that we want to bend is placed into a heating blanket.

This is a 5-layer sandwich. There is a strip of aluminum flashing, the heating blanket, aluminum flashing, the side and more aluminum flashing. The heating blanket isn’t directly placed onto the wood.
Now comes the most important part: The water. The water is kept in a bottle with a squirting attachment. The water is poured onto the wood in the aluminum sandwich. The goal is to flood the wood and saturate it as much as possible. If there is an ample amount of water the wood will not burn. The heat will spend all its time evaporating the water,
so, it won’t have time to burn the wood. The water acts as a heat conduit to evenly heat the wood. Water is continuously added. The heating blanket is turned on and put on a timer. Once about 5 minutes pass the wood sandwich is at a good temperature and begins to steam. Water is added where needed to keep it steaming.

An interesting point to note is that when the water is added the wood doesn’t bend under it’s own weight. Only after the side starts getting heated up does the side began to collapse. It’s easy to see that the piece is ready to bend because the wings not on the mold sag down from their own mass. This shows that the heat is a necessary component to softening the wood.
The steam permeates the wood to make it pliable. It is placed on top of the side mold and the screw with the shoe is placed above it. The screw is slowly applied to the wood until it starts to bend. During this phase, the screw is rotated until maximum pressure is applied. One of the techniques used is to screw it in, wait a few seconds then release the pressure and move forward repeating staggered steps.

Once the main screw is down the rollers are rolled over the top and lower bout. The pressure is slowly applied to bend the remaining section down into place. Then they are held at the end to keep it in place.
The bend is completed and now it’s time to make sure it stays this way. This is where the heat helps the most. It will dry out the wood and make it conform to existing shape. The blanket is left on for a few minutes then turned off. The sides are kept in the fixture for around 30 minutes until all the heat has dissipated. At this point the sides are removed and clamped into the form mold.


The sides are bent as well as the curfing. The curfing is already bendy because of the wood removed between each post. It’s still a good idea to send in through the bending machine to make it form the right shape. This is an easy job to do because it’s already flexible.

The wood is then returned to its mold and clamped into place. This is an important step to do to prevent the wood from relaxing back to its original position.

Conclusion

These are the two ways that we bend wood for guitars. One using only water and one using a combination of water and heat. It is possible to bend wood around soft curves using only water and clamps. It is also possible to bend tighter curves use fixtures and mechanical pressure. These methods are applicable to any way that wood is bent. First the wood is made to be hot and moist then it is conformed to a shape. It doesn’t matter what is being bent these are the steps to follow. Go slow when applying pressure and make sure that there is enough water in the system! It’s ok to break a few pieces. After all wood grows on trees.

This article is on my website at how to bend guitar sides

-- Portland Guitar https://www.portlandguitar.com/



3 comments so far

View Foghorn's profile

Foghorn

529 posts in 195 days


#1 posted 07-07-2020 02:57 AM

While I appreciate your post and the effort that went in, there are way too many wrong ways to do things that I don’t know where to start. Just a couple of things. A steam box should not be mentioned in the same sentence as bending guitar sides.

Soaking sides before bending is a rare occurrence and results in cupped sides that are difficult to level without making them too thin. A spritz of water to prevent scorching prior to bending on a pipe or Fox style bender is all that is required for most woods including rosewoods, maple or mahogany. Kerfed linings don’t need to be bent at all as the kerfs allow them to be easily formed. Solid and laminated linings can be bent along with the sides and binding or purfling.

Once again and without getting into way too many specifics, your techniques may work for you but are not the accepted methods with modern builders. Cheers.

-- Darrel

View Craftsman on the lake's profile

Craftsman on the lake

3397 posts in 4246 days


#2 posted 07-07-2020 05:05 PM

4” pipe, cap the ends off. Cut a one inch hole in the center of one cap. Put a propane torch in the hole. Soak wood in the bathtub with an inch of water and a little soap for a few hours. Rock the wood on the pipe adding water to the dried out spots with a sponge. You can bend rosewood, walnut, mahogany…. most anything else successfully with less effort and a simple tool. I’ve never had sides cup.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful.

View Foghorn's profile

Foghorn

529 posts in 195 days


#3 posted 07-07-2020 06:33 PM


4” pipe, cap the ends off. Cut a one inch hole in the center of one cap. Put a propane torch in the hole. Soak wood in the bathtub with an inch of water and a little soap for a few hours. Rock the wood on the pipe adding water to the dried out spots with a sponge. You can bend rosewood, walnut, mahogany…. most anything else successfully with less effort and a simple tool. I ve never had sides cup.

- Craftsman on the lake


I used to soak years ago when I first started. For the odd piece of wood, it seemed to be a necessity. I then listened to the pros on forums I used to frequent and became a “spritzer” :) Just enough to prevent scorching. It has worked very well for me for maple, mahogany and rosewood and definitely sped up scraping and prep sanding as a half a mm or less of cupping means a whole lot of scraping. It doesn’t happen every time, but I avoid it as it’s not necessary for me. I know that the vast majority of pros on the MIMF or OLF would concur. It certainly wasn’t something I discovered but am glad to learn from those more experienced. A steam box would be an emphatic no from any luthiers I know.

I made my first bending pipe with a 4” pipe bent into a teardrop shape with a 300 watt light bulb and dimmer. Still works great. I made a smaller one for doing cutaways. I built a fox style bender years ago as well and use it with the silicone heat blanket. Works great and you can bend your sides and bindings and purflings at the same time which is handy. I still use the pipes for one-offs or touch-ups.

-- Darrel

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