Build Techniques

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Bubba1960
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Re: Build Techniques

Post by Bubba1960 »

Ok thanks. Ill probly do that.
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Re: Build Techniques

Post by dragonfly »

new horizon wrote:Most likely a jig would work best.

I don't have much of an idea about jigs though, I was thinking of cutting a piece of wood out that takes the bottom and top sections of the tower and just gluing them together, but it seems kind of difficult to do, I can barely find the right size wood and cutting it perfectly to size would be difficult.
I have seen a jig from the 06 towers forum (I think?) that is pretty much three sticks of wood glued together, and you build it right on the jig. Wouldn't that be a bit inaccurate, and how would that work? Would you just glue your pieces right on, or would you build trusses and then brace it on the jig?
Part of the Towers event is to devise a functioning jig for your design. This one's up to you! And keep in mind in your designing how you plan on USING it; it's hardly helpful to copy a design and then realize it doesn't suit your building tactics or your preferences. Take some time on it: after all, it's what you'll be using to build a bunch of your future towers, so do it right! That's also not to say you can only ever build one jig, if you change your design or make it better, don't hesitate to take more time and make a new one that's perfect.
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Re: Build Techniques

Post by lllazar »

How does everyone lay their tower members on a template, for gluing? I hate thumbtacks because they always pop out - any ideas?
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Re: Build Techniques

Post by Littleboy »

lllazar wrote:How does everyone lay their tower members on a template, for gluing? I hate thumbtacks because they always pop out - any ideas?
Masking Tape works and putting two pins to make an x that goes right ontop of the member works to or you could use 1 pin and stick it through.
Masking tape works the best for me as it does not damage the wood or the members are held in tightly (problems with other 2).
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Re: Build Techniques

Post by S4BB »

We use a low tac double stick tape on our template, holds the wood in place while you glue, and then it is very easy to left the frame after it has dried.
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Re: Build Techniques

Post by Balsa Man »

dragonfly wrote:
Part of the Towers event is to devise a functioning jig for your design. This one's up to you! And keep in mind in your designing how you plan on USING it; it's hardly helpful to copy a design and then realize it doesn't suit your building tactics or your preferences. Take some time on it: after all, it's what you'll be using to build a bunch of your future towers, so do it right! That's also not to say you can only ever build one jig, if you change your design or make it better, don't hesitate to take more time and make a new one that's perfect.
1) Dragonfly is RIGHT-ON
2) Some of the folk posting to this board really know what they're talking about - Dragonfly being one - some.....way down the learning curve.
3) My experience is that a good (precise, and symmetrical), 3-dimensional jig is probably the most important thing for building a competitive tower. Building flat sides, and joining them without good 3-d jigging will leave you at the mercy of.....imprecision. If all or your legs are precisely, symmetrically aligned- whatever your design- the forces in them are minimized, because they're equally distributed. As the alignment varies from "perfect", one leg sees more force. The idea is to get to the lightest wood that will carry the load needed. If you don't have "good" alignment, you have to "over-build"- use wood heavier to carry the extra load.
4) The time put into figuring out, and building a "good" 3-d jig pays off big-time.

In the "Designs" thread, back on Nov 4, I posted a way to make such a jig. Lots of words; pictures would make it much easier to see/understand. Oh, well. As I said in the post, If at first it doesn't make sense, re-read, and re-re-read. This approach comes from, over the years, having tried/used a number of ways to "jig up" a tower. It is both the most precise, and least time-consuming approach we have yet figured out. Once you have the jig, tape leg sections on the edges of the "jig plates", then glue cross-bracing in, lift off. Each tower will be the exact same shape. That will give you a....massive advantage in refining your design (getting down to the lightest wood that will carry the load.

I'm sure there are other ways, likely ones that are even better. This way does work, though.
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Re: Build Techniques

Post by lllazar »

So im confused about how to find high strength balsa at low weight.

I have access to a good supply of balsa sheets and i can strip them pretty well, but im having trouble finding strong pieces.

If mass does not correlate to strength, what does? How can i possibly select strong pieces, even from such a large selection? I really do need a thorough answer to this, so thanks in advance.
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Re: Build Techniques

Post by jander14indoor »

You could of course test each piece, but unfortunately that just leaves you with a bunch of splinters that WOULD have made a good tower.

Obviously what is needed is a non-destructive measure that indicates strength.

Actually density (not mass) does correlate to strength. In other words, denser balsa is generally stronger balsa. Unfortunately this coorelation is pretty loose. Or said another way not a real strong coorelation coefficient. To build good structures you want to avoid the outliers to this coorelation on the weak side. To build GREAT structures you want to actively seek the outliers on the strong side.

Fortunately there is another non-destructive parameter to help with this. Stiffness. For the same density, stiffer pieces of balsa are generally stronger pieces of balsa. And for compression members, frankly stiffness is actually MORE important than strength as it dictates buckling limit, not strength.

Side digression in case I'm confusing folks in using STRENGTH vs STIFFNESS. In the lay world its common to use these terms interchangeably. NOT in the engineering world, they are very different things. Strength is a measure of maximum load a material can take before failing in some non-recoverable way. Typically the failure is either permanent deformation (in other words, it bent), or complete separation of components (in other words, it broke). Stiffness on the other hand is the amount a material flexes or deforms NON-permanently per unit load. Example, a normal coil spring, like a screen door spring. You can apply a small force and it will stretch out. Release the force and it returns to its original length. That's stiffness. Apply more force and at some point it does NOT return to its original length. This is bending failure. Pull still harder and the spring wire will actually break in half. That's ultimate strength.

OK, end of digression, how do we use stiffness to help select the unusually strong pieces for their density. Use the structural design models to determine how strong each piece needs to be. Use the available data on balsa strength vs density to select the density and cross section needed to carry the design load. Now, select a bunch of sticks that match that density and sort by stiffness. Give the most flexible pieces to the opposing team. Keep the mid-range stiffness pieces for non-critical parts. Use the stiffest pieces for the most critical parts. Alternatively, ONLY use the stiffest pieces and reduce the cross section accordingly!

Oh, how do you test for stiffness you ask? Dig into the archives a little, its there. Try key words like Euler.

Hope that's clear, if not, as usual, ask.

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Re: Build Techniques

Post by lllazar »

Ahah. Thats exactly wat i did for my first few towers. I assumed the really bendy balsa would be weak in compressiong so i pushed aside such sticks.

Thanks! Now the only part of my design i need to perfect is the base jig....this might take a while but im assuming its worth the time, thanks again Mr. Anderson!
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Re: Build Techniques

Post by Balsa Man »

As usual, Jeff is right-on.

Let me just add a few thoughts- first, around;

“Use the available data on balsa strength vs density to select the density and cross section needed to carry the design load.”

As often, this turned out a bit longer than I expected when I started; between Jeff’s excellent post and this, the "thorough answer" Illazar was looking for........

After.....lots of hours of web-digging, there is some, but remarkably little real data, in terms of column buckling strength, by density, by cross-section – for balsa, and for bass. There is some “tensile” and “compression” strength data, by density. The tensile data is directly useful, but that’s a different matter. The “compression” data is really resistance to crushing, and not really useable to column strength. Figuring out how to get “into the range”- a size and density that for the least weight “should” carry the design load is a major part of the challenge. The process of “picking sticks” comes after “getting into the range.”

Quick side note- the ongoing “balsa or bass?” question- in the context of legs- members under significant compression load. Young’s Modulus- the term in Euler’s Buckling Theorem that represents....inherent stiffness – is generally reported in the literature to be significantly greater in bass (on the order of 2x- but of course, it varies to some extent, with density, and the density range of balsa is a lot greater than that of bass)

So, onward. I’m aware of three ways to get a.....useful handle on density+cross-section that will/should carry design load. They all depend on/use the inverse square relationship of column strength to effective length- for a given.....piece of wood, if the effective length is twice as long, its buckling strength will be ¼ (1 over 2 squared), and if the effective length is half as long, the buckling strength will be 4x (1 over 1over2 squared). You can easily build a table in Excel to get a set of relative strengths for shorter and longer than a given length.

1) The best way – what will get you closest to “knowing what you need” is (even a relatively small amount) of real compression (column failure) data. Its a bit of a pain to get, but it can be done (there is a photo of a compression testing rig we built a few years ago in the gallery). Key is to have the test piece vertical, and the load applied precisely along its long axis. On this rig, we add water to the load container (1 liter = 1 kg....) The way the test arm is set up, load on test piece is twice the weight og the load container.

I think I posted a bit of data from such testing in the Bridge thread last year; a couple real data points for 3/32nd square bass; a piece from a 24” stick weighing 1.37gr saw buckling failure at 7.25 kg, at a length of 8.5cm; a piece from a 24” stick weighing 1.97 carried 11 kg at 8.5cm. These values are average from 4 tests (on the lighter) and 3 from the heavier. Small number, so statistics are weak, but variation from average was up to 9.8%. Something to keep in mind is that (at least from our testing over the years) is that for balsa the variability of (column) strength, at a given weight and cross-section is about twice as much as we’ve seen in bass.
If you put an inverse square relationship table together, it will tell you that the lighter wood would carry 5.8kg at a length of 9.5 cm, 4.75 kg at 10.5cm length, and 1.81 (1/4th of 7.25) at a length of 17cm (2 x 8.5cm).

2) A second way, If you did bridges last year/the year before, and kept the data.....you should have, is to look at what size/density (at what effective length) carried what load.

For other than high school seniors graduating this year- coaches and students- I.....would strongly encourage you to capture this information; it will get more and more valuable as you add to it.

2) , continued- Say, just using the same numbers from above - your bridge legs were 17cm long, braced at mid-point, (i.e., an 8.5cm effective column length); the design load on each leg at a 15kg bridge load was 7.25kg. You can use the inverse square relationship/table to infer that at an effective column length of 10.5 cm, that wood would carry 4.75kg. Gee, that’s right in the range for a tower leg on a 4-leg tower..... Now this approach doesn’t get you as close as actual compression testing. If your bridge held a full 15kg load, you don’t know how “over-engineered” the piece was, so maybe you start with a bit lower density, and work down from there...Or, if your bridge failed at....say 14 kg – if you happen to know/suspect it was the member in question that failed, a little bit more density should work. If you don’t know what failed, or it was a piece other than the one in question, you’re still in the dark, but you have a real starting point.

3) A third way is easier, but less precise “column testing” that will get you a lot closer than just guessing. You can do this by “testing” “long” pieces on a little scale. On a 24” or 36” piece, the force to buckle is very low. For instance, the 1.37gr/24” that tested at 7.25kg at 8.5 cm length. At 35 cm (about the length of a C-tower upper section leg), your inverse square table will tell you it will buckle at 0.428kg. With a couple triangles, or a plumb-bob (weight hanging on string) you can set up to push pretty close to straight down, on a stick, with the stick sitting vertically on a suitable scale- like a cheap pocket scale that measures to 1kg at 0.1gr increments
So, 3 different ways to get to a “this size at this density” answer, that you have a real reason to know it can work. We’re back to the question of how to pick the stiff sticks. There is, as Jeff notes, a lot of archive discussion on this. The third method described above certainly gives you objective/quantified data. Fanning sticks out over an edge- like a counter edge, and seeing which droop more or less is another; doing the scale buckling “test.”

And that gets us to the last piece of the puzzle - how do you get/pick the stiffest/strongest pieces cut from “stiff” sticks, and get to the lightest pieces that will “do the design job”?.
Short answer, with a good scale, a plan, and patience. First in the long sticks you’ve selected for stiffness, make sure there isn’t a missed “floppy” section; hold one end, take two fingers, put a little bit of bend in, and run your fingers along the length; if at some point, the bend angle suddenly increases, and then, further along decreases, you’ve identified a “soft; zone- pitch the stick, or mark the soft zone and don’t use it. The third stick stiffness test method above will usually pick up sticks w/ such zones, btw, Second, get an accurate weight on the sticks (to 0.01 gr at least- we actually use a scale at work that gives us to 0.0001gr....). From that weight, and the length, you get a grams per centimetre number- the average weight/density of the whole stick. Third, check for significantly heavy/light ends- mark mid point;of the stick; balance on a thin edge of something at the marked midpoint. If one end is significantly heavier, mark (highlighter/magic marker) heavy/light ends

Then cut pieces from the “stiff sticks” a bit longer than length you’ll actually build at – but cut them the same length. Mark (again, color coding- little marks works well), and weigh each; make a log/inventory. From weighing whole sticks, you’ll have an average gr/cm for the “source stick”, you’ll know what that average gr/cm would be at the length the pieces are cut to. Easy in Excel to have column that gives you the % above/below stick average. How many pieces (per piece you’re going to use? We’ve run in the 3 to 5 or 6:1 ratio; the more, the closer to optimum, with reasonable repeatability, you can get.

Looking, for a given “piece set”- say the 4 lower section legs of a 4-leg tower; a few strategy thoughts:

-beware the light outliers-2 or 3 out of a dozen that are....8, 9, 10% lighter than average
-consider a conservative build- a closely matched set to the upper end of the weight range; density does have some correlation to strength- good approach for a tower going to competition (like Regionals) and you’re pushed for time to build/test tower(s)
-select and build a tower for testing that “looks at”/tests the end-points of your weight range- like 3 matched legs toward the heavy end, and one light one (though probably not the lightest); test to failure w/ a safety tower- if the light leg blows, next tower gets 4 of the heavy-end legs; if it holds, next one gets 4 of the light end legs.
-As you build towers/use up your inventory, re-stock- identify based on tower testing sets for competition use.

So, enough for one day. I’m not suggesting that everyone doing tower “ought to” be doing ‘the whole nine yards laid out above. If you’re up for the work it takes to get seriously competitive, this will help get you there, though. If not, pick some tips that fit your time/budget/seriousness.
Have fun,
Len Joeris
Fort Collins, CO
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