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 Post subject: Burr on both sides
PostPosted: Wed Oct 09, 2013 11:38 am 

Joined: Tue Jul 24, 2012 1:28 pm
Posts: 408
I always start sharpening on the right side of the knife and of course raise the burr along the edge, flip the knife and repeat the process.
Has anyone ever noticed that it takes longer sometimes to raise the burr on the opposite side of the knife than it did when you first started.

For example, if I start on the right side with a Chosera 400 and raise a burr in 2 minutes for example (could be 3 or 4 or 30 seconds) then flip the blade to raise the burr on the left side.
There are times when this seems to take forever.
Do you guys ever experience this, if so, what is the reason?
If not, obviously I am doing something wrong.


I think what is happening is that I have removed the fatigued metal quickly on the right side and it has been "pushed" over to the other side (it's the burr) and when I flipped the knife I removed that burr but now the metal is not as fatigued, I've exposed some of the fresh new steel underneath and that is why it is taking longer to form the burr on the opposite side, the steel is putting up a better fight. Make sense?

Peter


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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 12:00 pm 
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Theoretically it should take the same amount of time (or roughly) to grind each side, pending the bevels were already even-ish to begin with.

So, a couple of possibilities:

Do you lap the stone after grinding one side? When using the process of grinding one side at a time, you have to remember that you started with a clean stone, probably freshly lapped, and it's going to cut much faster. After 2 minutes of rough sharpening the stone will load up a bit. I find getting the swarf off of the stone and giving it a quick lap before starting the other side to greatly speed things up.

Also, do you use 2 hands to sharpen instead of 1? Is one hand putting more pressure than the other. If you are using 1 hand, do you put more pressure on one side than the other? That usually isn't going to be much of an issue after you've sharpened for a while, but more often than not when first starting out you will make asymmetric bevels which will make things take longer than normal (although at your level experience I seriously doubt that is the issue, more or less for other people to read).

About removing fatigued steel: Generally speaking, unless there is a major chip then the majority of fatigued steel is going to be on the edge-of-the-edge. If you start to sharpen and hit the fatigued edge it will simply chip out the first time you raise the burr, so that really shouldn't be an issue IMO. Some steels have more plasticity (Globals) and hold on to a burr or damaged steel a little longer, so that can sometimes create a problem as well, but yet again I highly doubt this is the problem you are having.

I'd dare to say you should just try lapping the stone after getting the first burr and then see what happens. All things equal on the stone side then you can start to explore other possibilities. We WILL get to the bottom of this, I hope. :D :D :D



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Shaun Fernandez

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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 12:12 pm 

Joined: Tue Jul 24, 2012 1:28 pm
Posts: 408
Shaun thanks for the awesome response. (as always).
I should point out that this doesn't happen all the time, just every now and then.

Also, I don't grind for too long on one side if the burr doesn't appear quickly, I will flip the knife and work on the opposite side.

Now as for flattening, that is an excellent point, no I do not lap the stone after getting the first burr, I am pretty keen on keeping my stones flat but I've never lapped in between burrs so to speak. I'll try it out tonight, I have 14 knives waiting for me at home. It could be as simple as that.

As for pressure, yes two hands, I don't think that is the issue, it may have been at one point but not now.

Again, this just happens every now and then and it is likely something as simple as lapping as you recommend. I'll let you know if that did the trick.

( Also....I just realized that if I start on a very coarse stone like the Nubatama Bamboo 150, this doesn't happen, burr formation on both sides is usually very fast)

You're the man Shaun, thanks


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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 1:18 pm 
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Thank you, and a 'you're welcome.' ;) I know that you're a great sharpener already but I do throw in extra stuff sometimes for other readers at different skill levels, not that I think we misunderstood each other at all, I think we're on the same page. :D

But, one more thing about edge failure and burrs. There is the occurrence of edge failure when the burr breaks off. This happens on cheaper stainless most of the time, but also on carbon, and is usually the result of: 1) too low of an angle 2) too much pressure 3) huge carbides in the steel 4) Any combination of the these.

Trying to sharpen at too low of an angle will cause the edge to fail while sharpening. This can happen on carbon steel that has very low/no alloys as well as some stainless steels. On stainless steels in particular, if there is a lot of chromium and it is not a powdered or Swiss steel, it will likely have lots of grain growth in the Chromium carbides. This makes a weaker edge that's likely to fail in larger pieces, not conducive to making a thin edge. Too much pressure will cause the edge to roll too much and get damaged beyond (what should be) the burr. Any of these things will cause not only the burr to break off, but also the edge behind it, essentially dulling the knife instantly. And of course, this will cause you to have to sharpen longer. So, if this only happens on cheaper steels then you might be having a little bit of steel failure as well, in which case increase angle and reduce pressure. (I know you probably already know this, but more-so for other readers.) :)



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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 1:41 pm 

Joined: Tue Jul 24, 2012 1:28 pm
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Thanks again Shaun, I appreciate the input.
Yes I have a lot of experience but I also still have things to learn, I love the lessons and as you suggest, other folks will benefit.
You are never going to insult me by telling me things I may already know.

Pete


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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 4:10 pm 

Joined: Tue May 29, 2012 12:29 am
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You cannot have grain growth in a Chromium carbide or the result of Chromium content. That's just not at all how it works. Grain growth is a factor of how fast a steel is cooled. The major factor of what this effects is material strength.

As for the burr thingy,

You might be simply applying more pressure in your sharpening to one side vs the other. I know when I sharpen I can grind one side faster than the other because I can apply more pressure on my "strong side" which is usually the side where the edge is facing me. When the edge is away from me I cannot apply the same amount of grinding pressure in most cases or simply tend to not apply as much pressure causing a slower grinding action.


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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 6:19 pm 
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Well, as I understand it, grain growth actually occurs when the knife is being tempered. Leaving it above critical temp for too long will cause carbide growth, that's why custom makers make such a big deal about not going above critical temp or soaking for too long. Also, adding more chromium can increase this, but like you said it is not the determining factor. I'm not sure anything happens to the carbides when the steel is being quenched other than they get locked into place. Please tell me if it is otherwise.



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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 7:10 pm 
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Location: San Ramon Ca.
I've had similar experiences on the back or left side not getting a burr up as fast. I've always found it's some bad habit I've unknowingly gotten into.



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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 8:51 pm 

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Sorry, got it a little backwards on the heating and cooling part. The heating beyond the austenite phase to target temp is where grain growth is controlled. The longer the time spent at target temp the more time that is allowed for the grain to grow. Heating faster and cooling quickly will provide a finer grain structure. You don't want to hold at temp for too long because you grow the grain too much causing a very brittle steel.

The tempering phase is just to draw the hardness down from the quenched hardness where the grain structure has already been determined.

Truthfully this is all a bit over the top in sharpening theory, basically hardness and alloy distribution in the steel matrix, mainly carbide distribution, are the only factors that really have any effect on sharpening outcome.


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 Post subject: Re: Burr on both sides
PostPosted: Wed Oct 09, 2013 9:29 pm 
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It is a bit over the top and stuff like this isn't a factor 9 times out of 10, more or less just trying to cover all bases of theory for the sake of it I suppose. :)

But like you said, and I totally agree, hardness and carbide distribution have the most impact for sure when it comes to steel.



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