Failure mode of a hard-use folder

[Joe Talmadge]

Through proper engineering, a folder maker can decide how exactly he wants his knife to fail, if someone pushes the knife too hard. It can break at the lock, for example. Or perhaps he doesn't want it to break at the lock, so instead he beefs the lock up a bit, so now when the knife is put under pressure, the handles sheer or pop apart. Sometimes it's fun to try to guess where a knife might break ... hmm, giant thick blade, big beefy lock, but those handle liners don't look all that thick...

What do you think the right failure mode of a folder should be? You're cranking on it hard, how should it go? Do you want to see the handles flexing ominously before the thing breaks, so you have a visible warning that it's about to let go and can back off? Or do you prefer super-stiff handles and don't care if the lock fails without warning? Should the lock break, blade break, or handles fail?

 

[Buzzbait]

Off the top of my head, I guess that I'd prefer the pivot to die first. That way, under many conditions, the blade is left in the cutting substrate, and the handle stays in my hand. No damage done to myself.

 

[Emanuel]

None of the above options please!

I would prefer the manufacturer spend all his knowledge and R&D into simply making the strongest and most durable lock/blade/handles.

Ofcourse, I realize everything breaks given enough force on it, but I think a quality knifemaker shouldn't think about the right failure mode. He certainly shouldn't be building any weak points on purpose.

He should just make the strongest folder possible without any weak points.

 

[gaj999]

I think the failure mode is going to depend quite a bit on what the user is doing. Is he cutting hard, prying, twisting, or putting force on the spine of the blade?

I like thin blades that cut, so I'm willing to put up with the blade failing first. I like my locks to be secure so if I bump the spine accidentally I keep all of my fingers. I also want a lock to be secure when I'm using the knife in a stabbing mode for piercing heavy plastic and starting a cut. I couldn't care less about more lock strength than that requires. I don't like handle flex myself, but I tend to like larger, hand-filling handles anyway, so that's not usually an issue.

When you make a lightweight, thin bladed knife with no weak points that cuts like a demon and never needs sharpening no matter how many car doors I cut up I'm very interested.

Gordon

 

[Cliff Stamp]

What do you think the right failure mode of a folder should be?

The pins in the handle are designed just like a fuse and they break just before there is significant damage to the blade or handle. You then replace these and continue with no actual wear on the rest of the knife.

Note if you are prying with the tip of most knives it will crack long before there is any significant load on the pivot/handle. However you could use the same general idea to minimize damage and thus only lose X% of the blade before the pivot would release.

-Cliff

 

[STEVENKIM]

None of the above options please!

He should just make the strongest folder possible without any weak points.

A weird quote I learned from the officer corps that always stuck with me "Build a weakness into your plan."


Besides, any folder or manmade for that manner will have a certain breaking point. Not saying a maker should make a weak point on a knife on purpose, but he should be very aware of what the weakest point is or designate himself what the weakpoint is on the knife.

 

[3Guardsmen]

Sounds like you've been talking to Sal. I remember Cliff mentioning the idea of "engineered failure" a while back, during a review of the Chinook II (I think). Great concept! Kinda like "crumple zones" on a car.

I think a folder that will see hard use, especially in a survival scenario, should be engineered to break in a manner that will most likely not injure the user, and will allow at least the most important portion of the knife, the blade, to remain somewhat functional. A functional blade, minus the handle, can still be attached to a piece of wood, and can continue to serve as a tool.

One must remember that, in a survival scenario, any injury must be treated as life threatening, and therefore a folder that breaks in any area that is likely to cut or injure the user is unacceptable. If the lock fails (breaks), and the blade cuts, or worse yet, cuts off one or more fingers, the user is doomed.

I therefore would like to see the handle, on a hard use folding knife, start to give before the blade breaks or the lock fails. I have also noticed that some features in the design of folding knives make them less dangerous in the event of a failure. Sorry to wander a little off topic, but I thought this might at least partially pertain to the subject of this thread.

Designed for a possible failure?
One reason I prefer Spyderco folders is the fact that most of them have a "50/50" choil, where the lowest portion of the blade (the portion nearest the handle) is unsharpened, to serve as a place to position the index finger for detailed cutting. Should the locking mechanism fail on a Spyderco model with a "50/50" choil (or a Strider SMF, SnG, PT, AR, or GB for that matter), the sharp part of the blade will not make contact with a finger.

Some Spyderco models, such as the Endura and Delica, which lack the "50/50 choil", have an unsharpened "kick" that is long enough that it will assure the same safety; your fingers will survive a lock failure.

I have also noticed that folding knives with "flippers" on their blades can serve the same purpose. Take for example the CRKT M16 series of folders. The flipper would be the first portion of the blade to make contact with the user's finger. Granted, it certainly wouldn't feel good, and the grooves cut into the flipper would probably pinch like heck, but I can definitely see a flipper saving a finger from instant amputation should the lock fail.

Great topic, Joe! Thank you, too, for the very valuable knowledge you have given me in your various postings and FAQ's, here on BF, and on other forums as well!:thumbup:

Regards,
3G

 

[Halfneck]

I really don't like getting cut, as such my vote is for the handle to fail.

 

[gregvisser]

Makes sense for me to have the pivot fail.

My logic says: safety first, cost second, ease of execution third. Pivot makes sense in all three criteria.

Greg

 

[Hair]

I'd want the blade to break first. If the blade is the weak point, then it shows that the folder is practically as strong as a similar fixed blade.

 

[DaTrueDave]

One reason I prefer Spyderco folders is the fact that most of them have a "50/50" choil, where the lowest portion of the blade (the portion nearest the handle) is unsharpened, to serve as a place to position the index finger for detailed cutting. Should the locking mechanism fail on a Spyderco model with a "50/50" choil (or a Strider SMF, SnG, PT, AR, or GB for that matter), the sharp part of the blade will not make contact with a finger.

The flipper on the Kershaw Spec Bump is also a choil and, in fact, when closing it one handed, I unlock it and let the choil close on my index finger before shifting my grip to close the blade completely.

And I would vote (with Cliff Stamp) for the pivot to fail first, especially if it were designed so that the pivot could be easily and cheaply replaced with no damage to other parts of the knife.

 

[Joe Talmadge]

One reason I prefer Spyderco folders is the fact that most of them have a "50/50" choil, where the lowest portion of the blade (the portion nearest the handle) is unsharpened, to serve as a place to position the index finger for detailed cutting. Should the locking mechanism fail on a Spyderco model with a "50/50" choil (or a Strider SMF, SnG, PT, AR, or GB for that matter), the sharp part of the blade will not make contact with a finger.

I wonder about this, especially on knives with shallow choils, like the Calypso Jr or even the SnG, but I even wonder with deeper choils. If the lock went suddenly, it'd mean you had a huge load on it, and I'm not sure that the force from the blade wouldn't be sufficient to both move your finger and compress it enough to cut. I could be wrong there, just an uneasy feeling, I've let the Calypso Jr come down on my finger in the choil ... feels like I"d get cut if it were really hard.

 

[Joe Talmadge]

Sounds like you've been talking to Sal. I remember Cliff mentioning the idea of "engineered failure" a while back, during a review of the Chinook II (I think). Great concept! Kinda like "crumple zones" on a car.

Yah, I've read Sal talking about this, and seen Cliff and Sal go back and forth, but haven't really given it much thought. I came to the conclusion that ideally, I never want the lock to go. Cliff's description of the pins acting like fuses would be ideal.

Joe

 

[kresky]

Originally Posted by Emanuel
None of the above options please!

He should just make the strongest folder possible without any weak points.

It's not a question of weak points. It's a question of which point is the weakest. Every knife is breakable, it doesn't matter how much metal or engineering you throw at it, it will break under some strain. Taking this as a given, I think it's a very good question to think about the manner in which it will break.
If I had the choice of a knife that will break under 1,000 pounds of strain in such a way that it cuts off my fingers, and a knive that will break under 800 pounds of strain in such a way that it doesn't cause any problems, I'll take the second. Neither breakage is likely to happen, but I'll take a 2 in a billion chance of losing my knive over a 1 in a billion chance of losing my fingers.

 

[Blop]

At first sight i would say, the blade should be the first. That would lead to a lock and handle as strong as the folder would be as strong as a fixed bladed knife.

Second sight: Some steel grades do brake into pieces and with the right force, the parts may fly around and hurt the user. German knife magazin mentioned this as they were reporting their Strider vs. Extrema Ratio folder test.

So, only if a manufacturer can adjust the pivots strength in relation to the intended blade strength, the pivot as the breaking point would be more safe.

By theory, the pivot is an option, but i doubt, that will work.

My worrys on shattering steel, does count for fixed blades too. If a fixed breaks at the blade, same risk for the user.

On the other hand: If i use the knife in a cutting move (downward the cutting media) the blade should indicate if the knife is made for this, not anything else. Lateral stress should be indicated by a flexing blade or knife. If a prybar flexes, it surly is not the right tool for the job.

So, again, the blade should go before the handle, spine, pivot or lock.

Todays tolerances allow a replacement blade, so in case you can use the knife with a new blade.

 

[not2sharp]

None of the above options please!

I would prefer the manufacturer spend all his knowledge and R&D into simply making the strongest and most durable lock/blade/handles.

Ofcourse, I realize everything breaks given enough force on it, but I think a quality knifemaker shouldn't think about the right failure mode. He certainly shouldn't be building any weak points on purpose.

He should just make the strongest folder possible without any weak points.

I agree. Knives are not machines used in a uniformed way. There is load applied from all directions and it is hard to predict where the user will be relative to the knife. There is no safe way to break a knife; the closest to it would be to keep the blade soft and tough enough so that it bends rather then breaks. But, even that has its own risks; the blade can slip under pressure and stab the user.

n2s

 

[mnblade]

I really want to meet the person out there who - through legitmate use, even HARD use (notice I did not say "testing" or "spine wacking" or "abuse") - can make a quality modern folder fail. I'm talking Spyderco, Buck, Kershaw, Benchmade and so forth.