SG-250 Bond Strength - Too Strong For All But Simple Carbon Steels (softer)

[Thy Will Be Done]

I've been using this single stone for long enough to have used half its life, it's down to 9" from 10" diameter so I feel well qualified to say this.  It glazes too rapidly for anything other than very narrow bevels and/or very soft/simple carbon steels.  It is very well suited for that but anything like high alloy or wider bevels and it stalls pretty quickly.  I have been using the diamond truing bit to both keep flat and also to recut the surface as I find that the silicon carbide 'stone' they provide will simply not make the wheel cut aggressively enough to use as a grinder doing more than a simple touchup.  Changing geometry takes far longer than it should/could in many cases.

So, all that said I'm finally going for another wheel or I'm just going to get rid of everything I have that isn't simple carbon steel.  I'm wondering if the SB-250 is a similar proposition in terms of bond strength.  I know silicon carbide can be more friable aka crumbles more easily to renew cutting ability.  That said, I don't believe the black carbide is much more friable than the aluminum oxide used in the SG-250.  So, I'm wondering how well it really works on harder to grind steels or will it glaze similarly?  Which will still mean frequent trips to the diamond bit to recut the surface all too often to keep cutting aggression high enough to work at a reasonable pace without having to press excessively hard on the wheel, which I find is not practical as it causes muscle/body issues for me.

[Royale]

For context, I sharpen knives and scissors commercially, been doing it for about... 1.5 years, and now on my second SG-250 (already down to 230mm/9" 😭)

I've used my SG-250 to reprofile many cheap-ass soft steel knives, many exotic Japanese steels, all the way up to ZDP-189.

I used to grade my SG-250 coarse quite frequently for it to cut a lot faster? Then I realised very quickly that it doesn't wear down evenly, and any slight change in the diameter (like grading again) would start to create another facet on the bevel (usually at heights 3mm or wider)

I used to think my stone was always glazed, and wasn't operating efficiently. Then I realised that when grinding any secondary bevel height more than 3mm, the progress slows to a crawl (possibly because all downward pressure is now more evenly distributed over a wider area)


In summation, before you start discarding serviceable items, why not try grinding a narrower angle (12-15 DPS) on your harder steel knives to get a better feel of how the SG-250 cuts? I learnt the hard way that the lack of significant tactile feedback during grinding doesn't mean the grindstone wasn't working 🤦🏻�♂️

[Thy Will Be Done]

To be clear, the wheel is grinding virtually always. It's more a matter of how quickly that is important to me considering this is a low-RPM grinder which will never win a speed contest against something like a belt grinder.

I don't believe that many people realize this but a well conditioned surface on a 220 grit stone should literally cut tracks in the knife you are grinding.  The only time I can achieve this is using the diamond grading tool.  I just re-cut the surface again yesterday and it's back to cutting tracks again.

What does that mean exactly?  It means that placing a knife on the moving wheel and not moving the knife laterally will cut deep grooves in the edge immediately which make it difficult to even move the knife laterally again once they are there in the edge.  I mean immediately, as in under a second on the wheel.

Again, most people are using these wheels in a glazed and/or loaded state which significantly reduces cutting speed and cutting depth in the steel on a single pass.  In general, a bond strength which is too weak is much less of a problem than one that is too strong.  Sure, it will waste more abrasive though.

The problem is that the only good way to get the wheel back to cutting as a true 220 grit stone is perhaps a separate issue but the two are indeed linked and many modern steels and edge geometries seem to be a poor choice for this wheel, IMO. 

The problem is many knives are ultimately ground very thick and obtuse at the edge, which necessitated lowering the edge angle dramatically, often 2-3x lower than they come factory.  This also grows the width of the edge bevel dramatically which is indeed part of the problem as you said.

As the edge bevel width grows, the contact pressures on the wheel fall off dramatically and therefore does not reach the critical pressure of which new abrasive grains will be uncovered by shearing off worn abrasives.  So the stone simply loads and glazes rapidly and needs to be recut with the diamonds again.

[John Hancock Sr]

The bond strength is not the issue. The critical factor is the MOHs hardness of the abrasive compared to what is being sharpened. MOHs hardness is the resistance to scratching. In other words the sharpening material, the wheel in this case, has to have a higher MOHs hardness than that which is being sharpened, for instance the knife. The SG is Aluminium Oxide and is good for regular carbon steels.

In summary the abrasive has to be able to scratch that which is being sharpened otherwise the abrasive will glaze and not cut. It may cut but very slowly and require frequent re-dressing.

If your wheel is glazing then this is an indication that the steel is wearing out the grit in the wheel and is a higher MOH hardness than the wheel and this the wheel is unsuitable for sharpening this particular item.

The SB wheel is Silicon Carbide has a higher MOHs hardness and suitable for high carbon and alloy steels. Silicon Carbide is also suitable for Tungsten Carbide such as you find on some turning tools for instance. CBN and Diamond are harder again and thus also suitable for your harder steels.

I keep the SG for low to mid carbon steels and for harder steels such as special alloy and HSS I use my Diamond wheels but the SB is also an option.

[tgbto]

In my experience, the SG is okay for most stainless and carbon steels. Only carbide steels or one type of very scratch-resistant carbon steel require the SB or a diamond wheel. Could it be an issue with applying too much pressure on the wheel ?