Edge geometry vs Stress risers

[shane justice]

Guys,

I am getting into a project that requires delving into an area I am unfamiliar with.

The best way to start is to ask a question..

All things being equal...steel choice, thickness, blade profile, etc.

The blade would have sides that run parallel to each other, so that except for the edge geometry it is an even playing field.

Will a blade with a chisel ground edge be at greater risk than an edge with a symetrical Scandi edge, or a convex edge?

I put in a little drawing showing the idea better than my words can explain. The full convex is there just because that would be the sort of thing I would put on a forged knife.

Shane

PS PLEASE ask any questions that might help shed light on my less than precise explaination.

 

[James_Terrio]

Will a blade with a chisel ground edge be at greater risk than an edge with a symetrical Scandi edge, or a convex edge?

If it's ground at the angle shown in your drawing, yes, just because it's thinner. I'm not sure what you mean by stress risers, that's not really what's at play here.

 

[Ed Caffrey]

Its just me, but unless the only function was strictly chopping, I would never build a blade with parallel sides (equal thickness for the majority of the blade and flat)....and then it would be a convex arrangement. In all the drawings you posted, there would be so much cutting resistance...that it would be like trying to use a cold chisel type geometry for anything but chopping...and that would be much more difficult than if the blade had a full grind on it.

I may be wrong, but my guess is that you had someone ask you to create a blade for them with one of those types of profiles? If thats the case, I would try to talk them out of it....simply because one the blade is complete, its likely they will not like the way it works, and will be asking you to "fix" or "make it better", and it will be one of those blades that never goes away.

I'm assuming this is a knife we are talking about, but if not, please explain further. Much is going to depend on if this is a user type blade, its intended purpose(s), and the individual's willingness to acquire usability over some "cool" idea they had.

 

[Will Leavitt]

I'm not really sure what the question is... but like Mr. Caffrey said if it's someone asking you to make a blade with a specific grind, you'll probably need to find out the intended use. I know I would hate someone going around badmouthing about my blade because of their design.

 

[mathil]

I believe what he's trying to ask is which type of edge creates the most likely situation for cracking due to stress risers.

Stress risers are shapes in the metal that "focus" stresses put on the blade, and when metal eventually breaks, it breaks there.

 

[Will Leavitt]

I believe what he's trying to ask is which type of edge creates the most likely situation for cracking due to stress risers.

Stress risers are shapes in the metal that "focus" stresses put on the blade, and when metal eventually breaks, it breaks there.

Thanks, I knew what a stress riser is, having caused several the demise of several blades when I was learning to forge and heat treat.

I feel the intended usage would dictate the grind over the aesthetics. That being said, I would imagine a chisel grind would be more likely to be damaged by hard use due to the geometry focusing stress unevenly through the blade profile. I have no proof or even anectdotal reports of such a thing happening. Heck I'm probably wrong considering the stresses that planer blades go through.

 

[richard j]

the knife pictured here is a chisel grind and it held up great chopping some red elm and oak. last summer joe southerland chopped some 3" red oak limbs with it after i went through a piece of red elm about the same size. the edge held up great without any damage and still shaved hair from joes arm when he was done.

 

[shane justice]

You guys are probaqbly right. Stress riser is not the correct term. I saw a an explaination on Mr. Cashen site a while back, he was talking about "slip".

I will share more about this project as it comes to fruition. Right now I am sort of sniffing around for more information. The project will be a mid-tech deal using
either Sandvik 12C27(my choice) or S7. I need to make a tool that can stand hard use, be able to pry, (which I think, more than steel choice, is a function a geometry) and cut reasonably well.

The picture was simply a representation , not exact angles. I believe that edge geometry is best optimized at the grinder, rather than on paper. Did that make sense. Once in a while what I thought would work, didn't really sizzle until I had tweaked it some.

What I was concerned with was a blade failing when flex occured because of uneven loading on an a-symetrical edge geometry. Did that sound any better?

Ed,
For some reason I see a lot of makers building these "scandi" grind knives. As far as I can tell(not owning such a knife) those blades are basically flat sided with a bevel.

Possibly this is not the case.

Probably I need to back up and think a bit harder.

I have been looking at many of the "TACTICAL" knives for direction. The Besh Wedge is an exmple of a something different that I would never had considered.

I am only considering this project because my fine motor skills are diminished, and I simply cannot allow my dream to die, before it ever had a chance to really live.

This is a sort of teaser to see what I can and cannot do, and to discover what limitations I can learn to overcome and still be able to make a high quality knife.

Did that clear anything up?

Shane

 

[James_Terrio]

Shane, I think I understand you better now. You're right about scandi's,basically.

Your question about uneven stresses on an assymmetrical blade is interesting, but all other things being equal (as you wisely made clear in the OP) I'm not sure how much difference that would make.

I haven't used a chisel-ground knife, and honestly I'm not sure what they're for. For whittling or fine bushcraft-type work that would be cool, I think. If you only use your knife with one hand.

I don't know man, make test pieces with the different grinds and start breaking them. I think you're always going to find a convex edge is strongest, there's simply more steel there.

 

[Leu Custom]

I would imagine that it all depends on your heat treat and how well it is will depend on how well the edge holds up. How much force and shock the cutting edge gets and to what material.

They all look pretty beefy with maybe the exception of the chisel grind which isn't designed as a chopper anyways!

I think stress risers are mainly where joints are in a knife and where it might snap in half if used as a pry bar? (I could be wrong)

 

[Leu Custom]

the knife pictured here is a chisel grind and it held up great chopping some red elm and oak. last summer joe southerland chopped some 3" red oak limbs with it after i went through a piece of red elm about the same size. the edge held up great without any damage and still shaved hair from joes arm when he was done.

Almost looks hollowground on one side only Richard?

 

[shane justice]

Well, I do apreciate everybody weighing in...

I have abit of a physical limitation that may be overcomeable. Not sure yet.

I was hoping that a project like this might be just the thing to explore how bad or good thigs might be.

I have owned one tiny litle bitty Gerber knife...I think was originally designed by Ed Halligan...sort of a simple frame with a locking blade...it was a shar little booger...the chisel ground edge was a bear to cut with because it was set up for a right handed person so all I could do with it was cut circles the wrong way?

I wonder if I would find that in order to have a blade like this cut "REALLY" well it would have to be fairly thin...and not worth the trouble, so as to negate any ease of manufacture, over doing a traditionaly ground double bevel?

I will think on it some.


Shane

 

[wnease]

Regarding stresses in the chisel ground blade... in a long blade torsional stresses will occur that would not normally be present in a blade of symmetrical cross section (during cutting that is "deep", like 20 inhes of fiber, or flesh). The chisel blade acts like a wing instead of a rudder. Applicable more in long blades...