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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 4:25 pm 

Joined: Tue May 01, 2012 9:37 pm
Posts: 346
Location: Pensacola, FL, USA
Jason B. wrote:Correct!

Cutting is about geometry, how sharp a edge is is nothing more than personal preference.

When the edge and more importantly the blade is thinner the force pushing back on the edge apex during the cut is minimized reducing edge deformation and wear. As the blade becomes thicker the effort to make the cut and the forced produced at the cutting edge sky rocket.


Well, that's half right.

The effort to make the cut increases, but the actual force at the apex remains the same.


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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 6:17 pm 
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Joined: Tue Oct 23, 2012 1:14 am
Posts: 596
Location: San Ramon Ca.
Jason B. wrote:Correct!

Cutting is about geometry, how sharp a edge is is nothing more than personal preference.

When the edge and more importantly the blade is thinner the force pushing back on the edge apex during the cut is minimized reducing edge deformation and wear. As the blade becomes thicker the effort to make the cut and the forced produced at the cutting edge sky rocket.


I completely agree.

Something else to consider. What you can't control is once you give the knife back to the user / owner is how they use it.



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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 9:04 pm 
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More to this than simply angle, it's also steel, honing practices, and user.

IF the knife is already fairly thick behind the edge then some thinning will help, but other than that thinner edges do not last longer in my experience, they only make a knife cut better and make resharpening easier. Yes a thin edge takes less pressure to push but as stated the pressure at the apex is unchanged, and it is sacrificing toughness and assuming the user and environment are perfect. There is torquing in nearly every cut to a small degree along with board contact. Edge deformation happens much easier on thinner pieces of steel because they aren't as tough. You can keep pushing a thin dull knife through ingredients, but it won't shave hair to save its life and will even have trouble against softer ingredients. Deform the edge enough times and it will break off, and no amount of steeling or stropping can fix that.

A more acute edge will always make a knife cut better, but without a micro-bevel it's not as strong, that's why we micro bevel on really thin edges for longevity and the same reason why bushcraft knives use a 30 degree edge and not 15. Yes food is soft, but toughness is still needed.

I'd say try a less acute angle, or if the knife needs thinning then thin it but come back and add a micro-bevel. Sharpen under 15 degrees until the edge is met, refine the bevel, then come back with whatever finishing stone you want and add a micro-bevel at a higher angle. This should help you out.

If you edge is lasting longer than a day then I would rule out the wire-edge.



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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 10:01 pm 

Joined: Tue May 29, 2012 12:29 am
Posts: 872
Rick wrote:
Jason B. wrote:Correct!

Cutting is about geometry, how sharp a edge is is nothing more than personal preference.

When the edge and more importantly the blade is thinner the force pushing back on the edge apex during the cut is minimized reducing edge deformation and wear. As the blade becomes thicker the effort to make the cut and the forced produced at the cutting edge sky rocket.


Well, that's half right.

The effort to make the cut increases, but the actual force at the apex remains the same.



The key word is "as the blade becomes thicker"

The increased thickness causes you to apply more pressure to make the same cut as a thinner blade so yes, edge pressure increases due to friction.


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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 10:38 pm 

Joined: Tue May 01, 2012 9:37 pm
Posts: 346
Location: Pensacola, FL, USA
Jason B. wrote:
Rick wrote:
Jason B. wrote:Correct!

Cutting is about geometry, how sharp a edge is is nothing more than personal preference.

When the edge and more importantly the blade is thinner the force pushing back on the edge apex during the cut is minimized reducing edge deformation and wear. As the blade becomes thicker the effort to make the cut and the forced produced at the cutting edge sky rocket.


Well, that's half right.

The effort to make the cut increases, but the actual force at the apex remains the same.



The key word is "as the blade becomes thicker"

The increased thickness causes you to apply more pressure to make the same cut as a thinner blade so yes, edge pressure increases due to friction.



Yes, you apply more pressure, but no, it doesn't translate to the apex of the edge, rather it is expended in forcing the food apart.

What can happen is that the application of more force can result in the edge hitting the board harder as you finish the cut, thus negatively affecting edge retention.


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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 10:46 pm 
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So from what I understand, and thank you everyone for the discussion, the angle was probably not the problem. So in the past I took his knife to 8k on my Kityama. Should I have stopped earlier?
I am waiting until payday to get a 5k shapton pro, otherwise the next highest grit is my Green Brick, 2k.
What do you all think?


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 Post subject: Re: Longevity of the edge
PostPosted: Wed Jun 25, 2014 11:10 pm 
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From the 2k you probably could have just did a few stropping strokes on the 8k to refine the 2k scratches a little but leave some tooth. No need to work to try to get an 8k finish. I usually take my knives to a 3-5k finish depending on knife and steel and that seems to work very well for me.
I'll take a knife to 8k now and then for shits and giggles. It's razor sharp but it doesn't seem to give me the best "working edge". It looses it much more quickly than the lower grit edges.


Edit: Because my typing is as bad as Shaun's.


Last edited by Jeff B on Thu Jun 26, 2014 1:01 am, edited 1 time in total.


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 Post subject: Re: Longevity of the edge
PostPosted: Thu Jun 26, 2014 12:20 am 
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Yup, just a Jeff stated, try a little bit less refinement. There are cases where more refinement can actually stabilize the edge faster during cutting and lead to longer edge life, but for the most part a toothier edge is what we desire in the kitchen as it tends to slice into soft food better. Too toothy though and the knife will stick as you suffer from friction. This is why most of the time you will hear us saying stay between 1k and 5k.

If you can get a knife sharp between 1k and 5k, and still feel the need for more refinement, by all means do it, because at that point you understand your knife well enough to make that decision.

We more or less want to give people advice that will work for most users and at the very least get you started to understanding the limitations of your own equipment.

So again, it seems this argument may have gotten into the "perceived sharpness vs actually refinement" area again. If you sharpen a knife and the user says that a knife is going dull fast, chances are he is losing refinement at the edge for whatever reason (steep, wire edge, too refined). But, if a user says that a knife is sharp but still doesn't cut like it used to, then it probably needs thinned. Remember that not all knives last long with a 15 degree edge, different steels are capable of different things.

I wonder about making a sticky post on why knives go dull fast so that we may refer people to it in the future. Hmmm..... :ugeek:



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 Post subject: Re: Longevity of the edge
PostPosted: Thu Jun 26, 2014 1:36 am 
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Real fast, just wanted to elaborate on the argument of pushing an edge through:

There are 2 factors that determine how hard a knife is to push through an ingredient: Thinness and refinement.

The thinner it is the less force it takes for the bevel to slide through. The more refined a knife is, the easier the apex will push through and possibly less friction on the bevel itself.

Now the cons:

The thinner it is, the less strength it has. The more refined it is, the faster it loses slicing power (due to lack of toothiness), BUT, a more refined edge will last longer through chopping as they tend to stabilize faster, so for certain applications a more refined edge is desirable.

Want an edge that lasts with banging into the board longer? Higher refinement, higher angle.

Want an edge that will push through easier but not be as strong? Higher refinement, lower angle.

Want an edge that will slice better but not be as strong? Lower refinement, lower angle.

Want an edge that does most kitchen things well? Medium refinement (aka 2k to 5k) and medium angle (15-20 degrees per side). Any toothier and you get increased friction, any more refined and you lose slicing power faster. Any steeper and the edge gets weaker, any higher and you increase friction. A 220 grit edge at 30 degrees will slice for a long time, but it ain't winning no awards for "awesome edge of the year" I can tell you that much! ;)



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 Post subject: Re: Longevity of the edge
PostPosted: Thu Jun 26, 2014 1:54 am 
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good things to know. thank you, Shaun.

pat



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