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-   -   Why cant I bend sides? (https://www.acousticguitarforum.com/forums/showthread.php?t=567639)

David Bundrick 12-27-2019 03:12 PM

Why cant I bend sides?
 
I simply can't bend sides. I've tried a bending pipe, a steam box, and (currently) a silicone blanket, and the wood always cracks at least in the bend near the neck. I have two molds, one of solid mdf and one of mdf sides with dowels. I have tried various amounts of spritzing and even soaking the wood. I plane and sand the wood to 2 mm, and scrape it flat. I've used different woods from LMI and SM, plus spruce from Alaska. I've tried temperatures ranging from 250 to 375 (measured by a meat thermometer inserted into the sandwich). I can see steam generated, and give the wood time to soften before bending. With the blanket I use a stainless steel sheet on top, then the blanket, then the wood, wet and wrapped in foil. Clothes pins keep everything together. I work slowly and patiently when bending the wood over the mold. I let the wood soak at temperature and cool overnight. I've watched countless videos on the subject. I expected a learning curve, but not complete and utter failure. Wood is expensive, and I have lots of excess backs. I am not without woodworking skills. Can someone please help me? Everyone else seems to be able to manage this task; why can't I?

Shuksan 12-27-2019 03:29 PM

You might consider pre-treating your sides with a veneer softener such as Super-Soft 2. I use it and I know many other builders who do. I have yet to break a side when bending. https://www.veneersupplies.com/produ...nditioner.html

I know one very experienced builder who successfully uses Windex as a veneer softener when he bends sides. It's a cheaper alternative to Super-Soft 2.

jbbgibson 12-27-2019 03:48 PM

Too fast or too cold
 
My best guess is, in spite of your method you are either bending too fast or too cold. Some pics of your molds may help. Also you don't say if you are bending for a cutaway, which changes the equation. They can be difficult for even experienced builders.

The way I was taught and has served me well is as follows. I expect its unconventional and will generate some comments on its own.
  1. Spritz the wood and let it sit for about 15 minutes
  2. Make your sandwich as you have been, but DO NOT wrap it in foil. Foil is a heat barrier in my opinion and works against you.
  3. Start the heater and place your bare hand on the top metal slat putting slight pressure on the wood.
  4. As the wood heats you will feel it start to relax. This also coincides with the heat getting uncomfortable on your hand. As it relaxes use your hand pressure to help it through the bend.
  5. As it gets uncomfortable slide your hand up the sandwich a little at a time allowing the wood to relax and bend under the pressure.
  6. When you approach the center curve replace your hand with the brace from the mold and secure it. At this point you should be moving along pretty quickly.
  7. Continue with your hand pressure to bend the upper bout and secure it when the bend is complete.
  8. Generally I let it sit and "marinate" in the heat for about 15 minutes watching closely for signs of overheating. Then cool to room temperature.

Admittedly, this is not a "scientific" method. It requires you to develop a feel for the wood relaxing, which is not difficult. If you feel like you are forcing it, you are. Just let it relax under your hand pressure. You will recognize it immediately. Completing the bends takes me less than 10 minutes, and closer to 5 minutes most of the time.

Bruce Sexauer 12-27-2019 04:46 PM

If I put my hand on the SS slat when hot for more that a second or two I'd have a painful burn.

I preheat my rig when using a blanket. My entire fox bender process takes ten minutes on the timer, not counting preheating. Bending on the hot pipe takes about 20 minutes per side. A cutaway is another 10+ minutes.

If you are not scorching your wood slightly (very slightly) now and again, you are probably too cool.

These processes makes a lot more sense when you actually watch someone do in person.

phavriluk 12-27-2019 05:07 PM

A thought
 
How thick are the sides? Not far wrong at .080".

David Bundrick 12-27-2019 06:47 PM

Quote:

Originally Posted by phavriluk (Post 6250204)
How thick are the sides? Not far wrong at .080".

The sides are down to 2 mm. Next time I'll go a bit thinner. I'll also try hotter on the fear that my thermometer is not registering properly, although thjew stainless gets so hor I can't keep my hand on it.

mirwa 12-27-2019 07:31 PM

When i teach people guitar building we use a backing strap off stainless to trap the steam and help with the bend, when i do it myself i just use my hand as I find it quicker

2mm thickness is fine

I personally do not use any fabric softeners or veneer softeners etc, yes i have experimented with them but find them of no benefit.

http://www.mirwa.com.au/images/MKAGS14.JPG

Rudy4 12-27-2019 09:39 PM

Learn the ways of the wood.

I suggest you make up some expendable pieces and experiment to get a feel for how the wood reacts when you reach the temperature at which it will become pliable. Start with thinner wood, as thin as 5/32" until you start to understand the dynamics involved. Nothing wrong with breaking some wood, just do it on something that doesn't matter. (Do your practice like it does matter, though) I build banjos and the only way you figure out how tight you can tension a head is to break a few. This stuff is the "right of passage" into the world of making musical instruments.

I suggest you learn on a hot pipe so you can interact with the wood.

Keep the wood spritzed with water and rock the wood over the pipe so you introduce heat over a wider area. You'll be able to feel the wood sag over the pipe when it hits that critical temperature.

As the wood bends you can move on to the next area of the bend until your bend matches your form.

A backing strap will keep you from fracturing the bends, but learning how to do it without a strap will teach you a lot more. The hand over the back of the bend is an excellent way of feeling how the heat transfers through the wood. When you can't hold your hand there you've reached bending temperature (if you're fairly tough!)

As stated above, if you aren't at that point where you scorch the wood occasionally then it's not hot enough.

Keep at it, you're almost there!!!

nikpearson 12-28-2019 03:46 AM

One of the trickiest stages...
 
Bending sides is challenging if you want smooth curves. Iíve only ever used a bending iron similar to the one made Ibex but with twin heating elements.

Have found around 200 degrees centigrade or where any water dropped on the iron forms little sizzling balls before rolling off the iron and/or evaporating works well.

Also use some relatively thin leather gardening gloves so I can really get my hands around the wood without burning them. Bare hands gives more feel but at best is uncomfortable if the iron is hot enough.

Try to heat the section you want to bend along the flat section of the iron to make the wood more pliable. Once hot - you can feel this easily with gloves on - draw the wood along the curved iron section whilst applying pressure from behind with both hands.

My preference is to use little or no water as Iíve found this can lead to cupping and ripples in the bent side.

The advice above to practice on some inexpensive sides is helpful. Practice and experience are invaluable and youíll begin to get a feel for process even if this takes time.

If itís any consolation I still find bending sides to be one of the the trickiest parts of any build. But I have got better over time.

Good luck.

jbbgibson 12-28-2019 06:13 AM

Could have been clearer...
 
I should have noted that you donít want to keep your hand in the same place as it gets uncomfortably hot. That is your signal to move up the slat to where itís not uncomfortable. By the time you are approaching the top of the lower bout bend itís pretty much continuous movement. If wearing a light glove helps then do that. Itís really about developing the feel for when the wood is relaxing. The heat level just signals to me that itís about to happen.

Also, I bend sides at as close to .075Ē as I can get.

MC5C 12-28-2019 06:37 AM

All I can say is how I learned how to bend sides. By luck or wisdom I have yet to crack a side. I bend on a hot pipe, dry. All the water or steam does is improve conducting the heat to the wood lignin, and reduce scorching, and I find I have more control without it (I do wipe the wood with water, but that surface moisture disappears as soon as it hits the pipe, I kind of use it to judge the temp of the pipe). For me, the ah-ha moment was learning that the lignin in the wood, the part of the cell walls that makes the wood stiff, turns plastic somewhere between 250 and 350 degrees. I warm the section I want to bend on the pipe, moving it around, trying to heat a fair length of wood at once, and even heat the back of the bend. I want the whole piece, maybe 4" long, to come up somewhat evenly. I keep steady pressure on the wood, pressing in towards the pipe but also bending it with my hands. If all you do it press in towards the pipe I find I get kinks in the bend. There comes a point where the wood goes plastic quite suddenly, and I shape the bend at that point. The wood absolutely changes texture, releases all it's stiffness, and you have two or three seconds to work your will on it, then it stiffens up again, and you have either got you bend and go on to the next bit, or start over and fine tune the bit you are working on. A waist is easy, I have set up my bending tube to have the radius I want so I can take a little less care and get the wood to where I want it, a cutaway is hard and I take a lot of time getting the radius of the bends to flow into one another. Takes about 15 minutes to bend a plain side, and around a half hour to bend a cutaway side. Lower bouts are actually quite hard, you need to get a long section of side hot and plastic at once so you minimize kinks, and you never bend a lower bout by pressing onto the hot bending iron, you simply flex the wood in your hands around it.

Brent Hahn 12-28-2019 10:49 AM

Quote:

Originally Posted by MC5C (Post 6250495)
For me, the ah-ha moment was learning that the lignin in the wood, the part of the cell walls that makes the wood stiff, turns plastic somewhere between 250 and 350 degrees.

Okay, stupid question: why don't people just heat up their sides in an oven?

David Bundrick 12-28-2019 10:50 AM

Thanks to all for the great advice. I tried again today with a piece of scrap wood and had success. I ditched the aluminum foil and wrapped the wood in brown paper, which I wet. I lined the mold with stainless steel foil which, as a knife maker, I have for heat treating certain kinds of steels. Thus my sandwich was a stainless steel slat, the heat blanket, the wood (mayb e 1.8 mm) spritzed and wrapped, and the stainless steel foil lining the mold. I'm pretty sure the temperature was higher, because the aluminum foil over the wood was probably distorting the reading. Rather than using a piece of wood to bend the sandwich around the form as the bending machines seem to do, I used my hand protected by an old merino wool running sock - almost everyone mentioned getting a feel for the wood, and you're spot on. I did try the Windex but have no idea if it contributed to success. If I can replicate today's success, I'm home free, and the already completed tops, backs, and necks will soon be part of instruments. FYI, I'm obviously not a luthier, just a guy trying to make ukuleles for grandkids, but I might try a lefty guitar for myself. Again, thanks to all for your help.

mirwa 12-28-2019 05:30 PM

Quote:

Originally Posted by Brent Hahn (Post 6250700)
Okay, stupid question: why don't people just heat up their sides in an oven?

The heat in the wood is lost in the time it takes to get it out of the oven and onto a bending shape or form, same with using a steam box

Thick wood 1/2 an inch and above you can steam take out of the steam box and start shaping as the thickness of the wood allows the heat to be retained long enough to bend

Steve

Bass.swimmer 12-28-2019 06:51 PM

Chiming in at the end, I'd suggest practicing with either white oak (if you can get it) or maple, as those are the easiest to bend that I've found. Oak, if you soak it, bends like a dream. Maple bends easily as well, but it's a little touchy (it can scorch kinda easily).


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