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  1. #16
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    Oct 2009
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    Quote Originally Posted by qwertyu View Post
    Im wondering if cosman is correct in this situation - bobl has diamond wheel and is CBN often referred to as diamond?
    I know Bobl has already answered but I thought I’d add my two cents anyway. CBN is not referred to as diamond - it’s used specifically in place of diamond, in a situation where diamond’s performance isn’t that good (in an industrial setting at any rate), and the distinction between them is quite clear. And secondly, as Bob has already mentioned, diamond wheels are actually cheaper than CBN, not the other way around.

  2. #17
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    melb
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    Thanks for the clarification. Question on cbn, can I use it for kitchen knives which are stainless?

    Sent from my ONEPLUS A5010 using Tapatalk

  3. #18
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    Feb 2006
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    Perth
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    Quote Originally Posted by qwertyu View Post
    Thanks for the clarification. Question on cbn, can I use it for kitchen knives which are stainless?
    All the term "Stainless Steel" (SS) means is that it usually contains more chromium than other steels which makes it rust resistant (NB not necessarily rust proof). As a result there are almost as many types of SSs as there are "non stainless steels". The issue for sharpening SS on a CBN wheel is that most (especially cheaper) SS is surprisingly soft. This is bad for CBN wheels, just as regular mild or soft steels is bad for CBN for CBN wheel.

    The mechanisms that makes soft steel bad for CBN are:
    1) soft steel can "gum up" the CBN wheel reducing its efficiency - a bit like grinding Al a regular aluminium oxide grey wheel.
    2) more importantly, small bits of CBN can break off the wheel and embed themselves in soft steel so that instead of the steel being ground on CBN you have bits of CBN embedded in the soft steel grinding on the CBN on the wheel. This is why tool steel like HSS is fine whereas mild steel is not.

    As far as the SS goes, what it comes down to is the "gumminess" and "hardness" of the SS - the two factors are related with harder steels tending to be less gummy. If the SS knives are a cheap set of steak knives (with a Rocwell hardness [HRC] say of 45) then you should not sharpen these on a CBN wheel whereas a a high end kitchen knife should be fine. Surgical and dental SS ( eg HRC ~55) are regularly sharpened using CBN. There is also no single hardness that says - "This is fine" and "This is not" - the risk of damage just increases as the SS gets gummier and softer. Risk of damage also increases with use - if you accidentally sharpen a soft knife with Whether your knives are hard enough may be determined using a series of hardness testing files. Unfortunately these are not cheap (a set costs ~$100) but if you have a piece of steel that you know the hardness of you could use that to see if it scratches the SS or not. You could also make your own hardness testing file by hardening and tempering a regular file.

    BTW I use a 50 mm wide belt sander on a bench grinder to sharpen our SS knives is. I'm using a 240 grit belt to reform knives with damaged edges and a 600 grit to touch up these knives.

  4. #19
    Join Date
    Sep 2017
    Location
    Sydney NSW
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    Quote Originally Posted by BobL View Post
    ...
    2) more importantly, small bits of CBN can break off the wheel and embed themselves in soft steel so that instead of the steel being ground on CBN you have bits of CBN embedded in the soft steel grinding on the CBN on the wheel. This is why tool steel like HSS is fine whereas mild steel is not.
    ...
    Sorry mate, but your point #2 is the most improbable of all CBN myths I've ever heard.

    All commercial knife steels, stainless or not, budget and high-end, are OK to grind on CBN wheels - we've been sharpening stainless knives on CBN wheels since 2017 w/o any issues.

  5. #20
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    Perth
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    Thanks for the info.

  6. #21
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    Sep 2017
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    Sydney NSW
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    The capital difference between the diamond and CBN crystals is that when the diamond crystal fractures you lose it, but when the CBN crystal fractures it becomes sharper.

    The CBN crystals have better crushing resistance, and under workload the CBN crystals micro-chip, while the diamond crystals macro-fracture. With use, the diamond grinding wheel develops flat areas and you see drop in performance, while the CBN keeps grinding. When CBN crystals chip, the newly formed facets sustain their cutting ability. Compare the SEM:







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