It is not good for my ego to have the predictions mostly correct again, but this time there were things that surprised me a bit.
I have included a sharpening angle of 10° which I never use in praxis because it is not recommended for the N690 steel due to reduced edge retention at that angle (tendency to chipping), but that would not be a problem in this test and it is a data point of knowledge in case I make some carbon-steel sushi knives or razors in the future.
Now the boxplot:
© Charly, all rights reserved. Click for full size
So I got the prediction about how the cutting force will rise mostly correct. Mostly, not completely.
ANOVA test has found no significant difference between the first four angles but I am sure there would be one if I had performed more measurements and/or refined the testing method. The Lookandsee test does indicate a slow rise in cutting force from around 25 gf to around 50 gf.
The jump at 30° is a bit more sudden than I expected. I suspect that it is a fluke. And then the rise at 40-45° was a lot less than I expected. It seems that the 90° cutting edge is still significantly better than no cutting edge, which would be somewhere around 3-4 times worse with a cutting force of around 1000 gf. I did not expect that. The best-fit function is quadratic. This is less drastic than the predicted exponential growth, although still significantly faster than simple linear growth.
So in conclusion, it does appear that my opinion that whilst there is a difference at angles 10-25°, it is not big enough to matter for casual knife users is substantiated. The angle 30° performed slightly worse than I expected, and the angle 45° performed significantly better than I expected.
I am going to think about all this some more and then I decide how to proceed from now on.
Suggestion: quantify the reduction in cutting force achieved by slicing. Further suggestion: baseline force is blade applied to thread at 90 degree angle. Compare force required to achieve a cut at 30, 45, 60 degrees.
Just a thought. Interesting run of posts, thanks.
@sonofrojblake, good suggestions but unfortunately, I am not sure I can do this meaningfully. With a sharp knife or razor, if I drag it across the thread it cuts it so quickly that it often fails to register. I would need a significantly better force gauge for that. As for the cutting angles, I would have to build a guillotine of sorts. I might do that when the weather gets warmer and I feel like it but the odds are, I will have more pressing work by then. But I will keep these in mind.
Is this total included angle, or angle from the centre? I assume the later, but a lot of kitchen knife users report a difference between 15 and 20 degrees each side. I wonder if a practiced person can be a bit more sensitive than your setup?
I make carbon steel kitchen knives, so my approach is to take the edge as fine as I can before it chips out.
@dangerousbeans, it is an angle from the centre, so 15° sharpening angle means 30° cutting edge.
And even my setup can see a difference between 15 and 20 degrees, but it is a small difference. It is not statistically significant due to the small sample size (measurement count). And whilst the difference is there and it is possible that some users will notice it, I definitively think it is not big enough to be of concern for most people.
I think it’s a concern for a knife sharpener, seems like it’s a good thing to be aware of if you’re sharpening a knife. Even if for someone else, it results in a better product.
Back onto the discussion about sharpening methods: it occurred to me that Trizact belts are aluminium oxide abrasive, which isn’t the hardest abrasive. I wonder if a harder abrasive gets a different result in steels with high carbide volume like your N690?
Seems like something that’s outside the scope for you to test though
I don’t use a belt, I have one of these:
https://www.amazon.co.uk/Aluminium-sharpener-Constant-Grinding-machinewith/dp/B08SL8FNG1/ref=asc_df_B08SL8FNG1/?tag=googshopuk-21&linkCode=df0&hvadid=534951828415&hvpos=&hvnetw=g&hvrand=12165018268337730179&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9046500&hvtargid=pla-1170436603861&psc=1
Angle is finely adjustable and absolutely fixed, and you can flip the knife over very easily. It’s great. Sceptical, after reading this post series, of the need for the 1500 grit stone.
I was picturing a simple vertical push with a blade set at a fixed angle, rather than a drag as such. But hey, only if you have time.
@dangerousbeans
I agree that it is good to be aware of the angle at which one sharpens a knife but I do not consider it vital. And in light of these measurements, I will continue to tell those who are beginners in knife sharpening that they need not fuss about the exactness of their angle because anything between 15° to 25° will work just fine. Once they cease to be a beginner, then it is time to think about angles, etc. but I consider it counterproductive to intimidate beginners with such minutiae that make knife sharpening appear more complicated than it really is. As far as “better product” goes, there is a tradeoff with everything. A knife with a 15° sharpening angle has slightly better cutting ability than one with 25° but it also very probably has lower edge retention as well. That is why I sharpen camping knives at 20-25° and kitchen knives at 15°-20°. I do hope to think of a way to test edge retention with stuff available to me someday, so far I got no idea, unfortunately.
Trizact is not only corundum abrasive but is also structured abrasive and I think, subjectively, that it polishes slightly better than its grit equivalent. A45 is an equivalent of 320-400 grit but it gives a much smoother surface than 400 grit, more like 600 grit. As far as hardness goes, in most natural stones the abrasive function is performed by silica components that are even softer than aluminium oxide. It is beyond the scope of this test because I need to confine the tests to things that are relevant to me in my workshop. Hmm, I might need to repeat the test with my traveling sharpening kit with two whetstones and a strop now that I think about it. I will do so once I find a suitably blunt knife which in our household is not a given :).
@sonofrojblake, I know this type of knife sharpener and I think it is great for personal use. I am using a belt grinder because I need to sharpen knives not only reliably, reproducibly, and well but also quickly. I think the end 1500 grit stone still makes sense since the previous one is 600 grit but indeed do not think it is absolutely essential.
I intend to make a few of these knife sharpeners with pretty wooden components and a magnetic holder. Maybe if I got time, I am considering it for two years already. One like this would also allow me to eventually test more grits since it can be used with abrasive paper and I have greater variety and gradation of grits in abrasive papers than in belts.
@ Charly yeah i’m not sold on natural stones for high tech steels either.
This seems like classic science; always another variable to fiddle with, another dimension to explore :P