Knife Tip Edge Thickness - Tough Question

[herromynameis]

Looking for feedback about the tip of this knife design. It is fresh off milling and you should be aware of a couple things. 1, I mismeasured and it will have an extra 0.02 thickness across the whole knife once I remake it. 2, I'm going for more of a light edc/collector function than a hard use blade. 3, In it's annealed state (30 HRC?), the tip flexes under finger pressure.

-Steel - 1095 (Switching to A2 shortly)
-Blade Length - 3.7"
-Spine - 0.125" (I made an error, but ultimately the thickness would be 0.145". For reference, the spyderco paramilitary is 0.140")
-Grind - Flat, although spine does not follow to tip

Questions:
-I've made 3 marks on the blade tip. 1st mark to tip is 0.026" thick at the spine. 2nd mark is 0.044" at the spine. 3rd mark is 0.074" at the spine. Is this acceptable for any function? Again, all of this will have an extra .002 when I redo it.
-Any rules of thumb you stick to determining thickness/length to tip/angle?
-Will this design work for A2 or any other steel, or should I just increase the entire overall thickness to at least 0.16?

 

[Stacy E. Apelt - Bladesmith]

The thickness is OK for a light use or fine cut knife. I like your osoraku-zukuri blade shape. Beware that such a tip is fairly fragile if abused.
My concern is that you have milled it very thin and still have to HT and re-grind.
I would have only put in the distal taper and done the bevels after HT, but I don't mill my blades

 

[Nathan the Machinist]

If the tip flexes under finger pressure in the annealed state, it will flex under finger pressure in the hardened state. Steel does not get stiffer when it gets harder, it's yield point goes up and so the point where it crosses from elastic deformation into plastic deformation increases with hardness, but the elastic modulus is a constant.

I actually like a thin tip and thicker towards the ricasso on some knives like game prep knives where you might cut bone ahead of the guard and use the tip like a scalpel. But most knives the tip is thicker because that's the part people tend to pry with.

I think you're going to have trouble in heat treat with that design in 1095 but it would probably be fine in A2.

 

[herromynameis]

The thickness is OK for a light use or fine cut knife. I like your osoraku-zukuri blade shape. Beware that such a tip is fairly fragile if abused.
My concern is that you have milled it very thin and still have to HT and re-grind.
I would have only put in the distal taper and done the bevels after HT, but I don't mill my blades

Sensei Stacy, thank you for your input. I'm glad you think it could work. I'm still very slow moving in making and altering. After I saw the issue, I went on a hunt to find case study knives that assuaged my concerns, and due to the high volume of edc knives and their thicknesses these days, it made me wonder.

Heat treat warpage slipped my mind! I work a day job at home, and am trying to get the mill to do as much work as possible while I slave away at my computer. The thought was to bring it to the point where it needs just some hand finishing, or is adequate enough a finish to sand blast, then cerakote (I think I've seen 400 grit called out as acceptable, but not sure how this compares just yet)

 

[herromynameis]

If the tip flexes under finger pressure in the annealed state, it will flex under finger pressure in the hardened state. Steel does not get stiffer when it gets harder, it's yield point goes up and so the point where it crosses from elastic deformation into plastic deformation increases with hardness, but the elastic modulus is a constant.

I actually like a thin tip and thicker towards the ricasso on some knives like game prep knives where you might cut bone ahead of the guard and use the tip like a scalpel. But most knives the tip is thicker because that's the part people tend to pry with.

I think you're going to have trouble in heat treat with that design in 1095 but it would probably be fine in A2.

The saint of all things machining, thank you for the advice.

Good to know about the elasticity vs plasticity.

I guess with the blade style I'm going for there's no way to make it pry safe, but this tip does seem very fit for surgery.

This is very valuable insight on 1095 vs A2. I'm assuming the variables like surface to core temperature difference in oil quenching tempts the blade to warp more. I'm trying to get the mill to free my hands up. My first plan is for the mill to finish it to the point where sandblasting can remove the marks, then heat treat and cerakote it. I'm tempted to say the finish in this picture is acceptable based on my 5 minute experience, but am curious what you guys think. I haven't written off the idea of prehardening the A2 stock and hardmilling the blade to eliminate any issues like warping, but I'm sure I'd have to redo all my tool setting, and I don't know which process will end up more complicated.

 

[Crag the Brewer]

A2 Won't warp as much from heat treating.
I believe everyone was saying your grinds might be ok for A2.
I personally, would of made the edge around .015" before heat treating. Then do the final grinds After.

No benefit trying to machine the A2 hardened blank.
Wouldn't get very far.......

*A2 is air hardened

 

[herromynameis]

A2 Won't warp as much from heat treating.
I believe everyone was saying your grinds might be ok for A2.
I personally, would of made the edge around .015" before heat treating. Then do the final grinds After.

No benefit trying to machine the A2 hardened blank.
Wouldn't get very far.......

*A2 is air hardened

I had a feeling it would warp less... I also didn't want to mess with anti-scale coating etc with 1095. My thoughts for the .02ish thickness is mainly I want that part of the blade to err on the rigid side as I mill it down so that it won't chatter. I had an issue at first with the tool bouncing and punching through close to the edge.. Thanks for the advice, Crag! Cheers!