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Old 11-29-06, 07:10 AM   #1
BananaTugger
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What kind of wood should I use to make a catapult?

I have to make a catapult for my Principals of Engineering class, and we have to pick our own materials and construction methods.

I was thinking of something kindy heavy, like Cedar, but if might be to hard to work with.

What about Balsa? It's light and stiff, but I don't know how strong it is, and I'm afraid it might crack at the pivot points.

Where should I use glue? Does would glue work just fine or should I get that Gorrilla stuff?

Rosewood? That stuff is expensive, but it's strong. So is Cherry.

Plywood? Or is that too pedestrian? I am a Weight Weenie/OCP'er you know.

The catapult has to be powered by a spring, and is about three feet tall.

Thanks.
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Old 11-29-06, 07:33 AM   #2
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Are you allowed to build a trebuchet? The wood you use is far less important than the design (i.e., pine would be fine).
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Old 11-29-06, 07:34 AM   #3
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Are you allowed to build a trebuchet? The wood you use is far less important than the design (i.e., pine would be fine).
No, that's the next thing we're going to build, for now it's a spring powered job.
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Old 11-29-06, 07:39 AM   #4
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How big is it going to be? I'd say just go with some pine lumber. Cheap, strong, looks good...
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Old 11-29-06, 07:43 AM   #5
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How big is it going to be? I'd say just go with some pine lumber. Cheap, strong, looks good...
Three feet tall, minimum.

Width and length are up to me.

Pine... sounds good. I'll find a sample at the local 84 and see if it is suitable.
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Old 11-29-06, 07:47 AM   #6
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Oak is what traditionally used for the base, and spruce and fir were used for throwing arms.
Balsa has very little compressive strength, you can crush it in your hand, it's also expensive.
Poplar would work, it has less sap, a finer grain, and is a little easier to work with than pine.
A sanded ply would also be something to consider. It's strong, large, and can be relatively light. I would not recommend it for throwing arms though.

Yellow wood glue or Gorilla glue will work. In either case glue joints should be clamped or screwed together.

Luck,
--A
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Old 11-29-06, 07:52 AM   #7
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you need a material that bends without snapping...stay away from hardwoods....balsa is brittle and would just break probably.
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Old 11-29-06, 07:56 AM   #8
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Originally Posted by BananaTugger
No, that's the next thing we're going to build, for now it's a spring powered job.

a trebuchet can be powered by a spring -- i.e., one where the spring tension comes from being stretched, rather than compressed.
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Old 11-29-06, 08:00 AM   #9
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notrem, Notrem, NOTREM!!!
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Old 11-29-06, 08:05 AM   #10
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notrem?
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Old 11-29-06, 08:06 AM   #11
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Think: "The Shining"
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Old 11-29-06, 08:07 AM   #12
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Lol
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Old 11-29-06, 08:10 AM   #13
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Ideally, you'd want something with huge elastic modulus. I recommend carbon fiber.
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Old 11-29-06, 08:14 AM   #14
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Originally Posted by slvoid
Ideally, you'd want something with huge elastic modulus. I recommend carbon fiber.
I was going to suggest CF wrapped around/bonded to balsa wood.
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Old 11-29-06, 08:20 AM   #15
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Ideally, you'd want something with huge elastic modulus. I recommend carbon fiber.

only if you are using the wood to create the spring tension.
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Old 11-29-06, 08:21 AM   #16
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yeah, wouldnt it be the rope that creates the tension on a catapult?
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Old 11-29-06, 08:30 AM   #17
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How much weight do you plan on flinging? That is your criteria. My I not so humbly suggest ash? After all, it is what bats are often made of. You want a bit of rigidity that will spring back. Ash is also generally very straight grain. Hickory would probably do well as well - used a lot for handles on tools.

Want exotic? I think Mango would be something that would work well - it is used for ox yokes down here - pretty light, but very tough. If weight is no issue, I can suggest things that are one step away from steel.

Balsa down here is nearly free - 1 year from seedling is a 12" diameter tree. Not kidding. I wouldn't think of using it, it would collapse most likely. It is pretty tough though for such a soft wood.

You also might do some research into woods suitable for bows (as in archery) - the most famous is Osage Orange and for a long bow, Yew. Down here we have something called Peyibeja (I think I spelled it right) which almost looks like a palm, the original people here used it for making bows - incredible stuff. This will allow your spring to load the wood - not sure that is your goal. If rigidity is the key - then I might go with iron woods - check out Ipe (for something you should be able to get) - VERY hard, very rigid.

But I really don't think you could go wrong with ash - and it should be readily available. Also, hard maple (most dowels you buy will be of this) is a good choice as well.
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Old 11-29-06, 08:31 AM   #18
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Oh, I don't recommend a banana trees - won't work for sure.
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Old 11-29-06, 08:32 AM   #19
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notrem, Notrem, NOTREM!!!

Wannabe Notrem. Never. Nobody else can ever be.
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Old 11-29-06, 08:33 AM   #20
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Oh, I don't recommend a banana trees - won't work for sure.
And I was just about to recommend banana trees!
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Old 11-29-06, 08:33 AM   #21
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Oh, I don't recommend a banana trees - won't work for sure.

There are other similar "woods" that I would not recommend.
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Old 11-29-06, 08:42 AM   #22
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I wanna see you fling a piano with THAT wood!
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Old 11-29-06, 08:55 AM   #23
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I wanna see you fling a piano with THAT wood!

Only people who are very special to me get to see that.
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Old 11-29-06, 08:56 AM   #24
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Morning wood? *sits with eyes closed waiting for the abuse to start*
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Old 11-29-06, 09:01 AM   #25
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notrem, Notrem, NOTREM!!!
exactly what I thought when I saw this thread!
but noone could really copie him
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