Edge retention comparison : Sharpmaker CBN, Diamond, Medium (+ steeled edge)

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Edge retention comparison : Sharpmaker CBN, Diamond, Medium (+ steeled edge)

Postby Cliff Stamp » Fri Jul 18, 2014 7:03 pm

As noted in the CBN thread : http://www.spyderco.com/forums/showthre ... 4-CBN-Rods I had been using the CBN rods to sharpen various utility blades for tradesmen who had generally positive responses. Being curious I then did an experiment to compare the initial sharpness and edge retention with a common utility knife :

Image

Basic overview :

-edges were all sharpened at 15 dps
-sharpness was measured by slicing light cord under 50 grams of tension
-1/8" ridged cardboard (used) was sliced across the corrugations with a very short draw (2 cm)
-in total two kilometers of cardboard was cut
-results were partially blinded (the individual measuring the sharpness was not aware of how much cardboard was cut)

The results (red is edge retention, yellow is initial sharpness) :

Image

Summary :

-the CBN and Diamond rods are able to match the performance of the as-boxed edges
-the medium rods alone have high initial sharpness but lower edge retention
-the steeled edge is able to match the initial sharpness but the edge retention is even worse
-the edge reset on a Bester 700 and then finished on the medium rods is superior in both initial sharpness and edge retention

Explanation :

The steeled edge pushes the deformed material on the edge back into place but doesn't actually remove it and thus the edge is left very weak and brittle and it blunts very quickly. The medium rods have a similar effect but since they are abrasive they remove some of the damaged material and have improved edge retention over the steeled edges but are worse than the as-boxed edges in edge retention. The CBN and Diamond rods are abrasive enough to remove the damaged material and thus the edge is on undamaged steel and can match the edge retention of the as-boxed edge.

Now why is the medium rod finish after the edge reset the highest? Two factors :

-all weakened material is removed
-as the draw is very short this is close to a push cut and the ideal finish is likely not the most coarse

A few interesting questions :

-how would the fine rod edge retention be if they were used after the edge reset
-how would the medium rod edge retention be if the edge was first ground with the CBN rods

I would have looked at those as well but after two km of cardboard I ran through all the boxes of that type. I might look at those finishes later.

To clarify this is edge retention on a slice, it can't be extrapolated to edge retention in general. Edge retention on a push cut could very well suffer significantly with the CBN/Diamond rods, I do have some hard plastics to check this on.

As a few points of interest/curiosity :

-these are very cheap blades (less than $0.25 if you buy a large amount of them)
-the medium edge finish (after the B700 reset) could cut 500 feet of used/dirty 1/8" ridged cardboard cut across the corrugations and still wet shave arm hair
-the edges had no chips or visible deformation (except the steeled ones)

Five hundred feet of cardboard is a HUGE pile if the pieces are of any size, it is easily 1000+ pieces, and again the edge would still wet shave and this is a basic steel. Keep that in mind the next time you think that a pile of cardboard is an impressive example of edge retention.

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Postby sir_mike » Fri Jul 18, 2014 7:27 pm

So it looks like the CBN rods are alittle better than the diamond ones. It that your opinion?

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Postby JD Spydo » Fri Jul 18, 2014 7:31 pm

It's funny that you do your comparison with utility box cutting blades because I just did about 30 of them this week on my Spyderco DOUBLESTUFF stone. I actually had decent results with the Doublestuff stone and I use it a lot just to tough up my EDC folders as well and it does a fairly decent job in between my complete sharpenings.

I've been watching your threads on the CBN stones and I do find the information intriguing. I do plan on getting some of the CBN stones before the summer is over.

I've also had decent results sharpening box cutter blades on 2 different natural stones here lately. I've all but quit using natural stones on many of these steels that Spyderco uses but I still like the results on utility knife blades. If the box cutter blades are really dinged up badly I mainly use either aluminum oxide sandpaper or Silicon Carbide paper and that really sets them up for a great sharpening. I don't mean to drift off in another direction but I do find it an interesting coincidence since I just did a few of the box cutter blades here at the work shop just this week.

Also I would like to know how easy or difficult the CBN stones are to clean compared to the Spyderco ceramic?? Looking forward to the feedback you receive on the CBN stones.
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Postby MacLaren » Fri Jul 18, 2014 7:43 pm

Cool thread :)
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Postby sir_mike » Fri Jul 18, 2014 9:04 pm

JD Spydo wrote:It's funny that you do your comparison with utility box cutting blades because I just did about 30 of them this week on my Spyderco DOUBLESTUFF stone. I actually had decent results with the Doublestuff stone and I use it a lot just to tough up my EDC folders as well and it does a fairly decent job in between my complete sharpenings.

I've been watching your threads on the CBN stones and I do find the information intriguing. I do plan on getting some of the CBN stones before the summer is over.

I've also had decent results sharpening box cutter blades on 2 different natural stones here lately. I've all but quit using natural stones on many of these steels that Spyderco uses but I still like the results on utility knife blades. If the box cutter blades are really dinged up badly I mainly use either aluminum oxide sandpaper or Silicon Carbide paper and that really sets them up for a great sharpening. I don't mean to drift off in another direction but I do find it an interesting coincidence since I just did a few of the box cutter blades here at the work shop just this week.

Also I would like to know how easy or difficult the CBN stones are to clean compared to the Spyderco ceramic?? Looking forward to the feedback you receive on the CBN stones.


I too am very interested in this. Since the CBN's release, I have been trying to find out about these rods vs the diamond ones. :)

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Postby Brock O Lee » Sat Jul 19, 2014 3:30 am

Very interesting Cliff...

Intuitively your results and explanation makes sense to me.

Like you said, it would be interesting to see if Diamond (or CBN) reset + Med SM gives similar edge retention to Bester reset + med SM. I would assume so, since in both cases you first remove damage and then refine the edge for better initial sharpness.

2 km of cardboard, wow... :eek:
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Postby J D Wijbenga » Sat Jul 19, 2014 6:18 am

Very interesting, Thanks!

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Postby Cliff Stamp » Sat Jul 19, 2014 6:25 am

sir_mike wrote:So it looks like the CBN rods are alittle better than the diamond ones.
The CBN rods have slightly higher initial sharpness and the Diamond rods slightly higher edge retention but it wasn't a significant difference, it was within the random spread of the results.

If you look at the bars you will see lines drawn through the tops of them that go up and down. Those lines are basically how much confidence there is in the results, how much you expect them to vary. In order for a result to be significant, for the difference to be meaningful then it has to be much larger than those lines.
Brock O Lee wrote:resting to see if Diamond (or CBN) reset + Med SM gives similar edge retention to Bester reset + med SM. I would assume so, since in both cases you first remove damage and then refine the edge for better initial sharpness.
Yes, there is nothing special about the Bester I am just using it for the bulk of my current work. The bevels on these blades are fairly interesting however as they are not flat but hollow ground and it took 300-400 pps to actually flatten the hollow which isn't evenly ground along the blade. If I had more of the same cardboard I would have used a few more finishes. If I had to know I was going to do this much work I would have just random sampled from the beginning but I actually intended to just compare CBN to as-boxed, the others just sort of fell into place.
2 km of cardboard, wow...
Yeah and this was not even taking the edges to beyond shaving, cardboard simply that isn't that difficult to cut with a properly sharpened knife.
JD Spydo wrote: I've been watching your threads on the CBN stones and I do find the information intriguing. I do plan on getting some of the CBN stones before the summer is over.
The critical part about using these is how fast they recut the blades. It only takes 5 pps to get the edges almost back to optimal sharpness and cut off weakened metal. I do another 10 pps ultra-light trying to squeeze out that last bit of performance but practically this doesn't have much functional gain. In contrast any finer stone would take much longer and the blades would simply not be sharpened and just replaced.
Also I would like to know how easy or difficult the CBN stones are to clean compared to the Spyderco ceramic?? Looking forward to the feedback you receive on the CBN stones.
The diamond and CBN stones don't load up significantly, but again I use them with very light force. I have not cleaned the CBN rods yet. The finer diamond stones will load but it is light. On the MXF dmt plate I just spray it with water and wipe it with tissue paper after I am finished using it. With the fine DMT plate on occasion I rinse it with soap and water and scrub it with a toothbrush as it would just tear up paper if you tried to clean it that way.

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Postby _centurio_ » Sat Jul 19, 2014 9:01 am

Very nice and interesting experiment! Thank you.

The question I have is how much weak material of the edge has to be removed before sharpening to achieve high edge retention? As I understand for example using only the Sharpmaker for touching up the edge the edge retention will be low. Would it be enough to cut 1 or 2 times in the medium rods and then start sharpening to achieve high edge retention? Or do I have to use diamond/CBN rods to take away enough of the weakened edge before finishing on the medium rods?

2km of cardboard is insane... I can't imagine due to my personal experiences. Even my ZDP Spyderco blades got far beyond shaving sharpness cutting WAY WAY LESS cardboard even if I cut into a stone removing weakend material, made the burr smaller and smaller using very light pressure while finishing the edge. My mind is blown haha...

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Postby me2 » Sat Jul 19, 2014 12:43 pm

Just for clarification, 2 km of cutting was spread over all the blades used, right? Any idea what the steel actually is?

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Postby Cliff Stamp » Sat Jul 19, 2014 1:28 pm

me2 wrote:Just for clarification, 2 km of cutting was spread over all the blades used, right?
Yes.
Any idea what the steel actually is?
I have asked before, the general answer is high carbon steel, it sparks like it. They also have HSS ones but they are sold explicitly as HSS or bi-metal blades.

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Postby Cliff Stamp » Sat Jul 19, 2014 1:31 pm

_centurio_ wrote: The question I have is how much weak material of the edge has to be removed before sharpening to achieve high edge retention?
As much as was damaged - however you can't know that really so what you do is trial and error from experience. I have found that if I do 2-3 light cuts into a fine grit benchstone then unless the knife had visible damage the edge retention is optimal, grinding off more steel doesn't produce better results. What I would suggest is simply making 1-2 cuts into a stone and then 2-3 cuts and see if the latter helps, if it doesn't then just stick to 1-2 and keep in mind how heavy the knife was used when you go to sharpen. As well when you are sharpening there are obvious signs that the edge still has damaged steel :

-forms a burr really easily and large
-the edge tends to want to bend back and forth rather than be cut off

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Postby _centurio_ » Sat Jul 19, 2014 3:03 pm

Thanks for the information Cliff. Another question I have is why is the edge retention using only the medium SM rods lower than the as-boxed?

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Postby Cliff Stamp » Sat Jul 19, 2014 6:07 pm

_centurio_ wrote:Another question I have is why is the edge retention using only the medium SM rods lower than the as-boxed?
To clarify this was the method of sharpening with the medium rods :

-5 pps on the 15 to set the edge
-2 pps on the 20 to deburr
-5 pps on the 15 with ultra light force (5 to 10 grams) to maximize the sharpness

I experimented with slightly more/less passes (up to 15 pps) but it didn't do anything functional.

The critical thing is that the first two passes got the edge up to almost as-boxed sharp immediately and this is through just deforming the steel and pushing it back into place. This I think is the critical problem because it is leaving weakened metal on the edge.

I did try a run where I cut the edge off and then sharpened it on the medium rods and the results improved, but it takes the medium rods a long time to regrind an edge by themselves and they also load up and you start to get burnishing and the ideal way I find to use them is as a micro-beveling tool.

Now you might ask - well why did you use the bester, why didn't you use the CBN rods to reset the edge and them micro-bevel with the medium rods? That is an excellent question and the answer is that I didn't think about that part of the experiment well, it would have made more sense to do that. I just used the bester out of habit as it is the main waterstone I have been using the last year.

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Postby Cliff Stamp » Tue Jul 22, 2014 6:16 pm

Video over view :

[video=youtube;f-didYYIbgA]http://www.youtube.com/watch?v=f-didYYI ... e=youtu.be[/video]

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Postby _centurio_ » Wed Jul 23, 2014 11:23 am

Thanks for the vid.

Now I have a question which is very important to me: Is it possible removing the weakened material by only sharpening with the medium rods? Of course not only a few passes per side as you mentioned but let's say 20pps? Or better cutting one or two times in the rod, then sharpen the blade?

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Postby Jax » Wed Jul 23, 2014 2:05 pm

I would like to see the specs for a blade sharpened on the bested 700,then finished on the medium rods.
Or CBN to medium rods.
How would including the fine rods change the TCE?
Thanks.

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Postby Donut » Thu Jul 24, 2014 10:15 am

So, how much metal do you need to remove to give you a fresh edge?

Does the sharpening medium play a part in how much it deforms the edge? (Will diamond cut into the steel to remove the deformed steel better than the Medium Ceramic?)
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Postby Cliff Stamp » Thu Jul 24, 2014 10:35 am

Jax wrote:I would like to see the specs for a blade sharpened on the bested 700,then finished on the medium rods.
I am not sure what you mean by specs?
How would including the fine rods change the TCE?
Good question.

For long draw cuts in abrasive material then a coarse edge is preferred, the opposite for a push, this cutting is a blend between them.
_centurio_ wrote:Thanks for the vid.

Now I have a question which is very important to me: Is it possible removing the weakened material by only sharpening with the medium rods? Of course not only a few passes per side as you mentioned but let's say 20pps?
Yes, just :

-make a few light passes into the rods directly as if you were slicing it in half
-sharpen

Be expected to take a long time, as in a few hundred passes. You can speed it up if you press hard but it is very easy to press too hard, strain the edge and you have to start over. I am fairly convinced after seeing all the reports of people who describe very low performance in durability and edge holding that they are cutting with heavily strained edges.
Donut wrote:So, how much metal do you need to remove to give you a fresh edge?
All of the damaged part. There is no way to know (without high magnification inspection) what I suggest is to just do it and keep track. For example if I am sharpening a blade which just had regular work done and is blunt but not damaged then I typically do 1-2 passes on a coarse grit (cheap) stone. I then inspect the edge and make sure it is uniformly reflecting light, if it isn't then you have a hollow which you can correct by repeating the cutting.

However if I get a set of knives from someone who has used them to the point there is no edge at all, sharpness is < 1% and the edge shows visible damage then I do at least 5-10 passes and then assess the edge. As long as I can get the majority of the edge forming clean I stop.

If you pay attention to when you are sharpening you can tell as well because you if form an apex on steel which is damaged then it tends to very easy form a much larger burr, just compare :

Image

to :

Image


Now I am not saying you can't burr/damage an edge which had all the weakened material removed, just that it is less prone to it. In short the process is just do 1-2 destressing passes and see if it makes a difference. The next time do 2-4 and see if it helps. Stop when it doesn't do anything further and use experience on different knives, steels and users.
Does the sharpening medium play a part in how much it deforms the edge? (Will diamond cut into the steel to remove the deformed steel better than the Medium Ceramic?)
Yes, but just because it is more coarse.

I checked this as I was curious and the CBN has the same steeling effect. If I just do 2 passes on the CBN rods the edge is very sharp, but at that point while it did remove metal it also pushed a lot of it back into place. I was doing the same amount of passes with all, but of course the more coarse ones remove far more material.

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Postby Cliff Stamp » Thu Jul 24, 2014 6:56 pm

Here is another comparison but this one was more of a push cut. I was cutting thick plastic (screw ties) and measuring the push cutting sharpness. This cutting trial gave me a new appreciation for cardboard because :

-it is far more physically demanding
-the rate of blunting is extremely low
-blunting is extremely erratic
-it puts sharp shards of plastic everywhere


In general the as-boxed blades had more damage than the CBN rod finish, however the blunting was so erratic that I would not be overly confident that the same pattern would hold with extend work.


Image


This is a sample of what the edge looks like on the as-boxed edge after the runs, it is commonly dented/rolls. In contrast :


Image


This is the edge of off the CBN rods after the cutting.

Note that those little hooks are actually what was left over from the kind of damage from the as-boxed edge, they are not caused by the cutting itself. This is a sign obviously that I could have cut off more steel before sharpening and that the performance of the CBN rods was likely lower than optimal.


Image


The initial sharpness is lower with the CBN rods, that was not surprising because this is a push cut and that is optimized for a high grit finish. However the interesting thing is the TCE starts to get closer the more material is cut and it looks like if I continued that the CBN would overtake the as-boxed finish so the cutting lifetime would be higher.


It is not overly obvious why the sharpness on the CBN decreases slower than the as-boxed edge and there are a few interesting experiments that are suggested by this to try to determine the exact case :

-a longer cutting run
-a finer finish run (medium / fine grit rods)
-adjust the CBN rod angle to the as-boxed angle (a couple of degrees lower)



--


However the main question I wanted to do this type of cutting trial was to check if could the CBN rods be used to sharpen those utility knives if they were used for mainly push cuts on harder substances like plastic and the answer is definitely yes.

--

Note the Stanley knives have a primary hollow grind which is about 7 dps (if it was flat) and they have a secondary edge bevel of 12-13 dps.


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