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Old Jun 28, 2006, 01:20 AM
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ToyBoy's Avatar
Portland, OR
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What does "crack" mean? (balsa)

Yes, I am a total balsa newbie. Approaching 32 years of age and I finally decide it's time to start building balsa models. Anyway, I have this small rubber powered model kit here, and reading the instructions I notice "crack here" in a few places. What does this mean?

Are there any good resources on the web for balsa building newbies?

Thanks for suffering an idiot,
Murray
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Old Jun 28, 2006, 02:37 AM
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tim hooper's Avatar
Telford, UK
Joined Feb 2000
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Quote:
Originally Posted by ToyBoy
Are there any good resources on the web for balsa building newbies?

Murray
Murray,

How about this?

http://pldaniels.com/flying/balsa/index.html

tim
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Old Jun 28, 2006, 03:00 AM
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Staffs, UK
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More context might help but "Crack" in building instructions generally means what it does in conventional English. It's probably where there's a sharpish bend in a bit of wood and it means "Make a partial break in the wood by bending it sharply". When you try it you'll find the balsa doesn't completely break but just sort of changes direction . Doing it that way is a lot stronger than the alternative of trying to butt join 2 bits of wood together at an angle.

Steve
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Old Jun 28, 2006, 03:46 AM
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vintage1's Avatar
East Anglia, UK
Joined Sep 2002
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Yes. Cut partially through with a razor saw or knife, or juts bend, and soak the join in thin CA...


Generally used to bring the top spar in a wing down to the wingtip smoothly.
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Old Jun 28, 2006, 04:39 AM
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the wiz's Avatar
Bullhead City Az.
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here's a link i find helpful http://www.airfieldmodels.com/

the wiz
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Old Jun 28, 2006, 07:36 AM
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perttime's Avatar
Tampere, Finland
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You might get the "bend" also by pressing, on the inside, with a sharpish ruler before bending. Possibly less damage to the wood fibers.
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Old Jun 28, 2006, 11:37 AM
the-plumber
Joined Feb 2003
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Quote:
Originally Posted by ToyBoy
Anyway, I have this small rubber powered model kit here, and reading the instructions I notice "crack here" in a few places. What does this mean?

Are there any good resources on the web for balsa building newbies?
Dunno 'bout "crack here"; never seen that instruction, but my presumption is that it refers to wrapping balsa sheet around a tight radius and allowing the wood to fracture along the fold.

Might be worth trying a different method, spritzing household ammonia on the wood and bending it 'round the curve slowly. That's household ammonia as in "Parson's Household Ammonia", rather than some cleaning solution which has ammonia in it. The Parsons product doesn't have any perfumes or oils, and is about the strongest concentration of ammonia Joe Public can buy. This method works very well when you allow the ammonia to soak the balsa and bend slowly.

Otherwise, while not strictly a resources for balsa building, Thermal Thumbers are a prolific band of rubber builders, and it's likely they might be able to help.
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Old Jun 28, 2006, 05:32 PM
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East Anglia, UK
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No, bending will not do! You are trying to make an abrubt transition in a spar, not a gentle one.

In the good old days of balsa cement, a butt joint in end grain was a very weak affair..cracking the spar and rubbing cement in the crack as a simple and stronger way to achieve this.

Today probably sawing through and butting with CA or PVA would be just as good.
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Old Jun 28, 2006, 11:21 PM
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ToyBoy's Avatar
Portland, OR
Joined Nov 2004
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Thanks for all your replies. They confirm what I assumed but I wanted to be sure.

the wiz: I agree Airfield Models is a great site for info, and I in fact checked his glossary for "crack" before posting my question here.

the-plumber: That Thermal Thumbers site is really interesting - thanks!
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Old Jul 01, 2006, 02:21 AM
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ToyBoy's Avatar
Portland, OR
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Tim: somehow I missed your reply yesterday. Thanks for the link, looks like a very useful site.

Murray
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Old Jul 02, 2006, 10:13 PM
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The 'Wack, BC, Canada
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Toyboy, they truly mean bend until it cracks and then add glue. The idea is that the wood will fail but the fibers will all fail at various points at or on either side of the break. Then when you add a touch of glue the glue bonds the overlapping fibers to give almost as strong a joint as if it was not broken in the first place. This is a fairly common technique for the smaller rubber models like Peanut Scales and other models of similar size.

If you find that the wood does not crack with a lot of fiberous interweaving then you (or the kit maker) picked the wrong peice of wood for that part. You want to test break the wood at one end to test for the proper sort of fiber breaking BEFORE you build it into a side frame or other application. Substitute strips until you find one that has the right fiber makeup at least at the one end and look it over carefully to see if it at least LOOKS like it'll retain that charactarsitic to the point where the crackN'glue joint needs to go.

Even for models of this sort where you're not going to crackn'glue it's wise to pick highly fibrous wood for longerons and stringers since they are less prone to failure.
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Old Jul 03, 2006, 03:03 AM
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ToyBoy's Avatar
Portland, OR
Joined Nov 2004
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Bruce, thanks for that excellent advice. You're right, the model I'm about to start is a small one (walnut scale). I almost started it this weekend but realized that I don't have a building board, so it will be delayed until I buy or make one.

TB
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Old Jul 16, 2006, 09:39 PM
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USA, WI, Milwaukee
Joined Apr 1999
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The suggestion to partially saw through the piece is a good one. Andy Clancy used this method on his Lazy Bee kits. I have done this many times. Saw cut partially through the piece, "crack" it to the correct form, and then apply thin CA to the joint. Light, strong, and effective. Jim H
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Old Jul 17, 2006, 02:49 AM
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BMatthews's Avatar
The 'Wack, BC, Canada
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Newguy, sawing cuts the fibers and totally messes up the strength of the joint. Glue is NOT a substitute for overlapping fibers that are glued together in the long grain direction.

I've never seen one of Andy's plans but if he's suggesting this method then it is a weak point and not a strong point. Either stick with the proper crack&glue or make a proper joint with a long angled overlap where the sharp "kink" is required. The fact that this style of joint is holding up well is due more to good luck and low stresses than to good design.
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Old Jul 17, 2006, 08:32 AM
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willin's Avatar
In my Blu heaven! near Lincoln NE
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Mr Matthews is totally correct about the strength thing.

However with larger pieces of wood I have seen the instructions read "small cut on the top about half way through before you crack"
This is gone in order to be able to properly crack the thicker woods and allow tha upper half some room in which to move together and butt with the lower portion having all those overlapping fibers. I know it don't seem right somehow but it does work and still maintain a great deal of strength in the joint.

Probably not as strong as a simple cracked joint would.
I just can't see how it could. Keep, in mind that the thicker wood would not act the same and that the top of the crack(without the saw cut) would not crack properly but swimply snap in 2 because the top of the crack would act as a fulcrum.

Robert
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