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[Balsa Man]

Hey all- I got a jig finally We are using very light pieces now on the jig as we are confident in the build quality of the jig, but as it is a two piece tower jig, putting the top and bottom together is hard especially with the loctite CA glue I am using. I have been considering using wood glue and water combo or loctite and water combo to allow for more repositioning. In addition, sliding the tower off the jig is quite difficult- many bracings snapped in doing so- any advice to counteract that? Lastly, even though torsion isn't as big as a problem as bridges, I am currently using a Z design to cut mass and putting them in opposite directions so that four bracings meet at a single point. Should I use the Z design but in a parallel manner so that only two bracings meet at a single point? Using x's would conveniently take away that problem and increase torsion support but add on weight- so I am wondering as which of the options would be the best mass/strength decision. Thanks so much- we are selecting balsa pieces on BS now! (another homage to the forums)

Having a jig- excellent! Congrats.
It being really tricky to get top and bottom tower….sections together and lined up right; that’s one of the set of reasons I’ve noted, in a number of previous posts, for saying a 2-part approach is just not a good idea…. A reason I didn’t include in my list before is the problem you note of difficulty in getting the (narrow upper ‘chimney’ part, where the legs are close to vertical) off its jig. In a single truncated pyramid design (with a 10+ degree slant to the legs), and in jig for lower portion, with even more leg slant, having/getting ‘vertical clearance’ above ladder joints is easy; with near vertical legs, its very difficult, and repairing/replacing snapped braces from removal means additional glue/weight. In fact, what we ended up doing back in 2011/2012 to deal with this problem (when the tower rules meant you had to go with a 2-part design, unlike this year’s rules) was, for the upper part jig, creating a way to collapse the leg holding edges inward after chimney assembly; like this, showing one of the four jig plates that you have to hold the legs, and looking down from on top:

====== strip
C======+====Leg (JIG PLATE)
====== strip

Sorry for the crude drawing. C is the tower/jig center, Leg is the outer edge where the leg goes. + is a 3/32 x 3/32 bass stick. This jig was done in 3/32 plexi. First, the jig plates were cut to correct dimensions. Then, where the 3/32 stick is shown, did a straight cut from top to bottom, using a table saw with a blade that made a 3/32” wide cut. Then cut strips from the 3/32 plexi, ~3/4” wide, and glued them to the inside portion of the jig plate, so they created a slot for the outer portion to fit into. Tower assembly was done with the 3/32” sticks in place, and the outer portions of the jig plates pushed in so the sticks were tightly against the inner portions. When done, and ready to remove, pulled the sticks out the top, and pushed the outer portions of the jig plates in- then the chimney was free to lift off- no jig interferences with the bracing. A….royal pain in the tail to do the jig construction (have to be real careful to not get glue squishing into the slot area), but it worked.

Torsion (structure trying to twist) is actually a bigger problem in a tower that in a bridge, because of the much greater height. As I’ve said before, I don’t have a…’for sure’ engineering handle on optimal bracing designs, other than an Xs and ladders configuration, like I do on ladders and Xs and wood selection/specification to have sufficient buckling strength in the legs, using ladders and Xs for bracing. Still working on really figuring the optimal efficiency answer. You might want to go over the discussion on bracing systems in the 2011 towers forum that I posted a link to a few messages above. Then its build and test, and see if it works…. Doing that, it’ll be important to be able to see what breaks first- to understand the failure mode; the only two ways to do that is high speed (high frame rate) video, or a safety tower (a way to restrain the load block from above (when its being loaded from below), so that it can only fall…. like 1/8”. That way you can see what broke and how, without the tower being destroyed (when you can’t tell initial/primary damage from subsequent damage).
Hope this helps…

[BuildingFriend]

Hey all- I got a jig finally We are using very light pieces now on the jig as we are confident in the build quality of the jig, but as it is a two piece tower jig, putting the top and bottom together is hard especially with the loctite CA glue I am using. I have been considering using wood glue and water combo or loctite and water combo to allow for more repositioning. In addition, sliding the tower off the jig is quite difficult- many bracings snapped in doing so- any advice to counteract that? Lastly, even though torsion isn't as big as a problem as bridges, I am currently using a Z design to cut mass and putting them in opposite directions so that four bracings meet at a single point. Should I use the Z design but in a parallel manner so that only two bracings meet at a single point? Using x's would conveniently take away that problem and increase torsion support but add on weight- so I am wondering as which of the options would be the best mass/strength decision. Thanks so much- we are selecting balsa pieces on BS now! (another homage to the forums)

Having a jig- excellent! Congrats.
It being really tricky to get top and bottom tower….sections together and lined up right; that’s one of the set of reasons I’ve noted, in a number of previous posts, for saying a 2-part approach is just not a good idea…. A reason I didn’t include in my list before is the problem you note of difficulty in getting the (narrow upper ‘chimney’ part, where the legs are close to vertical) off its jig. In a single truncated pyramid design (with a 10+ degree slant to the legs), and in jig for lower portion, with even more leg slant, having/getting ‘vertical clearance’ above ladder joints is easy; with near vertical legs, its very difficult, and repairing/replacing snapped braces from removal means additional glue/weight. In fact, what we ended up doing back in 2011/2012 to deal with this problem (when the tower rules meant you had to go with a 2-part design, unlike this year’s rules) was, for the upper part jig, creating a way to collapse the leg holding edges inward after chimney assembly; like this, showing one of the four jig plates that you have to hold the legs, and looking down from on top:

====== strip
C======+====Leg (JIG PLATE)
====== strip

Sorry for the crude drawing. C is the tower/jig center, Leg is the outer edge where the leg goes. + is a 3/32 x 3/32 bass stick. This jig was done in 3/32 plexi. First, the jig plates were cut to correct dimensions. Then, where the 3/32 stick is shown, did a straight cut from top to bottom, using a table saw with a blade that made a 3/32” wide cut. Then cut strips from the 3/32 plexi, ~3/4” wide, and glued them to the inside portion of the jig plate, so they created a slot for the outer portion to fit into. Tower assembly was done with the 3/32” sticks in place, and the outer portions of the jig plates pushed in so the sticks were tightly against the inner portions. When done, and ready to remove, pulled the sticks out the top, and pushed the outer portions of the jig plates in- then the chimney was free to lift off- no jig interferences with the bracing. A….royal pain in the tail to do the jig construction (have to be real careful to not get glue squishing into the slot area), but it worked.

Torsion (structure trying to twist) is actually a bigger problem in a tower that in a bridge, because of the much greater height. As I’ve said before, I don’t have a…’for sure’ engineering handle on optimal bracing designs, other than an Xs and ladders configuration, like I do on ladders and Xs and wood selection/specification to have sufficient buckling strength in the legs, using ladders and Xs for bracing. Still working on really figuring the optimal efficiency answer. You might want to go over the discussion on bracing systems in the 2011 towers forum that I posted a link to a few messages above. Then its build and test, and see if it works…. Doing that, it’ll be important to be able to see what breaks first- to understand the failure mode; the only two ways to do that is high speed (high frame rate) video, or a safety tower (a way to restrain the load block from above (when its being loaded from below), so that it can only fall…. like 1/8”. That way you can see what broke and how, without the tower being destroyed (when you can’t tell initial/primary damage from subsequent damage).
Hope this helps…

Thank you for your advice! My friend suggested putting a small "sleeve" of white printer paper that was attached with a small amount of tape that was tautly stuck to it. After finishing building, take off the tape and slide the paper off. I will be trying that as the jig is already built and cannot modify it now with it being so close to competition time. With the bracing, we are actually not using any horizontals (or "ladders" as you call them). This is due to what we think will be lack of any compression (bowing inwards) occurring and using purely diagonal bracing. I don't know if this is possible on a two piece as my last tower failed due to bad connection. I would like your opinion on this and in addition, would lack of a horizontal member make the Euler's buckling theorem require more bracing to take in the lack of the ladder? (I assume the ladder is a lap joint?) but a Tappanzee coach suggested the ladder be a butt joint to accomdate for the possible bowing in of the tower. However, in the two piece, the chimney is under compression and only serves to get the force from top to the base so I don't know what bracing should be used. I think I will revert to a full X design rather than just diagonals but I will not include the horizontal- will let you know how it goes. Thank you for the advice again- we will use high speed camera to locate the reason of failure.

[Balsa Man]

So, why do you not think inward initiation of buckling will/can happen? The legs, in upper and lower sections are under axial compression load. They will, at some load, buckle. The direction of that buckling will be in the weakest direction; may be inward, may be outward- if instead of square cross section, one dimension is a few thousandths less, if there’s a slight bow in one direction, if there’s grain imperfection toward one side. One piece, two piece, doesn’t matter- the physics/engineering of bracing axially loaded columns (the legs) under compression is the same. Legs in both upper and lower sections ‘act only to get the load from the top to the bottom’. Ladders act to stop/control inward buckling; to be most effective they should between the legs (yes, butt jointed), so the loading they get as inward buckling is induced is axial.

In a ladders and Xs approach as I’ve described, the ladders only ‘work’ in compression, and the Xs (from 1/64th inch sheet) only work in tension (they have virtually no buckling strength). There are folk out there that seem to be having success using an Xs only bracing approach. Unless you find a way to be certain all induced buckling will be outward, these Xs have to have some level of buckling strength, i.e., have to have a minimum cross section >1/64th”, maybe 1/16”.

The aspect of Euler’s buckling theorem that comes into play in bracing approaches is how ‘end conditions’ affect ‘effective length’. I’ve discussed this in some detail in previous post; worth reviewing. A ladders and Xs bracing approach results in end conditions for the braced leg segments that approaches “pinned-pinned” end conditions- this makes the ‘effective length’ of the braced section shorter, and, hence, its buckling strength higher, than if the end conditions were something other than pinned-pinned. This is where the advice to take the buckling strength you measure in a stick by ‘single finger push-down’ testing, and multiply it by 2.3 to get the buckling strength from which you do inverse square calculation to get the buckling strength of a shorted braced segment comes from. Still working on fully understanding the numbers, but I believe the end conditions in an Xs only approach are….less effective, and the 2.3 effective length multiplier (good for pinned-pinned conditions) needs to be smaller. Some data I’ve seen suggests something on the order of 0.5, and that means a lot tighter bracing interval. You’ll have to play with that to see….

[EastStroudsburg13]

Scores from UPenn:

1. 2153
2. 1981
3. 1863
4. 1791
5. 1762
6. 1756

16th and higher were above 1000. 35 total teams competed.

Also of the top 6, 3 earned the bonus and 3 did not.

[Balsa Man]

Scores from UPenn:

1. 2153
2. 1981
3. 1863
4. 1791
5. 1762
6. 1756

16th and higher were above 1000. 35 total teams competed.

Also of the top 6, 3 earned the bonus and 3 did not.

Hey, thanks!
These numbers for B or C?

[EastStroudsburg13]

Scores from UPenn:

1. 2153
2. 1981
3. 1863
4. 1791
5. 1762
6. 1756

16th and higher were above 1000. 35 total teams competed.

Also of the top 6, 3 earned the bonus and 3 did not.

Hey, thanks!
These numbers for B or C?

These are all C division numbers.

[StormTJ]

Is a score of 2560 in division C good?

[Unome]

Is a score of 2560 in division C good?

This is certainly good enough for top 15 in Nationals; likely top 10, possibly a medal. This would win most state tournaments, and still probably finish top 3 in the most competitive ones.

[Random Human]

Can someone post a picture of their jig? I don't build off jigs but my teammates do. From what I've seen and heard, I don't think that what you people are doing are the same as my teammates. It'll be interesting if someone would be willing to post a picture of a jig.

[S4BB]

Here are a couple of current ones, made from 1/4" MDF, for both B and C division.