The Importance Of Knife Blade Hardness

Part III

I'll discuss the importance of Rockwell Hardness for the knife performance, based on the true story :) That is, whatever experience I have gained from my own knives and hardness testing.
Gator, 04/10/2009

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Rehardened And Polished Benchmade 710HSSR and Kershaw Shallot CPM S110V

As usual this kind of projects tend to go longer than planned. At least I don't remember ever completing customization or modification project on time :) A lot of factors interfere, but majority is still sending blades to someone and getting them back. Anyway, as you know I wanted to polish one M2 blade myself and the other two were going to knifemakers. Original plan was to send them to Tom Krein for regrind and polish. Sadly, he doesn't do polish. So, I've had to find someone who'd do that for me. Long story short, I just posted a message on asking if someone would do this type of work. So, I've sent those two to another maker, Richard J. Meanwhile, during this search I've sat down on the weekend and polished 64.5HRC blade myself.

To be honest the polishing job was hard as hell. I clearly had no idea how hard it would be to work on M2 steel hardened at 64.5HRC. As you can see on the photo above, the last knife, at the bottom is the one I polished myself. Rough grind lines are still visible and I wasn't really able to get all the oxidized metal out either. I thought I was equipped pretty well, various abrasives starting with 08 grit and going all the way up t0 100 000 grit. Obviously you guess higher grits weren't really the problem, as it is right now the blade still needs some major work with the low grit abrasives, and I just can't make myself to sit another day with them, Eventually I will.

For the record... I started with 80 grit, which was a sponge like abrasive thingy from 3M. Worked with it until all the abrasiveness on it was gone. That amounts to approximately 2 hours. Then went with another abrasive block, 120 grit. In 3 hours that was gone too. And after that next 4 hours I was working with 220 grit sandpaper. Then I've decided I've had enough of rubbing and went with 600 grit for short time, then 15µm microabrasive film, then 5µm, 2µm, 0.3µm and then I've decided to call it a day. Literally, it was late night by then. At the moment, the blade has ok satin finish. The problem is that factory knives like my Benchmade 710 don't have very smooth surfaces, one reason is that it costs more to produce precision surface, because of the more wear on belts and other tools, second reason is that coatings stick better to the uneven surface. So, whenever you strip the coating you're left with relatively rough surface, which you can see on the third blade in the photo above, and if you go to previous pages and see original photos right after hardening, another M2 blade has the same grind lines, as it had BT-2 coating too. Kershaw blade however, has much smoother surface, it was satin finish originally. Well, hardening did mess things up though. Ok, enough about my knife polishing hassles.

As I said, I've sent two blades to Richard. CPM S110V blade from Kershaw, originally 59HRC, and M2 blade, originally 60HRC, both were rehardened by Phil Wilson to 64HRC. So, the job for him was quite hard, even if he had better tools. The trouble is, with those blades you can't use high speed rotary tools, at least not at high speeds, it'll overheat the blade, thus ruining the heat treatment, and I was very anxious about that part, obviously after spending time and money to get the blades rehardened to 64HRC, I'd be really upset if the treatment was ruined again :) So, poor Richard had to work at very slow speeds with his grinder and the whole thing took few weeks. Obviously, my 2 blades didn't take few weeks of grinding, it was few days spread over those few weeks, to be precise 4 weeks. And I got them back around the end of June 2009. After examining those blades I was satisfied. They're not exactly showcase mirror finish, but definitely better than just satin finish. Richard was still unable to remove grind lines from M2 blade, but now they're more like hairline marks than thick lines. Much better than my own work.

What next?

- Well, next is two things. The same two blades are going to Tom Krein, who is really famous for his regrinds, beginning of July 2009. Which is next week. After examining them Tom and I will decide what exactly needs to be done. My goal is to make efficient cutters. Those two blades are already at very high hardness, so naturally thin edges and thin grinds would be very good choice for making a good light cutting knife. As for the #3 blade on the photo, M2 refinished by me, so far it's been my EDC and edge holding is exceptionally good. Right now it's around 24° angle edge total(vs. 40° originally) and takes very keen edge. No chipping that I can talk about so far. I'll update Benchmade 710 HSSR review with the edge holding testing data later, and as for the plans for this particular blade, eventually I'll get to it and give it better finish. Next update as I get those 2 blades back from Krein.

Regrinds are back

- Well, this update was very long overdue, but my Interactive Knife steel composition chart project was taking pretty much 100% of web development time I could allocate :) Hence the long delay. Although, it wasn't just me. Regrind process took a lot longer than I have anticipated, I've sent them around august, and got them back on 11/12/09. I've assembled the knives on November 14, that is two days later, and started carrying Krein regrind blade the same day. And it has been my EDC since then. Tom Krein had the same problems with regrinds as Richard J. had with polishing and cleaning up the blades. Well, obviously so, since the hardness was the same. Anyway, both blades got hollow grinds and at the moment both have 15° per side edges. During almost 4 months I've used all three blades (Kershaw Shallot and both Benchmade 710-s) a lot. After all both Benchmades were my EDC for few months each, and I was trying to use CPM S110V shallot as much as I could for testing and utility cutting at home. I've accumulated good amount of the test data, and I was diligently keeping records. Obviously I won't write 20 page long essay :) But, the next part will be more or less detailed description of the cutting tests and use data.


Last updated - 05/19/19