Towers B/C

[bernard]

I do a lot of laser cutting. For projects that need to be precise, laser cutting is not recommended. Chemical inconsistencies in wood will make cuts microscopically inconsistent, creating weak points in your structure. If you need to strip wood, use a bladed wood stripper.

I'll definitely take your word for it, since you're speaking from experience, but can you elaborate on why there'd be a difference? The wood would have small inconsistencies regardless of how you cut it. Does the laser vaporize some trapped water causing mini-explosions? I would have just assumed that the laser would essentially function as an even thinner blade.

A blade physically splits material into pieces. A laser cutter burns the material holding two pieces together. Lasers are capable of both engraving and cutting, but if precision matters, you might mill instead. Burning can be inconsistent if your material is inconsistent; it's why some choose cast over extruded acrylic when laser cutting: the process for manufacturing cast acrylic results in a more uniform yet expensive product.

Note this is science and I'm making claims without quantitative data. For data, uou could test laser cut vs. blade cut bridges and compare.

[Llamastwaimzjf]

Hey, I'm Jessica. This is my first year with towers last year I did work on bridge building. How do you think is the best way to build the 4 supports connecting at the top square?

[embokim]

I found that balsa is actually better to use for joints, just because bass is denser, and glue won't easily penetrate through, into the wood creating a weaker bond. Plus, has anyone made a triangular base and top yet?

[dholdgreve]

I found that balsa is actually better to use for joints, just because bass is denser, and glue won't easily penetrate through, into the wood creating a weaker bond. Plus, has anyone made a triangular base and top yet?

I would think that in order to "do" a triangle shaped tower, you'd need to use triangular shaped columns... Not just triangle shaped columns, like you'd buy at a Hobby store that has a 90 degree corner and (2) 45 degree corners, but triangle shaped columns that have (3) 60 degree corners... (So that the faces of the columns would align with your bracing planes) ... To the best of my knowledge, these are not available anywhere, so you would need to make them yourself. Making these columns yourself will take specialized equipment and extensive time. In addition, you are no placing the entire load on 3 bearing point instead of 4, so I'd think your columns would be carrying about 25% more load, so they would need to be bigger. Another major factor is the bracing itself... In a tower that is square or rectangle, the adjacent braces are 90 degrees to the face plane. In a triangular shaped tower, the adjacent brace planes are only 60 degrees to the face plane, making it less effective. You may be able to work with a triangle that is 90 -45-45 degrees, but with the braces being at different angles to the face plane, I would not expect it to do well...

Bottom line, a whole lot more work, and I can't see where there would be any advantages gained... Heavier columns, less efficient bracing...

[dholdgreve]

Hey, I'm Jessica. This is my first year with towers last year I did work on bridge building. How do you think is the best way to build the 4 supports connecting at the top square?

Hmmm... Who said supports are required? Think about it

[Balsa Man]

I found that balsa is actually better to use for joints, just because bass is denser, and glue won't easily penetrate through, into the wood creating a weaker bond. Plus, has anyone made a triangular base and top yet?

Agree w/ dholdgreave's take on triangular/3-leg approach. One other important thing to note (and its been noted before, so maybe you missed it), when you refer to 'base and top', that's something that was necessary in 2012 (last time towers were the structure), because rules said the tower had to fit in an 8cm circle above a specified height above the base, so a base with strongly angled legs, and a narrow/small 'chimney' going up from it. There is no such constraint in this year's rules, no reason to build it that way, and only disadvantages in trying to go that way- it will be heavier, and much harder to build precisely.

[Llamastwaimzjf]

Ohhhh true. What design would you recommend on braces? I mean if you have to many braces that is a lot of unnecessary weight, but if no braces at all then weaker tower.

[Balsa Man]

Ohhhh true. What design would you recommend on braces? I mean if you have to many braces that is a lot of unnecessary weight, but if no braces at all then weaker tower.

See long, detailed answer to the same question you asked in the Measuring/using buckling strength thread........ See all the discussion in this thread about figuring out what bracing interval will work with what strength of legs.

[baker]

Ohhhh true. What design would you recommend on braces? I mean if you have to many braces that is a lot of unnecessary weight, but if no braces at all then weaker tower.

See pages 10 and 17.

[baker]

Balsa Man... I hope people appreciate the work you do! Your research shows hours of testing, stuff I always wanted to do but never had time to. Sometimes hard to follow but I love the conclusion. Because of years of hands on building with different sets of students I can see your data follows our experience of test and rebuild and test again.

In 2012 we used the 3/23 sq Bass for the base legs as we kept breaking the 1/8 sq balsa. In the top we used 3/32 balsa which worked very well. This year we used 3/32 sq bass for legs but the mass of the tower came in just over 13 grams, too heavy. I like to have new students build a little heavy to get some build experience and an ego rush from success . From there we then start to bring the mass down. Different cross section or density.

So from the lasted data posted, I see legs - 1/8 x 1/8 sq, 10-12 # density balsa legs with bracing intersections at about 12 cm (5 inches).. Also, I know from past posts about how important bracing is, I'm interested in your opinion about which pattern you like best. 'X' or 'V' bracing.

Thanks, Baker.
I always enjoy your insights. Its a pleasure to share the.....basic data we've accumulated; helps everybody up their game, and do some real engineering, and less shooting in the dark.

On legs, 10-12#/cf translates, for 1/8x1/8 to 36" stick weights between about 1.45 and 1.75gr. At 1/5 bracing intervals for a C tower meeting 29cm circle specs (12.27cm which is I assume your "about 12cm), you're definitely in the right ballpark. Specifically, a 1.52gr (to the light end of the 10-12#/cf range stick calculates out to 4572gr buckling strength at 12.27 braced intervals (which is force on each leg, +20% safety factor, for a 15kg tower load).

On bracing, see attachment on my post on October 21 (pg 10 of this thread); provides a pretty detailed discussion of the "Xs and ladders" we use- your middle figure. What's different from most in implementing this pattern is the use of 1/16th wide 1/64th" strips, put on pre-tensioned, for the Xs. I'm not 100% convinced its the very lightest way to go, but it works......

On the 13gr 3/32nds bass tower, that weight sounds about right. With 1.52gr/36" legs, ladders and Xs, weight calculates out to a tad under 10 grams. We'll see, one of these days, if reality aligns with all the calculation.

Len Joeris
Fort Collins, CO

Time for a Balsa Man reality check....
Built for bonus, 5 cm square at the top and 22 cm square at base
Legs -1/8 x 1/8 inch, 12#/cf balsa, 1.27 to 1.51 grams / leg (yeah, one was stripped a little heavier)
Ladders - 1/8 x 1/8 inch, 7#/cf balsa, 0.73 to 0.90 grams / side
X bracing - 1/64 x 1/16 inch, 8#/cf balsa, 0.35 to .53 grams / side
ladder intersections at 12 cm
After glue and some extra bracing at the top, final mass was 10.82 grams. Carried the full 15000 grams with no problems. With bonus final score 1571.16 I think being a little more careful and consistent with material weights we could have brought the mass to just below 10 grams. Will probable try 10#/cf for the legs next. All materials were stripped from 4 inch wide balsa and there was still variations in #/cf pieces. Darn close to Balsa Man's ball park guess... I'm just impressed that the legs held as well as they did.