I think it depends more on the way you use or intended uses than the steel in this case If you are going to use it for splitting wood or rough use ... most any steel type whether a basic 1095 or M390 would benefit from not such an acute angle ... but for a real slicer the M390 an like steels will take at least 15 degree 30 inclusive if not even slightly lower angles ... and preform extremely well ... where some softer steels may start to dull incredibly quickly at acute sharpening angles.
^^yup, yup ^^^
I wasn’t going to answer after it dawned on me this question is specifically about M390 etc.
The one example I have is pretty thin behind the edge (15 thou) and fairly stock bevel grind ~15° per side. The knife performs brilliantly while doing cuts that tend to roll or make lesser edges / alloys sick.
So my M390 is perfection around there abouts.
What I was doing to say and I think I will blunder on is :
When I took charge of my Dad’s pocket knives after he passed it surprised me how he sharpened them. First let me say that he was no dummy with his hands. He was a highly experienced steam fitter working with dams and steel mills and coal fired power plants. He was a farm boy and he could build a barn, lay brick and block and was always lovingly tuning and maintaining all his stuff.
So . . .
how did he sharpen ? And he never offered to teach me. With him either I got it from watching or I taught my self. I had a lot of independence growing up but not much tutelage. Double edge sword . . . and quite a dull sword until I finally got this sharpening thing together seriously when I was in my thirties.
The way he seems to have sharpened and I rarely saw him do it, was he would put a sheet of wet or dry sand paper on an inches thick steel block that he had and now from looking at the blades I see he just laid the side of the knife on the paper and sharpened SUPER SHALLOW to the edge. He appears to have always left them toothy, the grind of the grit can be easily seen on the sides of the blades. Must be 600 or even 400 grit. Worked for him. Granted this has nothing to do with M390 and I am getting to that . . .
He didn’t sharpen the lawn mower blades that way; they were at a stock angle just as you would expect. I got VERY familiar with that lawn mower once I got big enough to mow. He was very generous with his lawn mower. He “Let me” use it every week for years and years. What a guy huh ?
In the photo is my stock PM2M4; I just polished the edge. My way . . . sorry Dad.
Now usually it is my habit to reprofile thinner and sharpen the edge thinner but first I use the knife and see how she does. This knife works perfectly for me so I left it and just polished it.
The tiny Buck was one of his knives sharpened by him as described.
The Swiss Army knife at the bottom in the photo I use for a specific task at work and have pretty much sharpened way back to the sides of the knife like he did but polished, again sorry Dad.
The point is . . . what I am trying to say is : try the knife and grind it thinner until it cuts like you want or there is a problem and then steepen the edge its self a little. There are many variables.
Trial and error are the best indicator when it comes to cutting edges for hand held tools. Machine mounted cutters cutting specific materials at specific speeds and feeds . . . now that is pretty much nailed down to certain parameters.
Hand held edges . . . well . . . I would go really shallow and steepen the edge if really needed.
. . . oh . . . and carry a pry tool :
