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Old Feb 22, 2011, 11:44 PM
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Help!

Bending sheet balsa


Greetings all.
On my current project, I need to bend a piece of 1/8" balsa around a circular former which is 42mm (1.41") in diameter. I have been trying to figure out the best way to do this. I vaguely remember hearing that ammonia was a good way to make the wood pliable enough to bend it around a fairly tight radius without it cracking along the grain. Is this the best method out there besides planking?
I was also considering using 2 pieces of 1/16", one on top of the other, but i would be concerned about the weight increase as well as making sure that there was adequate bonding between the two sheets.
Anyone care to give me a point in the right direction?

Thanks all!
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Old Feb 23, 2011, 01:46 AM
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thats a tight radius to bend sheet . i would use 2 laminations of 16th . soak balsa in water and tape around a round former of maybe 2.5 inch diameter . let balsa dry before removing from former . now wet outside face and it MAY not crack when reduced to 1.4 in .there is minimal weight gain with a laminated 1/8 piece versus solid 1/8 . the lam' will be the stronger of the two .due to the small radius , I would make two round formers and plank these with 1/8 square or 1/8 x 1/4 .chamfer strips to fit neatly . add strips in turn on opposing sides to avoid twisting . formers can be removed after sanding if required .
Old Feb 23, 2011, 01:59 AM
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Light balsa and tape


Plane_tech,
Good advice so far - I would be tempted to laminate two thicknesses of 1/16" as well.
Two tricks to help with tight radius curves:
  • pick really light (soft) straight grained balsa, and
  • apply tape to the outside of the curve before bending - this prevents splitting and can be removed after glue has set.
Laminating will add negligible weight if you use white or aliphatic glue and spread it thin, rubbing the pieces into contact so any excess comes out the edges to be wiped off.
Cheers,
PeteM
Old Feb 23, 2011, 05:14 AM
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Ditto on the laminating two pieces. Coat both sides with Tightbond then squeegy most of it off. Not only does the glue adhere the wood but being waterbased it causes the grain to raise and intertwine in the glue joint, making a real strong joint.

Rick
Old Feb 23, 2011, 09:45 AM
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All good advice above. IME, the most critical factor in doing something like this is selection of the wood. This can easily make the difference between success and failure. Obviously you need to avoid very hard or C-grain balsa, but (surprising to some) overly soft balsa can also have very poor bending characteristics. Some just seems to be "brittle", some is OK. Do some testing first.

And presumably this is a simple curve. A compound curve will require planking of some kind.
Old Feb 23, 2011, 10:02 AM
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In addition to the great advice above, try using 90% isopropal alcohol. I use this when bending wood, and find it allows for tighter bends than ammonia, smells a heck of a lot better, and dries rather quickly. No need to soak the wood. I cut down the dip tube on a sprayer from a window cleaner or such to fit the alcohol bottle; it screws on the alcohol bottle and allows for easy application.
Old Feb 24, 2011, 12:44 AM
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Quote:
Originally Posted by Harry D View Post
All good advice above. IME, the most critical factor in doing something like this is selection of the wood. This can easily make the difference between success and failure. Obviously you need to avoid very hard or C-grain balsa, but (surprising to some) overly soft balsa can also have very poor bending characteristics. Some just seems to be "brittle", some is OK. Do some testing first.

And presumably this is a simple curve. A compound curve will require planking of some kind.
Yes it is a compound curve. It changes from a 1.41" diameter to a rounded square former with a 0.75" radius on the bottom two corners, but over a span of 8". See attached image.


Quote:
Originally Posted by Petem View Post
  • apply tape to the outside of the curve before bending - this prevents splitting and can be removed after glue has set.

PeteM
I am guessing in strips parallel to the grain?


Quote:
Originally Posted by rcav8r2 View Post
In addition to the great advice above, try using 90% isopropal alcohol. I use this when bending wood, and find it allows for tighter bends than ammonia, smells a heck of a lot better, and dries rather quickly.
I would agree on the smell issue and luckily we have that in 55 gallon drums at work. How long do you typically allow it to soak thru prior to bending?
Old Feb 24, 2011, 02:48 AM
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Crikey!


Re the tape - from memory the strips were across the grain, but looking at your formers I think bending is not going to work.
Welcome to your first planking job!
PeteM
Old Feb 24, 2011, 10:58 AM
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Originally Posted by plane_tech View Post
I would agree on the smell issue and luckily we have that in 55 gallon drums at work. How long do you typically allow it to soak thru prior to bending?
Not long really at all. As soon as I spray it on one side it starts to curl. I use this to my advantage by only spraying on the side I want it to curl towards. On stubborn pieces I will spray the other side.
Old Feb 24, 2011, 12:23 PM
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Quote:
Yes it is a compound curve. It changes from a 1.41" diameter to a rounded square former with a 0.75" radius on the bottom two corners, but over a span of 8".
Then you will need to use some form of planking, or at least more than one piece of balsa. Good luck. It's not too easy (or much fun, IMO).

Would stringers (1/8" square or something like that) be an option?
Old Feb 24, 2011, 09:02 PM
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Quote:
Originally Posted by Harry D View Post
Then you will need to use some form of planking, or at least more than one piece of balsa. Good luck. It's not too easy (or much fun, IMO).

Would stringers (1/8" square or something like that) be an option?
It would be an option yes, but had a "lightbulb" moment today when I was cutting the wing cores. What I might do is figure out how I am going to split up the curve so that the balsa is not twisting too much, then, once those are dry fitted, use a piece of foam to make a male and female mold of sorts, then soak the wood in the alcohol, and use the mold to form the balsa to the correct bends. On top of that was going to step down to 1/32" balsa and just do 4 sheets to get the desired thickness.
Just an idea, I am not sure it is going to work. If that doesn't I might be looking at planking or just turning the foam mold into a plug to make a glass part.
Old Feb 24, 2011, 09:21 PM
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Quote:
...turning the foam mold into a plug to make a glass part.
Fiberglass is a good way to go, if the application is right. After more than 50 years of balsa bashing, I tried foam-and-fiberglass construction for the first time a few months ago. It was a fuselage with nothing but compound curves. The original designer had planked his, but I just couldn't see myself doing that. Fiberglassing worked out really well. It wasn't difficult, the end result was all I had hoped it would be, I learned a lot, and I really enjoyed doing something new and different. Build thread here https://www.rcgroups.com/forums/show....php?t=1327129 if you're interested.

Let us know how your project turns out, in due course.
Old Feb 24, 2011, 09:57 PM
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Thanks Harry. I will keep you posted as to the outcome. I am picking up the formers from the laser cutter tomorrow... that is if I get the drawing finalized tonight.
Old Feb 25, 2011, 10:56 AM
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Hay Harry D; I checked out your thread on the Cobra. Boy is that turning out GREAT. Will be following with interest.
Old Feb 25, 2011, 05:48 PM
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The secret is just to use good, soft wood with a clear grain, and to boil it in water for about 10 minutes, returning to the water if necessary. You can make tapered bends quite easily in this manner (I've bent 3/16" balsa around a .75" radius). With the right piece of wood, this is a piece of cake - assuming you are patient.

Hopefully this weekend I'll be doing some balsa molding (compound curves) for an ultra-micro model this weekend, and will do a build thread later in the process.

Rather than bending over an open area, consider making yourself a foam mold. This will support the balsa between the formers so you get a nice shape.

Andy
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