Wednesday, May 29, 2013

All on the EDGE! - Edge durability

 On my last post of "All on the EDGE!" I have concluded that "don't get bothered with sharpness of a knife that much!". If the sharpness is not that important, then what should we look for in the knife???

 When I pick up a knife, I usually check how it feels in my hand, the length, shape of blade, the make/finish, blade geometry and steel. Then if the knife is mine to do what ever, I will put it to some test.
 Many of those I have mentioned are simply for my preferences. Like feels, length, shape and finish, all of these are for comfort and everyone has own opinion on it and has nothing to do with the edge or most importantly with the quality of the edge.

 Quality of the edge... on the other word, the "Edge Durability" and this "Edge Durability" is the most important in any knife and should look for when picking one.

What is the Edge Durability?
 The edge durability is simply how tough the edge is, in other words, "How long it will retain the cutting edge".
 What it meas is that from the moment of completed sharpening, use it for some length of cutting task and see how much job does it get done before it requires the resharpening.

 Here is an extreme case as an example:
a tuna can knife V.S. an Axe. Both did cut the paper well but what happens if continue cutting?

 As you can see, the tuna can blade is already struggling on the second cut and not able to cut paper after from the third cuts. On the contrast, the axe goes on cutting for.... well that will take too much work to find out.
what if I put those blades to wood cutting test? I think the results are obvious, axe will fell few trees easily and the tuna can even have problem making decent shaving on a stick.

How to check the durability?
 For quite sometime, I was wondering how to test the durability in comparable measure. and here is a method I came up and have been using it called "Durability scale"

> Take Deer antler and start making approximately 1 cm length shavings with the knife under testing.
> The each cut is made using exactly same section of the knife.
> Count the number of shavings the knife can make before it dulls.
> Repeat this with three different sections of the knife. (if the knife is larger than about 15cm, do on 4 - 5 different sections.)
> Take average of counts from all tested sections and divide by 10 is the Durability scale.
 --- Note: ---
 - I usually stop cutting when the count reaches 100. If the blade still can go on after 100, the result will marked as 10+
 - It is also important to observed the condition of damage.

Name of Blade
Blade made from tuna can
Edge rolled badly.
YP puukko
Edge rolled. Easy to restore with fine grid.
Svord peasant knife
No damage
No damadge
 Opinel No.9 (with modified to convex cutting edge)
10+ but little dulls

!! Note !!
The test is performed with default / factory edge (if not specified  and the edge angle and geometry does affect the result. Use this information as reference to the original edge geometry of each blades.

Where the Edge Durability comes from?
 There are three elements affects to the edge durability.
1- Material
2- Manufacturing.
3- Edge angle

 Above these three, Manufacturing is the one user can not do anything about it. it is totally up to makers' hand and is very difficult to manipulate or see. It is basically how correctly forged and/or heat treated.

 Material, is also difficult to change as a user, but has choice of selecting what material being used. Different steel has different characteristics. one can be harder or springy than others.
Harder the steel, easy to achieve thin / sharp edge but prone to chipping. More springy, difficult to achieve sharp edge and prone to roll / bent but the edge does holed longer time.

 Material and Manufacturing will set the base durability of the knives' edge. The material will determine the theoretical characteristics of edge can be achieved and the manufacturing will make steel close as possible to that theoretical characteristics.

But there are some case, the manufacturing skill has made characteristics of steel exceeding the theoretical characteristics.  This done by the thing often referred  as "smith manipulating the steel grain". Basically it means that by forging well, the grain of steel sifts around to line up in such that best suited to be an edge.  this level of manufacturing only comes with master smith level of skill and hard to come by in current popularity on Stock and Removal custom knife making and mass-manufacturing of knives.

On the steel and manufacturing, there are not much room for user to make change in edge durability, but with the Edge angle, user can easily manipulate it by grinding and sharpening.

Generally, larger the angle, durable the edge is, but compromising the cutting ability, because there is more resistance when cutting in.

 Lets talk more about the angle, since this is the one user can do.
Giving the same condition of material and structure of steel, difference of the edge angle determines the durability of the edge.
 The theory is very simple. thicker the material, stronger the structure. if the edge angle is bigger, naturally there are more materials supports the cutting edge, means stronger edge. But we must remember, "Thinner edge is sharper edge", so need to keep this angle within the resealable range.

Larger angle has more materials (red area) behind the cutting edge(the tip point of angle) .

as for the reference, here are the approximate "cutting edge" angle of the knives used on durability test.
Name of Blade
Cutting edge angle
YP puukko
Svord peasant knife
 Opinel No.9 (with modified to convex cutting edge)

Now keeping this in mind and think of Puukko. In-general, puukko has about 20 degrees or less angle on cutting edge, while others have generally30 - 40 degrees. But puukko can withstand the use on material like wood and so on. This means, to make usable puukko, the smith need to lean some level of decent knife making skills. if not, the edge will chip out or roll up very soon when user cut in to wood. Or, cutting in to antler without any damage on edge would be impossible...

Why  "Edge Durability" is most important?
 Combining all these three elements, can get / manipulate the edge to needed durability. But why is it so important?

 In elementary level, edge durability is needed, because cannot make cutting edge without it.
other than that, more durable the edge is, longer time you can go on working without fixing the edge. Also, it gives a knife capability of cutting varieties of materials like paper to antler. This is important especially if the intendid use of the knife is utility knife or camp knife. Because for the utility usage, the maker and the user do not know in advance, what the knife will be cutting.

 Other than the practical importance, I think the durability of the edge can tell us the skill of the maker. If putting the edge angle a side, only the maker can manipulate the durability. so, if the the knife has got appropriate edge durability, then the skill of the maker is surely skilled. And if it is consistent between different pieces he/she makes, then it is a trustable craftsmanship.

So, more durable the better blade?
The answer is NO! If you are talking only utility knife, then perhaps it is. But if talking about all knids of blades / knives, then there are so many different blades designed to do very specific tasks. Some are purely after the best possible sharpness. to achieve it, the blade needs to be as hard as it possible can.
on other case, the durability is more important than having sharp edge. On this case, the  blade is not hardened much. in fact it is quite soft that can even file it with metal file.

On the end, it is a tool, it need to do what its intended to do. So, choose well what we / you want from knife. there is no such knife as "One does it all".

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