tongueriver
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Dogstar is emphatically correct on all counts. Additionally, we must remember, if we are to survive in this world, that every profession sees its share of incompetence.
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My newest tool, the Mega Pry!
- Thread starter olaf_anderson
- Start date
Dogstar is emphatically correct on all counts. Additionally, we must remember, if we are to survive in this world, that every profession sees its share of incompetence.
I am a licensed structural engineer, Summa Cum Laude in structural engineering, 38 years experience, and there is no gray area here, it is clearly weaker in any form of bending with the grooves milled in it. Any person with a proper background in structural engineering will tell you this. If they tell you otherwise then that is evidence of what kind of background they have.
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So strength to weight ratio would be improved as with a distal tapered knife blade?
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^^He is right. Having said that, the ball groove thinngo looks awesome and DOES increase the strength to weight ratio, and considering people have to carry this I would say it is a good thing. Also from the looks of the groove from the photos they are quite shallow and would produce a negligible effect on the tool's strength.
Also, why should we be nit picking over this, the guy came up with what looks like an awesome tool and a great idea, we should be encouraging not disouraging.
James.
Also, why should we be nit picking over this, the guy came up with what looks like an awesome tool and a great idea, we should be encouraging not disouraging.
James.
I like his tool. I even said so. As for "nit picking," facts are important. The spread of misconceptions is best nipped in the bud, no matter what or where.
Very cool concept. I like the idea of having a prybar that can be compacted into an easy to carry package. I travel a lot with work and have need for such things when out in the field. As far as the misconception that the removal of the material makes it stronger, it really does need to be clarified to both Olaf and anyone else that erroneously believes it. Not because one wants to discourage Olaf, but, believe it or not, because it actually benefits him and others, offering constructive criticism so that he can design and market his innovative designs based on accurate science.
Since removal of material in the manner Olaf describes would make it lighter, yet still maintain strength, I am wondering how much could be removed to make it lighter for traveling, yet still be functional for medium duty jobs.
Since removal of material in the manner Olaf describes would make it lighter, yet still maintain strength, I am wondering how much could be removed to make it lighter for traveling, yet still be functional for medium duty jobs.
fishface5
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trying to grasp the physics of I-beam/fuller I found this helpful video
https://video.search.yahoo.com/vide...a&sigb=130v2mu7g&hspart=mozilla&hsimp=yhs-001
An I-beam gives more strength than the same amount of material made into a solid beam, but less strength than a solid beam the same size as the I-beam. If you need strength level X and don't have space limitations, a bigger I-beam will meet X using less material than a smaller solid beam would need. BUT NOTE that "fullers" such as used in knives don't operate like I-beams for purposes of prying, because they are on the wrong "sides" of the tool, they are on the faces that are being stretched/compressed. They decrease rather than increase the surface area on the outermost planes, weakening the structure. They do however reduce weight more than they decrease strength, but only up to a point so with diminishing returns it can be taken too far.
As for the tool it seems like a cool idea but I was wondering who would find it's relative compactness worthwhile over a full bar. But LabMonkey's post shows that there is indeed a market for such an item, at the right price of course.
https://video.search.yahoo.com/vide...a&sigb=130v2mu7g&hspart=mozilla&hsimp=yhs-001
An I-beam gives more strength than the same amount of material made into a solid beam, but less strength than a solid beam the same size as the I-beam. If you need strength level X and don't have space limitations, a bigger I-beam will meet X using less material than a smaller solid beam would need. BUT NOTE that "fullers" such as used in knives don't operate like I-beams for purposes of prying, because they are on the wrong "sides" of the tool, they are on the faces that are being stretched/compressed. They decrease rather than increase the surface area on the outermost planes, weakening the structure. They do however reduce weight more than they decrease strength, but only up to a point so with diminishing returns it can be taken too far.
As for the tool it seems like a cool idea but I was wondering who would find it's relative compactness worthwhile over a full bar. But LabMonkey's post shows that there is indeed a market for such an item, at the right price of course.
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Well actually this isn't correct either. The material being removed is at the "extreme fiber" and thus the material providing the most strength. Removing it removes a higher percentage of strength than the percentage of weight it removes, reducing the strength to weight ratio. But it is still stronger than an equivalent weight bar that has no grooves (the equivalent weight bar would be thinner overall.)
For an example- here are some numbers.
Case 1: 1" x 1/4" bar
Section modulus (strength) = 0.0104 in^3, cross sectional area = 0.250, strength/weight = 0.042
Case 2: 1" x 1/4" bar with 1/4" x 1/16" rectangular groove milled in each 1" face.
Section modulus - 0.0081, cross section area = 0.219, strength/weight = 0.037
With the grooves- strength went down 22%, weight went down only 12%, strength to weight ratio went down 11%
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OK, So kindly tell me about this concerning a distal tapered knife. Does this in fact improve the strength to weight ratio of the blade by distal tapering it, over another blade of equal weight that isn't distal tapered?
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I am not a structural engineer, however, this is a great idea and you are getting solid though somewhat callous at times feedback. Good job sir, keep up the good work!
It appears you are only comparing section modulus in the "weak" direction... like a knife being used to pry. Here, the I-beam is at a disadvantage.
If you compare section modulus in the the "strong" direction, like a knife being used to cut:
Case 1: 1" x 1/4" bar
Section modulus (strength) = 0.0416 in^3, cross sectional area = 0.250, strength/weight = 0.166
Case 2: 1" x 1/4" bar with 1/4" x 1/16" rectangular groove milled in each 1" face.
Section modulus - 0.0413, cross section area = 0.219, strength/weight = 0.189
With grooves, strength decreased 1%, weight went down 12%, strength to weight ratio increased 14%
Actually, I see why you left this part out, as it applies to the half of the "Mega-Pry" that is shaped like a chisel. However, the strong axis analysis applies to the crowbar half of the "Mega-Pry." I don't want people to get the impression that an I-beam is only weaker than a solid beam. It is weaker in one direction, but stronger in the other. Otherwise, it would not have been so widely used in construction.
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We are talking about cross sections of different shape ("I" vs. rectangle). Distal tapering keeps the same cross section (a rectangle for knives), but shrinks the cross section as you approach the tip. Distal tapering has more to do with cantilever beam theory, where the tang of the knife is considered to be "fixed," while the tip is allowed to bend freely. There is less stress at the tip, which allows for less material to be used. Now, if you are prying, the tip becomes the fixed end, and then you know what happens next.
BTW, I'm not the greatest engineering degree holder to walk the earth, far from it. I'm just trying answer your questions as best I can.
Yes, bending in the weak axis direction will be the critical direction and I assume people would use the product for prying either direction. And actually when I worked in a garage I used a standard crowbar most often in the straight direction rather than the hooked direction. Strength of the bar in the strong axis direction would not be critical since the hook itself would then be the limiting factor.
Strength to weight ratio in the strong axis direction is a different story- removing material on the sides is removing the material that is doing the least for bending strength and thus removing that material improves the strength to weight ratio.
This depends a lot on how the knife is used and how forces are applied to it.
Considering prying, where you stick the tip of the knife into something and then pull sideways on it: A tapered blade will have better strength to weight ratio because you have removed material that isn't contributing to strength. In other words if you are prying the blade wants to break near the tip, the remainder of the blade is stronger than it needs to be.
Considering some other form of loading, maybe incidental loads that would occur if you are hammering with the blade or batoning it through a log- you could have high bending forces at various points along the blade and thus a tapered blade would probably be more prone to breaking.
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Wow, it looks like I was absent from the internet for too long. I admit that it made no sense to me that the ball groove would make the shape stronger but at the time it seemed wise to defer to the fabricator with decidedly more experience than myself. That being said, I prefer the look without the groove but I find that many people like its aesthetic and reducing some weight is a good thing. The prototype is still not back yet and I haven't heard from the facility in a week as of tomorrow. I know that my primary contact there is at SHOT show so I may not hear from them for a few more days. Soon as I have it in hand I'll be spending several days trying to kill it. If all goes well then I'll have new pictures and the project on Kickstarter will launch. I appreciate all the spirited replies here, some of the material is outside my experience but I value the insight. As yet I have not found a suitable carry method for them, am trying different ideas with Kydex, rubber O rings and sewn Nylon pouches. The price is still going to be somewhere between $100 and $150, not all costs have been nailed down by the fabricators so we'll see. Thanks for the support and stay tuned!