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Hello folks
I've bent my first pair of sides. Sapele thinned to .085". I used the fox style bender that I built along with a foil wrap, aluminum flashing slats and a heating blanket. I aimed for 310F for a bending temerature but my heating blanket thermostat doesn't keep the temperature very steady. After each side's first bend I saw a lot of spring back. So, I put the sides back in the bender with the cauls in place for a 2nd bend. This time I kept them hot for around 30 minutes. They still have significant spring back. They were lightly sprayed with water both times.

What can I do differently to help these sides maintain their shape?

How much is significant spring back? Are you able to get the sides into the mold without a lot of force or are they straightening quite a bit? Are you letting the sides cool clamped in the form?

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Sounds as though by the time you are beginning the bend, the water is gone. which will usually result in excessive spring-back. We begin the bend as soon as we see steam rising out of the bending package, rather than waiting for a certain temperature. The 310 degrees F number sounds like a peak temperature, rather than minimum bending temperature where the waist caul may be moved down and the bouts bent. Using an envelope sounds beneficial, but we've seen issues with printing on softer woods like mahogany and khaya, and inconsistent drying...sheets of foil cut to size works as well and encourages drying.

Finally, consider using a water reservoir (kraft paper) to get a more consistent water quantity when bending woods that can be difficult benders with too little water and fiber collapse on tight contours with too much.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Super Soft II helps.

Sapele is notorious for spring back. The way I deal with it is to run the side though multiple heating cycles. Once the side is bent I reduce the heat to 280° and let it cook for 20 minutes, then let the side come to room temp and cook for another 20 minutes. I repeat this 3-4 times. Scorching can be a problem especially if you don't have good temp control. I prefer to place my blanket under the side, not on top. I don't have a scorching problem but if I did I haven't rendered the side unusable by having the scorching on the outside.

Cheers, M

stopped using super soft windex does fine
you need 375F 310 isn't hot enough
let set till room temp

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hmm, I wrote a response and it didn't seem to work. Trying again.

Thanks for all the replies. Some answers:

It will fit in the mold but nowhere near the correct contours. I originally just had a spreader for the upper and lower bouts but because of these sides, I made a pinching waist clamp. The sides completely cooled in the mold every time.

I thought about that maybe there wasn't enough steam left but I figured the 2nd time through, with the heating beginning after the sides were fully clamped in the mold should take care of that possibility. I may try some kraft paper instead of foil.

I also may give Super Soft II or Windex a try. Just use that instead of water?

I am planning on getting a Harbor Freight router speed control so that my blanket will maintain a temp rather than constant ramping up and down. I will also try going hotter, up to 375F.

for Supersoft II you need to apply it about 12 hours before you bend so it can soak into the side. I've never applied it to a partially bent side though, I don't know if it will make it start to straighten. I don't know about windex this is the first I'm hearing of it's use for bending.

Not to hijack the thread but, John do you jut use regular windex and wet the sides. Is it really as effective as SSII for figured sides? I am of the impression that SSII is actually magic. Any issues with the blue dye on light woods?

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Sapele can be very cranky.

Assuming it is not figured....

Just try this and see how it goes.

Wet the wood down good. Not damp - wet....
Damp Craft paper top and bottom
Aluminum foil top and bottom
Bending slats per usual
Wood clamped snugly into the bender

Disconnect the controllers and temperature deals.

Get your oven mitts on.
Plug the blanket straight into the outlet and let the package start heating up.
Once it gets "Hot enough" - you will feel the wood "Relax" to your hand pressure. Bend the wood quickly into the form and clamp it down.

Now cook it until it stops "sizzling" and let it cook a few minutes longer.. Unplug and let it cool down over night.

Check and see how it went.

Here's the thing... The trouble is that you were working to times and temperatures. You need to let the wood tell you it's ready.. And doing it with your hands on it - you will feel it relax and then you can get going.

My biggest trouble sources side bending are:
Too dry
Too cold
Heating up too slow or or working too slow

I might try that next time, but these sides will fully clamp down into the bender right now. They just won't stay in that shape.
I just picked up the router speed controller so I will have better temperature control. I will go hotter and see how that does.

The router speed controller doesn't work as I had hoped. The temp doesn't level off, at least below 375F. It just seems to take longer to get to temp.

I finally got some time to redo one of the sides last night and let it cool overnight. With more heat (375) and cooking it for 2 20 minute periods at 280 it came out much better. There is still some spring back at the lower bout but I think it will be fine as is. Thanks to all for the help.