buckthorn wrote:does each slice represent the retention achieved by using only that abrasive
What's different about the relationship between those first two steps and all the subsequent steps.
That 36 grit stone isn't an actual sharpening stone, I used it as a point of curiosity, it is a dressing stone intended to reshape other abrasives, usually power grinding wheels. It has an extremely strong bond and the abrasive isn't very friable and thus it just wears smooth.
The abrasive is simply huge, I can't even shoot it at 50X magnification as all you see is one grit, I have to turn the magnification way down to show the structure of the stone as the grit is so large you can see the individual ones easily by eye :
Now there are two possibilities as to why the edge retention decreases when you go from the 120 Sigma Power to the 36 grit dressing stone :
a) the too large grit itself has become a negative
I can't see immediately why this would be the case, but I just list it as it could be possible.
b) the edge isn't well formed even though it is sharp
This is where it gets a bit technical and it has to do with how very coarse edges blunt. Here is a typical very coarse edge :
The way this blunts is by a sort of staging :
-the tips wear and round
-the inside of the scallops wear
-finally, large flat spots are produced as the tips and scallops are worn away
Here is the worn edge :
While there is still some jaggedness remaining you can see most of it is worn away and the edge has lost most of its sharpness at this point, < 1.5% of optimal.
Now here is what I think is happening with the dressing stone :
Imagine a serrated blade which is fully sharp, now imagine two others :
-one which has fully sharp tips but blunt between the tips
-one which has fully sharp between the tips but the tips are themselves are blunted
If you imagine for example drawing either of those partly sharp blades across a piece of paper then they both will cut it readily because either the tips or the scallops cut it and thus if you just did a quick paper cut with all blades they might appear to be all sharp. The only way you could tell which one was really sharp was to cut something with a small movement and measure the tip sharpness and the scallop sharpness directly.
Now what I think is happening is very similar with the edge left by the dressing stone that it is sharp a little in the same way but not fully sharp as is the edge on the Sigma Power 120. I think this happens because the stone simply doesn't cut well and you can feel a lot of deformation vs abrasion. There is no way to check this directly as it isn't like that serrated blade where you can actually cut something with the tips or scallops as in this case they are all tiny, < 10 microns.
There are also a few other things such as I think the edge on the dressing stone is a bit over strained, again due to deformation. And as always I would want to run all of this data a few times before I would be really confident in the results.
At this stage though this is all just a hypothesis, in order to check it out I need to get some other very coarse abrasives and see what happens, where they fall. I have some loose 36 grit Silicon Carbide and I intend to check that and a few others. What would be interesting to see happen would be if the performance took a strong negative past some grit point and the edge retention decreased. I can't immediately explain that so it would be cool to see it happen and they try to figure out why.