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PostPosted: Mon Feb 22, 2021 5:34 pm 
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When bending rift (~45 degree) sawn sides, WHY do they tend to cup?
Is there any way to stop this happening, or at least minimize it?

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PostPosted: Tue Feb 23, 2021 9:23 am 
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Brazilian Rosewood
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What species of wood? Mahogany and maple are usable off quarter than most woods. 45 degrees isn't that big a deal. What was your bending procedure?
Too much water can cause cupping so knowing how you did this and what species can help

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PostPosted: Tue Feb 23, 2021 10:44 am 
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Black Limba.
SS 2 overnight then dry, then a light spritz water and wipe off, dampened wallpaper lining paper both sides.
(Edit) Heat to 250, bend waist to within ~1/2", lower bout, upper bout then set waist all the way, them up to 360, immediately down to 250 for 15 mins.
Cycle to 250 for 15 mins x three times to set/dry.

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Last edited by Colin North on Tue Feb 23, 2021 12:02 pm, edited 1 time in total.

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PostPosted: Tue Feb 23, 2021 11:56 am 
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Brazilian Rosewood
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limba can be a pain
SS2 overnight may be your problem. I stopped using that and went to windex with Ammonia D
Too much water can cause this.
next time do this

damp paper not wet spray the wood with windex and place a 2 lb weigh on the wood far enough off the support but not so close to the end that it falls. In about 2 to 3 min the weight will start bending and it will fall so that tells you the bending temp of that wood and bend lower bout first upper bout then waist take heat to 325f and then set to about 250 to 275 for 10 to 15 min and let cool

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These users thanked the author bluescreek for the post: Colin North (Tue Feb 23, 2021 12:04 pm)
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PostPosted: Tue Feb 23, 2021 11:59 am 
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It's more of a general query about cupping really, my first side is on the way and seems to be doing well from a quick look after the second drying cycle.

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PostPosted: Tue Feb 23, 2021 4:25 pm 
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Brazilian Rosewood
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not all wood is homogeneous so you must be careful too much heat too much water are the main reasons of cupping

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These users thanked the author bluescreek for the post: Colin North (Tue Feb 23, 2021 4:45 pm)
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PostPosted: Wed Feb 24, 2021 3:45 am 
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Just took the first side for the mini jumbo shape (L-0, 1926-'33) out of the bending jig after overnight and it looks good, no cupping.
Would still appreciate any other input anyone has on the causes/cures for cupping when bending rift-sawn sides.


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PostPosted: Wed Feb 24, 2021 3:38 pm 
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I wonder if a dual blanket scenario would minimize it? Could it be that when the blanket is only on one side it's drying out one side faster than the other and then that gets set in with the bend?

I have no idea - I'm glad it came out right though!

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PostPosted: Wed Feb 24, 2021 3:45 pm 
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wood is stupid get it hot it bends more important is the blanket wattage and temp you are bending at.
Moisture is often the big factor on cupping. over wetting is a big no no
after 20 years of bending I have learned some wood just doesn't want to be a guitar.

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PostPosted: Wed Feb 24, 2021 4:09 pm 
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Brad, I think you hit on an important part of the bending process. Is one side wetter, or wet longer causing the cup and then getting set by the heat?

"I wonder if a dual blanket scenario would minimize it? Could it be that when the blanket is only on one side it's drying out one side faster than the other and then that gets set in with the bend? " Brad C. I was thinking the same thing.

And John's points about lots of variables going on in the process are right on.

I say a prayer every time I bend.

I bent a bunch of padauk sides that gave me fits until out of desperation, I ended up submersing them in water for 1/2-1 hour before I got it to bend. Others woods bend dry. Some come out of the bender with undulations in the middle of the side like bubbles. Sometimes even on perfectly quartered wood.

gaah I wish it were easy and consistent.

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PostPosted: Wed Feb 24, 2021 4:34 pm 
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bcombs510 wrote:
I wonder if a dual blanket scenario would minimize it? Could it be that when the blanket is only on one side it's drying out one side faster than the other and then that gets set in with the bend?

I have no idea - I'm glad it came out right though!

You could have something there. This has crossed my mind. After reading a post from someone using 2 blankets, I started using the original incandescent bulbs underneath with the blanket above, and I think it may help even out the drying process.
This is the first set I've bend that's been rift sawn since I started doing this, and I was a little concerned..

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PostPosted: Wed Feb 24, 2021 7:34 pm 
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I’m not sure I understand the notion of how drying one side more than the other can cause cupping and that cupping gets set in the bend. I understand how boards cup when the sides dry unevenly but that is when they are not constrained in a bender. When the sides are bent and held in the form, they can’t really cup even if the wood is really wet on one side and dry on the other. It might cup after you remove it but then that cupping isn’t being set in by the bending.

I sometimes use a bender and sometimes a pipe. I don’t see cupping when I use the bender even though I am only using 1 blanket and it is only the 2.5 watt per square inch variety. I get the sides up to temp so that steam is escaping and bend. Then I let it “cook” for a little while. At the thicknesses we are using, that should be plenty of time for the whole side to be at temp and giving off moisture at about the same rate. Then I let it cool in the bender. Then I hear it up and let it cool down once or twice before I take it out (usually the next day). I don’t get any cupping and the sides are dry. More importantly, they have had a chance for the moisture level to spread evenly throughout the thickness. I think this is because I 1) let them cool and stay in the bender for a long time after and 2) my sides and craft paper are wrapped in foil so the exposed side isn’t losing moisture faster than the bender side.

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PostPosted: Sun Feb 28, 2021 3:54 am 
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Could anybody explain what Windex is? Is it a usual window cleaner which includes amonia.
Can I replace it by a usual amonia cleaner or what substitute would you recommend?Cant get windex here in germany.
would like to give it a try and compare it to my usual bending process with dest.water and aluminium foil.
Thank you
Achim


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PostPosted: Sun Feb 28, 2021 7:24 am 
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Brazilian Rosewood
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yes window cleaner you can make a mild solution try about a tablespoon to a cup of water

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PostPosted: Sun Feb 28, 2021 9:44 am 
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Brazilian Rosewood
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If it is the ammonia in the window cleaner that helps the wood bend, why not use a dilute solution of ammonia? Does Windex have additional things in it that help make wood pliable?
Ammonia is also good for cleaning the "grunge" and brightening up old brass tuner plates.


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