Balsa or Bass

[lllazar]

So you cut your wood from sheets of balsa - what did you use to get straight pieces of specific size?

Also - how do u measure tensile strength? I thought there was a correlation between density and TS, but apparently not.

[old]

So you cut your wood from sheets of balsa - what did you use to get straight pieces of specific size?

Also - how do u measure tensile strength? I thought there was a correlation between density and TS, but apparently not.

We cut our pieces out of board with a small table saw. I believe you can buy them from Micro Mark but ours came from the local hobby store (actually it came from Germany and ended up in the local hobby store). I have seen good ones for somewhere between $100 and $200 dollars, although ours did cost more than that .

There is definitely a general correlation between density and both strength and stiffness but the correlation on individual pieces is very weak. In other words, while it is absolutely true that denser balsa is generally both stronger and stiffer than lower density balsa, it is also true that the variation from one piece to another is often greater than the general density vs. strength correlation. If you are going for the gold then you will need to probably cut 10 or more pieces for every one you intend to use. Then you will weigh each piece and test each piece for the characteristic of interest for that particular application. So if you are building your tower with 4 legs you may need to cut 40 or 80 of them, then weigh each one and test in in compression and see when it buckles (which is related to stiffness not strength). When you are satisfied that you have found 4 pieces that are strong enough to support the necessary load (you can use one of the structural simulators listed on the SO website) and are the lightest possible, then you can move on to the next piece. Calculating the required stiffness of cross bracing can be difficult because it requires that you know the Young's modulus of all the pieces, which is so variable for balsa as to be essentially undefined, so you have to mostly just guess and test the full structure (or a piece of structure that has bracing).

When we were using this technique a few years ago we could predict within perhaps a kilogram or so when our bridge or tower would fail. If you just build with random (untested) pieces you simply cannot predict reliably when one will fail. You could build a tower with just one piece that won't hold and the entire structure will fail. Conversely you could build a tower where almost every piece was heavier than necessary to support the 15 KG load.

[tying15]

I prefer bass. Amazing structures have been built out of both. It's not the wood, it's how you use it.

Very wise and I've only done science olympiad last year so I consider myself still a newbie. So thank's for all your suggestions.

[lllazar]

So you cut your wood from sheets of balsa - what did you use to get straight pieces of specific size?

Also - how do u measure tensile strength? I thought there was a correlation between density and TS, but apparently not.

We cut our pieces out of board with a small table saw. I believe you can buy them from Micro Mark but ours came from the local hobby store (actually it came from Germany and ended up in the local hobby store). I have seen good ones for somewhere between $100 and $200 dollars, although ours did cost more than that .

There is definitely a general correlation between density and both strength and stiffness but the correlation on individual pieces is very weak. In other words, while it is absolutely true that denser balsa is generally both stronger and stiffer than lower density balsa, it is also true that the variation from one piece to another is often greater than the general density vs. strength correlation. If you are going for the gold then you will need to probably cut 10 or more pieces for every one you intend to use. Then you will weigh each piece and test each piece for the characteristic of interest for that particular application. So if you are building your tower with 4 legs you may need to cut 40 or 80 of them, then weigh each one and test in in compression and see when it buckles (which is related to stiffness not strength). When you are satisfied that you have found 4 pieces that are strong enough to support the necessary load (you can use one of the structural simulators listed on the SO website) and are the lightest possible, then you can move on to the next piece. Calculating the required stiffness of cross bracing can be difficult because it requires that you know the Young's modulus of all the pieces, which is so variable for balsa as to be essentially undefined, so you have to mostly just guess and test the full structure (or a piece of structure that has bracing).

When we were using this technique a few years ago we could predict within perhaps a kilogram or so when our bridge or tower would fail. If you just build with random (untested) pieces you simply cannot predict reliably when one will fail. You could build a tower with just one piece that won't hold and the entire structure will fail. Conversely you could build a tower where almost every piece was heavier than necessary to support the 15 KG load.

Wow...i wish we had enough money to do that. And when you say test each piece, wouldn't the piece break in the process of testing - leaving you with no wood?

[jander14indoor]

Unless you are using bass, there is NO need for a mini-table saw to strip balsa. A sharp knife and a good metal straight edge are more than adequate. A properly tuned wood stripper (I think they may be up to $7 or $8 now) will pay for itself FAST by avoiding the cost of buying sticks.

Now, taking the time to individually select wood, and throwing out 90% as garbage, that can't be avoided at the highest performance level. You can use some of the less good wood for other purposes (jigs, gages, training structures, etc), but you will still find some stuff useless.

One way to reduce your loss rate significantly is to be selective in what you buy. If you can review the wood first, it won't take long to find that most sheets are useless for SO purposes. Too heavy, too wavy grain, grain defects, too soft, etc. Of course this only works if you have a local supply source, hobby shop, Michael's, Hobby Lobby, etc.

Note, your loss rate goes down as your performance goals go down. In other words, you can win most regionals with a much smaller waste.

Jeff Anderson
Livonia, MI

[blue cobra]

...A properly tuned wood stripper (I think they may be up to $7 or $8 now) will pay for itself FAST by avoiding the cost of buying sticks...

May I recommend from personal experience the Master Airscrew Balsa Stripper. I believe it comes to $11 and change with shipping.

[lllazar]

How do you use the balsa stripper exactly?

[AlphaTauri]

First, you find a nice workbench or piece of scrap wood, so you don't accidentally carve up anything nice. Then, set the balsa stripper to the correct width for the sticks you want to cut (anything thinner than 1/16 might be a little tough, though), and score the balsa. Don't try to cut through the whole sheet on the first try; take a couple of passes, lowering the knife blade with each pass until you have cut through the whole sheet. (This is why you need a workbench or scrap wood, if you set the blade too deep and score the cutting surface.)

At least, this is what I do.

[old]

We never had very good results with the balsa stripper except on very thin wood. I have heard that it can work but it must take steadier hands than either I or my partners had.

As to the comment about the cost of doing all this selecting. It is true that the saw was expensive but the wood cost very very little. I estimated that we spent less than $100 over a period of about 5 years of bridge tower and boomilever building. We bought bulk balsa sheets from internet sources (just search balsa) and made everything from the sheets. Remember that a good bridge or tower will only have about 5 grams of wood in it (hopefully less) so even if you throw away 10 times that you are only talking about 50 grams of wood. A good 1/8" 3"x36" piece of balsa will cost only a couple of dollars at most, and that piece will weigh about 25 grams (of course it could be much more or somewhat less depending on the density of the piece). So even with the wood you will throw away you will only need perhaps $10 worth of balsa to build a National class tower. If you include the cost of building 5 or 10 towers you should still be under $25 - $50 because you can buy the stuff cheaper when you buy in bulk.

I have to disagree about the comment suggesting that the design is less about what wood you use and more about what you do with it. Both are very important. You can pick your way through a mountain of balsa, or bass or spruce or eucalyptus, or whatever, and pick out the highest strength to weight ratio pieces, but then build a poor design with poor workmanship, and still end up with a lousy structure. But just as surely, even if you are the best designer and builder in the world you are not going to win with a tower built of wood that is heavier than what someone else is using. It is possible through very careful design to build a tower using high density wood (like bass), and still have it turn out lighter than someone else using lower density wood but it is difficult. With the higher density wood you will have to work with extremely thin pieces, which makes buckling (compressive) loads hard to handle (not impossible but difficult). In some tower designs a few years ago we had over 100 pieces of 1/64" balsa to help connect several beams into a cross braced girder. Each piece was so light that our 0.01 gram scale could just barely read them so we had no way to find which were lighter. In the end the cross bracing never failed, just the main beam (from buckling).

[jander14indoor]

How do you use the balsa stripper exactly?

OK, this comes up every year. Remember I said "well tuned" balsa stripper.
The master airscrew stripper is a nice product for the price, but low price brings low tolerances. Out of the package it works OK, but not great. Blade tends to wander, isn't straight up, etc.

So, what do you do? Tune it! Here's how, starting from easiest to hardest.
First, make sure you are always using a fresh, sharp blade, you can only strip so much wood with a blade before its too dull to cut.
Second, take that pointy #11 blade and break off about 1/8 inch or so of the tip. That's the first cause of wandering, its too flimsy.
Third, set the blade so it only cuts just a hair over HALFWAY through your sheet stock.
Finally, (and this is the trickiest, you get a lot of improvement in performance from the first three) square up the end of the beam the blade clamps against. This is a die cast part, so there's a little draft (angle off square) on this face to allow easy removal from the die. I have a jig I use in my model making specifically designed to sand the end of sticks square. I used this http://www.fourmostproducts.com/index.c ... &product=6 You might find it handy to make better glue joints, but its more nice to have than critical.

OK, now that you've tuned your stripper, how to use it.
Start by selecting good wood. Besides density and thickness, you also need straight grain. Curvy grain will have odd spots of weakness, avoid it unless you need a curved piece (unlikely) and then cut along the grain to get your curve.
Next prepare the sheet.
- Its harder to strip a long piece so if you don't need it, cut that 36 inch sheet in half to an 18 inch piece. For the tower legs, cut it to maybe a centimeter or so longer than the longest pieces. Use the short end for bracing.
- Again, refer to the grain. It might be straight, but slant along the factory sawn edges. Those edges might also be dinged from handling. Take a nice long steel straight edge and a good sharp knife. Trim the edge to follow the grain, straight and square. Your stripper will follow every imperfection in this edge, eventually exagerating them resulting in wavy sticks.
- Like another poster said, find a nice smooth surface to work on. With the blade set to half the sheet stock's thickness, you actually don't need to worry about damaging it.
- Now, prepare yourself. If you don't pay attention you won't get good results. You may also slice your fingers. Stand so your arms are free to move without hitting things, especially for longer pieces. Short pieces can be stripped with just wrist or forearm movement, but long pieces need freedom for the arm to work. Not hard, just smooth.
- Secure the sheet with one hand (or maybe tape, or a jig) so it doesn't move. Note, I've never really found stripping to be a two person job, the person holding the sheet moves unexpectedly to the person doing the strippping. You might have more success, but...
- Start at one end, and run the stripper down that prepared edge in a smooth continuous motion keeping it pressed tight to the sheet (oh, you did set it to cut the width you wanted, I hope). You've cut halfway through.
- Flip the stock over end for end and run the stripper down that prepared edge again. Your strip should now pop free.

For hard or thick stock you may need to vary this a little and only cut say 1/4 through at a pass. Don't cut deeper by resetting the blade though, too time consuming.
Instead, set it to cut half way as before, but pick up the LEADING edge of the stripper as you run down the sheet the first pass just scoring the wood. Then run it down again with it against the table cutting fully half through.
Alternatively, set the stripper to cut only 1/4 way through the sheet. Strip as before, but the strip will still be attached. Take a piece of sheet half the thickness you are trying to strip and set the original sheet on top of it with the edge clear of the underlying piece. Run the stripper down the top and bottom again to complete cutting through.

With these techniques (and yes, a little experience, but I can get newbies up and running pretty fast) I can strip almost ANY wood. Course stuff harder than balsa takes much longer. But frankly, if I can't do it in four cuts, that wood is probably too dense for this event ANYWAY.

Hope that's clear

Jeff Anderson
Livonia, MI