Calling all steel experts ... or anyone with some knowledge!

[coyote711]

I just tried the trick, over the last couple of days, of leaving a Case knife half open to soften the snap of one of the blades. It worked. What exactly did I just do?

On farm implements we have some parts we call spring steel - under pressure it will give, but not break, and then snap back into place. (I imagine similar to the leaf or coil springs on a car or truck). I'm assuming the backspring on a knife operates under the same principle. I don't know if it's the composition of the steel, or the heat treating, or a combination that gives it its "springiness", but would I be doing any permanent damage to a knife by doing what I just did? And what exactly did I just do - rearrange the molecules of the steel? Weaken the temper? And not that I would ever do this, but what if you forgot and left a knife in this half open position for a month or two? Would you end up with a knife with absolutely no snap at all? Is this procedure progressive and time dependent?

Sorry if these sound like stupid questions, but they are honest ones. Just curious.

 

[cnas122]

I've had the spine of several knives break on me over the years, but have no way of knowing whether it was just due to defective manufacturing, heat treat, etc. I tend to think it's best not to put undue pressure on the spine...but that's just IMHO

 

[Chewy1]

Whatcha got here is set. As in "it didn't spring back all the way cause'n it got some set in it." Set is purty hard for lots of folks to git, but in easy werds the spring doesn't spring back all the way. We say that's set. Here's to hopen I didn't make it too difcult on ya.


Seriously, I have absolutely no idea why this happens. Let's say you put your knife in a vice and bent the blade half a degree. You could probably do this a thousand times without seeing the blade return to anything but 100%. Now, if you overbend the blade past what it's designed to handle, say 15 degrees, you'll see it take a permanent angle change...it won't return to straight. If a knife spring is operating within it's optimum stress range, my thought is that you won't see any permanent set and it won't get any easier to open your knife. However, if your backspring is not sufficiently flexible, it will take a set if left half open and will open easier the next time. I would think that over time, these knives could see more backsprings break because they've been overstressed just by opening and closing them. But I'm am completely guessing!

 

[ohski]

I tried this with a new custom I got... Didn't work...! 3/4 open for over a week, nothing.

Ski

 

[coyote711]

Whatcha got here is set. As in "it didn't spring back all the way cause'n it got some set in it." Set is purty hard for lots of folks to git, but in easy werds the spring doesn't spring back all the way. We say that's set. Here's to hopen I didn't make it too difcult on ya.


Seriously, I have absolutely no idea why this happens. Let's say you put your knife in a vice and bent the blade half a degree. You could probably do this a thousand times without seeing the blade return to anything but 100%. Now, if you overbend the blade past what it's designed to handle, say 15 degrees, you'll see it take a permanent angle change...it won't return to straight. If a knife spring is operating within it's optimum stress range, my thought is that you won't see any permanent set and it won't get any easier to open your knife. However, if your backspring is not sufficiently flexible, it will take a set if left half open and will open easier the next time. I would think that over time, these knives could see more backsprings break because they've been overstressed just by opening and closing them. But I'm am completely guessing!

Are yew pokin' fun at me 'causen I'm jest an ole farm boy from Ioway? Darned high falutin' Ohioans!

 

[Chewy1]

Why didn't anybody tell me this here was a test? Was an Ioway? "Margaret! Git my dicktionery. I'm thinking this feller jest calt me a name. Was an Ohian?"

 

[DennisStrickland]

springs may take a very long time to 'relax' some .consider the magazine springs in pistols, they do relax some but usually that takes years. i'm surprised the metallurgists have'nt chimed in since they could put light on this subject. i'm relatively sure that the stress over long periods of time & use incurs metals work to harden & eventually fail.

 

[artmichalek]

springs may take a very long time to 'relax' some .consider the magazine springs in pistols, they do relax some but usually that takes years. i'm surprised the metallurgists have'nt chimed in since they could put light on this subject. i'm relatively sure that the stress over long periods of time & use incurs metals work to harden & eventually fail.

Absolutely not. Work hardening requires plastic deformation (meaning you have to bend enough that it doesn't spring back), which does not happen over time. If a spring is deforming enough to work harden, it's going to do so the first time it's deflected. The "relaxation" in magazine and knife springs is the result of wear on contacting surfaces. No metal will creep at any kind of temperature low enough to safely store a knife or gun. The only way that prolonged static stress below yield and at room temperature will cause a piece of metal to fail is if there's already a crack in it. The stress can pull the crack open allowing the inside of it to corrode, which then makes the crack grow. Most of the time when you find a knife with a broken back spring, this is what happened.

I can't think of any reason why leaving a knife half open will change the snap. The best I can figure is there was a bit of buffing compound left on the tang or back spring, and just the act of opening and closing the knife a few times wore it off.

 

[ken erickson]

I can't think of any reason why leaving a knife half open will change the snap.

This has been my experience also. I used to work on and replace heavy tension v shaped leaf springs used in side lock quality shotguns and drillings. Most of the time when a spring failed I could contribute it to corrosion.Let me say that this is assuming the spring is heat treated to proper spring hardness to start with. I have taken apart many of these firearms and I am sure many had been stored for days, months, years in the "cocked" or compressed position without me being able to say they lost any tension from new. In the past I have thought about building a testing station using a low rpm motor with a cam that duplicates the lift that a typical slipjoint imparts on a backspring, add a counter and let it run till it breaks(if ever) to see how many cycles a typical backspring will endure before failure.

Interesting subject.

Ken

 

[coyote711]

Thanks Art & Ken - interesting stuff. :thumbup: The knife I did this on was an older Case knife and it sounds like there must have already been a crack/corrosion on the backspring. That makes sense to me, as I have tried this same "trick" on a couple of new knives, which resulted in no change at all to the snap.

p.s. Over the course of the past few months I had flushed out the tang & backspring with various solvents and cleaners, so that is why, based on your information, I suspect it was a cracked backspring rather than crud or buffing compound left in there.

 

[artmichalek]

In the past I have thought about building a testing station using a low rpm motor with a cam that duplicates the lift that a typical slipjoint imparts on a backspring, add a counter and let it run till it breaks(if ever) to see how many cycles a typical backspring will endure before failure.

I think that what constitutes "typical" is the most interesting part of that question. As long as the tensile stress (which is not hard to calculate) in the spring stays below around a third of the yield stress it should go forever. Changes in the pin layout and spring thickness can give a big range of stresses for the same tang force and lift, which could make certain patterns much more prone to breakage.

 

[DennisStrickland]

thanks guys for adding to my knowledge. i assumed that the metal got "tired"

 

[Chewy1]

A follow up question then. Could a certain manufacturer have a batch of steel with a very low "yield stress"?

 

[joe-bob]

I know that it doesn't make sense that springs should give or sag, but the suspension springs in passenger cars do it all the time, and it only takes a few years.

For racing cars, we found that some brands of calibrated "racing" springs were unuseable, because they would take a set after using, so the ride height could not be set reliably because it would change. We attributed this too poor quality, although we never did scientifically test them...it was cheaper to simply buy a different brand.

On a knife, I would guess that either the spring is not at the proper hardness, or that it is much too close to it's maximum deflection.

 

[artmichalek]

A follow up question then. Could a certain manufacturer have a batch of steel with a very low "yield stress"?

A bad batch of steel or botched heat treat is entirely possible.

I know that it doesn't make sense that springs should give or sag, but the suspension springs in passenger cars do it all the time, and it only takes a few years.

For racing cars, we found that some brands of calibrated "racing" springs were unuseable, because they would take a set after using, so the ride height could not be set reliably because it would change.

Passenger car suspensions are prone to both rust and contact surface wear. For a brand new racing spring to behave that badly, the adjustment ring must have been moving around on its own.

 

[joe-bob]

Passenger car suspensions are prone to both rust and contact surface wear. For a brand new racing spring to behave that badly, the adjustment ring must have been moving around on its own.

That's why we always use twin spanner nuts and lock them down. That's how we knew the springs were junk. (I never said I didn't know anything about racing cars. )

 

[malextwo]

Auto knife owners are always encouraged to store the knife open. Am I to understand that that it technically not necessary?

Mike

 

[dalee100]

Hi,

Technically, if the springs is made right and the load is proper for the spring, closed storage shouldn't hurt it. But such mechanisms are hand made and hand fitted by in large. So there can be large variations in construction quality. And flat springs are notoriously difficult to make consistently. That's why coil springs have supplanted flat springs where ever possible.

dalee

 

[neeman]

I work in Hi-Tech.

We have some very expensive floatation devices with coil springs.
They definetly have a break in period and 'set', and need to be adjusted after a certain amount of time.

 

[artmichalek]

I never said I didn't know anything about racing cars.

Sorry, I didn't mean to imply that. It's just that coil springs are supposed to be proof loaded at the factory to eliminate any chance of plastic deformation. Any variability must be coming from something else in the assembly.

Auto knife owners are always encouraged to store the knife open.

Sounds awfully hard on the pockets