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 Post subject: Re: I am losing my needle point tips.
PostPosted: Wed Jun 18, 2014 12:12 am 
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So to clarify terminology a bit there is the width of the left and right bevels and their ratio of sizes determine blade asymmetry. Then if you measure the distance from the top of the left bevel to the top of the right bevel. if I read you correctly, you are referring to the bevel width as well. Perhaps bevel thickness might be a better term for this.

Now if the knife at the spine were uniform from the handle to the tip of the blade in thickness, your first observation would be true, but fortunately the blades of most knives have a distal taper that compensates for this so that the blade's bevel thickness is approximately equal along the edge going around the tip so that if we were to measure thickness at 1, 2, 3 etc mm from the edge perpendicular to the tangent at each point including the curve at the distal end of the knife it would be roughly consistent. This tapering solves the first problem. The second instance is a bit too confusing to visualize, but again if the blade has an appropriate distal taper it is also of no concern.

The important point is to maintain your sharpening angle perpendicular the the tangent of the blade's curve for the entire length of the blade. For the straight part of the blade it is simply perpendicular to the blade, since the tangent of a straight line is the straight line itself. Then as you follow the curve the tangent is as shown in the video so the knife is rotated accordingly.

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 Post subject: Re: I am losing my needle point tips.
PostPosted: Wed Jun 18, 2014 11:38 pm 

Joined: Sun Aug 11, 2013 11:00 pm
Posts: 37
I don't think I made myself clear.

I'm not referring to left/right asymmetry at all. I'm considering each bevel (left and right) separately. Just concentrating on one at a time. I'm only concerned with the visual width of the edge bevel. The distance from where the cutting edge bevel begins to where it ends at the cutting edge. So something like 1/8 to 1/16" for a typical blade.

I'm referring to the tendency for the bevel width to get more narrow, or more wide, as the bevel goes through the curved portion of the blade, and nears the tip. In my experience it tends to get wider near the tip if one holds a "constant angle" relative to the abrasive. Again, this is my observation based limited experience with a device that holds the stone at a constant angle while you hold the blade parallel to the table. I'm open to the idea that my observations are wrong because my technique was bad.

You're right that "distal taper" thins the stock of most blades at they near the tip. But is it enough to compensate? The only way to be sure would be to measure the thickness at the shoulder of the edge bevel (from left to right through the stock of the blade). Measure it at the heel of the blade and then measure it a few mm back from the tip of the blade. If they are the same, they my argument holds no water. If the thickness is greater near the tip than near the heel, then the effect I'm describing most certainly exists. I wish I had a digital caliper right now!

Again, I may be chasing an effect that doesn't exist. If I am, I'm curious about what I've observed in sharpening a LOT of blades with my WSKO. Namely that I need to alter the edge angle more dramatically as I near the tip than one would expect if I was simply rotating the blade in it's plane. I'm actually pretty careful about maintaining the normal line of the blade parallel with the center line of the belt or bench stone when I sharpen. I got that idea several years ago (before I saw your video) and it always just seemed "right", so I've been doing it that way for a while now. But the angle difference I'm describing in the last 1/4 to 1/2" of most blades goes beyond following the normal line. At least that's what I think.

Thanks for the discussion.

Brian.


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 Post subject: Re: I am losing my needle point tips.
PostPosted: Thu Jun 19, 2014 3:32 am 
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So two 'side points' - one for the Ken Onion and one for the EP.

On any belt grinder, you should not take the tip past midway on the belt a narrow KO belt, or a wide 2" or more belt etc. This is especially true for high speed fixed rpm units like the 1x30 grinders. Why? because you will burn the tips and cause MORE wear on the tips. This may be what you are seeing. Also I want to emphasize going around the curve at right angles to the tangent of the blade rather than grinding perpendicular to the spine all the way to the tip.

On the EP as one goes around the curved tip one is getting closer to the 'mast'. This would make for a more obtuse angle. This is a relatively small effect.

Beyond that, we would have to sit in front of a grinder or stone to figure this one out. Time to get the calipers out and start writing formulas ...

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 Post subject: Re: I am losing my needle point tips.
PostPosted: Thu Jun 19, 2014 12:07 pm 

Joined: Sun Aug 11, 2013 11:00 pm
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Addressing your two points:

1. I'm pretty good at making very sharp tips either by using stones or a grinder. Not trying to boast; just saying I'm not rounding tips at *all*. As you said, stopping with the tip touching is one of the big keys. I was trying to describe how I have to increase my angle from blade to belt *more dramatically* in order to actually touch the tip to the belt. This produces a very sharp point. Maybe it's just me "following the curve", but the motion seems like more than that. I'll have to test and take video or something.

2. I guess I wasn't clear about how I rotate the blade. I'm doing what you do in the video. I'm following the normal line of the blade edge. That is, the line that is perpendicular to the tangent of the edge. As you have said. I don't pay much attention to the spine unless I'm grinding directly on the spine for some reason. :)

I'm not sure we're making much progress here. Maybe I'll make a new thread if we both feel like it would be helpful. Again, thanks for the discussion.

Brian.


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 Post subject: Re: I am losing my needle point tips.
PostPosted: Sun Jun 22, 2014 2:03 am 

Joined: Mon Jul 22, 2013 2:04 pm
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Does the tip look almost like it is forming a "D"? (a loupe is a very useful tool for this and sharpening in general)

I've found on some knives this happens partly because the angle that the tip and upsweep are on are different than the rest of the knife and so a constant sharpening angle will not proportionally remove metal until the edge is set to a constant angle. This is a reason I really like the EP as it allows me to set a 100% constant angle on the knife with ease and very quickly. After that initial sharpening, it makes upkeep much easier. In some cases, a microbevel can make upkeep even easier, although I've not ever used it on a kitchen knife and can't say if microbevels are a huge plus or negative for these sorts of knives.

In terms of getting the tips sharp again, the trick is removing some metal from the top (spine) rather than just sharpening, and removing metal not just from the area immediately above the tip, but the metal leading up to it for a proportional finish. A lot of people forget this step and just work the knife from the edge side, which can mean removing more metal and tips that look funky when completed. Progressive 'shaving' of metal off the spine allows you to make the tip fix look natural (and perform well).


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 Post subject: Re: I am losing my needle point tips.
PostPosted: Tue Jun 24, 2014 7:45 am 
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Brian - a fun discussion. We should do a video phone call (on Skype) and wave some knives around. I'm at k_schwartz . Looking forward to it.

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 Post subject: Re: I am losing my needle point tips.
PostPosted: Mon Jun 30, 2014 4:36 pm 

Joined: Thu Dec 26, 2013 6:34 pm
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Ken, that video was really interesting. But most guys I've seen sharpen with the blade at an angle to the stone so that it is never perpendicular to the tangent of the edge, like this guy.

http://www.youtube.com/watch?v=GB3jkRi1 ... 79F53216AB

Can the same information still be incorporated into that approach?


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 Post subject: Re: I am losing my needle point tips.
PostPosted: Tue Jul 01, 2014 4:04 am 
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In most instances he is still sharpening perpendicular to the tangent to the edge. You will also notice that he is very one handed (not at all ambidextrous) and rather dogmatic in his approach. There is no one ideal way to sharpen, just as no two people's signatures are ideal (ie identical). I could nitpick about his technique, but that goes away from the point of striving for consistent technique. YOUR consistent technique.

You can approach at different angles to help distinguish the scratch patterns of individual stones which involves a different approach for each grit. This involves the ability to adapt to the task and even in some instances the shape of the stone.

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 Post subject: Re: I am losing my needle point tips.
PostPosted: Thu Jul 03, 2014 9:25 pm 

Joined: Thu Dec 26, 2013 6:34 pm
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Thanks Ken. I am still very much a noob at sharpening, but the approach in the video has gotten me the best results so far. That's why I've stuck with it for a bit. I do like the ambidextrous approach, but I'm a guitar player and my hands are very used to specializing in different tasks. I was watching a Murray carter video that other day on maintaining a convex grind and was impressed by the ease with which he switched hands.


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