What are the attributes of a high performance knife?

Ed, the reason I asked about strength is that it is what you have to sacrifice both materials and geometry wise to get a high flex point. A differential tempered blade for example is a lot weaker than a fully hardened blade. A distal taper, or dual taper as you have commented on recently, does allow a very high degree of flex, but does so by reducing the cross section in the extreme parts of the bend and thus maximum deformation that the blade has to take to accommodate a given curvature. This also directly weakens the blade. I have seen very cheap blades that could easily flex to 180 degrees repeatedly without fracture. They did this by having a full flat grind, distal taper, and thin stock and thus would see very low curvature even on high angle bends . However I could easily bend them the 180 degrees with just wrist torque. Now as to which is better strength or flexibility, that is up to the user, my point is simply that there is a tradeoff and it would be informative to know its extent.

In regards to the flexibility in general, you might want to start looking at curvature radius instead of the angle the blade is bent. For example a cheap Ontario machete will bend to 180 degrees, does this indicate a large degree of flexibility in the steel (high elasticity and ductility) ? Well no, not really. In fact I have even taken some machete brands to past that, approaching a full circle, touching the handle off of blade and they are actually even usually fully hardened. However there is a more critical factor than the inherent properties of the steel. The blades can be so far bent without fracture because of the extreme length of the blades involved (18+"). The curvature induced is actually less extreme than a 90 degree bend on a much smaller blade.

In general, the following are what I would call high performance, meaning that it approached the best that I have seen so far, or is at the limit of what I think is possible based on what I have seen :

Sharpness :

- <50 g to cut light thread
- push cut through /4" poly with 1000 g

This is about 2-3 times as sharp as a "shaving sharp" production blade like Cold Steel, Spyderco etc. . A blade that can do the above will shave free standing hair, push cut very light paper, etc. . This is at the level of sharpness of razor blades I have tested.

Shallow cutting ability :

- rocking push cut through 3/8" hemp with <15 lbs of force
- cut 1" off the end of a 1" hardwood dowel in 2-3 slices. Users strength level comes into play here obviously, this number is based on mine

Chopping ability (deep cutting performance) :

-get 50% better penetration on fresh pine than a Gransfors Bruks Wildlife Hatchet on a per weight basis without excessive binding in the wood

Point penetration :

-2 pages per gram of knife weight into a phone book, vertical stab
-dig through a 2x4 in under 20 stabs and 2 minutes, must be able to fit a 1x1" piece of board in the hole and rotate it around

Point strength :

-must not break nor bend in the 2x4 digging
-suffer only minor impaction or bending on heavy stabs into bone

Blade strength :

-must not break nor bend in the 2x4 digging
-torque fracture point >3000 in.lbs

Blade flexibility :

-should be rated by induced curvature not raw angle of bend to elimine skew due to blade length influence
-for example a 10" blade should be able to bend to 60 degrees without taking a set
-should flex significantly past this point without breaking only taking a set (a 10" blade should flex to 90 before fracture)
-geometry is as important here as the ductility and elasticity of the steel

Handle ergonomics and security :

-can do all of the above without discomfort or loss of control even when grip is soaped/oiled

Steel aggression :

-at a high polish (8000 grit waterstone, ultra fine ceramic etc.) can slice through the 3/8" hemp with significantly less force than it takes to do a push cut, about half is a decent ball park.

Class I edge retention (deformation) :

-with a high polish (last paragraph), can cut through 254+ pieces of 3/8" hemp rope without exceeding 400 g on the thread cutting, nor 4 cm on the poly. The latter is the level of sharpness that it takes to slice photocopy paper without tearing. The slicing of the hemp is performed over 2" of blade with an equal vertical draw along the cord, not a push cut. I think 510+ might be possible with some of the higher steels, I have not looked at them in detail yet.

-with a coarse finish (600 grit DMT), must do the same but on this time cut through 510+ piece of the rope. I think 1022+ might be possible with some of the higher steels, I have not looked at them in detail et.

Class II edge retention (wear) :

-cut a decent amount of abrasive material like fiberglass insulation or used carpet, have not done enough work to judge good performance in this field yet

Class III edge retention (fracture) :

-1000+ heavy chops into medium woods without losing the ability to slice photocopy paper
-will not suffer visible edge damage on knots in such wood

Class IV edge retention (corrosion resistance) :

-will perform highly after soaking in salt water, have not done enough work to judge good performance in this field yet

Edge durability :

-can chop heavy bone without visible damage, method and bone type are critical
-can function as a bill hook on hard/frozen limbs with a 2' handle extension, no significant edge deformation (sub mm)
-heavy chops into concrete will only result in edge damage, no penetration into primary grind. This is for those times when you accidently whack the blade into something that you wish you hadn't

Sheath functionality :

-holds the knife securely and at the same time is easily released and sheathed

Sheath durability :

-can take repeated drops on concrete from 25 feet in sub-zero weather (-30)
-will not deform in high heat
-will not rot in high humidity
-will not take anything beyond surface wear in very abrasive conditions

I have seen all of the above in various knives, not all in the same knife. Some I don't think may even possible in the same knife and a lot of it is user dependent (flexibility vs strength). The biggest problem is cutting ability vs durability. For example to get the high level of cutting ability as specified by the above (15 lbs on 3/8" hemp for example) you have to go with very thin and acute edges, that is going to take a very high end steel to withstand the bill hooking. Some are not relevant to various knives. A high performance fillet blade for example has no need of a 3000 in.lbs torque limit, nor does it need to be able to be used as a bill-hook etc. . There are aspects I have ignored in the above and some of the tests are really vague as I have not done enough work with various blades to know what is high performance is yet. Suggestions welcomed as always.


-Cliff
 
Hello Cliff: You bring up some good points. The exact geometry, length of blade and photos of the test Blade will included be in Knife Talk in the next issue of Blade (about 30 days from now).

We must remember in this discussion we have examined only a spelcific aspect of what I call Functional Balance of the using knife. Every aspect of the knife must be considered in relation to What For? Why? and How?. To evaluate a knife one the basis of only a few aspects is like judging a concert on a single note. The functionaslly balanced knife is much more than a tee totter balance point, but much more significantly the whole knife carefully evalauted in relationship to man and task.
 
Cliff did you really mean torque or bending on your 3000 inlbs. Sorry to nit pick but can't help it this time (being a structural engineer), there is quite a bit difference. Some of the strain/thickness/radius/length relations are linear some not, anyway it should be taken into account if bending is used as a test. It is a very practically oriented test and a usefull criterium for heavy use knives but both material and geometry should be considered. I once got into somewhat heated discussion on the ABS bending test, the opposition chose to totally ignore the geometry.

I like Ed's attitude on the why's, there are naval guns and 22's.

TLM
 
TLM, the 3000 in.lbs is the amount of torque necessary to get the blade to bend through its thickness to the point where the elastic limit is exceeded, the plastic limit is passed very quickly for most high strength steels, with a few exceptions. This is again just a rough figure, and it applies to a break through the main body of the blade. My main point with this statistic, is not the number itself but simply that it is an important aspect of a knife and one that would be valuable to know.

Specifically on the strength vs flex issue, I would rather have a level of strength that I could not exceed, rather than a high level of ductility that would keep the blade from breaking once it has been bent. It is far from trivial to straighten a bent blade, and the ones that have a very high fracture point tend to be able to get there easily for the reasons stated in the above. As well, I have seen blades that had this level of strength, and at the same time a high level of toughness, could take repeated (100+) hammer hits to the spine for example.

That is the critical perspective, whenever you increase the performance in one area there is a opposite effect being induced somewhere else. It is very important to know both where the weakness is being induced and to what extent. A blade that was very strong, but fractured very easily if it was hit by a hammer I would not want in a survival/emergency knife. So you just can't increase the strength without considering the consequences, same with everything else.

... the opposition chose to totally ignore the geometry.
Click to expand...

Yes, blade geometry is easily as important as steel in regards to blade performance. Thus it is far from trivial to draw conclusions on the properties of steels based on how knives perform unless the geometry is nearly identical in the knives used, or the effect of the geometry is well known and thus can be normalized out. The latter is far from trivial.

And yes, as noted in the above, I would not "pass" nor "fail" a knife simply based on how it performed on the above work. You also need to look at what it was intended to do. For example, any knife that I have had for any length of time will get altered so as to maximize its performance when I use it, this tends to have the opposite effect on how it performs for others as there are counter balances to everything.

-Cliff
 
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