Aia's Boomilever Guide

= Introduction =



The goal of this guide is to help new and intermediate builders contend with the complexities of the boomilever event. I built towers in 2007, boomilevers in 2008, and bridges in 2009, so I hope to offer advice from my own experiences in the balsa building events. DISCLAIMER: The advice I offer here is derived from my own experiences. There are doubtlessly better designs and better building techniques out there.

= Helpful Links =


 * A slideshow of the 24 booms I built during the 2008 season
 * A slideshow of the boomilever I used for the 2008 National Science Olympiad
 * A slideshow of boomilevers from past competition years
 * A link to the boomilever discussion board when it becomes available

= Building Techniques = adfasdf

Balsa vs. Bass
I have seen the “bass vs. balsa” question asked countless times on the forum. In my opinion, neither one is inherently better than the other. I’ve seen nationally medaling structures that were all balsa AND some that were all bass. However, based on availability alone, I do believe balsa is a better option than bass for most builders.

For comparison’s sake, I will list out the advantages and disadvantages of each type of wood.


 * Balsa's advantages
 * Lighter
 * Commercially available in many sizes
 * Much easier to sand, cut, and glue
 * Balsa's disadvantages
 * Lots of variation in density through one stick
 * Non-parallel grain


 * Bass's advantages
 * More uniform density
 * Bass's disadvantages
 * Heavier
 * Available in fewer sizes and sold in fewer locations
 * Typically sold as 24” sticks instead of the 36” sticks that balsa is sold as

For the boomilever event, I found that balsa and bass each had a role to play. I typically used bass tension members and a small amount of bass in the lamination of my base. I used balsa everywhere else.

Balsa’s variation is both an advantage and a disadvantage to a builder. For example, I quickly found that 1/16” x 1/16” sticks that were 36” long could weigh anywhere from .3g to >1g. Using the .3g sticks for my cross members immediately lightened my structure without compensating any strength. Although variable density proves advantageous to the builder, it also carries the danger of non-uniform density. Let me illustrate this issue with an experience from my tower years:
 * During the 2007 tower season, I generally chose my four legs by picking two 36” sticks of an equal weight and cutting them down to size. Let’s say each 36” stick was initially 1.5g. After cutting the balsa down, my four legs were then .7g, .7g, .9g, and .5g. Obviously, the .5g leg will be the first to fail and this is not ideal.

To contend with the variability of balsa, I suggest recording the weight of each piece (not each stick) of balsa you use in your final structure. For example, record the weight of each tension member in your boomilever, the mass of each component of the base, etc. This will help you maintain tension members, compression members, etc. of a similar density. These values also provide an easy way to quantify the distribution of weight in your structure, and allow easy replication later.

If you purchase balsa on your own, I also suggest looking at balsa grain as a means of balsa quality control. In my experience, I’ve found that balsa has two different types of grain:
 * 1) Well defined grain with distinctly visible lines
 * 2) A blurry grain

Some balsa sticks have both types of grain, depending on which face you are looking at. As a general rule of thumb, the lines in the well-defined grain should ALWAYS run parallel to the wood, especially in balsa that will be used in compression members. Balsa tends to shear along grain lines, and I found this especially true for boomilever compression members. Looking at balsa sticks before purchase also allows you to note any major defects, such as crookedness or splintering.

Tools
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Glue
I used the following glues while building my boomilevers.
 * Purple CA glue: I used purple CA (cyanoacrylate, 5-15 sec) glue on 90% of the joints of my boomilevers. It's somewhat viscous, so it is easy to apply. This glue works best on lap joints.
 * Application: When I first started building, I used to put a small dab of glue (about the size of a penny) on wax paper and use a toothpick to apply glue to the appropriate location. In time, I eventually switched to squeezing glue directly from the bottle onto the wood. With practice, you can carefully get the exact amount of glue that you want with no excess.
 * Availability: I could only find this glue at hobby stores. At one point, I did try buying Krazy Glue (another CA glue) at a large craft store, but it was never as effective as what I found at the hobby stores.
 * Helpful hint: I found that the 1 oz bottles (instead of 2 oz or larger) were the easiest to use. Firstly, the bottles were easier to squeeze without using too much glue. Secondly, I did discover that glue could dry out in the bottle over time and become hard and thick. By choosing 1 oz bottles, I could usually use all the glue before it hardened.


 * Blue CA glue:  I used blue CA (cyanoacrylate, 1-2 sec) glue on butt joints. It's very watery and difficult to apply. In general however, it tends to give stronger joints than the purple CA glue because it soaks into the wood. In my experience, it dries almost immediately after application.
 * Application: I always applied blue CA directly out of the bottle. Because it's watery, it comes out of the bottle very quickly. I would suggest practicing application prior to gluing anything onto your structure.
 * Availability: Like purple CA, this glue is found at hobby stores. I could find no equivalent at major craft stores.
 * Helpful hint: Blue CA gets very hot as it dries, so try not to get it on your hands.


 * Gorilla Glue: I only used gorilla glue while building boomilevers. Even then, I only used it in a few select locations on my structure (primarily the base). Gorilla glue expands as it dries, ultimately growing to 4-5x its original size. Unlike the CA glue, gorilla requires clamps after application and a 24hr drying period.
 * Application: I never squeezed gorilla glue onto any structure directly. After creating a dab on wax paper, I always applied it using a toothpick. Until you become familar with gorilla glue, always apply LESS than you think you'll need. The glue expands to an astonishing amount.
 * Availability: Gorilla glue is available nearly anywhere- craft stores, hobby stores, hardware stores, office supply stores, and even some grocery chains carry it.
 * Helpful hint: Avoid plastic clamps if possible. I used rubber tipped clamps (seen here: http://gallery.scioly.org/details.php?image_id=2025) that didn't indent the balsa.

=Major parts of the Boomilever= asdfasdf

Compression
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Tension
In my boomilevers, I eventually chose to use 3/32" square bass tension members. I tried using balsa, but it routinely broke early on in the loading process. My tension members were forced to "curve in" because I wanted to minimize the size of my base. I think that this added curve made the balsa tension members much more prone to shearing apart. I also tried using smaller bass pieces for the tension members, but I still seemed to have the best success with the 3/32" square.

I also found that the lightest bass pieces, as long as they were 3/32" square, could withstand the full load plus some. When purchasing wood, I suggest bringing a scale and only buying the lightest bass sticks. Although bass doesn't vary in mass nearly as much as balsa does, I could still find pieces that were significantly lighter (.4-.5g) than some of the others. A good rule of thumb is to save your best/lightest bass members for competition structures.

Distal end
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Base


The base is arguably the most important part of the boomilever. Even if you have the best boomilever ever, you won’t get far with a weak base. The following instructions show how to build a functional, mid-weight base. I used this style of base for 90% of the boomilevers I built in 2008. If built correctly, this base weighs ~1.5g and will support 18-19 kg.


 * 1) Laminate a thin sheet of bass with a thick sheet of balsa: I used 1/16" bass and 3/16" thick balsa. Typically I would cut the sheets of balsa/bass down to size prior to doing the actual laminating. Use gorilla glue for the lamination, clamp the result, and wait 24 hours. Remember to use less glue than you think you'll need- gorilla glue expands A LOT.
 * 2) Drill three holes: Make a hole for the bolt in the center of the base, and then drill two other smaller holes on either side of the first hole. I suggest penciling in the desired locations of the holes before doing any drilling. Remember to account for the size of the washer. Also, drilling from "bass to balsa" will prevent the drill from shredding the wood.
 * 3) Shape the two small holes appropriately: Using sanding sticks and an exacto knife, carefully carve out the small holes. Reference the side view diagram below: these small holes will need to accommodate a tension member and a triangular piece of bass on either side of the tension. As such, the holes will appear small in the front and larger in the back. Use the bass side of the lamination as the "front" of the base. Try to minimize the size of your holes for the best result; however, don't worry if the holes are a little larger than intended.
 * 4) Add glue to the small holes: Situate the tension members, triangular pieces, and base in place. Make sure that you have already used CA glue to glue the triangles to the tension. Then add gorilla glue- again, use less than you think you'll need. Once you add the gorilla glue, you'll need to let the base sit for another 24 hours. After drying, you'll probably have extra glue that has puffed out around the front of the base (like in the picture above). DO NOT SAND IT OFF. It doesn't look pretty, it doesn't look professional... but the last thing you want is to accidentally sand your tension members and create an unnecessary weakness. Gorilla glue seems to form a hollow matrix inside once dry, so I think it only adds a few hundredths of a gram anyway.
 * 5) Sand your base: Now is the time to sand the backside of the base to ensure it will sit flat against the testing wall. When completing step 4, I had the boom in a "standing" position on my building surface, so the base was already in a flat position with minimal sanding needed. I also sanded the corners of my rectangular bases to reduce weight. At this point, your base should be complete.



=Frequently Asked Questions= asdfad