Elevated Bridge B/C

[sewforlife]

What wood could you possibly be using that's so hard to cut? It seems ot me, any wood that you would need a razor saw to cut, would not be appropriate for SO structures... No?

It wasn't exactly the hardness of the wood, it was more like the bass was really dense, and my blade was pretty dull, also my patience was wearing out.

Does anyone know if exact-o knives are supposed to snap under pressure? I don't think that's a good sign.

All X-Acto blades will snap under enough pressure. Even the ones in the large handles, the blades aren't much bigger. To hold an edge, steel must be very hard, which means it is also brittle, not ductile. Takes more force to break, but goes bang quick when it dies, not much bending.

So, NEVER force a cut. If its not cutting easy, cut more often, don't do it all at once.

Or better yet, instead of cutting large/hard pieces with a knife, consider a razor saw instead, see: http://www.hobbylinc.com/htm/xac/xacx75300.htm for one example. Cuts faster with MUCH less effort, and much less damage to the wood. Also easier to control. All in all, safer and faster.

Thanks,

Jeff Anderson
Livonia, MI

thanks. I might consider buying one of these.

[jander14indoor]

What wood could you possibly be using that's so hard to cut? It seems ot me, any wood that you would need a razor saw to cut, would not be appropriate for SO structures... No?

Not really commenting on what wood appropriate to the event, just how to cut the wood described safest, fastest, most accurately, etc.

And just my experience, but:

I find large soft balsa easier/more accurate/quicker/less damage to cut with a razor saw than knife. I can cut soft balsa quickly with a sharp knife and crush the ends terribly. I can take many cuts with a sharp knife and not damage it. Or I can cut it quickly, accurately, and without damage with a razor saw faster than a knife. Dull blades are right out for soft balsa for anything other than destroying good wood.

Almost any size but the smallest bass I can cut it much faster with the razor saw, and even with the small stuff I don't save much time with a knife. For any size at all, the razor saw beats a knife in bass or spruce by miles. Frankly for large hard balsa a razor saw beats a knife.

<SNIP>It wasn't exactly the hardness of the wood, it was more like the bass was really dense, and my blade was pretty dull, also my patience was wearing out.
<SNIP>

Hardness and density tend to go hand in hand with wood.

NEVER use a dull blade, leads to excess force which just leads to cuts. Almost every cut I've had with an X-Acto was due to a dull blade, not a fresh sharp one. Plus it tends to butcher the wood. I buy blades in the hundred pack now. Not so much because it saves money (it does) but to make sure I'm never tempted to use a dull one because I'm down to my last blade. Blades are cheap, cuts painful, stitches painful and expensive! I could replace every blade I've used in the past 40 years ( I couldn't say fi... fif... fift... a whol bunch of years of life) by avoiding ONE trip to an emergency room (spurting blood in front of a Wright Stuff class and 7 stitches!).

When patience is out, its time for a break (that emergency room trip wasted a LOT more time). The rest of my injuries with X-Actos (not related to dull blades) were directly related to being in a hurry. Heck, even some of the dull blade cuts were a result of being too impatient to change a blade.

Be safe, change blades often

Jeff Anderson
Livonia, MI

[sewforlife]

Thanks for the advice. I need to buy a new exact-o-knife.
I'm testing my three point load bridge tomorrow, which decides if I'm on the team or not. My brother and I came up with this really nice design that distributes the weight evenly. When we tested it the first time, the bottom piece of balsa was a squiggly line sorta like this: ~~~~
Can someone explain what that was?

Wish me luck.

[jander14indoor]

<SNIP>
When we tested it the first time, the bottom piece of balsa was a squiggly line sorta like this: ~~~~
Can someone explain what that was?

Wish me luck.

I'm assuming it got squiggly while loading? Did you run an analysis on your bridge? Is this member loaded in compression or tension?
If compression (which I'm guessing), its getting read to fail by buckling. If your bridge failed there, you need to increase the cross section of the member while not increasing weight. Think Box beam or I beam. Or you need more cross braces to shorten the unsupported run.
If tension, this probably indicates you are getting twisting from the joining members

Good Luck

Jeff Anderson
Livonia, MI

[nejanimb]

If it's a bottom piece, it probably is a tension load or some kind of bending - I can't think of any designs that would load a bottom chord in pure compression. More than likely that's because of uneven load due to having that triple-point design, or some issue in construction that yielded unevenness.

Good luck with your test! I hope you make the team.

[sewforlife]

<SNIP>
When we tested it the first time, the bottom piece of balsa was a squiggly line sorta like this: ~~~~
Can someone explain what that was?

Wish me luck.

I'm assuming it got squiggly while loading? Did you run an analysis on your bridge? Is this member loaded in compression or tension?
If compression (which I'm guessing), its getting read to fail by buckling. If your bridge failed there, you need to increase the cross section of the member while not increasing weight. Think Box beam or I beam. Or you need more cross braces to shorten the unsupported run.
If tension, this probably indicates you are getting twisting from the joining members

Good Luck

Jeff Anderson
Livonia, MI

It was in compression. Yeah, it started to be like that after around 5 pounds or so. Yeah, I'll keep in mind an I beam. thanks!

If it's a bottom piece, it probably is a tension load or some kind of bending - I can't think of any designs that would load a bottom chord in pure compression. More than likely that's because of uneven load due to having that triple-point design, or some issue in construction that yielded unevenness.

Good luck with your test! I hope you make the team.

Actually, I tested in the middle, so it wasn't really an uneven load. Probably the construction.

Does anyone know exactly how long at min. I should dry my bridge before I test it? (24 hours at the least I'm guessing?)

I'm going to run through my testing.
We loaded at point C. My bridge had a mass of 8.5 grams. (My stomach was dropping by the second, of course)
As we loaded, I had a feeling that it wouldn't do as good as my test at home. At home, I use a smaller bucket, a 3-ish gallon bucket, and my bridge held about until it was full.
In the actual test, I felt my bridge snap WAY too early.
It turns out, that it held about 4.5 kg.
That gave it an efficiency of 522 (about 80 points better than our test at home-I'm happy about that. )
Thanks for your advice. The bridge place 2nd out of 6 partnerships

[smitherzz6]

im doing this event and i dont know the matierials i need can you help?

[croman74]

Look at all of the links posted on the original post of this thread. They should all help.
Has anybody been building and testing bridges with the new rules?

[smartkid222]

im doing this event and i dont know the matierials i need can you help?

for building you just need wood, glue, things to make jigs/plans out of, rulers, razor blades, sandpaper, stuff like that.
is that what you're asking?

[AlphaTauri]

Look at all of the links posted on the original post of this thread. They should all help.
Has anybody been building and testing bridges with the new rules?

I've built one and am working on plans for a second. I haven't tested any yet because SO here doesn't start until November. The one I have made is 3/16 and 1/8 balsa with lateral bracing made out of 1/4 by 1/16. It's basically like the one you see on the EB rules, only with more of those diagonal things that keep the elevated span from collapsing (I can't remember what they're called).
__________________
/_ \_ /_ \_ /_ \_ /_ \ <- these things

Does anyone know a good way to get CA off your fingers? Or a way to avoid sticking your fingers to the bridge in the first place?