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I continue to get, what I consider to be excessive springback when I bend my sides in a bender. I use a Fox style bender with a heat blanket oriented steel slat - heat blanket - side (wiped with a slightly damp cloth wrapped in aluminium foil) - steel slat.
I keep the temperature of the heat blanket around 300 degrees F. measured with a surface thermometer. I have tried as high as 400 to 450 degrees F and as low as 250 degrees F, all with the same result. I heat the sides for 15 minutes and after it has cooled completely I reheat again for another 15 minutes. Then I let the side sit in the bender overnight. Pictures to follow.

OK here we go again,

picture of the bender

 

bent side compared to bender

 

bent side compared to mold and other side finished with a bending iron

 

550 degrees F is just a little too hot. Ah well, ***** happens.

Ron, this is a good question, as I often have the same problem, I will be watching this thread closely

I know you can compensate your form to "over bend" your sites, but it looks like you need more than that.

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Ron-
Unless the wood is very curly, and very thin, I use more water than the 'damp cloth wipe' would indicate. I get the side WET (with a sponge and a bucket) before I wrap it. I like to see/hear steam coming from the wood when it bends.

This also helps (a bit) to prevent burning as well. Your pic of the broken sides look like a classic 'too dry to bend' example. You can only get away with less water if you have a very resinous wood like EI Rosewood, in my limited experience-even then, water helps.

I think of the 'second heating' cycle as the 'bake to help dry' cycle. I usually take the hot side right from the bender to the mold and clamp it temporarily in the (separated half) mold until I 'get around' to unwrapping completely, trimming, etc. Same with bindings/purflings.

BTW, in spite of the enthusiasm for digital thermometers, I've not found them that useful. I started with a (hand) bending iron, so a splash of water on the steel slat, and watching/listening seems to work for me. If it gets too hot, a quick spray of water will cool things down quickly.

Good luck-I'm interested in reading the responses you get.

Cheers

John

[QUOTE=Ron Belanger] I heat the sides for 15 minutes and after it has cooled completely I reheat again for another 15 minutes. [/QUOTE]

For me, it's more like 5min max (at 80-100% heat) to bend, then
15 min at 40% to reheat (also using a blanket).

John

Hmmm .. I usually only have to lightly touch up the waist area after bending a side - the uper an lower bouts seem to fit with minimal springback. I am using a blanket as well, 2.5 per sq inch Omega ... running thru a cheapo light dimmer set at about 3/4 of the dial - more than that is too hot and will scorch/dry out wood (wrecked a set of qiulted sapele, one mad rw and a couple others I cant remember before I figured out what was wrong - I used to use a big variac to drop the voltage around what I thought was 115). As with John, I have no idea what the actual temp is, but its definitelythe sweet spot - I have bent everything from ebony and afr blkwood bindings to braz, eirw, mad, sapele, curly koa, coco .. all at the same setting. Spritz with distilled water, wrap in foil and go - you can hear the water sizzling inside the foil. Takes abotu 6-7 minutes form startup to bend a side ...

What kind of wood is that Ron that sprung back so bad ??? mahogany ???

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Thanks for the responses. The wood is Mahogany and it is .078 in thick.

As Tony has, I have also spritzed the sides so they were very wet, but I then got some rippling in the sides, particularly with Bubinga.

John my thermometer is a surface thermometer consisting of a spring made of two different metals. When it is placed on a hot surface the difference in expansion moves the dial to the correct temp.

 

John,

regarding the scortched side. It was in the bender at max temp for more than 12 minutes.

John,

One question. Are you timing your 1st bend from when the blanket reaches it's max temp or from when you turn it on. I make a sandwich of the steel slat, blanket, foil wrapped side and steel slat and lay it on the bench together and bring the heat up to the desired temp. Then I put it in the bender and clamp it in place and time it from there.

 

This will probaly add to the confusion but, I have had some spring back problems but it is very inconsistent. On my current instrument one side came out fine but one just doesn't want to cooperate. It is wavy sapele so maybe one side has more curl but it doesn't look like it. I spritz my sides with supersoft, wait till it drys the I spritz with water and put them in aluminum foil and turn the heat blanket on high. I spritz the blanket with water if it gets to hot. I have never burned a side so I must be lucky or doing something right. I let it cool and give it another heat cycle for ten or so minutes after the bend is done to help it set. I am also going to be watching this thread with interest.
Bob

Ron, my last three have had very little springback. Mahogany/satinwood/mahogany. I mist both sides (about .075") with water, wait 30 seconds and remist so it all looks the same colour, then wrap in foil. My mold is wood, then slat, blanket, foil wrapped wood, and the last slat facing up, and it's all clipped together. I turn the 2.5W omega blanket on, it usually takes about 10 minutes before I can see a little steam, then I leave it about 5 more, always sniffing the steam to make sure it isn't smoke. Shut it off to cool down for 30, turn it back on for 10 and leave it over night. I have a feeling the steam is a key part of the process.

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David White, Toronto

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[QUOTE=Ron Belanger]

John,

One question. Are you timing your 1st bend from when the blanket reaches it's max temp or from when you turn it on. I make a sandwich of the steel slat, blanket, foil wrapped side and steel slat and lay it on the bench together and bring the heat up to the desired temp. Then I put it in the bender and clamp it in place and time it from there.

[/QUOTE]

Ron-
My bender is modified to use the 'Doolin'(?) method where there is tension on the slats during bending and the waist is bent last, so the sequence is a bit different.
I load the (foil-wrapped) wood into the bender and clamp at the neck end. Then I switch on the blanket at 100% (sometimes I use 2 blankets-the sequence is the same), and use some clothespins to 'clamp' together the slats, wood, and blanket(s) so the wood is being heated. This only takes a few seconds, and I check to make sure that things are still lined up OK. I then bend the upper bout by hand (gloves on) - 15-30 sec?- and then put tension on the slats. The lower bout usually curves down at this point. Then I add the waist clamp and crank down (slowly) on the press screw till the waist is just about totally bent. Then I snug up the tail clamp and finish with the waist bend.
Here's the difference between our methods: Once the waist is bent, I turn the blankets OFF or down to a very low heat. The blanket is definitely off by the time 5 min is up. It sounds like you give the sides 15 min+15min of heat once they are bent. I'd definitely try more water and less heat if I were you.
Steam/water is a good way of moving heat energy into the wood. Using the bending iron, I've sometimes wrapped a wet rag around a piece of binding to get it steamed and 'started' if it just didn't want to bend. Of course, too much water and you can have problems with rippling in figured woods. I've also had bad luck soaking wood for long (hours) before bending- it seemed to weaken the wood (walnut and basswood linings) and it cracked/pulled apart/crushed in the bender.

I use a solid bending form covered with sheet metal (not rods with side panels) so my form probably helps to keep things warm for a bit longer than the rod-type.

I definitely don't think the wood needs much more heat once it's bent on the form. When I'm hand-bending, I put the wood to cool in the mold once I've got it to the shape I want. Same idea goes with the blanket bender, except that I add (like you) another cycle to dry out the wood. I had some problems with curl(across the side) with one set of flat-sawn sides so I think I'll add a few more cauls/clamps to my form if I run across more flat-sawn stuff, to help keep things flat during cool-down.

All this self-confident advice will guarantee that the next time I bend anything, I'll hear nothing but snapping and cracking! It's still a mystery to me, in large part.

Cheers

John

You may just have a very stubborn set of sides, especially if your technique has worked well for you before.

Once again, thanks for all the replies. It seems that I have a few new techniques to try out. I will keep you all posted.