Yup, that'll work. There are other ways that will, also.dholdgreve wrote:You are correct, with towers, one could build a single 3-D jig to assemble the entire structure around, then slip it off when complete... with Booms, think 2-D... build each side independently, verify they are exact mirror images to each other by holding them next to each other, then connect the 2 sides... The trick is how to build the 2 sides exactly the same... Think template...fanjiatian wrote:Any ideas on building a jig for the trusses?
For towers it was easy because you could just slide the structure off but you can't do that with boomilevers unless if you leave one side unconnected.
This is a good example of a question – and there are a myriad of such questions for the various S-O events – where the answer, or more correctly, the path to the answer, is “Sure it can be done; you do it by applying critical thinking skills.”
The process and the path it takes you down is iterative (“....ok that’s not going to work because..., need to circle back...”), and it’s like peeling the layers of an onion (“...ok, that’ll all work, but now I need to figure out how to solve this new problem I can now see...”). There are many different....detailed answers; how, exactly to do it. Countless ways to skin a cat. A few suggestions and.....guiding principles.
What the ultimate answer is, for you, or anyone else, depends first on a number of... let’s call them up-front constraints:
-How much time you’re willing to invest,
-How much money you have to invest,
-How precise is good enough (knowing that precision takes time and money, but/and, higher precision allows you to be more competitive - see past discussions of "geometry control")
-What materials and tools you have or can get access to, and,
-How good your skills are at using the materials and tools you have to work with.
With those constraints “pencilled in”, the path to the answer starts with problem definition; the question, “What is it I have to do, what do I have to make, to accomplish what I need/want?” Form follows function.
To find the path to the answer, you need to back up, recognize, and let go of preconceptions; “you can’t do it”, “you can’t do it unless x, or y or z”
Start with the presumption, “I can do this.” Put your up-front constraints to the side for a bit. Assume you have any materials you might need, and a magical tool that can make anything, any shape you need. Then figure out what this theoretical jig looks like; what form(s) to accomplish what function(s). Then bring those up-front constraints back onto the table; put them up against the theoretical jig; how can I make or get this part; how can I get as precise as I want; ok, how ‘bout the next, and so on. In this process, it can be very helpful to do some “prototyping” and “proof of concept” work; putting....something physical together with easily worked materials you have on-hand, or can easily and cheaply get; to see how an aspect works, what it looks like; to uncover any, “oh, that won’t work, because... issues.
If/when you work through this process, you get to a viable definition- “I need to make these pieces, and put them together these ways to get what I need.” For each, look at what can I buy/get that will work, and what do I have to make. For those things you have to make, what’s the best option you have (in terms of time, precision, etc.). If what you have available won’t work, or won’t work as well as you’d like, think about new, outside resources you might bring in- shop/shop teacher at school, a friend/team mate with a parent who has a good shop, etc. Iterate through all the pieces; make sure it will all work together.
OK, now from theory of approach to ....an outline of one (of many) possible approaches to a solution.
First, have to make a couple assumptions/clarifications. When you refer to “putting the trusses together”, I presume you’re talking about a boom compression member that is essentially “a tower on it’s side”, more specifically, the chimney of a tower- 4 legs, joined by bracing. When you note this was easy with a tower, I’m guessing it was easy because the legs “leaned in”; closer together at one end than the other. The form -jig – for the tower was a tapered solid....thing. Legs positioned on the 4 edges, bracing pieces put on between the legs. Because of the taper, when you lift from the (smaller) top, the edges of the form move away from the legs; the assembled tower lifts off easily (as long as you didn’t glue the tower to the form/jig when putting it together. One possibility is a tapered boom compression member; truly a tower chimney on it’s side.
There are, I’ll argue without going into detail, performance advantages to be had with parallel legs. I have to presume your take “can’t slide the structure off without...” is in the context of a.....non-tapered structure; where the....core form, the jig, is a straight sided, square piece; 45+ cm long, with a cross sectional dimension from outsides of an adjacent pair of legs 5cm or less. Your experience, or assumption, is that if you assemble compression member/tower tightly on this form (which you want/need to do to get the legs straight and symmetrically aligned), you won’t be able to slide it off (even if you’ve....protected against glueing structure to form/jig). I agree that, at best, it will be a pain in the you know what to get the structure off the straight (solid) form.
OK,....form follows function...
So, putting pre-conceptions and constraints aside, what if, once the structure was assembled on the jig, if you could make the jig shrink- collapse inward, have a smaller cross section; move in away from the legs and the bracing pieces... that would make it easy to take the structure off; that would accomplish what you want.
OK, how could you make that happen?? Hmmm... what if the form, rather than being one solid piece, was multi-piece?
if you could somehow pull out a piece or pieces from the middle, so that the corners along which the legs run were free to move inward, away from the legs,... that would work.
So, if you had an inner piece, let’s say for purposes of discussion, with a 1” x 1” cross section; it could be 1” steel or aluminium bar, or 1” aluminium box, or a 1”x1” piece of wood, or a 1”x1” piece of foam, or a 1”x1” cardboard assembly (in descending order of cost and likely precision), what sort of outer pieces would you want/need – that would position the legs, and allow the 1”x1” core to be pulled out in a way that freed those outer pieces to be free to move inward?
OK, how could you make that happen?? Hmmm... if you put pieces of angle iron along the corners/edges of the core..... Side dimensions would need to be less than ½” (so when the 1”x1” core was pulled, they’d be able to move toward each other); let’s say 3/8ths or ¼” side width. Hmmm, a quick google says you could order online, Home Depot and Lowes only seem to go down to ½” width. If you have access to a table saw, you could get some ½” aluminium, and trim the width down a bit. You could get some thin wood and glue up wood angle iron pieces (metal will be more precise, though).
However you go to get there, what ever dimensions you want to use, when you have a core + 4 angle iron pieces with a width less than half the width of the core running along the corners/edges of the core, you’re good to go. Put the structure together. Pull out the core, the angle irons pieces fall/come out, and you have a nice, straight, symmetrical compression member.
There will, of course be a few .....implementation details you’ll need to work through, but they’re trivial once you’re on to a basic approach that will work
Have fun!
