Primitive Archer
Main Discussion Area => Bows => Topic started by: Blacktail on November 06, 2011, 09:39:46 pm
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how do you guys go about doing a sg test on wood...what is the process....and is there differant ways of doing it....john
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John, typicaly you would oven dry a 1 cubic inch piece of wood and then float it in water. Slowly lower it into the water and note what percent floats. If 30% is above water your wood is .70 sg
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Take a cube of any size, or a rectangular piece, or any piece that you can measure and calculate the exact cubic dimensions. Measure it in metric, figger out how many cubic centimeters (or cc's) you have. Then weigh it out in grams. A perfectly square cube is easiest.
Now, divide the grams of wood by the cc's of wood. That tells you the weight per cc.
For example, it measures 4 centimeters per side and is cube shaped. 4x4x4=64cc. It weighs 48 grams. 48 divided by 64 = 0.75, so your specific gravity of that piece of wood is .75, got it?
Specific gravity is based on water, the metric system is based on water, too! One cc of water weighs exactly one gram. The specific gravity of water is 1. If it floats, the specific gravity is less than one, if it sinks it's specific gravity is more than one.
You can call me Mr. Wizard, if you like.
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I like that Mr wizard. I never knew 1 cc of water weighed one gram, very helpful.
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This may be a dumb question, but what is the signifigance of specific gravity when it comes to building bows? ???
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I like that Mr wizard. I never knew 1 cc of water weighed one gram, very helpful.
;D ;D
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This may be a dumb question, but what is the signifigance of specific gravity when it comes to building bows? ???
Woods with higher specific gravities are more dense, and tend to make better bows, because they resist bending. That is, until their SG is >1, when they become too heavy. Like teak, or ironwood. The wood is so heavy, the limb mass becomes too great. Woods like osage, with a specific gravity of .75 - .80, make great bows. Soft woods like pine, with a SG of <.40, don't make great bows. SG is a guideline. Yew is an aberation. It has lower SG, but can be fairly dense, and as we all know, makes a great bow. Beside SG, properties like a wood's elasticity are important factors.
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Thank you so much for the explanation and patience with a newb like me.
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In the lab where I work I have an oven for moisture testing of wood. I take sample wood (unimportant shape and dimensions), let it 24h on 100C to be absolutely dry and then place it under water in laboratory glass. When I know its volumen I measure its weight and calculate SG.
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Once it's dry, you just need a long rectangular piece of the wood that you can slip down into some water in something similar to a test tube which is just wide enough to allow the wood to float upright. The percentage of it that is below the water line is your SG, simple as that. Measure the cm below water line divide by total length of wood. I've even premarked the wood into ten equally spaced marks to make it quick and easy. The longer the piece of wood the more accurate you can be with it. This can be actually by far the most accurate way of measuring sg that i've seen. They have something similar to this in TBBI.
Drop the wood carefully and slowly so it never dips below it's flotation equilibrium, and the water line left is your mark. Even if the wood leans a little, you just take the midpoint between the deeper edge and shallower edge for your underwater length. All this is provided your piece is a regular rectangle.
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Well sure, stixman, if you wanna do it the EASY way! >:(
Don't let Specific Gravity be the master of your decision making protocol. Osage is dense, but yew is definitely not a dense wood. You wanna see a fight, call out the yeller wood guys against the orange wood guys and stand back!!!
SG within the normal range in a species is a more credible deciding factor. The more dense a wood is FOR IT'S SPECIES the better the bow wood. A great yew stave in the heavy end for it's species is still much lighter than the lightest of osage...and the yew will beat the tar outa the osage in that case.
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I would think measuring is the most accurate, I often use water just for an estimate, it is close enough I believe.
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stixman, why it must be rectangular, I do not see reason? Any other shape (round or what) will do the job, right? If you use needle to hold it under water it will not change volumen of water pushed up and you will have accurate datas.
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A cylinder will work as well, or even a triangular prism (any shape that has a base/cross-section that is a regular polygon), but notice in this method you're not pushing it under water to determine its volume, you're simply lowering it until it floats by itself. Measuring volume and mass by hand have inherent instrument and human error, so the more steps and the more human, the more error ::)
anyhow, you'd get close to the same either method, direct SG measurement is just easier IMO. There's a reason SG exists, and it's due to the ease and accuracy of the method. It's a fundamental rule of physics that applies (as Badger noted), that if an object floats in water, the percent of it's volume that is under water is exactly it's SG.
If you're using a tree stave instead of lumber, it's also difficult to cut a piece that is measurable with a ruler unless you do cut it down to dimensions. I just take a sliver of the wood that is long and has the same cross-section all along it's length. Longer pieces have less error than shorter pieces.
JW, sorry, i'm a little lazy at times ;) I totally agree with you on the SG issue, there are wonderful relatively low-density woods as well as high-density woods, and my experience with oak has been as you mentioned-- higher SG oak performs better than low SG oak :)
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You can measure it or you can look it up on line. SG is the same numerical value as density...just without units. I know it varies from tree to tree but build in a little wiggle room in the design. The less dense the wood the wider it needs to be. The one time I calculated the density I sawed out a rectangular piece; I measured the length, width and height in cm. I multiplied the values together. I weighed it and converted the weight to grams. There is the density and the sg as I mentioned above. The value will be slightly off as Badger mentioned because of the moisture content but close enough for my needs. Jawge
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Stixman, you are very wrong. You can not always get symetrical shape for SG testing and errors are large while floating method. The way I am talking about is much more accurate, doesn't affected by human.
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The problem with any shape other than rectangular is that you have a real difficult time determining what percentage is under water.
I use George's method, look it up on line to get a close SG. After I have worked a specie of wood a couple of times I can usually tell if it is dense for its species by the way the tools work.
Like was said, there are exceptions so trying new woods even if the SG is not what you expect is a good idea. Yew has a low SG but is good as mentioned. Ipe has an SG at or over 1 many times but rather than being to heavy, it is incredible.
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Ultimately, when you get to the bottom of the sheet of paper and have covered every last inch with calculations, graphs, and notations....what are you gonna do?
BUILD A FLIPPIN' BOW AND SEE IF IT SHOOTS! ;D Right?
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Druid: I respectfully disagree. Any time you use an instrument, there is inherent error in the instrument, and there is always inherent human error in reading any instrument, even a simple ruler. The more measurements you have to take, the more the error is compounded for any method. My method is not error free, i just feel it has less potential error than other methods if done correctly, and doesn't require a sensitive scale or precision ruler. But that's more science philosophy than anything.
That being said, whichever method you use, the error involved won't affect the results so much as to make you work your bow differently. All you really need is to get an idea whether your stave is low, high, or average for it's species. The difference would have to be dramatic to affect your treatment of the wood. I like the method i mentioned because it's quick, simple, and accurate enough for the bowyer's purposes. I haven't found it very difficult to find/cut pieces suitable-- from lumber or staves. I've only done SG testing twice, more out of curiosity than anything else.
I truly admire the work you do on your bows, they are always very impressive, so let me emphasize this as a respectful disagreement ;D
But in the end I agree with JW, let's go shave some wood 8)
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...there is always inherent human error in reading any instrument, even a simple ruler.
Ain't that the truth! I went home the other day and used the cm3/gram=SG formula and was consistently getting results about 30% higher than I should have been. Of course, that's probably because I was using a tape measure with 1/16" increments and then converting to metric. Guess I shouldn't be trusted with a simple ruler ;D
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g/cc or centimeters cubed for density as it pertains to SG. Jawge