CBN vs Diamond

[Thy Will Be Done]

So it appears not everyone is in agreement as to which is best suited to use on the Tormek.  Tormek emphatically states diamond was chosen over CBN because it's best abrasive for this purpose.  Vadim (RIP) at Knife Grinders states the exact opposite when it comes to their grinding wheels.  Confused yet?  I am, I am in need for something to grind HSS and similar high carbide knife steels.  Which will I be best served by?  I will need a coarse wheel for reprofiling and finer for bevel setting.

https://tormek.com/en/knowledge/the-tormek-way-of-sharpening/sharpening-with-diamond-grinding-wheels

http://knifegrinders.com.au/CBN.htm

[Ken S]

In my opinion, the answer is not a simple A or B. There are many pros and cons to consider. The best answer may be a combination of both, perhaps supplemented with other options.

I remember when Vadim first posted about using CBN wheels. This was several years before Tormek introduced its diamond wheels. The choice back then was CBN or the conventional matrix wheels. Vadim had trouble with his aluminum CBN wheel dishing; however, that must have eventually resolved. In truth, both diamond and CBN are more fragile than conventional grinding wheels and require more careful use.

CBN wheels were designed for high speed dry grinders. That does not mean that they will not work with the Tormek, only that it is a secondary function.

I wanted to do some testing for the forum. As Tormek was not selling either CBN or diamond at the time, I saw no conflict of interest. I contacted the two principle CBN vendors, wanting to borrow some wheels. I advised both that I intended to test them both dry and wet. One emphatically insisted that his wheels could only be used dry. The second told me that using them with water should not be a problem. I don't think he ever tested this. His steel wheels are dynamically balanced with small drilled holes. With plain water, these drilled holes rusted quickly. Also, his wheels have a one inch bore. This allows several different diameter reducing bushings. His reducing bushings are of excellent machining quality and reasonably priced. Unfortunately, they are not of stainless steel and rust quickly with plain water. Using a solution of Honerite Gold prevents rusting. I consider my wet tests successful whel Honerite Gold is used. (In fairness, I will say that I think the first vendor was overreacting to warranty complaints from users who carelessly were not using anticorrosion solution and just left their wheels sitting in water. I don't believe there would be a problem if the wheels were used carefully with solution and properly stored dry.)

Returning to pros and cons, I consider the dust from using either wheel dry a major con. I also consider the value of an anticorrosion  solution in keeping the wheel clean as outweighing the small cost of the compound.

I have one CBN wheel with radiused corners. This is a feature not presently available with diamond wheels.

My newer diamond wheels have a larger side flat area than the CBN wheels. This might be an advantage in knifemaking or sharpening scande grinds.

CBN wheels are available in 80 grit, whereas "coarse" with diamond wheels means 360 grit. For more occasional heavy grinding, low cost conventional wheels are available in 46 and 80 grits.

Do not feel obligated to "match" wheels. Choose the wheel which bestmatches your desired function. That may be one of each or one with a conventional wheel .

This is just my opinion; I am sure you will read others. By the way, Vadim's research was tragically cut shortby his early death. In one of his last videos, he was in beta testing with a 1200 grit diamond wheel. Sadly, we will never know the results of these tests.

Ken

[Thy Will Be Done]

Another question.  None of these wheels make mention of which grit grading scale they are measuring in.  One cannot make a good choice without this information to know what the actual grit range is in microns.  I've seen Vadim write that the Tormek wheels are FEPA and the Sun Tiger wheels are JIS.  Can you clarify this here?

[3D Anvil]

Here's an interesting quote from a company that sells both diamond and CBN grinding wheels:

"Diamond is composed of pure carbon and is produced under high heat and pressure. When Diamond is used to grind hardened steel it causes a chemical reaction and essentially begins to form graphite and lose its strength.

So what makes CBN better for working with hardened steels? CBN is not composed of carbon atoms, instead it is formed with boron and nitride under pressure and heat. Grinding hardened steels produces high temperatures, CBN has a great thermal conductivity, keeping it cool under extreme heat."

But ... my impression is that this applies only, or mostly, to high-speed, high-temperature grinding.  I don't know if it has any impact on a Tormek, with its low rpms and low temperatures.

[Thy Will Be Done]

Here's an interesting quote from a company that sells both diamond and CBN grinding wheels:

"Diamond is composed of pure carbon and is produced under high heat and pressure. When Diamond is used to grind hardened steel it causes a chemical reaction and essentially begins to form graphite and lose its strength.

So what makes CBN better for working with hardened steels? CBN is not composed of carbon atoms, instead it is formed with boron and nitride under pressure and heat. Grinding hardened steels produces high temperatures, CBN has a great thermal conductivity, keeping it cool under extreme heat."

But ... my impression is that this applies only, or mostly, to high-speed, high-temperature grinding.  I don't know if it has any impact on a Tormek, with its low rpms and low temperatures.

Yes, I am aware of the heat issue with diamonds and I do agree that it's likely not a factor with Tormek wheels as the heat simply will not accumulate.

Definitely CBN is a no brainer for high speed grinders.

[Ken S]

The quote is confusing and does not apply to the Tormek. For an accurate picture, watch this video:

https://youtu.be/38c95D6z5D8

Håkan Persson, superabrasives expert and Tormek CEO, gives a very balanced comparison between diamonds and CBN. If you watch the CBN vendors' youtubes, you will find that they use high speed dry grinders. (They recommend half speed "low speed" grinders for beginners, but use the high speed grinders in their own work. CBN is the logical choice for higher speed dry grinding.)

This does not apply to low speed wet grinding. I suspect whoever posted the quote does not understand Tormek sharpening.

Ken

[Thy Will Be Done]

The quote is confusing and does not apply to the Tormek. For an accurate picture, watch this video:

https://youtu.be/38c95D6z5D8

Håkan Persson, superabrasives expert and Tormek CEO, gives a very balanced comparison between diamonds and CBN. If you watch the CBN vendors' youtubes, you will find that they use high speed dry grinders. (They recommend half speed "low speed" grinders for beginners, but use the high speed grinders in their own work. CBN is the logical choice for higher speed dry grinding.)

This does not apply to low speed wet grinding. I suspect whoever posted the quote does not understand Tormek sharpening.

Ken

So it's interesting to note that he claims here the reason for diamond being chosen was that it's harder than CBN.  Vadim says the reason he used CBN was because it is more impact resistant.  That is an interesting property to go off of because as far as I can tell there is little impact to the abrasive as much as just slow wear (which would lend itself to hardness being the most important factor to longevity).

It seems the reason plated diamond plates/stones wear out has mainly to do with excessive pressure causing the shallow plated diamonds to become dislodged from the surface.  Would having a more impact resistant abrasive change this fact at all?  It seems no because Vadim still recommended using light pressure.  There are other purported benefits he claimed with CBN, shorter burr formation, etc.

[Dontheo55]

I don't know, but for knife sharpening, I have three CBN wheels.  They are just a single layer of CBN material but for knives, I have not seen any wear.  180 grit is for re profiling.  Maybe not the correct word but when their are chips md other problems this squares it away.  A 600 grit is used for general knife sharpening and when I really want to get the perfect edge I go to the 1000 grit. 

As for wear I have not detected any.  Such a light touch is used.  Just make sure you take them out of the water once done and you dry them well.  I leave mine out on the bench overnight before I pack them back in the box.  For knives, this is all you need.  I go to diamond for carbide cutting tools and a better bonded CBN wheel when grinding tools steel and HSS.

[3D Anvil]

So it's interesting to note that he claims here the reason for diamond being chosen was that it's harder than CBN.  Vadim says the reason he used CBN was because it is more impact resistant.  That is an interesting property to go off of because as far as I can tell there is little impact to the abrasive as much as just slow wear (which would lend itself to hardness being the most important factor to longevity).

There is impact every time a diamond or CBN crystal hits the blade apex -- at least in edge leading sharpening.

[agwinner1]

I bought four CBN wheels on Amazon direct from China, 80, 150, 400 and 1000 grit, $299 each. I'm using Bora Gold fluid. I am a pro, I'm working farmers markets and doing pop-ups and some restaurants. I usually have 20-30 hours of work a week in season, I've lost count of how many pieces I've sharpened. I wish I had the CBN wheels from the beginning, I went through three stone wheels, struggling to keep them true, getting uneven results (although my customers have been very happy with my work all along.) Stone is great for small volume, not acceptable for high volume. I am working much faster now than befofre. I have had them now for four months, holding up beautifully. I figure Vadim worked with this factory for his wheels, but they offer fewer than he did, two with round shoulders and two with square.

[John Hancock Sr]

Interesting discussion. The hardness of CBN and diamond is quite close and both are much harder than would you are sharpening so I don't think that this is really and issue. However the article linked here https://www.sciencedirect.com/topics/engineering/cubic-boron-nitride#:~:text=Abrasives%20and%20abrasive%20tools&text=The%20popularity%20of%20CBN%20is,grinding%20high%20quality%20tool%20steels.

indicates that "...Due to chemical–thermal degradation, CBN wears five times more rapidly than diamond..." when water cooled. This is due to the layer of boron oxide that is formed is water soluble. So maybe that is why some manufacturers do not recommend wet grinding for CBN.

To be brutally honest for most of us either is perfectly fine, depending on your requirements. My last diamond wheel I got $130 off the odds which made it nearly on par with the cheap Chinese CBN so a no brainer for me.

[LeU]

I was looking into CBN wheels also, and actually the text @JohnHancock refers to (I can only see the abstract of the article) says:

In air, CBN forms a stable layer of boron oxide that prevents further oxidation up to 1300°C. However, this layer dissolves in water, so CBN wears more rapidly when water-based fluids are used than with neat oil fluids.

This made me wonder: has anybody here used oil instead of water, even with a standard SG wheel and would there be any benefit in doing it?

[Ken S]

Tormek mentions that their ACC solution can be used with their standard grinding wheels. I think we must overlook arguments pertaining to high speed grinding.

I also have faith that Tormek, with Håkan's lifetime of super abrasive expertise, had good reason to choose diamond wheels. Tormek could have chosen either.

I am convinced that this not an either/or issue. Diamond wheels are obviously not a good candidate or higher speed dry grinders with the BGM-100. Or, for any wheel coarser than 360 grit. I have 80 and 180 grit CBN wheels. That does not prevent me from using 360/600/1200 grit diamond wheels. There is no need for all the wheels to match.

 Agree that either will work.

Ken

[Thy Will Be Done]

Tormek mentions that their ACC solution can be used with their standard grinding wheels. I think we must overlook arguments pertaining to high speed grinding.

I also have faith that Tormek, with Håkan's lifetime of super abrasive expertise, had good reason to choose diamond wheels. Tormek could have chosen either.

I am convinced that this not an either/or issue. Diamond wheels are obviously not a good candidate or higher speed dry grinders with the BGM-100. Or, for any wheel coarser than 360 grit. I have 80 and 180 grit CBN wheels. That does not prevent me from using 360/600/1200 grit diamond wheels. There is no need for all the wheels to match.

 Agree that either will work.

Ken

Well it seems that Tormek obviously have their reasons and I would say that there may be an advantage but as you say they lack the real low grits.  I don't find this to be an issue because actually I'd be more interested in the higher grits as I don't believe it's needed that you shape most any steel tool (HSS and all) on diamond/CBN.  Where it seems critical is on the higher grits with those harder to grind steels as the abrasive size starts to approach the size of the actual carbides or aggregates of those carbides.