Balsa or Bass

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Re: Balsa or Bass

Post by blue cobra »

jander14indoor wrote:...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.
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Re: Balsa or Bass

Post by lllazar »

How do you use the balsa stripper exactly?
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Re: Balsa or Bass

Post by 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.
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Re: Balsa or Bass

Post by 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).
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Re: Balsa or Bass

Post by jander14indoor »

lllazar wrote: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

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Re: Balsa or Bass

Post by lllazar »

Wow thats a lot thanks Jeff, a few more questions -

How should i break the 1/8 of the blade off? Isnt it sharp :shock:

And how do i "set the blade so it only cuts just a hair over HALFWAY through your sheet stock"?

Thanks! I feel like a newbie all over again :)
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Re: Balsa or Bass

Post by brad123664 »

Does anyone know of a good website to purchase known density balsa wood. I found http://www.specializedbalsa.com/ but they are very pricey.
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Re: Balsa or Bass

Post by quizbowl »

brad, is that you? did mr. diers tell you to ask it?
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Re: Balsa or Bass

Post by jander14indoor »

lllazar wrote:How should i break the 1/8 of the blade off? Isnt it sharp :shock:
Carefully? Safety glasses would be good. I use two pair of pliers. Hold blade firmly with one so only 1/4 to 1/2 an inc of tip is exposed. Grab the tip you want to break off with the other. Give the blade a sharp bend like you want to fold it in half. The metal is very hard and brittle, it will break like glass, kind of. If you are unsure about this, or tend to be clumsy, have someone with steady hands, or a teacher do it. Not as scary as it sounds, but yes, the blade is sharp so be careful.
lllazar wrote:And how do i "set the blade so it only cuts just a hair over HALFWAY through your sheet stock"?
Depends on the stripper you are using. If its a Master Airscrew, the blade is held on the end of the arm by a plate secured by two screws. Get a piece of hard sheet stack half the thickness you are preparing to strip. Loosen those two screws so the blade can be slid around. Set the end stripper on a flat surface, place that half thickness wood under the blade so it slides it up off the surface. Resecure the blade holding screws. May take a time or two to get it just right, this thing is good, but not a real precision piece of equipment. Most other strippers will be set similarly.
brad123664 wrote:Does anyone know of a good website to purchase known density balsa wood. I found http://www.specializedbalsa.com/ but they are very pricey.
Unfortunately, if you want it cheap, you don't get to specify density. On any site. While it doesn't take long to sort through your stock, asking the supplier to do it adds labor someone has to pay for and makes the remaining stock LESS valuable. You can save some money by only specifying ranges of density, but if you want specific densities, the prices go up fast.

Hope that's clear.

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Re: Balsa or Bass

Post by old »

brad123664 wrote:Does anyone know of a good website to purchase known density balsa wood. I found http://www.specializedbalsa.com/ but they are very pricey.
Essentially the only thing you get when you pay more for balsa is known density. You are paying for the work involved in weighing and marking the density. Personally I don't see a lot of advantage to getting known density balsa and here is why. 1. You don't really know what density you will need until you test the pieces. 2. The density varies so much even within a single piece that the density figure you are getting is only an average of the densities of the various parts of the board. The one exception to the known density balsa question is when you know you need some very low density balsa. There are a number of companies that sell very low density balsa as a special item. One company calls it contest grade, while others just call it low density or actually specify that it is guaranteed to be below a certain density (usually somewhere in the < 4-6 lb/ft3 area). Buy a few boards of low density balsa, slit it or cut it with a micro saw and then test each piece for weight and strength. The cost per stick will be very low and you will have a much better idea of actual density and strength of the pieces you have.

By the way there are companies that sell balsa to the highly specialized F1A rubber powered plane market. They have balsa as thin as a few thousandths of an inch and of very low and measured density. There is even a company that sells this type of balsa tested for stiffness (Young's modulus), although their measurement is subjective and rough, but the cost is astronomical. Just for fun we bought a couple of pieces of the stuff 5 years ago. The cost was something like $25 for a 1.5" x 18" piece that was 0.024" thick. We never could find a use for it but it sure was fancy.
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