Thinning, what is it, and why is it important

I've seen a few references to thinning the blade during knife sharpening, however I can't find any information on what this means, why it is important, or how you would go about doing it.

So please can someone enlighten me?
:beg:

over time, this is what will help you maintain edge geometry over time... i have some images on my work computer that would help out with this, but as i'm not there, it might be a few days before i get to post them.

as your sharpen your knife, the edge works its way down to the spine. Knives are designed such that they taper from the spine down to the edge and repeated sharpenings will eventually cause the area above the edge to be thick and as a result, cause wedging. hope that helps

I would add that just a few sharpening sessions without thinning will cause a noticeable loss of performance. The good news is that thinning behind the edge is very easy and give almost an instant performance boost. It's done by sharpening at a little more acute angle than the one you used with the very edge. Use a marker to see where you are removing material.
When you're dealing with asymmetric knives (left side flat, right one convex) thinning behind the edge is normally done at the right side only; the flat side should remain flat.

Maybe this image will help:



It is reproduced from a good article on sharpening which is worth reading:

http://forums.egullet.org/index.php?/topic/26036-knife-maintenance-and-sharpening/

Rick

Thanks for posting that link PT, that guy should probably write a book on kitchen knives!

Dusty said:

Thanks for posting that link PT, that guy should probably write a book on kitchen knives!

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:rofl2:

Thx for the link, it was enlightening. though didn't really cover thinning I think. I wonder if we can get permission to reproduce that article and add it to the encyclopaedia here

dragonlord said:

Thx for the link, it was enlightening. though didn't really cover thinning I think. I wonder if we can get permission to reproduce that article and add it to the encyclopaedia here

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What about thinning isn't covered by this paragraph?

"The back bevel also solves one of the great problems with V-edges, the fact that the metal behind the edge gets progressively thicker as the knife is sharpened over time. The knife doesn’t cut as well and becomes harder and harder to sharpen. The answer is to grind the shoulders off the edge at an acute angle, i.e. add a back bevel, then reestablish the primary bevel."

Rick

I would be interested in seeing how different people carry out thinning. I have done it pretty heavily to a couple of my knives and it worked out great, but I have seen other people with different ideas about how it should be done and certain ways to do it. I used trial and error basically and apart from the scratches, it worked out great. It also added much needed convexity to the left sides of my knives due to me being a lefty

The thing with thinning is that there are at least two different schools of thought, regarding it: Some people thin a little each time they sharpen (I believe Murray Carter is one of these). Whereas, others thin behind the edge only when they feel it is needed (cutting performance deteriorates, and is fixed with thinning). Of course, to further complicate matters, many of us thin a knife upon getting it to what we presume should be a good angle, based on steel, hardness and reputation of the maker's HT. This method will get you to "maximum performance", but will leave you with some weak edges until you find the right primary bevel, over time (and excessive steel consumption).

Pensacola Tiger said:

Maybe this image will help:

It is reproduced from a good article on sharpening which is worth reading:

http://forums.egullet.org/index.php?/topic/26036-knife-maintenance-and-sharpening/

Rick

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I get it. Makes perfect sense.

Most effective thinning I have done required refinishing a blade afterwards.

The way I do is to put the knife flat on the stone (Beston 500 is my stone of choice), and by applying pressure with your fingers at above the edge area, thin 1/2 - 3/4" above the edge on both sides of the blade. Thinning convex blades is fairly straightforward, as you have the angle built in, so to speak. Other types of grinds will require some small adjustments.

I typically thin till about .005" on the edge, then I put an edge on it on Bester 1200 and thin a little more on Bester 1200. Then I refinish the knife starting with 220 grit automotive sand paper, following by 320, 400, 600 and 800. I get a very fine satin finish and an improved performance.

M

I have read someone say that you should never actually have the edge touching the stone when thinning, it should just be behind the edge or you will screw up the knife. However when I did it to my knives I rolled the edge in the process, but afterwards it took literally seconds to put the edge back on. I don't really see how hitting the edge does anything that bad to the knife. Has anyone else heard of this?

When your knife is thick behind the edge, for you to touch the edge when thinning, you will have to lift the spine quite high. If you follow the grind angle the knife has and remove metal along the edge you should not hit the edge, until your knife is thinned enough -under .005".

I got to admit that some Japanese knives will require a lot of thinning, and some folks simply remove a little bit of metal each time, rather than doing it all at once, as I suggested here.

M

stevenStefano said:

I don't really see how hitting the edge does anything that bad to the knife. Has anyone else heard of this?

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Fear of overgrinding.

Pensacola Tiger said:

What about thinning isn't covered by this paragraph?

"The back bevel also solves one of the great problems with V-edges, the fact that the metal behind the edge gets progressively thicker as the knife is sharpened over time. The knife doesn’t cut as well and becomes harder and harder to sharpen. The answer is to grind the shoulders off the edge at an acute angle, i.e. add a back bevel, then reestablish the primary bevel."

Rick

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Ok, didn't associate it with thinning as I read it.:whistling:

stevenStefano said:

I have read someone say that you should never actually have the edge touching the stone when thinning, it should just be behind the edge or you will screw up the knife. However when I did it to my knives I rolled the edge in the process, but afterwards it took literally seconds to put the edge back on. I don't really see how hitting the edge does anything that bad to the knife. Has anyone else heard of this?

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+1 to Benuser's comment on overgrinding. The other problem is it is simply a waste of your knife to hit the edge when thinning. Once you hit the edge, you are sharpening, which shortens the life of your blade. If you are going to put a new edge on it afterward at a different angle, you are essentially resharpening twice (or more depending on how much material you remove at the edge when thinning). Hopefully, I'm making sense.

i prefer to thin a little every time i sharpen. i make it come as part of the routine and never have had a problem with a knife being to thick behind the edge.

as stated above, there is a drawback of scratches. i dont mind them as it gives the knife character

Marko Tsourkan said:

Most effective thinning I have done required refinishing a blade afterwards...

I typically thin till about .005" on the edge, then I put an edge on it on Bester 1200 and thin a little more on Bester 1200. Then I refinish the knife starting with 220 grit automotive sand paper, following by 320, 400, 600 and 800. I get a very fine satin finish and an improved performance.

M

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I was going to put up a post w/ a couple of questions about thinning as I've recently thinned one of my knives, so this is timely for me.

First to Marko - when you're using sandpaper, are you using it wet, what are you using as a backer, what direction are you sanding in, and are you sanding the entire blade face or only the area thinned?

Next question in general - when thinning, do you first thin each side and do a progression on stones or sand paper, and then go back to the bevel(s) and start over with the stone progression, or do you hit the bevels on the same stone the thinning is done on, just changing the angle?

I thinnned my Yusuke by using a Best-whatever 500, and GS 1K, 4K, 8K (it's all I have right now). It turned out well, the finish the GS8K leaves is shiney and the scratches cleaned up well. I think the only issue I had with the appearance is there is not a clean line between the newly thinned area and the factory grind - no shinogi line so to speak, but then again gyuto's (typically) don't have a shinogi line, so I expected the there to be some variation in finish. Now I can see why Marko refinishes w/ sand paper. Cheers! mpp

I use automotive paper with spray window cleaner for lubrication. When I thin, I only thin with one stone - Beston 500, and then remove scratches with 250 grit paper and go onto finishing with higher grits paper. I only go to 800 grit, as after that a satin finish turns into a polish.


I clamp a 2x3 board in a vice and clamp a knife to it with a C-clamp. For a push stick, I use a piece of phenolic long enough to hold with it two hands. The stick is outfitted with some cork backing to conform to the blade's geometry. I sand in back-forth motion.

Look under a bright light at your blade to see if you left any scratches.

Once you get to your last paper, go one pass per sheet, in one direction only.


M

Dusty said:

Thanks for posting that link PT, that guy should probably write a book on kitchen knives!

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He did.

http://www.amazon.com/dp/0061188484/?tag=skimlinks_replacement-20

Here's half of the rough grinding I did for a thinning job today. Notice part of the original bevel is still intact. No wasted metal. The second pic is the side I'll work on tomorrow, probably.

Thanks for the input guys- i was gonna get around to asking all thos someday. Not at all what i thought thinning was.

Good stuff, Marko. One thing that I would add is that a lot of guys will go one grit higher than their desired level of finish and then drop back one to to the final sanding. I use one of the 3M hard rubber sanding blocks that you can get at Home Depot for that with a long strip of paper. They are fairly stiff, but have enough flex that if you press down, you will be in contact with the entire blade surface on pretty much any normal flat or convex ground blade. I use the front edge of the block and scoot the paper after every stroke. I typically use cutting fluid for the lower grits and Windex for the higher ones. For that final straight sand, I use dry paper.

Marko Tsourkan said:

I use automotive paper with spray window cleaner for lubrication. When I thin, I only thin with one stone - Beston 500, and then remove scratches with 250 grit paper and go onto finishing with higher grits paper. I only go to 800 grit, as after that a satin finish turns into a polish.


I clamp a 2x3 board in a vice and clamp a knife to it with a C-clamp. For a push stick, I use a piece of phenolic long enough to hold with it two hands. The stick is outfitted with some cork backing to conform to the blade's geometry. I sand in back-forth motion.

Look under a bright light at your blade to see if you left any scratches.

Once you get to your last paper, go one pass per sheet, in one direction only.


M

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