Blade Geometry For a CMP-M4 Chef's Knife?

[huntingsgr8]

I've got some steel on order and among others I picked up a bar of CMP-M4 1/8x36"x2". I intend to use it to make a couple of chef knives, but having never used the steel, and there being very little information about its use in knife making, I figured that this would be the place to ask a few questions about it. Now ordinarily when I make chef's knife out of W2, I profile, rough grind to .030"-.040" at the edge, I heat treat, straighten, and finish the grinding, and convex the blade so that the edge is <.003" and the spine tapers from full thickness at the tang to .050"-.060", 3" down the blade (on a knife with an 8" blade) and to about .020" a half inch before the tip. I opt for plunge-less grinds on chef knives.

I think I already know the answer, but can I go that thin with M4 and expect the blade to hold up, or will it just chip? I temper my W2 knives to around 63 hrc (according to the HT data sheet), I'd like to leave the M4 at least that hard. Can you recommend a more suitable geometry for this steel on an 8" chef knife? Will I be able to do any grinding pre HT on this steel, or should I just suffer through doing all the grinding after its hardened? Can this steel be drawn back the way more conventional ones are?

 

[samuraistuart]

With M4 and any air/plate quenching steel, you can take the edge down very thin prior to HT. Not quite what I would call "final dimensions", but very close, as in sub 0.010". Before I started doing my own HT, I sent a petty to Peter's in CPM M4 that had an edge that was about 0.010" with a full flat grind and there were no issues at all. Came back straight and I LOVE that knife. 64HRC is a nice target hardness for CPM M4. Once heat treated, you'll want to grind it to final dimensions, but that won't take long. What I do with all of my kitchen knife steels with alloyed steel is basically grind it to a zero edge (safely without ruining the temper), to where you basically have an edge angle that is formed by your primary grind, which ends up being about 2°-3° per side, depending on spine width and blade height. Then I back that zero edge off with a piece of 220 grit paper to establish a "flat" that just almost reflects light, do my hand sanding, then sharpen.

IMHO, 1/8" is thick for a chef's knife, but it isn't too bad. You can draw back the spine (I assume that is what you meant), but I wouldn't recommend it. It's a chef's knife, not a competition chopper.

 

[huntingsgr8]

Thanks for the reply. I agree, 1/8" is thick, I had ordered 3/32, the plate was marked 3/32, but it mic's at .130 or .135. That warehousing snafu really messed with my belt allocation for that "run" I guess you could call it. My main reason for drawing back the spine is actually fairly simple, it's not that I'm worried so much about the blade breaking, but I got sick and tired of screwing around with shimmed tempers and the like. If the spine is drawn back, I can straighten the buggers out cold on an anvil between tempering cycles, and when they're thin and finished, warp is a lot easier to discern, so I can straighten them just by hand at that point. It takes me 5 minutes to draw back the temper, and saves me hours of conventional straightening.