What am I doing wrong? (sharpening w/ high grits)

[Grease]

I can make a knife shave with a 400 grit DMT stone. I can make it shave more easily with a 600 and 1000 grit DMT stone, or my set of arkansas stones. I can even make a knife shave with a 600 grit belt on a grinder.

So why can't I sharpen a knife with a higher grit stone or belt? I have one of the DMT extra extra fine stones, the 8000 grit one. I have a 6000 grit belt. Both of them leave my knives duller than when I started...


Why?

 

[willis]

Maybe raising the angle to get the same feedback?

 

[HeavyHanded]

I can make a knife shave with a 400 grit DMT stone. I can make it shave more easily with a 600 and 1000 grit DMT stone, or my set of arkansas stones. I can even make a knife shave with a 600 grit belt on a grinder.

So why can't I sharpen a knife with a higher grit stone or belt? I have one of the DMT extra extra fine stones, the 8000 grit one. I have a 6000 grit belt. Both of them leave my knives duller than when I started...


Why?

When freehanding, feedback will diminish a lot as you climb to higher grit surfaces. As Willis says, you might be elevating the spine unintentionally to get more feel, or the lack of feedback has you rocking the bevel and thats resulting in a broader edge. The margin of error for a "flat" bevel is determined by the size of the abrasive, and moving from a rougher finish to a finer one will always reveal a slightly convex surface. If you progressively work from the apex each step, the edge will rapidly grow more broad. To a lesser extent it will also grow more broad if you work from the middle point to the apex.

I remind myself to always work from the shoulder to the apex, or if a convex I'll work from a point well back from the edge and advance toward the edge from there.

Also have to change your cut tests somewhat, or at least how they're done. A rougher finish makes an edge that can catch more easily and will have a large advantage when drawn across a sample material. As the edge finish becomes more and more fine, it will lose this drawing advantage and shift to more of a pressure cutting mechanism.

Lastly, on most surfaces it is very important to reduce pressure considerably at the higher finish levels.

The following is from an older post on freehand sharpening but applies to this.

Thought I'd add a little of my own theory to the discussion. I've long felt that since tactile and audible feedback represent the only useful sensations to work with when freehanding to precision, the limitations should be fairly easy to identify. I do not believe one can readily discern between angle variations that fall within the margin of error defined by the abrasive. By this I mean for example, say a 40u abrasive is making 5u troughs, the minimum possible amount of variation is going to be just under 5u. At higher angles (where you are crossing the theoretical plane of that bevel side) you will feel the edge catching 100% on the abrasive. At lower angles you will either feel it catch on the shoulder, or there will be a complete absence of catch along the edge. Keep in mind, you can feel the difference whether moving into the abrasive or moving away from it, though there is a bit more feedback when edge leading.

With a convex you can still feel the variation, so the lack of a shoulder to frame the level of feedback is nice, but hardly essential.

In my diagram, #1 shows the underlying undisturbed steel that falls 100% within the grind troughs. The zone to the outside of this is the region where the troughs are, so only a percentage of steel remains. This also illustrates why, when at the grinding phase, burr removal is limited to material that falls outside this region - any unsupported steel (bur) that is within this region will have to be removed with a loose abrasive or one on a conformable surface, as it will be bordered by shielding projections.

#2 shows how the abrasives can tear troughs out of the steel within the margin of error based on abrasive size. For the most part, any passes that fall within this amount of deviation will go largely undetected to the senses.

#3 is a funhouse blow up of the possible grind path deviations, and

#4 funhouse blow-up shows where the highest percentage of steel will remain after the grinding process and how that convexity is already built into the scratch pattern.

The size of the abrasive is going to regulate this to some extent - virtually every time you freehand to a finer abrasive, you will initially uncover more convexity than you thought was there from a visual examination of the coarser bevel. It doesn't matter what the absolute abrasive size is. In the diagram #2, that bevel to the naked eye is going to look flat even though from a percentage of material removal, its already convexed but "hidden" within the grind troughs.

This is why I always remind myself and recommend to newbies, always work from the shoulder out. This is the most reliable way to maintain the original edge geometry as you refine the scratch pattern. If there's no shoulder (full convex) work the back bevel for awhile before advancing on the apex. Its possible to "ride" the convex portion and grind it down to meet the target geometry, but now you're working from the area with the least possible tactile feedback - keeping in mind that with every jump to a finer abrasive there will be less feedback anyway.




Hope this makes sense...

 

[awestib]

I remind myself to always work from the shoulder to the apex, or if a convex I'll work from a point well back from the edge and advance toward the edge from there

Do you mean that literally (you feel the shoulder) or figuratively (imagine where the shoulder is) Martin? When I do my grinding/sharpening on Non-Washboard devices, I often do a few very light "beginning strokes" to feel where I am and then go forth and back along the edge. I noticed that I subconsciously try to feel the shoulder first so I know I am too acute. My finishing strokes are usually edge leading single strokes along the whole bevel and the bevel glides easily only if I have low enough pressure and if I am right on. It's almost as that the bevel floates on the bit of water (on diamond stone) or oil (if Crystolon) that is on the surface. Again the shoulder-feel is a good starting point.

 

[HeavyHanded]

Do you mean that literally (you feel the shoulder) or figuratively (imagine where the shoulder is) Martin? When I do my grinding/sharpening on Non-Washboard devices, I often do a few very light "beginning strokes" to feel where I am and then go forth and back along the edge. I noticed that I subconsciously try to feel the shoulder first so I know I am too acute. My finishing strokes are usually edge leading single strokes along the whole bevel and the bevel glides easily only if I have low enough pressure and if I am right on. It's almost as that the bevel floates on the bit of water (on diamond stone) or oil (if Crystolon) that is on the surface. Again the shoulder-feel is a good starting point.

In general, I literally start out on the shoulder and advance the scratch pattern across to the apex. In my experience, this tends to keep the bevel at the target angle. Since there's always a bit of deviation, and the tendency is almost invariably for the angle to increase, all the small correction I do that might make it more acute on paper tend only to maintain it at a steady angle, especially on regular V bevel edges.

This assumes the angle is a good target to start with. On some Scandis this will lead to an inclusive that's a bit unrealistic for doing any work, but I'll frequently convex them a degree or so anyway - - but only from the original shoulder. If the edge has gotten more obtuse by following a different point along the original bevel face, the terminal angle is going to creep that much larger when slightly convexed and require a lot of recovery work later.

Once the bevels are established at a good angle, in my hands if I don't work from the shoulder out at every transition I'll frequently make the angle slightly larger. Catching it only as it becomes obvious at the higher grit values where it cannot be easily fixed. On some of my Scandis that have been around for a few, I begin to phase out the convex as the bevel gets "worn in" at a good angle anyway, approx 25-26 inclusive rather than the 22 or so that it comes with from the factory.

Occasionally I have to make an edge a bit more broad due to it being thinned out a tad, mostly just on my convex edges - this is a good problem to have. Much easier to fix than having to thin the thing out.

Martin