Fox Bender - what am I doing wrong?

[Paul Burnside]

I didn't think the learning curve would be so hard. I'm bending straight-grain mahogany and getting a LOT of springback on both bouts and the waist. Here's my setup:
MDF forms cut 0.10" tighter than my outline.
Silicone blanket with an LMI controller.
Sandwich consists of (top to bottom)
1) galvanized aluminum flashing (I know, spring steel is preferred, but also pricy)
2) wet contractor's paper
3) wood (thickness sanded to just under 0.09")
4) aluminum foil
5) blanket
6) flashing
This was the arrangement I decided on after having several sides crack on the upper bout. For some reason with this arrangement it doesn't crack.
Blanket is heated to 325F. Bending on the waist starts at about 280F.
Once the bending is done, temp is reduced to 215F for an hour, then allowed to cool to room temp.

I'm still getting a lot of springback. I've tried reheating the side with more wet paper and the blanket on top of the wood for the second heating, but no luck. I've also tried reheating without the bottom layer of flashing - no luck. Could the lack of spring steel be making that much of a difference?

Thanks in advance.

[Mario Proulx]

Put the blanket -under- the wood....

Here, see this quickie video I made some time back. Keep it simple!

http://youtu.be/sUJbB0pa7Zs

Honestly can't remember the last time I cracked a rib, and very minimal springback, if any.

[Mario Proulx]

And on the subject of springback, see this one for why I -want- some springback....

http://youtu.be/DIwYWift3_w

[Paul Burnside]

Blanket is under the wood, the only separation is the aluminum foil, which is there because the blanket itself is causing some staining of the wood.

[Rodger Knox]

.09" is a little thick, but it should still bend OK...

[Mario Proulx]

Some folks use parchment paper between the blanket and the wood. I can't understand why you get staining, though; I'll bend a set of Cocobolo ribs, then use the same blanket to bend maple ribs the next day, without ever cleaning it.

I just caught that you're using aluminum flashing. That's a no go! You really need something heavier as your upper slat; something that will retain heat, otherwise, you're only heating the inside of the rib, and not the topside. Even thick aluminum would lose its heat too quickly(that's why aluminum is used for heat sinks, radiators, cooling fins, etc...). Find a HVAC shop, and ask them to cut you a slat from some .020" stainless steel if they have any, or at least some heavy gauge galvanized tin. Don't worry about the zinc, because you're not going to get it hot enough to create fumes.

[Alain Lambert]

I think you have 2 problems
1)
Your temperature may not be high enough.
I cook my sides for 10-15 minutes with just a little less temp than the bending temperature. As long as you have water in the "sandwitch" there will be no scorching. Watch the steaming, when it stop, give it about 10 minutes more than let it cool in the mold.
2)
Try not tu use the wet contractors paper, Wet the wood (slightly) instead. As mentionned previously, as long as you have water the wook temperature will only be 212F(even if the blanket reads 325) It only get higher when the water is gone and the wood dries.

[Chuck Tweedy]

One other thing - Mahogany is not the easiest wood to bend.
It is notorious for springing back more than other woods.
I have 3 guitars in process right now, and bent all their sides in the same way ( 3 different shapes, but the same blanket/method)
One is Honduras rosewood, one maple, and one mahogany. Only the mahogany one had significant springback.

[David Malicky]

With the blanket between the wood and form, and aluminum on top, the outside of the mahogany is probably not so hot, as Mario mentioned. Can you check the temp on the outside of the wood? For the outside slat, stainless steel conducts less than steel, and of course, aluminum.

For slats, the best I've found is McMaster's full-hard 0.010" 300-series stainless -- $32 for 100", item 53755K19 on this page:
http://www.mcmaster.com/#catalog/118/3740
You could go thicker, but it would probably take a set in tight bends. 0.010" stays almost straight for my forms.
100" gives 3 slats -- 2 on top would give more insulation and support.

As Chuck mentioned, Mahogany is often difficult. You could also bump the temp hotter, briefly.

[Tom Frei]

I had horrid spring-back with my first sets. Toddstock did a great video on side-bending on youtube. Coudnt believe you use such a small amount of water. My last Mahogany set using this method did not spring back at all, had to pop it off the mold. My problem is fitting it to the negative mold. Have to redo the positive I guess.

[Paul Burnside]

Incorporated all of the above suggestions, including using 26-g stainless steel. Still no luck. I may have to finish this set out on the pipe.

[Mario Proulx]

Sometimes, we run across some very ornery wood that simply refuses to cooperate; don't feel too bad. And yes, it's always good to keep your pipe-bending chops up; a touchup here and there is sometimes needed, and I still bend my 12 fret guitar ribs on my 14 fret benders, then re-bend the upper bout on the pipe to fit the 12 fret form. Keeps my pipe-bending chops fresh....

I will echo what's already been mentioned, though; 0.090" is rather thick, and unless you're getting some slight scorching, you can, and should use more heat. Put away the temp probes, and use your instincts.

[Rodger Knox]

Remember the stiffness is proportional to the square of the thickness, so thinning from .09" to .07" will make a huge difference.
The last couple of sets I took down to .06", I suspect that's close to what Mario uses. Both bent easily with no problems. One was mesquite, the other was highly figured birdseye maple, not generally easy woods to bend.

[Paul Burnside]

So I went with the common denominator: all of the side sets were cut from the same billet. So I picked up a nice piece of padauk, made some side sets, and they bent beautifully. So all of you who suggested that there are some pieces of wood that just WON'T bend were right on the money. Thanks everyone for your suggestions. Hopefully I'll have some pix up soon.