Sir Joe wrote:
[...] but if I try to sharpen and SEE what happens I would have a hard time trying to get a piece of truth out of that mess of variables which is a new knowledge which I do not yet master. I would have no idea what is going wrong, where does the problem comes from.
Anyone with even a basic understanding of science is always in the same position, it is only the ignorant that make conclusions rapidly. Lets say you get a new knife, the Spyderco Duper in S666V, you sharpen it at 15 dps with the UF stones and you do a little work with it pruning some vegetation. Now the knife goes dull rapidly, what is the cause?
-the polish is too high?
-edge angle isn't optimal
-sharpening technique was off, left the apex/edge fatigued
-cutting technique was off
-wood was dirty
-the edge/apex actually hit / contacted something else that you didn't notice
-the initial edge was over heated and needs to be cut back
and the list goes on. How do you determine what factor is causing the performance loss? Well it is a step wise process where you slowly build up experience by changing various aspects and noticing patterns that develop. Any time you get frustrated you find someone else interested in similar, discuss your results ask questions and see if any helpful suggestions can be made.
I would say that cutting plastic is mostly push, would you agree? Specially for harder plastic, but also the soft plastic sack for soil, I tend to open it by stabbing it with the knife and then lifting the open part a bit so put the "to be cut" part under tension, and then just pressing a bit with the knife.
In general yes, it is pretty hard to actually saw cut plastic which is essentially what it means to slice.
So, would I benefit from a UF rod?
I don't think you are likely to see any benefit from the fine rod, the materials sound too dirty. Even if you are push cutting, dirty materials are so abrasive that they wear an apex very fast and hence you will very rapidly lose an ultra-high polish. In general, unless you are cutting very clean and non-abrasive material then moving above a Spyderco medium is not likely to see an increase in edge retention, though you likely may notice a small increase in initial push cutting performance. But you also have to consider the time it takes and if you enjoy sharpening. There are people who like sharpening as much or more than using knives so sitting for an hour producing perfect bevels is something they enjoy.
The problem is, for me it is as Jack said: I had just today this experience of making a knife duller with fine stone than it was with coarse stone.
Why does this happen? How is it even possible? It makes no sense to me. If the edge is sharp, and the fine stone just takes the teeth away, how can I manage to make the thing dull??? What am I doing wrong? What should I change?
When you use an abrasive on a steel then three things in general happen :
-the steel is deformed, ploughed
-the steel fractures, cracks
=the steel is rubbed and compacted
With coarse abrasives mainly the first two happen and it leaves the edge with an apex which looks very much like an irregular serrated finish :
As with any serrated finish, they don't need to actually be sharp to cut because of all those points and scallops will cut on a slice regardless. However as the finish is raised then these are all cut away and for the knife to actually cut well the apex has to be free of deformation, burrs and generally make a nice straight, continuous and very fine line.
Here is the simplest method to apply a very high polish, it is almost fool proof :
-use a King 1000 or other similar stone (weak bond, forms thick slurry) to over grind the edge
-apex bevel with a fine Spyderco, ultra- light, water on the stones, alternate passes, 5-10 pps
The reason the King works so well is that thick slurry grinds into the apex as much as it does on the sides and thus it keeps the apex free of any burr, deformation or general damage. Keep grinding until the edge starts to slice newsprint well, it will be difficult to get it to shave or push cut newsprint (unless you remove the slurry) but at this point it is perfect for the application of the apex bevel as it will look like this :
That is 121REX which is a duper-stuper steel, ~70 HRC, full of carbides, and none of it matters because a stone like the King ignores all of that due to the high friability of the abrasive. All of that swirling mass of slurry means it will cut off the burr and leave it perfectly ready to take an apex/micro-bevel.
Now what most people do is something like :
-grind the edge with the coarse stone
-grind the edge with the medium stones
-grind the edge with the fine stones
-grind the edge with the UF stones
This is possible, it can work, but it is the hardest and least efficient way to make the apex take the UF finish. If you have a girlfriend and she has jeans you don't like then that method is like saying "Hey, those pants make you look really fat, you might want to cut back on the pizza.". This can work, it might make the jeans stop having an appearance, but it is also likely to be frustrating and it takes a fair amount of skill and charm to make it work.
If you are not interested in making things difficult, then plateau sharpen and micro-bevel :
http://www.youtube.com/watch?v=mSt2mCWGFII
It was not even a super steel. It was a **** Opinel 8 (I hate this knife so much, I cannot understand why people like it).
It has one of the slimmest grinds on a production knife and has high cutting ability. It is easy to grind though can be difficult to sharpen because the hardness can be a bit lower than what people are expected to and that combined with the very thin section means you can push the metal around easily vs abrading it.