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High-Performance Blades Sharing ideas for getting the most out of our steel.

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  #31  
Old 04-19-2006, 09:20 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Quote:
Originally Posted by SamLS
A leaf spring is held on two ends and loaded in the center. The ends are allowed to pivot. So in practice you would place the handle and tip of the knife on two blocks and load the center of the knife. How would a knife encounter that loading in practice? A distal taper would help that loading scheme but how does it apply to using the knife? I suspect a distal tapers true purpose is something other than strength.
SamLS, I agree that you do ask very good questions. I feel distal tapers effects are most evident in weight dispensation when the blade is long enough to notice (8" or longer in my experience), and in somthing as long as a sword it is absolutely critical. There is a lesser effect that is not as self evident in load distribution, as I can tell that kbaknife has it pegged quite well. If the sides of a blade run parralel then the thickness will be constant from tip to tang, giving an equal chance for any portion of the blade to be the point of yielding in a bend. Distal taper will create a situation in which any point thicker than the fulcrum will have an decreasing chance of yielding, while any point beyond this area will have a better chance but has a continuosly decreasing mass so the inertia in a strike (real world use) is decreased.

The conflict with the ABS test and this concept, is that the load is at the handle end, the fulcrum is the sole concentration of force and the thinner areas beyond are entirely static and out of play, it replicates absolutely nothing that occurs in actual cutting, chopping and other real world uses, unless one refuses to own a crowbar.

Another property that distal taper profoundly effects are the impact effectiveness and subsequent vibratory reaction of the blade in your hand. No I will not be using the word "harmonics", as I believe that is a very hyped up way of approaching simple concepts like a sweet spot. A huge difference will imediately be felt when cutting with long blades with inadequate tapers, they will bang like a bar of steel while flopping and bouncing about in a most unrully manner that will evetually tire your arm and hurt your hand. Well tapered geometries will negate these effects and are capable of concentrating the cutting advantage in those natural fulcrum areas we know as the sweet spots. I agree with Ed that this feature is almost lost entirely on a 4" hunter though.

As for impact testing, I agree with your analysis that it is much more relavant to the heavy impacting of chopping, so much so that I do have my own impact tester to study these effects. Such standardized tests are best used to determine material properties and the effects of heat treating upon them however, since the geometries are already determined by the Charpy or Izod specimen standards. I am currently working on a standardized impact test that will work directly upon blade edges and tell me most of what I need to know about how knives can handle the rigors of use. Too often folks like to use the concept of knife cross sections changing things to where standard tests are not applicable, I believe this is a flimsy excuse that doesn't hold water well, since steel is steel, the materials and their properties are the same and quantifiable, the cross sections just concentrate or lessen the effects of these universal factors.
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  #32  
Old 04-19-2006, 03:32 PM
SamLS SamLS is offline
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I understand what your saying Kevin. I suspected the distal taper was used to tune the center of percussion ( the techno geek name for "sweet spot" ) for swords etc. I suspect the location of the first two fundamental frequency nodes may also play a part. Proper tool design is a science all in its own and knife and sword making is no different I appreciate being able to discuss such on an open forum with experienced individuals. I have to give the ABS folks credit for the bend test though, I know they take some flak for it. I've seen alot of pocket knives with tips broken off and I've broken a hunting knife or two myself trying to split the pelvic bone on a deer.

Distal Taper:
How much is the blade actually tapered? Does it run right to the point or is it half the thickness of the blade. Let me know I'll draw up some models and post the results. Leaf springs are designed such to gain deflection for a given amount of stress ie tailoring the spring rate.

Last edited by SamLS; 04-19-2006 at 04:20 PM.
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  #33  
Old 04-21-2006, 04:39 PM
SamLS SamLS is offline
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I did some FEA's on a blade model. A distal taper blade is weaker no matter what the loading is than on that is not, and makes sense due to the lower area of the cross section for an 8" blade length. But, a distal taper can be effective at moving the stress concentration away from the tang simply due to the flexibility it adds to the blade meaning you'll bend the blade before breaking off the handle.
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  #34  
Old 04-22-2006, 02:00 PM
Ed Fowler Ed Fowler is offline
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Good Thoughts Sam:
This is why I went to what I call the modified Price Grind. The bladesmith can dictate strength, where and what for and how much. The geometry of complex convex grind is very stimulating.
You might like a book titled Cats Pars and Catapoults by Steven Vogel, all kinds of thoughts about the designs of nature, what for and why.
These are variables that need a lot of discussion, all too often we simply accept what is popular rather than explore and know why first hand.
Thanks for your interest and contribution.
Ed


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  #35  
Old 05-24-2006, 09:37 PM
Ed Fowler Ed Fowler is offline
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Some more information that came to light last night concerning the use of the torque wrench. We were testing some new events on a 5160 blade of Butches.. It cut very well, completed 10 edge flexes and no chip. The blade was about the same size and geometry as the one earlier reported on this thread.
Butch flexed a 5160 blade back and forth through six and one half 180 degree flexes. One side of the etched blade showed banding that is very close to the Wootz structure, the other side of the blade had some obvious banding but not nearly as much. Stretching the 'wootz' side through a full flex required 75 foot pounds, while stretching the side that did not show as much banding required 55 foot pounds to flex.

I have seen many blades that etched differently from side to side, but never did I feel that there would be a difference in resistance to flex. This information is based on only one blade, and the next blade may be different, but ---

I posted these results in order that those of you who are interested in making high endurance performance blades may find this information interesting.

The point is that the more we develop means of evaluating the characteristics of the blades we make, the more we may have the opportunito to learn.


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