Sharpening M390 is puzzling

Spats McGee

Spats McGee

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Jun 6, 2019
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I have all of one knife in M390. When I got it a few weeks ago, the edge didn't feel right. I don't mean that it was dull, but more like it wouldn't 'grab' paper to cut it. If I could get it to grab, it sliced OK. More often than not, though, it seemed to just slide off the paper. I thought, "Hmm. Maybe too polished?" So I put it on the Hapstone and went 200/600/1000 at 23* per side. I figured that would be a good, working angle. It seemed OK, but not great. Then I stropped it and it suddenly seemed duller.

So I went back to the Hapstone, 200, then 600, and stopped there. It was reasonably sharp, so I left it there for a while. Last week, I decided to take it down to ~19*. Because I had lowered the angle, I went 200/600/1000, and got it reasonably sharp. Once again, I stropped it and BLAM! Sharpness gone. OK. So now I seem to have a stropping problem. I was thinking I used too much pressure. So I went back to the Hap again, 200/600, touched everything up, skipped the 1K, and a few extremely light passes on the strop. (Flexcut Gold, rough leather on balsa wood, FWIW). This time, at the very least, I didn't appreciably dull the edge. Granted, I was also unable to determine if my stropping did any good.

But I'm confounded. Why is M390 giving me such trouble? I've tried to do my due diligence on M390 and, by all accounts, it should take a good edge and hold it pretty well. Particularly in light of the fact that this is a light-duty office knife. It's not like I'm out batoning black walnut or anything.

No, I don't have any pictures of the edge right now, but I can try. What I can tell you is that everything looked right. I Sharpied the edge, got that worked off, got a (tiny, but discernable) burr, etc.

Can anyone shed some light on what's going on with my M390? Any tricks for this steel that I need to know?
 
What knife is this? 23° per side is a pretty steep angle unless this is some kind of thick survival knife. I almost universally sharpen EDC knives at 15° per side. All else being equal, a more acute angle will cut better.

Aside from that, I haven't found anything particularly unique about M390. It's relatively wear-resistant, so it won't develop a huge burr. I would strop it at the exact sharpening angle.
 
Since it looks like the issue comes up after stropping, I'd first look at an alternative compound.

M390 doesn't have a ton of vanadium. But it still has enough, at 4%, to affect sharpening and stropping choices of abrasive. The Flexcut compound is mostly aluminum oxide. I'd use a diamond compound instead, for the sake of the vanadium carbides in the steel.

The fact that the edge is failing after stropping is a strong hint that the stropping isn't currently doing enough to clean up the edge of the burr and further refine the edge. That may be due to the relative weakness of the AlOx-based compound to properly handle the carbides at the edge. The loss of sharpness after stropping is a common result of this, as the carbides at the edge tend to get rounded off by the less-hard compound, rather than thinned cleanly. A switch to diamond (or cbn) would likely produce much better results, and more easily.

Alternatively, work to further refine the edge as much as possible on the hones, to thin the burr some more - as much as you possibly can. Then strop on something bare, like clean paper over a hard backing, to strip the remnants of the weakened burr away. This is the approach I take more often, as I prefer not use compound much at all for stropping.
 
3D Anvil said:
What knife is this? 23° per side is a pretty steep angle unless this is some kind of thick survival knife. I almost universally sharpen EDC knives at 15° per side. All else being equal, a more acute angle will cut better.

Aside from that, I haven't found anything particularly unique about M390. It's relatively wear-resistant, so it won't develop a huge burr. I would strop it at the exact sharpening angle.
Click to expand...
It's my new Valet.
https://imgur.com/a/vTlxwXs
So far, I've been reluctant to take edges (much) below 20*, but I think i just need to get over that. I'll go ahead and take it down a couple more degrees and go from there. I clearly need to up my stropping game.
 
One of my concerns about going 'too shallow' on the angle is that my Hapstone (bought used) already has some scrapes on the clamp where a previous owner tried to go too low. Nonetheless, I'll drop it a couple of degrees and see what that gets me.
 
I don't think that would be a problem on a Hapstone, but set the angle and then check to make sure the stone's not hitting the clamp before you go to town.

My Hapstone R1 can get down to about 10 deg. without hitting the clamp.
 
Changing angles is interesting, but not while your problem solving this specific issue. Agree with Obsessed with Edges, stop using ALO on strop, and use paper on hard surface, it's a good test if you don't have diamond emulsions. Your multiple passes are cleaning edge from any heating effects left over from factory heavy handedness. Are you going light of hand during final passes?
 
Spats, does your system adjust angle of stone carrier to knife clamp? If so, incorrect settings may be why it got scratched up in past.
stupidly worded, I meant does stone carrier also have a second adjustment besides stone angle that presents stone higher or lower to knife.
 
I have that same knife and sharpen at 18° on a KME with little to no stopping afterward.
It gets wicked sharp and holds a good edge.
If you continue to have trouble you might try sharpening at 17 or 18° then put on a micro bevel at 20°.
Good luck..
 
scottc3 said:
Spats, does your system adjust angle of stone carrier to knife clamp? If so, incorrect settings may be why it got scratched up in past.
stupidly worded, I meant does stone carrier also have a second adjustment besides stone angle that presents stone higher or lower to knife.
Click to expand...
If you're referring to some kind of fine angle adjustment gizmo, there is one available, but I don't have it yet.
 
I won't be doing any sharpening tonight, but I did put the Valet in the clamps to see what I have to work with. As far as I can tell, I can get down to 15*, maybe even high 14s, before I risk grinding the clamp. So I guess I'm headed for ~15* next time.
 
Hey Spats McGee Spats McGee , it's a beautiful knife that Valet and M390 should give you screaming sharp edges with diamond abrasives.

With the grind of the Valet, your Hapstone will be able to go much lower than 15 dps before hitting the clamps. The only limit you may find is the thumb stud. Remove the thumb stud or you'll likely hit that when sharpening at 15dps. Right where the thumb stud is, would also be the perfect location for your primary clamp with the second clamp right about where the swedge blends into the flat section.

Try using it without any stropping, a slightly aggressive edge on M390 works well.
 
777 Edge said:
Hey Spats McGee Spats McGee , it's a beautiful knife that Valet and M390 should give you screaming sharp edges with diamond abrasives.

With the grind of the Valet, your Hapstone will be able to go much lower than 15 dps before hitting the clamps. The only limit you may find is the thumb stud. Remove the thumb stud or you'll likely hit that when sharpening at 15dps. Right where the thumb stud is, would also be the perfect location for your primary clamp with the second clamp right about where the swedge blends into the flat section.

Try using it without any stropping, a slightly aggressive edge on M390 works well.
Click to expand...
The main problem I have with going lower than 15dps is that I have trouble seeing what's going on in the space between the clamps and stone. I've been clamping right up against the thumb stud so that I have a consistent clamp position every time. With all of that said, I'm sure I can get the thumb studs off if I decide to go lower than 15*.

I'll try that.
 
Like someone else said, you may be leaving a burr on the edge. That can result in behaviour like you described. This could possibly be exacerbated if there was something that went wrong with the heat treatment on that particular knife, possibly leaving excessive retained austinite. Which might explain why it didn't come particularly sharp from the factory.

If it's a burr, try going back and forth between stones and strop a few times. That usually works well freehand at least, or you could even do something like running the edge through wood or cork to pull off remaining burr.

If it isn't a burr, and you are either changing your angle when stropping, or you are pressing (extremely) hard with the strop, you may be rounding over the edge.

I highly suspect it's a problem with burr removal, if not, then possibly a one off bad heat treat, if not that, then the edge is getting rounded (pretty rare with a guided sharpening system) .

I really doubt it has anything to do with the type of abrasive in the compound you are using.
 
Blank, even a bad heat treat would take a good edge, it just wouldn't hold it.

Are you measuring stone angles when you switch stones or strops? different stones have different thickness's so the angle changes. And yeah, strops take a very light touch. Otherwise they tend to create a concave edge.
 
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