Build Techniques

misterlee
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Re: Build Techniques

Post by misterlee » February 2nd, 2011, 3:46 pm

The rules say we can laminate the wood. What does that mean? I assume it's used in a particular way in the context of wood... as in, it's not talking about taking pieces of wood and running them through a poster lamination machine. :P

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Re: Build Techniques

Post by deezee » February 2nd, 2011, 6:03 pm

i think it is like bending the wood or treating it with something... it makes it a lot stronger but A LOT heavier
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Re: Build Techniques

Post by lllazar » February 2nd, 2011, 6:44 pm

You basically take two lighter, smaller strips of wood and glue them together, to make a stronger piece of wood. It's difficult to make it work right, takes a lot of experimentation.
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Re: Build Techniques

Post by soccerkid812 » February 2nd, 2011, 7:31 pm

whats the benefit of two smaller pieces of wood vs one larger piece of wood?

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Re: Build Techniques

Post by lllazar » February 2nd, 2011, 9:44 pm

I'm really not the best person to explain the details because i don't really laminate, but the idea is that you will get a lighter member at no sacrifice of strength - wait for another more experienced person to answer your question too though.
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Re: Build Techniques

Post by jander14indoor » February 3rd, 2011, 7:11 am

I'll take a try. Note, do a little digging, this topic has been discussed well many times in the past. Don't forget last years archive.

First, laminating two pieces won't accomplish much, though it may occasionally be useful.
Second, it doesn't have to add a lot of weight, depends on glue technique/practice, but it certainly can. Whether the weight gain is worth it becomes a system question.

OK, some basic theory.
Laminating is all about helping with bending or buckling resistance, and nothing about load carrying along the beam. Doesn't affect tensile or compression strength, though it can affect buckling which affects system compressive strength. Frankly, for towers it is probably (but don't take that as certain) not as useful as it was for bridges.
When you bend a beam, the stress is not evenly distributed through the beam. Depending on geometry and material properties, the material near the center of the beam is contributing nothing to bending resistance, might as well not be there.
This is why you can take advantage of laminating as a system. The geometric effectiveness of a given piece of the cross section is about the 3rd power of the distance from that neutral axis.
It's why so many real structures (think bicycle frame) are made with hollow tubing for immense bending strength at light weight.
So, you can increase bending resistance without increasing weight by messing with the geometry of the beam or the distribution of its material properties across the beam to get the good material well away from the center, while still connecting it to act as a system. The first results in things like hollow tubes or I-beams, the second is where laminating comes in.
OK, lets look at a practical example.
Base case, simple beam, say solid 1/8 square, 15 lb/ft3 density. Its too bendy for the location you have.
You could increase its size to ¼ by 1/8, to increase stiffness 8 fold (along the wider direction) but you've doubled the weight.
You could use lighter wood, say 7.5 lb/ft3, increase size to ¼ by 1/8, no weight gain. But the wood isn't as stiff so you only increase stiffness by 4.
If that's enough, great, go that way. But if you need more, lets try laminating.
Take a couple of 1/8 by 1/32 pieces of that 15 lb wood. Laminate (glue) them to the narrow edges of a piece of 1/8 by 3/16 4 lb/ft3 balsa. If I've done my math right, allowing for about 10 percent of the weight to be glue (reasonable), that piece weighs the same as the original piece, and has a stiffness increase of 5.5. This is because we've moved the nice stiff 15 lb wood away from the neutral center of the stick and replaced all that stuff by VERY light balsa, which needs do little work.
Lets try a little more extreme case. Use two pieces of 1/8 by 1/64 15 lb wood. Now you can use a 5/16 by 1/8 4 lb balsa to hit the same weight and you have a piece just short of 9 times as stiff as the original, for the same weight!

Hope that all makes sense,

Jeff Anderson
Livonia, MI

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Re: Build Techniques

Post by lllazar » February 6th, 2011, 5:39 pm

How do you guys make sure that when you place the loading block on the tower, it is perfectly parallel? I hate premature breakage :/
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Re: Build Techniques

Post by icyfire » February 22nd, 2011, 8:24 am

Paradox21 wrote:I cut my wood a little longer than I need it, then I sand it down to the correct length, however, I am having problems getting each end to be sanded to the perfect angle. Does anyone have tips on how to ensure that the correct angle is achieved?
To be honest, I find that cutting wood with a knife will give me a better angle than sanding. Normally I wouldn't have this problem, as I would lay a piece of wood down and carefully cut it. I don't mean to use the blade of the knife and press down; that would be just breaking the wood unless your knife is extremely sharp. If the angle is close, I will use it, however if it is off I just use another piece of wood (if the angle is completely off). Honestly, I don't find that sanding is very useful; maybe to level off an uneven base of top of the tower.

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Re: Build Techniques

Post by The Architect » February 22nd, 2011, 8:39 am

misterlee wrote:The rules say we can laminate the wood. What does that mean? I assume it's used in a particular way in the context of wood... as in, it's not talking about taking pieces of wood and running them through a poster lamination machine. :P

Thanks!
it is when you take the original piece of wood but you take it apart and make the grains face different directions. Since the grains don't all follow the same direction the strength of the wood is increased
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Re: Build Techniques

Post by T-B » February 26th, 2011, 8:22 pm

I made a 3D form out of heavy display board to use as a tower jig -- my plan was to build directly on it, then lift the tower off. I've seen other people post that they are doing something similar.

Right now I have a top section form and a bottom section form. They are beveled on the bottom of the top and the top of the bottom, so I was planning to glue them together creating one form for the whole tower. But now I'm wondering whether it will be a problem to get the finished tower to slide off the 51 cm form. It is covered with packaging tape so it won't get glued to the form, but will the jig just drop out or will it be hard to remove?

Any thoughts on that? I didn't do a cool thing like BalsaMan did last year, but I did get precise cuts so it is pretty good. I don't want to screw it up now.

I really like the idea of connecting the top and bottom and building on the form, especially since I don't have any great ideas on how to join the two sections if I do it separately. BTW I'm in Division B.

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