Designs

[mrsteven]

I'm not going to quote that whole thing simply for logical flows sake.

I would disagree with the statement that .3 g/36 stick is too low. I use just that and I find that it still isn't what breaks. I started at .4g/36'' but moved down because my towers always broke at joint never at the bracings in the chimney.
However your 6g tower is much more impressive than mine. I'm at ~10 grams per 70cm per 15 kg holding. And at 15 it breaks at the joint, however each time I know that when connecting them its slightly offset which there lies my issue. So I could still even then lighten up most of the supports (possibly even main 4 members) and probably gain similar results given I can finally attach them correctly.

I'm hoping states side I can get ~110 regionals scoring (I could convert... too lazy) score. No clue if that will place at state for IL C div but hey, its where I can see me at since no way I'm changing my design again and dealing with making a new one in inventor.

On the comment of 1/8 V 3/32 I'm on 1/8. Mostly because I have so much of it...

[Balsa Man]

Julian,
You're at a seriously competitive level, sir; congratulations!
A few thoughts that will hopefully help you in your quest to pick up a bit more.

Stripping - there's nothing magic or difficult in cutting strips. Tried a stripper some years ago, and found it was more difficult, and less consistant (as in getting straight cut at a consistent width) than a straight edge. First, you want a.....proper straight edge. We use a ruler from a square; steel, about 1/16th thick, an inch wide, with nice square edges. Second, you want to be stripping at only the length you need - the longer you go, the more difficult it is. Work on a good cutting surface. Cut your sheet into pieces that are desired length. I'll discuss doing pre-tensioning in a second; the length we are working with this year is like 20.5cm. Put the sheet section down on your cutting surface. Get/have a piece of wood, or metal, that is the thickness/width you want to cut to. We're running at a bit over 1/16th, so use a 1/16th square brass tube piece as a guide piece. Lay the ruler on the sheet section; lay the guide piece against the edge. Adjust things so you can see just a bit of the edge of the sheet sticking out under the guide piece- so the amount sticking out is the same along the length. Put your thumb and a finger firmly on the ruler. Move the guide piece out of the way. Take a fresh, sharp razor blade between thumb and forefinger, holding it perpendicular. Concentrating on keeping it up against the edge of the ruler, gently run the blade down the ruler. Do it again, and maybe a 3rd time. Do not try to make the ...full cut the first pass. With high density, the hard parts of the grain structure will tend to deflect the blade - multiple, gentle passes.

Taper- the numbers I provided earlier were approximate. Threw the calipers on this morning. The diagonal measurement at base of chimney is just under 8cm- like 7.95. The side length of square is 5.6. At the top, the diagonal is about 6.2cm, with a square side of 4.6. So, the amount of lean-in toward the center- which is measured on the diagonal, is right at 0.9cm.

And then pre-tensioning.... I was thinking of waiting a week or two for a detailed explanation, but what the heck; glad to answer your question.
First, worth taking a look at a video one of our assistant coaches did at regionals and posted to You-Tube - http://www.youtube.com/watch?v=sTfH2YeV6OI
In this you can see how the TSHD and ladders are configured- ladders between the legs, flush with outer surface, with Xs on top, spanning the butt joint between ladder and leg. Second, you'll see that our Team 1 tower ran regionals at a fairly heavy weight (12.96gr). Two things going on here- first, its been a couple of years (my youngest son graduated a couple years ago) since I've had builders willing to put in the time for real development/reginement (build/test/build, etc). Our....accumulated data (and good jigs) allows us to jump right to a reasonably competitive level. This was the only tower they built, finished the week before, and went right to regionals w/o testing. Do I recommend that, no; I work with what/who I have to work with. Second was a decision to 'go conservative' for regionals- not pushing the limits, based on what our design data information base was telling us on densities/strength vs design loads. We'll be lighter for State, but not into the range you're working in.

Second, wood for Xs as we use them- yes, Specialized Balsa sells 1/64th sheets and you can specify the weights. We're using 3"x36" sheet at between 10.5 and 12.0 grams (thats btw 24 and 27gr/cu ft density- the same density as 1/16th at 0.9 to 1.0gr/36"). Obviously, its nice to be able to go down to Specialized and sort through sheets in the right weight range to get....good looking grain structure. If you want to go this route, I'd suggest calling Jake at Specialized, tell him what you're looking for- good straight grain structure for thin tension-bearing strips.

So, exactly how to put pre-tensioned Xs on, solo. We're not talking a lot of pre-tensioning for Xs; the key is enough to have no slack, and just a little tension, and to have that tension even/consistent. What they need to hold is the initial buckling outward load. As I've discussed, balsa stretches/elongates under tension. The higher the density, the less that elongation is. On elevated bridge 2 yrs back, the main tension members (paired, 3/16ths x 1/64th at 11.5kg) were stretching about 2.3mm- on the pre-tensioning bow, they plinked like a violin string; put on pre-tensioned, they "humped" the bridge, then as it got loaded, it settled back into straight/design configuration. So, in a tower chimney, we're talking around 1kg. (for the bottom Xs in the base, btw, we're going 1.5 to 1.75, and a width of 3/32nds- they have to hold bottom of leg spreading tension- a lot more than initial buckling loading).

We use a "pulling tool". It takes quite a few words to explain how- a video would be easier, but..... just read thru, re-read- play with things, and it'll make sense.
You'll need a base plate. We're using 1/4" plexi; the white-coated shelf board you can get at Home Depot/Lowes also works- you want a smooth hard surface that CA will stick to reasonably well, and that you can clean off w/ a razor blade. In overview, what you do is glue one end down to the plate; you use a spring mechanism to pull tension on the other end. With the tension on, you glue a bow to the strip. Then you cut the strip out beyond the ends of the bow. What you end up with is a tensioned strip held on a bow. You then glue that tensioned strip onto the tower, and when the glue's set up, cut it off the bow- as in cut at the outside of the legs.
So, first, figure out your "working length" - your longest X, plus enough length for pulling and bow attachment. What you do is glue one end of the strip down to the plate- a decent-sized drop of CA. What you need at the other end is a "puller." We use a piece of 1/8th brass rod in a hole drilled in the plate to hold one end of a spring; the spring is like an inch and a quarter long. The "puller piece" is made out of 1/4 plexi - a square about 3/4 x 3/4", cut to a "U", where one side of the "U" fits the loop in the end of the spring. You want to position the brass rod (relative to the end of the strip opposite the end you've glued down to the plate) so that when you pull the U -stretching the spring- with a pull of about a kilo, the bottom side of the U is over the end of the strip by enough to provide a glue zone. You want to mark that "pull point'; better yet, put a "stop block" in that you pull the U to, then push down onto the end of the strip. Put down a little piece of waxed paper, under the "glue to the U" zone, so you don't glue things down to the plate. With the spring pulling on one side of the U, and the strip glued to the bottom of the U, the U will want to rotate - you'll need to keep a finger holding down the side attached to the strip. Then you put a bow on it. For bows, we're using 1/8th square bass- the long part long enough to provide a strip between the bow ends that is 1-2cm longer than your longest X piece. On each end of the bow, glue a 1/4" long piece of 1/8th on one side. The strip glues to these end pieces. Before you "pull" a strip, 1) put little pieces of waxed paper on the plate where the bow ends come down on the strip, 2) put a good drop of slow CA on both end blocks of the bow. So, you have the strip under tension on the puller, holding the puller U-piece down with one hand; with the other hand, grab the bow, and put it down on the strip. Hold it in place till the glue goes off. Then cut the strip beyond the outside ends of the bow. Its then ready to put onto the tower. You'll want to make 4 bows. The Xs, btw, go on after the ladders are installed. Using a razor blade, clean off excess CA at the "glue-down end" after doing each one. Also clean off the bottom of the U-scraping away the wood, and glue build-up).

To do a "level" of Xs, make up four 'loaded" bows. Put glue on the tower where the Xs will attach. Use slow CA. It helps to pre-mark the zones the Xs go on/over. The Xs should overlap the butt joint of ladder to leg. Bring the strip on the bow up against the tower. Don't push against the tower too hard, or you'll put a bow in the tensioned X piece- at each end, go in with a "push rod'- we use a 3" piece of 3/32nds steel or brass rod- use it to push in on the glue zone. When the glue's gone off, cut the strip at the outside edge of both legs. Put Xs on in "opposite pairs" - on opposite sides, one running one way, the other the opposite way- don't cut the strips until both are in-place (to avoid pulling a twist/rotation/spiral) into the chimney. Work from either the top down, or bottom up- we go (and I recommend) bottom up. When you move up/down a level, one end of the next Xs goes onto the legs, and the other end goes over the previously installed Xs of the adjacent level.

Now, depending on how your jigging works, doing the bottom Xs will require something different/additional. If, as we have, your jig is on a base plate, there is no way to get the bottom end of an X on bow up against the bottom end of the leg...... We do a "pull-in-place" at the bottom. First, at the bottom, we have a temporary set of ladders in-place- snug slip fit, with little pieces of Glad Press-n-seal around the ends so it doesn't get glued in when you put the xs on. This holds/keeps the positioning of the bottom legs (for attachment to the base), so the tension of thee Xs dosn't distort things. They come out after the chimney is glued to the base.

To pull in place, (and we do this at both bottom of chimney, and bottom of base) we use a piece of carbon fiber strip (its a bit less than 1/8th inch wide, like 0.030 thick- stiff but springy; you could use wood or plexi- you'd have to experiment to get the right flex.... Its about 3-4 cm longer than the X. One end is cut to a tapered point. It goes in a (1/4") hole drilled in the jig base. That hole (there are two for each leg end) is positioned so that when you put one end of the "pulling stick" in it, the stick can run/lay against the legs on the line you want the Xs to run. The other end has a "glue block" attached- a piece of 1/8th square bass, about 1/4" long, mounted at a bit of an angle. What you do is glue the bottom end of an X onto a leg, positioned so it crosses the adjoining leg where you want it (crossing the leg/lader joint). Using like thumb and two fingers of one hand, bow the pulling stick, so that the glue block can move into position to attach to the unglued end of the X. You'll need to play with length to get the amount of pull right. Before you position and bow the pulling stick, put some glue on the glue block. Bring it into contact with the upper end of the X. Using either accelerator, or tweezers, get the upper end glued to the puller. You can then let your fingers that have been holding the bowing loose- the X is in tension. Put glue on the leg/ladder (as described above), glue the upper end on...

That's how we do it. Hope this helps you if you take on going this way. have fun, and good luck. Keep us posted on what you decide to do, and how it goes.

[havenguy]

I am going to test my first square base in about a year, and I have a few questions:

1) Previously, (with rectangular bases) I have been making my legs rest on two level desk that are 20 cm apart. However, I am wondering if, on a square base, this will make a difference, since two whole sides will be resting on the desks. It seems like this could change the outward movement of the legs on the base.

2) How far should the legs of the base be from the hole? Currently, the 1/8" legs just clear, but I don't know if it would be a disadvantage.

3) At what point does a square base become better than a rectangular base? For example, would a 22 by 9 rectangular base (tapering to a 5.5 by 5.5) be better than a 15 by 15 square base (both at 20 cm tall) in terms of weight outweighing stability. (I just want an opinion, not necessarily scientifically proven)

[Balsa Man]

The greater the angle of the legs (away from vertical), the greater the force they see at a given load.
A square base allows you to position the leg bottoms at the midpoints of the sides of the 20x20 hole, so measuring across the diagonal, the bottom ends are 10 cm from the centerline. Measuring from 1 leg to the adjacent leg, they're a bit over 14cm apart. If you have the leg ends 20cm apart (1 leg to adjacent leg), then along the diagonal, the legs ends are about 14cm from the centerline.
Assuming the legs slope in to a 5.5 x 5.5, the angle on the legs in the first case is about 24 degrees; in the second case almost 45 degrees. The force on each leg at a 15kg load is about 4.11 kg in the first case; in the second, it is about 5.27kg. The first case allows you to use less wood (shorter legs), and lighter wood.

[havenguy]

Sorry, I don't think my question was clear enough...

The legs of my square base are 14.5 cm apart, but while testing, I just push the two desks I use for the 20 by 20 cm hole together to about 13 cm. This means that I don't rest the legs on the midpoints of the 20 by 20 cm hole, but I rest all the legs on the desks that are pushed closer together. I am wondering if this will make a difference.

Anyway, I tested the tower and it did pretty well, but I was wondering if stronger crossbeams but main leg members that are 3/32" (as a pose to 1/8") will make it lighter, and if it is worth it to change the legs to 3/32".

[Balsa Man]

OK, understand on the desks setup. Make a difference in what? as long as they're level and in the same plane- and stay that way in loading, the tower doesn't see anything different than sitting on a single piece with a hole in it.

On your second question, there's not a simple, straightforward answer, and we - you - don't have the info to know; to calculate anything. It's all about buckling strength. There are numerous previous posts on how that works - Euler's Buckling Theorem. Buckling strength depends on a) exposed column length - the bracing interval, b) the cross section, and the inherent stiffness, which generally depends on density. What leg material will "work" depends on all three factors. If you replace 1/8th with 3/32nds of the same density, buckling strength goes down significantly; to get it to "work", you have to tighten up the bracing interval (shorten the exposed column length. Making the bracing stronger, but at the same interval gets you nothing; it will fail/buckle between the bracing. If you go to higher density 3/32nds, at some density, it will carry (at the same interval you're using now). What that density is, whether using it will get you a lighter tower, you'll have to figure out yourself. I can tell you that in a C- tower, 3/32nds bass, at a density of 1.4gr/24", with midpoint bracing in the base legs, and an 11cm bracing interval in the chimney can work. That says in a B-tower- 3 ladders in the base, 10.8 exposed column length, it will work, and in the chimney 4 ladders-10cm interval, its more than enough.

[thsom]

I was wondering, what would be the effects of this:
Instead of placing the x's on the base on the same upper side of the main members (so that the cross members overlap and one bends over the other), what if you put one diagonal of the x on the upper side of the main members and the other diagonal of the x (going the opposite direction) on the bottom side of the main members so that there would be a gap the width of the main members in between the pieces, would this harm the construction in anyway, less stability or less strength (buckling, and contortion) because there would be no contact between the diagonals and no glue connecting them (like there would be if they overlapped). I figure this would be 1: easier to construct (less messy, less risk, neater), and 2: possibly more stable. However, I could be completely mistaken.

[noobforce]

I was wondering, what would be the effects of this:
Instead of placing the x's on the base on the same upper side of the main members (so that the cross members overlap and one bends over the other), what if you put one diagonal of the x on the upper side of the main members and the other diagonal of the x (going the opposite direction) on the bottom side of the main members so that there would be a gap the width of the main members in between the pieces, would this harm the construction in anyway, less stability or less strength (buckling, and contortion) because there would be no contact between the diagonals and no glue connecting them (like there would be if they overlapped). I figure this would be 1: easier to construct (less messy, less risk, neater), and 2: possibly more stable. However, I could be completely mistaken.

Actually, I personally think that it would be harder to attach one bracing on one side of the main member and the other one on the other side, because it would be kinda difficult to create the two halves of the tower before you attach it using a jig.

But the strength would be the same, I believe.

[thsom]

Great! Any advice about this from other experienced grad's or builders too? (like mrsteven, slm, lkn, or balsaman?)

[Balsa Man]

I agree with the apparent difficulty building, but that depends on how you're doing your jigging.
One of the advantages of thin strip Xs is you don't have the bowing you get with, say, 1/16th square Xs. That bowing means there's a tiny bit of "excess length- if the leg starts to bow out at attachment point, it will be able to pull the bowed X out a tiny bit, which may be enough for the leg bowing to "happen"- to go into column failure. Pre-tensioned strips allow essentially no motion.
You mentioned glueing Xs where they cross- you don't want to do that.