CBN vs Diamond?

[Antz]

Sorry for all my newbie Tormek questions but is there a noticeable difference between CBN wheels vs diamond wheels? Does one preform better than the other? I'm asking because the CBN wheels from wood turners wonders are considerably cheaper. Another thing is I read the CBN should be used only dry, so will using it with water affect the grind? I'm primarily going to use them for knife sharpening when I run into super steels and I also have someone asking me to do their planer and jointer blades which I know would take a very long time on the aluminum oxide wheel.

Thanks
Antz

[Jan]

Diamond is at room temperature about two times harder than CBN, but losses the hardness very rapidly with temperature rising above 700⁰C (1290⁰F).  For temperatures above 1000⁰C (1830⁰F) CBN performs better than diamond. At low temperatures (water cooled grinder) diamond wheel performs generally better than CBN wheel.

The situation is different in high speed grinding. Diamond abrasive, thanks its hardness, is suitable for machining all carbides, but is not suitable for machining steel. The reason is following: at high temperatures produced in high speed grinding process, steel extracts carbon atoms from the diamond abrasive, which results in eroded diamond grit.

CBN abrasive, in contrast to diamond, contains no carbon atoms which could be extracted, and for this reason is CBN better suitable for high speed machining of hardened steel and HSS.

Jan

[John_B]

Antz's post raises a question in my mind. How many knives made of the super steels requiring diamond wheels do you actually see? I have not had anyone bring me one for sharpening. I also wonder if most of these knives are bought more by collectors and are rarely used and would never require sharpening.

[Ken S]

Your question is complicated by a lack of good information. This is not intentional, however, using CBN wheels or diamond wheels with a Tormek is an after thought. The CBN market is primarily woodturners using eight inch high speed dry grinders. The Tormek market uses primarily the traditional Tormek grinding wheels.

When I started researching CBN wheels several years ago, I corresponded with Dave Sweitzer of D-Way and Ken Rizza of  Woodturnerswonders. I trust both of them, however, both of them inadvertently gave me inaccurate information regarding using their CBN wheels wet with the Tormek. I attribute this misinformation to well meant high speed dry grinding backgrounds.

Both brands of CBN wheels work well wet with a Tormek, The keys to successful use are using an antioxidant solution and not leaving the wheel in water.  Diamond wheels have the same anti  oxidant and dry storage requirements. When used this way, I have had good success with both CBN and diamond wheels. Ignoring these two usage requirements was what caused Woodturnerswonders to void the warranty if used wet. The problem is not the wheels; the problem is carelessness.

We have been brainwashed with the "need" to have perfectly flat, polished backs on our bench planes and chisels. Once you learn David Charlesworth's ruler trick, you will never again waste time flattening the entire back of a plane iron. Only a millimeter or two is needed. This is easily achieved with a bench stone. For fine work, a chisel back should be flat and the part near the edge highly polished. Rougher work is not nearly so critical. How many chisels do we really need for hand cutting dovetails. Chisels are marketed in sets. How many of those dozen chisels do we really need. If we buy top quality, factory flat chisels, and only a very few sizes, we will have no need for side grinding with either CBN or diamond. In my opinion, side grinding is a marketing strategy designed primarily to sell more expensive wheels.

My work with CBN and diamond wheels has been almost entirely with searching for a coarse wheel for the Tormek. The SG is frustratingly slow for heavy grinding. The problem is the grit size, not the grit material. Norton 3X grinding wheels in 46 or 80 grit make formidable grinding wheels for the Tormek. Tormek has never been able to get past its one wheel made more versatile with a stone grader philosophy. In my opinion, a proper coarse grit wheel, either Norton, CBN or diamond, eliminates the need for a dusty dry grinder conversion.

I am withholding my thoughts on the SB blackstone until I have a chance to use an 80 grit diamond dressing plate with it. (Wootz' recent post) This may solve the glazing problem which has caused me to sideline my SB.

I now feel confident that the historic lack of a coarse grinding stone has been solved with any of the three options, I will start thinking about finer stones for day to day sharpening. My gut feeling is that the SG, if kept properly trued and dressed, is quite adequate for most users, including me. Anyone who is actually doing high volume knife sharpening might think a diamond or CBN wheel in the four to six hundred grit range is essential. As long as that "need" is only a future possibility, I would stay with the SG and develop your skills. I would follow the forum closely for diamond plate options to the stone grader.

I think we are still in early days with both diamond and CBN. Chances are that future developments will cause your next wheel not to be your "forever" wheel. For anyone looking to buy a second wheel in diamond or CBN, I would look for a simple, straight wheel in a fairly coarse grit, the 360 grit in diamond or the 180 or 80 grit in CBN. For those of us with a T4, the DWC-200 is a real joy to use (360 grit diamond wheel marketed for the T2, but quite usable with the T4).

Between diamond and CBN, I don't feel there is a bad choice. Both work well.

Ken

[Antz]

Wow thanks Ken and Jan, those responses were perfectly worded and answered all of my questions. Very much appreciated, now I have the info I need to make a good choice for my next wheel. I'll probably hold off like you said Ken and develop my skills more before purchasing one but I'll take both of your advice when I finally get around to buying one. Hope those responses also help others wondering the same things as well.

Thanks
Antz

[Jan]

Antz's post raises a question in my mind. How many knives made of the super steels requiring diamond wheels do you actually see? I have not had anyone bring me one for sharpening. I also wonder if most of these knives are bought more by collectors and are rarely used and would never require sharpening.

John, you are correct that the number of knives really requiring diamond wheel is quite small. They are often knife blades with higher content of vanadium carbides which are harder than Al2O3. Although those knives can be sharpen with SG stone, the results are not fully satisfactory.

From practical point of view it is very convenient that the diameter of diamond wheels does not change. This simplifies the bevel angle setting greatly.

I always use my diamond wheel to sharpen masonry drill bits.

Jan

[RichColvin]

I always use my diamond wheel to sharpen masonry drill bits.

Jan,

I'd love to see a video of this.

Kind regards,
Rich

[Jan]

Rich, your interest pleases me! 

I do not have a video, but some pictures of my homemade jig for drill bits are at the end of this thread: https://forum.tormek.com/index.php?topic=3226.0

The picture shows normal drill bit sharpening, masonry bit requires greater clearance between the jig and the wheel side for the excess of the carbide insert. A new stone is so aggressive, that several touches of its side by the tungsten carbide tip are sufficient.

Jan