Sharpening dynamics

M

MJB

Joined
Nov 16, 1999
Messages
105
I've been using a professional model Edge Pro for a few years now...very happy with the sharpener itself and the results. Lots of friends are too.

For some time, I have been examining various edges under 30-50x magnification, sometimes >100x when I have access to my friend's microscope. I have also used other sharpeners, steels, diamonds, etc. After experimenting a fair amount, this is my quandry:

Under high magnification, many hair-popping-sharp edges (even brand new blades just received from the makers) look like plowed fields, with countless parallel scratches in the steel, resulting in an undulating, saw-like edge. It looks like what you might call micro serrations. The angles for the blades are from 18 to 22 degrees. These edges will frequently pop the hairs right off with no problem.

On the other hand, I have worked on edges with stones progressing from 220 grit to 1200 grit, then waterstones at 4000 or even 8000, then stropped with 10,000 grit chromium oxide, until they have a mirror-like polished surface and a very smooth, straight edge (under 50x mag). These edges are also at 18 to 22 degrees.

The rougher edges are fairly easy and quick to produce and, if shaving is an effective measurement, they are also sharp. Other tests, like grabbing the fingernail, also seem to indicate a sharp blade.

However, the highly-polished, straight edges take some time and don't seem to shave or grab the fingernail quite as well. These mirror edges last well and are easy to touch up, but don't seem to possess the raw cutting power that the rougher edges have.

Have the expert sharpeners in the forum noticed this too or am I missing something? Is it just a waste of time to work for the mirror-polished, precise edge? It seems that the quicker, easier job would meet most folk's demands just fine, but those mirror edges sure look nice.

Any thoughts?

Mike
 
I recall A.G. Russell was selling some knives a few yrs ago that were made of some type of cast steel that supposedly had all of these micro serations that were supposed to make it cut better. Maybe there was something to that. I am not an expert sharpener but I have noticed that with some of the stainless steels such as ATS-34 or CPM 440V it's pretty difficult to get as nice and smooth of an edge as with the carbon steels such as Carbon V or M2. Still, the stainless stuff cuts just fine. I recall reading in some knife mag that a "rougher" edge actually cuts certain materials such as rope better than a more polished edge. I do know that if your shaving with a straight razor, you had better have that nice, stropped edge or you will think you are scraping your face with a broken bottle! I guess I always thought that with knives the advantage of the polished edge is that tiny little pieces of steel would be less likely to get torn off, resulting in premature dullness. Just my 2 cents worth . . .
 
MJB :

First off that is a very solid post on a topic that isn't in general well known.

Is it just a waste of time to work for the mirror-polished, precise edge?
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Increasing the polish of an edge will in general raise the push cutting ability while decreasing the slicing ability. Highly polished edges are also more durable in regards to impacts and less metal is removed with each sharpening. most cutting is a mixture of push cuts and slices so which edge is best overall depends on what you are doing and how you are doing it.

For more detail, this has been discussed frequently in the past. To go way back to the source of most of the internet based discussion, Mike Swaim did a lot of work comparing highly polished to rougher edges and this was discussed on rec.knives many years ago. Joe Talmadge also mentions it in his knife FAQ and he has also quantified the performance to no small degree including a unique dual edge sharpening process.

-Cliff
 
I am no X-SPERT, but I have found the same results as you. A very polished edge just doesn't work well for me, I like my edges with some bite.
 
I just wanted to add that after hearing about the phenomenon from reading Joe's FAQ, I started going with rougher edges. But I have recently found that I went with TOO rough an edge. I sharpened my MEUK on a "medium" waterstone (I don't know the grit, but I think it is 1000 or 1200) and that finish was way better for my cutting needs than when I has just sharpened it on my rough stone.

I lie my SAK because of the 2 blades. I have 1 sharpened with a toothier edge, and one with a more polished edge. It sure is handy having both a push cutter and a slicer in your pocket!
 
I've found that it all depends on what you cut. I use an Edgepro Professional, myself.

If I was looking for a good all-arounder, including dressing game, I might leave a toothy edge. In fact, my original AR edge was toothy, and I was quite impressed.

But for push cuts, trimming thread, caping and anything with paper, I like a mirror edge. Further, I just put a mirror edge on that same AR and I'm going to try it for a week.

One friend is a chef, and she prefers a toothy edge for wet vegetables. Another friend is a test engineer and he prefers a toothy edge for cutting plastic pipe. I kind of like the feel of a mirror edge slipping through paper on a push cut. Go figure.
 
Except for shaving a little tooth on an edge is almost always preferable. Unless you have your blade clamped in a fixture you will always apply a little bit of transverse slicing action when you cut. A little bit of tooth will cause the edge to apply higher pressure to a scattering of points along the cut line. This will start cleavage of the material and weaken it allowing further cutting to procede more evenly.

With shaving you want to avoid the edge snagging in your skin and drawing it up to be sliced. In that application a toothy edge will tend to pierce the surface and cause it to bunch up and ramp up into the edge (ouch). The toothy edge may still cut the hair a little easier than a smooth edge, but you may have trouble with scraping or nicking your skin if you shave your face. If you are trying to seperate muscle fibers without cutting them you might want a smooth edge and a rounded point for much the same reason.

I usually do only casual shaving with my knives so I leave a little tooth on them. I want to minimize friction on the sides of the blades so I polish them and then go back and put a little tooth on the final edge.
 
Many thanks for the comments on the polished vs. rough edge.

You can't imagine (well, maybe you can) how relieved I was to learn that I wasn't doing something incorrect or stupid.

Except for a few cases, I will be able to save some real time in the future by backing off a bit on the polished edges.

This forum is a great resource for learning from experienced, knowledgeable folks.

Mike
 
Good thread guys..., and yeah.., a topic that isn't so well understood for the most part...., thanks!


"Hunters seek what they [WANT].., Seekers hunt what they [NEED]"
 
Crayola :

But I have recently found that I went with TOO rough an edge.
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All slicing contains an element of push cutting ability, and since rougher finishes degrad push cutting performance there will come a level of roughness at which slicing will be degraded because the push cutting component has lost more performance than has been gained by the aggression on the draw. The level at which this happens depends on what you are cutting. For some materials the limit is really high, on others it is very low. With hemp rope for example 100 grit AO belts, which are many times more coarse than any benchstone, give very high slicing performance. The same finish isn't able to slice very fine materials as the teeth catch and drag. It has to do with the inherent strength of the material being cut and the volume, how resistant is it to being moved and/or compressed. In general, the lower this is, the higher the finish it requires.

-Cliff
 
Cliff,

exactly! And what it really boils down to was that I needed to take a look at what my EDC needs are, and sharpen accordingly. What I had done is read tons on how great toothy edges are and changed the edges on my knives based on that info. I should have used that info as a guide, but changed my edges based on how they were performing in the materials I cut! :)

Now that I have that down pat, I need another knife nut problem to wrestle with! Any ideas?
 
Hi MJB. In our testing in the early '70's, we learned that the "toothed" edge will almost always cut more aggressively than the polished edge. Even when shaving with a straight razor. The act of "stropping" is wisping out the burr on a relatively soft steel (straight razors). Under 50X, it looks very ragged (the burr) and that is what seemed to shave the best. (I shaved with a straight razopr every day for more than 5 years). re-stropping the razor, rather than sharpening usally worked best, sharpening only once or twice a year.

With knives, we learned that the rougher edge cut better, but the polished edge stayed sharper longer. after much testing, the logical conclusion was a serrated edge that was highly polished. That cut the most aggressively and stayed sharper the longest.

That is why Spyderco introduced serrated edges back in 1982. The Sharpmaker also resharpened these serrated edges by polishing inside of the tool which was possible due to the small radius found on the corner of the triangular shaped sharpening stone.

IMO, your evidence and thinking is clear.

sal
 
Sal Glesser :

With knives, we learned that the rougher edge cut better, but the polished edge stayed sharper longer. after much testing, the logical conclusion was a serrated edge that was highly polished. That cut the most aggressively and stayed sharper the longest.
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A serrated edge which is left coarse will out slice one that is highly polished on most materials (ropes and such) and retain this higher level of cutting ability for much longer, many times to one. Of course it doesn't push cut nor handle impacts as well, but serrated patterns in general are very poor at this compared to straight edges. In general rougher edges will have greater edge retention in slicing than polished ones, but lower edge retention (and cutting ability) on push cutting.

As was brought up on rec.knives awhile ago, if your knife is optomized enough for the task at hand, the edge doesn't need to be very rough at all for the cutting ability to be high. An Opinel for example even at a mirror polish will cut through ropes with ease, as it can straight push cut even thick cord easily, with less force for example that serrated edges on other blades with heavier edge profiles.

-Cliff
 
I'm glad I read this post. I thought it was just another one of those "How do I sharpen my knife?" or "What's the best sharpener?" type threads :rolleyes:. Very informative--thanks, guys.
 
Being a happy owner of an EdgePro Apex, I initially put mirror edges on all my knives. Like most of you pointed out, these surgical edges weren't much good for slicing but fine for push cuts. Since I use my blades mostly for slicing, I have gone back to rougher edges to obtain that "micro-serration" advantage - - but with a twist.

After sharpening with a 180x or up to 220x stone, I finish off the knife with a few passes of the 3000x AlO2 polishing tapes on each side. This means skipping the EF and UF, 320x and 600x stones. The AlO2 tape takes away what little burr wire remains on one side of the blade, and adds the polished looking edge. However, the tiny grooves from the coarser stones are still there.

By doing this I am hoping to retain the advantages of the micro-serrations for slicing, yet put a high polish finish on those tiny serrations so they will last.

The Question: Am I fooling myself, or really optimizing the cutting potention of my blades?

Opinions?

TT2Toes
 
TT2Toes, Try testing how many feet of cardboard you can cut with your knife until you can't cleanly slice copy paper. First test your regular method then try sharpening your way including burr removal with the tape, but then go back and finish with 320 grit (just don't do too many honing strokes which would raise another burr). You tell us which seemed to cut better and which seemed to last longer. (I don't know in general and it may depend on your blade alloy in particular).
 
TT2Toes :

After sharpening with a 180x or up to 220x stone, I finish off the knife with a few passes of the 3000x AlO2 polishing tapes. This means skipping the EF and UF, 320x and 600x stones. The AlO2 tape takes away what little burr wire remains on one side of the blade, and adds the polished looking edge. However, the tiny grooves from the coarser stones are still there.
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This is quite an imaginative solution. It is a well known effect with sharpening chisels because people don't lap the back. Thus you get very coarse grinding ridges intersecting with a polished surface. Lee documents it in his book on sharpening.

By doing this I am hoping to retain the advantages of the micro-serrations for slicing, yet put a high polish finish on those tiny serrations so they will last.
Click to expand...

You will get a slicing ability lower than the coarse finish alone, but a higher push cutting ability. It will be very similar if you finished completely on an intermediate grit. I looked at this effect when it was suggested 3-4 years ago on Bladeforums. A poster suggested sharpening one side of the bevel x-fine and another x-coarse. You get something that reacts like a medium finish, but is much more complicated to maintain. Many makers do this as well, first sharpening the bevel with a decently coarse belt and then finishing with only 1-2 passes on a buffer. This isn't enough to completely polish the bevel, but does tone the roughness down.

You can also try completely polishing the edge bevel and then using a coarse hone on a high degree setting to add a small coarse micro-bevel along the edge. This won't however be as aggressive as if you honed the entire bevel with the coarse hone because the higher angle setting makes more shallow teeth (the aggression is proportional to the tooth depth which is proportional to the angle).

Sal :

How do you do your push cut tests on both plain edges and serrations?
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There are a number of ways, you can for example push cut cardboard, but I usually don't have a lot of that and thus tend to cut wood instead. You can push cut thick rope as well, just remembering to only cut as deep with the plain edge blades as the serrated ones, as the latter are limited to the depth of the scallops, unless you suspend the rope and don't cut on something. I also sometimes cut rubber tubing. For larger blades I also use chopping to induce the blunting, this is usually done last as it can damage the serrations readily with most patterns. Edge degredation by testing push cutting ability on light thread and then slicing aggression on 1/4" poly and 3/8" hemp.

This isn't the critical factor though with serration patterns from my persepctive though as they perform poorly here compared to plain edges. What is more important is how they slice various materials, on what class of materials do they outperform a similar ground plain edge, how is the relative durability (usually far better for the serrated blade), and how is the relative life of the cutting ability.

-Cliff
 
Thanks Cliff_Stamp and Jeff_Clark.

You will get a slicing ability lower than the coarse finish alone, but a higher push cutting ability. It will be very similar if you finished completely on an intermediate grit. I looked at this effect when it was suggested 3-4 years ago on Bladeforums.
Click to expand...

Should've figured some other forumite had tried this already. I may look into Lee's book (Title??) on sharpening. Thanks for the tip.

I had noticed on my S&W HRT fixed blade, that while the edge had a good mirror shine to it, there were tiny parallel grooves on each side. This blade had remarkable slicing ability, straight from the box. I figured it was due to a coarse belt followed by a light buffing at the factory. So, I thought this may be a good recipe for my other blades too. I'll read more and try to do some tests.

Thanks again.

TT2Toes
 
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