Try this corrected cnc code for megamega:

Attached files bridge_replaced zero radius.cnc (155.8 KB)Â

 

just in case it wont cut right.... finished product should/would have looked like this...

Attached files

 

Ok, megamega, I think I got you sketchup file fixed. Have to tidy up and will post it in a bit.

 

megamega, here it is. It looks ok with ncplot, so hopefully works with mach3, also. All you got to do is reapply the phlatlines. I've already rearranged everything, to now fit completely inside the safe area. I just redid 1/4 of your openings with bigger radius and copied over to the rest. It doesn't get the zero radius error, any more.

I think the problem was your 1/16" radius arcs. Since we use a 1/8" bit, the offset would be 1/16". And for inside cuts, that resulted in a radius of zero in the gcode. And I believe this is what caused the error in Mach3. The outside cuts were not effected, because the offset would be added. So the outside cut gcode would have a radius of 1/8" for a 1/16" radius arc.

-Kwok

Attached files bridge2.skp (548.9 KB)Â

 

What a neat contest!
This was far better then the stick wood bridges we had built in HS.
And it really showed off the constitution of the foam we build with.
As the contest went on it became apparent to me that I had under designed the top of my bridge.

BUT

The really neat part of this contest was the limited material.
Myself I went through 3 other designs that I wanted to try but the problem soon became the limited material (two of these were heavily supported on the top). But, then there was the consideration on the amount of time it would take me to design, as well as the possibility that someone else may not assemble it correctly or possibly have trouble (which looking at the build job MnT were doing was unfounded, you guys were great).

Hats off to Kwok for taking the time to do exactly what I was too lazy to do above!
And I can’t wait to see how Gefahren accomplished getting the parts onto one sheet with the height that he obtained. I knew from the start he’d be trouble!

I estimated that foam under tension would break apart before it would fail in compression. Boy was I wrong about that! While foam is very strong under compression, it is also apparently very strong in tension. I think that with a little bit more foam on the bottom the arch bridge that Tiger made would have been strong enough to walk on.

I think it would be neat to do this contest again to see how people can improve on designs on a really fun (and educational) contest!


60:1 weight ratio… that’s just silly!
Great Job!
:mrgreen:

 

To be second place to 22.4lb with 22.2lb sucks!!! I demand a recount. Where is my lawyer, I go to the supreme court.

TV, I had the bottom stringer doubled. In hindsight, I should have tripled it and it would have taken 30lb with no problem. That was the only reinforcement that I had, BTW.

 

It was a fun contest and I learned alot about real structures. It's not really about winning or losing. I think my main problem is, I treated it like a 2D homework problem. My main goal was 1) to use up all the foam. The idea being there shouldn't be any weak members, as in everything should be double layers. And 2) to build the strongest 2D side sections I could. Well, I did but didn't think in 3D. And I didn't think anything about the top cross pieces other than using them to keep the sides from falling over. But then I didn't have any foam left to do anything about it, even if I wanted to do something better. I am starting to think that most of these type bridges failed in torsion, because as I remember from school, torsion acts at 45 degs. Which means one should have diagonal bracing at the top? The interesting thing is Gefahren's bridge had the same type of non diagonal bracing as mine and looks like one less brace. But I did notice something very unique about his design. Actually 2 things. I would really like to see how much foam he used up.

I actually looked up the strength property of eps foam and the info was pretty sketchy. But it seem to say that foam was stronger in tension than compression. That's one of the main reasons I felt I could go with 1 layer in the outer diagonal braces which I believed were in tension. If you guys don't know about truss, then check this out. It shows what I mean about some members are in compression and some in tension. And it show the highest load is at the very top in compression: http://www.garrettsbridges.com/design/trussdesign

Actually, one of the biggest things I learned was, how close you can layout your parts. It finally came down to me decreasing the size of the bridge, until all the parts fit. Well, it was a good learning exercise. But, don't really want do it this way, again.

Ok, was the idea to post all our plans in the architectural forum. Should we start a thread and start posting? [edit: never mind, I see people have already started posting there designs: viewtopic.php?f=204&p=17824 ]

-Kwok

 

In reality, every material is stronger in tension than in compression. The most obvious example is to use a single piece of thread: You might cut your hand if you pull on it, but if you push on it, it will begin to fold (fail) with nearly zero pressure. I remember seeing a slow-mo video of a concrete beam in a similar bending test, to failure. The video clearly showed the TOP of the beam failing in compression first, allowing the rest of the beam to bend and fail.
When we test a design for strength at work, whether it's wood or titanium or unobtanium, we use the Instron tester and put the sample under compressive loads every time. That's mostly because our hydraulics can easily provide the 20 tons to push the sample pieces to failure, but the tensile loads would need to be three to ten times that. Also, when things break in tension, pieces go flying everywhere - compressive tests are much easier to "contain".
Great contest!

 

I guess it's my turn to eat crow. I said a few things about the rules, but in a nice way, I thought. But I can see it's bad for a moderator to appear gripping about what is a fun thing and so many have worked so hard to contribute. And also disrespectful to our host, Mark and Trish. I would like to offer my apology and try never to have it happen again.

-Kwok

 

Kwok, that's the spirit. Even if Tigers bridge falls into a gray area, I would rather have seen him compete then to sit it out. I know we all spent a lot of time thinking and deciding, and it would have been a shame to see anybody’s efforts go to waste. It was very cool how everyone stepped up and helped Megamega, I wish I was able to contribute as well, but instead I sat lazily in my living room watching, trying to keep my eyes open…

Dorsal, what you say may be true for some specific materials however is not the case for all.

Wiki shares this information, which is typical.

And that theory is what I based my efforts on.
Unfortunately bridges were failing from induced torsion, which results in another “shear” stress. Shear result sin both tension and compression, and it takes a careful analysis to determine which actually caused the failure. (many many factors will cause the torsion, an ever so slight lean to the platform for instance, or even a slight wobble to the bucket, or even internal stresses inside the foam caused when it was originally made).

 

I didn't mind anything grey about Tigers bridge, all I kept saying to myself after seeing it was that I should have thought of that. I really thought his roadway would fall out of the bridge before the sides would split. His also looked better than mine!

If you look back at the video on how my bridge failed, the top structure did fail in tension, the compresive load was trying to push the top either out or in and the shape made it easier to go in, so when the top stringers split everything else followed. So both of the top bridges failed in tension whereas most of the others failed in compression. Or at least that's how I see it! :geek:

I've already improved my design and am sure I can get several more pounds on it, Just waiting for the next bridge building competition!! :!:

 

Show? Oh no, I've got planes to fix!@!!

 

Well, except for generating Phlatcode for it, my improved design is done, and I only had to give up a half inch of height for all the improvements. Now, if I can only find some way to get it cut out to test it...
My 2 year old daughter only wieghs ~30 lb so it might be possible to actually have her in a bucket hanging from the bridge! I need to hurry before she hits another growth spurt...

 

id be willing to bet money that if two sheets were allowed, with all the knowledge gained from this competition someone could make a bridge hold ~60lb
that would be too cool
although it would need a lot of money and material and glue...
i guess that wouldnt work

 

Well, I can't cut it out myself, but I could send you the code if you want to test it on one of the future live shows. I know right now you have the toledo show to worry about but after that it could just be a part of one of the wednesday shows.

Of course if you do another competition in the near future I could wait for that, but otherwize I'm just impatient to see what it can hold!