Thinning - What's the deal.

Hi there folks, thank you for being here at my Blog.


     Sharpening knives as a profession leads me down many different but related paths, a lot of things pop up over the years, sharpening terms that I either ignore or feel  compelled to understand. Generally, I don't like general statements such as " A 1,000 grit edge will last longer than a polished edge"

     Seriously?  How can one make that statement and it is one I read often,  and I admit the my issue with this particular well known "law" is that I cannot get a grasp on the physics behind that, I don't see the reason.  Also, folks tend to repeat what they have heard or read somewhere, I do this, I have repeated things when in fact, I didn't know what I was talking about. Maybe I just wanted to sound intelligent but I think this is a common thing for folks to do.  Now I am just talking about sharpening knives, this must be true in everything in life, "you need to learn to play guitar on an electric guitar because it is easier to learn, then you can go to an acoustic guitar". Things like this,  peoples opinions become laws that we spread and people think they must be true.

     Gees, you must be wondering what the heck this has to do with thinning, "where is he going with this"?       ...............It's my Blog, I can ramble:)

     Thinning:

 What is it, Why is it and How is it?

    Most of you know that over time, as the edge of knife is sharpened and tiny amounts of metal is removed, and I mean tiny, the knife becomes thick. It can be as sharp as a new razor blade but still not perform as it should because the area directly behind the edge is too wide.  Now we can see this with some brand new knives as well. I remember buying a set of kitchen knives as a gift in Dubai, which I didn't take a good look at until I got home. I ended up throwing them away because of the very poor design, way to thick.

     The thickening process starts early and progresses over time, (or it comes with the knife as I said) so when you are considering purchasing a new knife, look carefully at the geometry of the blade, a nice thin blade is going to cut very nicely. As you can imagine, thin blades mean you are not forcing as much metal through the product you are cutting. Of course thin is good for kitchen knives, hunting knives and such need that support behind the edge and you generally don't cut up carrots with your hunting knife.

     Thinning is a process that is meant to retain positive slicing geometry, it is something that will change the appearance of your knife and there are some very important things to consider when thinning, you need to be aware of the negative aspects of thinning before proceeding. You'll be able to weigh the Pros and Cons and decide for yourself if it is good for you and for your knife.

     Generally thinning is good, it can have a major impact on how you feel about your knife. Remember, it can be extremely sharp but useless for certain tasks. Unless you have purchased an already thin knife than it is something you can consider.

Thinning

     In the photo above you can clearly see that the process of thinning involves the removal of the metal directly behind the edge, the metal that is making it thick in the first place. You can also see that the primary edge can be sharp but ineffective. Also, you can thin the entire knife a little or just the area behind the edge.  (This is more of Relief Angle and I will explain.)

How is it done?

20 deg Sharpening Angle

     Let's assume that I am sharpening the Grohmann Chef knife in the picture at 20 deg per side. (I am freehand sharpening so this angle may not be 20 exactly, it doesn't matter, it is the appropriate angle for this knife and I can hold this angle from heel to tip on both sides).

     Now if I continue to sharpen this knife at this angle over and over the Primary Edge will move up into the thicker part of the blade, the area behind the edge that is there to support the edge. This is not a good thing to do, just sharpen it over and over exactly at the same angle without taking this thickening process into account.

    I can do one of a couple of things: I can and should thin a little as I sharpen the knife, I should work at maintaining good cutting geometry by adjusting the angle a little to keep that knife from becoming thick but this has to happen early on in the knifes life. In most cases, I get knives that are already thick and I need to either just sharpen it as is or thin it.

    To thin a thick knife I have to adjust the sharpening angle in order to be able to grind the unwanted metal away and this isn't easy, the idea behind it is easy but the process itself is not that easy, it is a lot of elbow grease and takes patience and courage.  ( I will explain the courage piece)

Adjusted thinning angle

    The POSITIVE effects of thinning are immediate to the user, the knife will perform better (assuming it is sharp) because the blade is thinner. So to thin a thick knife I would need to decrease the sharpening angle significantly and I would sharpen/grind at this angle until I reached the edge of the edge. The knife would be thinner and very likely exceptionally sharp.

That's the good news, here is the bad news:

     When you reach the stage that the thinning process is not a continual gentle process, when it comes to the stage where it is all done in one step the knife undergoes changes. It will look different, it is quite possible to scratch up the blade, and you will will scratch it, it can't be sharpened at a greatly reduced angle and not be scratched. However, that can be fixed by polishing out the scratches.

    The biggest problem and it is a big one, is that the thinning process and significantly reduced the strength of the edge, it will be sharp yes but the edge retention is going to take a big hit. So you will find yourself having to hone your knife a lot to keep it sharp. Some people thin their new knives, especially the older European knives that are often thick to begin with. Just be aware that you are adjusting the ability of the edge to hold, that already thin strip of metal, the Primary Edge, has just lost a lot of support behind it.  So sharpening this knife is going to be a chore.


(When I say it takes courage, I mean the knife that you are used to seeing is going to look different)

Is there a better way?

        Well you could start with a thin knife and keep it thin by maintaining the geometry of the knife throughout it's sharpening life. Shun knives for example are thin but even they can get a little thick if not sharpened properly.

    Of course dream knives like these come nice and thin to begin with, their cutting power is astonishing to be honest.  However, I have seen a Fujiwara that was left to get dull over a period of a few years that had lost all of it's grace, a shame really.


     A Relief Angle is perhaps the way to go and this is what a Relief Angle is, sometimes just called a Relief.

     If I sharpen my knife an 20 deg per side, I could create a Relief by first sharpening at 15 deg per side. Now the key is that I would not grind the metal away until I hit the edge, so I am not sharpening at 15 degrees, I am creating a Relief which is also thinning the blade. Now I can go to town on this Relief and polish it to my hearts content.  The knife is still dull, that's fine, I am not sharpening I and building a Relief by grinding at a 15 deg angle, one that is at least five degrees less than the Primary Edge Sharpening Angle. I need to stop grinding before I hit the edge, basically, I need to leave some room to sharpen the knife. Once this is achieved I can SHARPEN my knife.

    Now I raise my angle to 20 deg and start my sharpening process on a 1,000 grit stone, I don't want to go to coarse here, I need to be careful not to cut into that beautiful relief angle I  spent so much time creating and it is like a mirror.  The primary edge is now going to be simple to sharpen and I can sharpen this to 5,000 or 2,000 grit. The knife now has two angles on it and if I wanted to create a Micro Bevel it would have three but we won't worry about that at the moment.

     The beauty of a Relief Angle is that from this point on, I don't need to work at that 15 deg angle, I just have to sharpen the Primary Edge now at my 20 deg angle. ( If my Sharpening Angle was 15 deg my Relief Angle would be 10 deg).  ALSO, that Relief Angle is still going to provide support to the edge itself.

     This all sounds real easy eh?  It isn't, doing this by freehand takes some skill, I need to hold the knife at a consistent 15 deg angle on both sides then repeat that process at 20 deg and be careful not to lower that angle so that I am disrupting the Relief.  This is where the Edge Pro comes in nice and handy. I can lock in that 15 deg angle and from from that point I can put away the EP and finish the knife by hand (at the sharpening angle) or just finish it on the EP. Not everyone has an EP of course and not everyone cares about this entire process. I am simply describing an alternative method to improve the performance of your knife without compromising the strength of the edge.

    Now Ben Dale, the brilliant sharpener and creator of the Edge Pro starts his Relief at the Primary sharpening angle, the 20 deg angle first and then goes down to the 15 deg. If he does it this way, it is pretty safe to say it is the way it should be done.

Again, not all knives should be thinned.


I really hope that I cleared up some things, I did for myself, that happens a lot when I write things down, everything comes together.

Respectfully and with gratitude,
Peter Nowlan