CBN Rods

[Cliff Stamp]

As an update, I recently did an experiment to look at the edge retention on a push :


-cutting done with the as-boxed finish and resharpened with the CBN/15 dps
-used screw ties were used as the blunting media
-cuts done almost on a push (very little draw as it is hard plastic)
-push cut Espirit thread to determine sharpness

I expected that the CBN would be behind because the finish is much more coarse and while it does well for slicing it would not do that well for push cutting. However the experiment hit a snag because :

-cutting thick plastics causes a very low rate of blunting
-the blunting is not at all consistent

I needed to make a 1000 cuts into the screw ties (like zip ties but fairly heavy and are used/dirty) to even produce a significant loss of sharpness and even then it was only on certain parts of the edge, the vast majority of the edgeis at almost as boxed in sharpness after the 1000 cuts. But with two runs done with each there is a still a significant difference in how they behave :

-the CBN has a lower initial sharpness

There is no surprise here it is push cutting sharpness and that increases with polish, but what was not expected is :


-the edge durability is higher with the CBN


Now I have only done two runs with each, so this still may be nothing but random variation but :


-both runs of the as-boxed showed the edge reflecting light in spots which were clearly visible by eye
-under magnification you can see the edge is clearly dented/bent
-this starts to set in at a few hundred cuts and just gets worse


In comparison :


-one run of the CBN showed no light reflecting through the entire 1000 cuts
-the second one showed two small spots barely visible


Under magnification the difference is quite obvious - which raises the question as to why it is happening? There are three possible causes :


-the more coarse edge of the CBN is doing more of a slice and stabilizing the edge in the cut
-the 15 dps angle is slightly higher on the CBN than the as-boxed edge (a couple of degrees)
-the initial edge is power sharpened and over heated/damaged

I think what is happening is a combination of all three effects. This can be determined through further experimentation :


-resharpen with the CBN at the exact same angle as the as-boxed, just tilt the Sharpmaker to lower the angle slightly
-constrain the cutting to a pure push cut (rocking cut)


If neither of those made a difference then it points to power sharpening being the issue, this would then be checked by comparing the edge on the fine grit rods which should show an improved finish over both the CBN and as-boxed. However I don't actually have enough material to do any of the above experiments so that will have to wait until I get it (which is just random).

[Mjc1973]

Cliff,
I am not really in a position to help right now but is there a way we could donate testing materials i.e. Cardboard, plastics.
I find your information very interesting and when I am able I would like to help. Maybe there are people on this forum who could help right now but just need to know the best way to go about it.

Thanks, Jamie

[Cliff Stamp]

... is there a way we could donate testing materials i.e. Cardboard, plastics.

I cut up what is disposed of as garbage. I looked into many years ago into buying actual materials but I decided against it for a number of reasons :

-I cut up a tremendous amount of material
-the raw cost of the material
-the disposal of the waste

The cost is a factor because lets say instead of using those screw ties I bought actual zip-ties then it would cost $50 in raw material just to look at those two finishes (mainly because of how often I repeat something just to make sure). If I want to look at other finishes it costs $25 for each one. Therefore I either spend $100 on zip-ties or I spent $100 on two stones and use them as part of a further experiment and I just use screw ties as I get then randomly.

As well, while I am not as extreme as many, I don't like the idea of buying something just for the sake of turning it into garbage. All of the material I cut has already been selected for disposal and by cutting it up a lot of the time this is avoided but at worse case scenario I didn't make any more garbage.

I appreciate the offer but would ask that if you wanted to help the best way is to review/offer feedback. While I am fairly careful, and often very critical of what I do, anyone doing work is going to be biased in many respects so independent commentary is always of use and much of what I do comes from questions/commentary.

[Cliff Stamp]

The simple things can often not be :



This was dropped off to sharpen, it would appear to be quite simple :

-edge is < 0.003" thick
-steel is 5Cr13

However :

-there are multiple bevels, one is > 25 dps
-the primary grind is really uneven
-the edge was used to the point it is completely flattened

I had to apex the edge and then cut it off twice before it would form clean. 1000 pps on the CBN rods (light force, 1/2 lbs but small contact area meant the pressure was high 10-20 psi) to set a clean 15 dps bevel, then just 5 pps on the medium rods at 20 to set the micro-bevel.

[Mike157]

Great information as always Cliff. Your generous sharing of the methodical testing you do has greatly increased my understanding of steel, edges, sharpening, and sharpening methods. All of this had made me more willing to use my knives instead of babying them quite so much and has added significantly to my enjoyment and satisfaction when using them. Your work is appreciated! Mike

Ha! And now I'm off to sharpen a Swiss Army Knife!

[Cliff Stamp]

Thanks. I was helping a friend out yesterday and he was throwing out :



So I now have some ultra-abrasive material I am sorting out as I want to run a couple of trials to check an idea I have about edge retention (and ape stability) in ultra-high carbide steels.

[joelozzy]

Guys,

Is someone able to point out the pros/cons of the Diamond vs CBN rods for bringing back dull knives than utilising the medium and fine stones.
I've read the thread but I don't believe there is a clear answer on which rod is superior in terms of durability and grit etc.

[sir_mike]

Guys,

Is someone able to point out the pros/cons of the Diamond vs CBN rods for bringing back dull knives than utilising the medium and fine stones.
I've read the thread but I don't believe there is a clear answer on which rod is superior in terms of durability and grit etc.

I am waiting for the answer too! I even posed the question but didnt get a definitive answer! :)

[Cliff Stamp]

I am waiting for the answer too! I even posed the question but didnt get a definitive answer!

They have similar grinding ability and finishing ability, have similar requirements for lubricants.

I will break both of them shortly to see if the durability of the grit is similar or not, however please be aware that this is one sample so may not be representative in general. The only reason I have not done it already is because quite obviously once I do it I can't use the stones any more and there are still a few things I am curious about.

[sir_mike]

I am waiting for the answer too! I even posed the question but didnt get a definitive answer!

They have similar grinding ability and finishing ability, have similar requirements for lubricants.

I will break both of them shortly to see if the durability of the grit is similar or not, however please be aware that this is one sample so may not be representative in general. The only reason I have not done it already is because quite obviously once I do it I can't use the stones any more and there are still a few things I am curious about.

Please dont break one on my account. I am just curious to why Spyderco decided to come out with the CBN when they have the Diamond ones already, especially if there really isnt much difference in them in the way of sharpening results, time or whatever! Seems like there has to be something special with them as a selling point over or vs the Diamond ones!

[sal]

Hi Joel,

Welcome to our forum.

With regards to your question, the simple answer is we don't know. The whole CBN thing is new and we have no history. That's why Cliff has been playing with them for a while. Often times, Spyderco will do experimetal work on new stuff (steel, CBN, etc.) for which there is little or no history. Our forum has some real afi's and they are a good source for "real world" testing. We can do lab testing, but as we all know (or should) labs do have their limitaions. I imagine we'll know more in the next couple of years.

Cliff has been gracious enough to do some testing and share his findings. Hope you enjoy your time here and get to gain knowledge as well.

sal

[Cliff Stamp]

It is an interesting question, just realize when you are saying which one is more durable that is just asking which one breaks first, the only way to know is break them and find out. I want to do a little more comparative work however before I do that.

[joelozzy]

G'day Cliff and Sal,

Thankyou Kindly for your detailed response. Am definitely enjoying the forum thus far.

I've put an order in for the CBN rods.

By durability I was referring to the rods lifespan + need for ongoing cleaning. No need to break one!
When you say similar requirement for lubricants, what do you recommend? Or are they O.K to use dry?

Cheers guys

[Cliff Stamp]

If you are talking about durability in regards to how long will they last if you don't damage them, it is a very long time. I have diamond plates which are 15+ years old and I use them almost every day. As a loose estimate, my 600 DMT plate has about half a million passes on it and it still cuts with the same grit pattern that it did initially. However I use it with water (treated with a rust inhibitor) and with very low force (1/2 lbs, 1.5 to 3 psi). If you press hard and/or don't use lubricant then you can tear out the grit which can kill a plate very fast. The biggest mistake a lot of people make is when they first get the plates they use cheap knives and a lot of force to try to smooth them out, this damages the surface readily. The only people who actually wear out diamond plates are those that use them as lapping plates and do a lot of sharpening. But just remember they are actually grinding aluminum oxide and silicon carbide the things used to grind steel. People who use diamond plates in that manner often remark they can wear one out in a year or so, but again they are doing a lot of lapping on very hard surfaces.

[Patko]

What is with the cleaning of CBN rodes? Is it like medium rodes or maybe they don't need to be cleaned at all?

Thanks,
Patko

[Cliff Stamp]

All stones need to be periodically cleaned, however the more coarse ones will tend to load less. You can actually just use a soft brush on the CBN rods once dry. Periodically flush them in use with a cutting fluid to reduce loading as well.

[Bill1170]

Cliff, what exactly do you mean when you talk about "breaking" the CBN or diamond rods? I would guess this refers to sharpening with high force to tear the abrasive particles off the substrate, if so, it would be great to somehow quantify the force required. Having once ruined a coarse Diafold with excessive pressure, I'm curious to know if CBN performs differently under identical abuse as diamond.

[Brunzenstein48]

The hardness of both materials may be similar at room temperature. With increasing temperature the hardness of diamond decreases exponentially and falls way below CNB properties as these remain unchanged even when heated to tenthousands of degrees.
Recognizing that and Cliff Stamp's formula (on another thread) showing the brutal force on a corner of the rod on a micrometer level, temperatures may (or may not) rise for a very short period of time to extreme levels. For the diamond hardness the timeframe of a few millisecond may be enough to level it way below what one expects. Therefor diamonds may (or may not) wear off especially when force is applied - not because the particles are rubbed off but the diamond itself becomes physical unstable when confronted with heat.

Just a thought why Spyderco may have brought such similar products to the market

[Cliff Stamp]

Therefor diamonds may (or may not) wear off especially when force is applied - not because the particles are rubbed off but the diamond itself becomes physical unstable when confronted with heat.

There are a number of things which happen to both abrasives at high temperatures, both are made from Carbon for example and both can suffer diffusion based wear at high temperature, hence why proper coolant is necessary in grinding. In general, you rarely want to dry grind (as you don't want to breath in the dust from those abrasives). Is it possible that in hand grinding you could reach very high temperatures (600+C) and the abrasives could suffer chemical wear or physical degradation of properties? This is possible, however just consider that the steel you are sharpening is likely less resistant to temperature than the abrasives and it would get damaged long before the abrasive would be degraded.

[Brunzenstein48]

... both are made from Carbon for example and both can suffer diffusion based wear at high temperature, hence why proper coolant is necessary in grinding.

Not so fast Cliff!
Diamond is made out of 100% carbon with all its known properties. CBN is a completely other story - it will not blink or change even at thousands of degrees - not so diamond. Knife steel consists on a micro level so to speak of carbon particles in a variable binder material and a whole bunch of other alloys - so that is not necessarily a race with equal head-start.
Question remains - is it clever not only to work with slow pace but also to use (also on the sharpmaker) some sprayed on water to the rods as intermediary/coolant?