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  #1  
Old 05-28-2024, 11:20 AM
viento viento is offline
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Default Bending "sandwich"??

Until now I have bent the sides for my acoustic guitars using a heated tube and then an electric bending iron.
However, a few types of wood are not entirely happy with this and tend to crack in some places.
That's why I think of using a more friendly side bender like the legendary Foxbender .

About that I have an important question :

What should the „sandwich“ look like for bending the sides ?

A stainless steel sheet at the top,
then below the electric heat blanket and
then the wood to bend and
another stainless steel sheet at the bottom?
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  #2  
Old 05-28-2024, 11:32 AM
runamuck runamuck is offline
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Quote:
Originally Posted by viento View Post

What should the „sandwich“ look like for bending the sides ?

A stainless steel sheet at the top,
then below the electric heat blanket and
then the wood to bend and
another stainless steel sheet at the bottom?
Yes, that will work. Many people, myself included, wrap the wood in kraft paper that is damp.
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  #3  
Old 05-28-2024, 12:36 PM
difalkner difalkner is offline
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Why not put the silicone heating blanket below the wood? Heat rises, so why put the blanket above the wood?

I've only bent a few sides on the bender I built and I placed the blanket below the wood just for that reason, that heat rises. They bent just fine but I've wanted to ask the pros why the blanket goes on top.
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  #4  
Old 05-28-2024, 07:05 PM
Fathand Fathand is offline
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I spray wood with water, wrap in tin foil then put blanket on top. If there's a tighter bend I add a strip of .012" aluminum on top. I know people use spring steel or stainless but I had the aluminum handy ( dryer vent), it seems to work.

Last edited by Fathand; 05-28-2024 at 07:13 PM.
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Old 05-29-2024, 11:16 AM
Howard Klepper Howard Klepper is offline
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Originally Posted by difalkner View Post
Why not put the silicone heating blanket below the wood? Heat rises, so why put the blanket above the wood?

I've only bent a few sides on the bender I built and I placed the blanket below the wood just for that reason, that heat rises. They bent just fine but I've wanted to ask the pros why the blanket goes on top.
Heat does not rise. An area in a fluid that is warmer and hence less dense than it surroundings rises in the fluid with reference to a gravitational mass that is called "down." That is called convection. But the heating in a guitar bender is almost entirely by conduction, and for conduction the spatial relation to the major source of gravity does not matter. I put the blanket on top so it will heat the inside of the sharpest bend (the waist) the most. The wood that is in compression from bending seems to need more heat than the wood which is in tension.
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Old 05-29-2024, 02:11 PM
difalkner difalkner is offline
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Originally Posted by Howard Klepper View Post
I put the blanket on top so it will heat the inside of the sharpest bend (the waist) the most. The wood that is in compression from bending seems to need more heat than the wood which is in tension.
This part makes a lot of sense so I'll try it that way next set I bend. Thanks, Howard!
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  #7  
Old 06-12-2024, 02:59 AM
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Mark Hatcher Mark Hatcher is offline
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Quote:
Originally Posted by difalkner View Post
Why not put the silicone heating blanket below the wood? Heat rises, so why put the blanket above the wood?

I've only bent a few sides on the bender I built and I placed the blanket below the wood just for that reason, that heat rises. They bent just fine but I've wanted to ask the pros why the blanket goes on top.
Convection heat rises, more accurately, hot gases and liquids tend to rise. A heat blanket on a mold doesn’t really rely on free flowing gases or liquids. Conduction and radiation is how the heat is transferred in the sandwich and they don’t care about up or down.

One reason to put the blanket on top is if it is on the bottom you would have to accommodate its thickness to keep the bends accurate.

Another reason is the blanket is less likely to mark the wood from the top. I think this happens because the mold acts as a heat sink and keeps things cooking longer than you may expect.

Mark
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Old 06-23-2024, 03:24 AM
viento viento is offline
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A couple of questions :

How hot does the heat pad have to be?
How long does the bending procedure take?
I'm a little worried that the 2mm nut sides could break.
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Old 06-23-2024, 06:20 AM
Fathand Fathand is offline
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Quote:
Originally Posted by viento View Post
A couple of questions :

How hot does the heat pad have to be?
How long does the bending procedure take?
I'm a little worried that the 2mm nut sides could break.
Temperature depends on wood species but 250 to 300 F is approximate.

Once up to temp about 2-5 minutes. I start bending when I hear the damp wood sizzle. If you take too long it can dry out and burn.

They can break if wood has run out, is too thick, bend too tight, too fast, is too cold.

Distilled water minimizes water stains.
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Old 06-23-2024, 07:17 AM
viento viento is offline
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Fathand, Thanks for your friendly information!
I will give my Foxbender a chance (and me too) and start bending my wood bindings first to get used to the procedure of using that machine...
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Martin D28 (1973)
12-string cutaway ...finished ;-)
Hoyer 12-string (1965)
Yamaha FG-340 (1970)
Yamaha FG-512 (ca. 1980)
D.Maurer 8-string baritone (2013-2014)
and 4 electric axes
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Old 06-23-2024, 05:10 PM
Neil K Walk Neil K Walk is offline
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Here's a pair of videos I did of my third build using a bending machine that I build myself for about $100. The most expensive part (apart from the heating blanket) was the crank for the waist caul.

The back and sides on No. 3 are black walnut. They're .080-.085" thick. I spritz the wood with glass cleaner with ammonia just before making the sandwich. I wrap the wood in the same parchment paper that my wife cooks with and determined that undyed paper was better. I use blue steel slats and place the heating blanket on top.

The bending process takes a half hour and generally begins around 240 deg F and doesn't exceed 305 deg F. I keep the bent slats under heat for 10 minutes to set the bend then unplug it and let it cool under tension. I also leave it sit for at least 12 hours then put it in the mold.

Part 1
https://youtu.be/goljrocMwfA

Part 2
https://youtu.be/IM1UBkEgPvI

Summary: if I can do it you can do it. Some woods are harder than others. Walnut was easy. I've also used the fox bender to bend my curly maple binding and the results were actually better than when I did a previous set of maple bindings with a heated pipe. My first build was mahogany and it cracked on me. My second was east Indian rosewood which was a dream to work with. My fourth will be mahogany one again and I hope that what I've learned prevents a repeat of my first failure.
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Old 06-23-2024, 06:01 PM
viento viento is offline
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Quote:
Originally Posted by Neil K Walk View Post
Here's a pair of videos I did of my third build using a bending machine that I build myself for about $100. The most expensive part (apart from the heating blanket) was the crank for the waist caul...
Neil, thanks for your detailed information, especially for the videos!

I´m not yet bending because I'm struggling with the numerous functions of the digital controller. With trial and error I might be able to get it right in the coming week.
If you like, you may have a look at it: https://www.keenovo.com/Keenovo-Digi...ler-Manual.pdf
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Martin D28 (1973)
12-string cutaway ...finished ;-)
Hoyer 12-string (1965)
Yamaha FG-340 (1970)
Yamaha FG-512 (ca. 1980)
D.Maurer 8-string baritone (2013-2014)
and 4 electric axes

Last edited by viento; 06-23-2024 at 06:23 PM.
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  #13  
Old 06-23-2024, 11:13 PM
Fathand Fathand is offline
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Quote:
Originally Posted by viento View Post
Neil,

I´m not yet bending because I'm struggling with the numerous functions of the digital controller. With trial and error I might be able to get it right in the coming week.
If you like, you may have a look at it: https://www.keenovo.com/Keenovo-Digi...ler-Manual.pdf
Thanks for this. It looks just like the controller I bought from aliexpress that came with no instructions. Maybe I can do more with it now.
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Old 06-24-2024, 09:02 AM
Neil K Walk Neil K Walk is offline
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I may be playing with fire but I don't have one of those controllers. I use watch the temperature like a hawk and turn it down when the numbers start to climb too quickly. There's typically a good 10-15 deg F swing in temperature for each adjustment. I also have one of the cheaper nylon heating blankets that I have to install the plug to.

BTW, I'm also using a steak thermometer - the same one that I use when I'm grilling chicken on my propane grill. I can't say whether or not I've used cooking spray as a softening agent! /s

Apologies to the serious and professional luthiers out there. I was raised to be overly frugal. That's kind of how I got into building and why I haven't flipped guitars to turn enough of a profit to afford a Holy Grail guitar. Mine are more "carpenter's cup" guitars.
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  #15  
Old 06-24-2024, 06:00 PM
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Quote:
Originally Posted by viento View Post
A couple of questions :

How hot does the heat pad have to be?
How long does the bending procedure take?
I'm a little worried that the 2mm nut sides could break.
You are wise to worry about breakage!

I also wrap the wood in damp parchment paper. The moisture not only softens the wood making it pliable, but it buffers against rapid heating/cooling.

I take about 15-20 minutes bending black walnut or hog for OM sides. Since my bender is plenty wide, I throw in my maple binding strips and shape them as well. That REALLY simplifies the binding process!

Finally, i usually get the sides bent, then shut off the heat, and let them sit until morning. That helps set the shape, and it has almost no memory of its straight self. Some argue that having some memory tension enhances the tonal qualities of the finished guitar, but I never believed that.
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