Angled Lock Face vs. Radiused Lock Face on liner and frame locks.

[Fred Sanford]

So what is the deal with a flat lock face vs. a radius lock face on a liner lock or a frame lock?

AFAIK, Chris Reeve, Spyderco, Benchmade, and a few others use a radiused lock face. Kershaw, ZT, Strider, Hinderer (I think) use flat angled lock faces. I made my own little model out of some pieces of paper that I cut up and pinned the paper lock bar in place and as it travels it seems that the only good way to make it contact the blade tang the entire time is to cut a radius. With a flat angle the lock bar only touches the blade tang entirely at one point. Before that point and after that point the lock bar only touches with it's corners. In the past I have had a few knives that I have been able to put pressure with my hand on the back of the blade and then watch the lock bar slide across the blade tang (lock face) in the direction of unlocking. I have seen this only on knives that have had either poorly radiused tangs or flat angled tangs. It seemed that it would be due to only a corner touching the blade tang and physics. The fact that you put pressure on the blade which then puts pressure on the lock bar and then if it is not square, the lock bar slides in the direction opposite the force.

With all that said, why do people make liner locks and frame locks with flat angled lock faces? Recently I heard that Strider changed to that from a radiused face. Seems like it's going backwards to me.

This is not a bashing thread on Strider or any other company that uses the flat angled lock face. I'm just looking to understand why they use it instead of a radius?

If you fix a point (the non moving end of the lock bar) and you move the other end of the object (the locking part of the lock bar) it moves in a circle. It creates a radius. So shouldn't the lock face match the radius of the moving bar?

I will post an illustration later for anyone that doesn't understand.

Thanks!

 

[Reeek]

Please forgive these two exceprts from past posts of mine because they are a little out of contextt with your thread but not completely. These do describe my take on the new trend by some MFG's and makers to a non-radiused lock bar interface and why I think it works for some knives while other knives still work well with a radius tang grind. These are copy and pastes of my past replies in other threads. This is my take on the matter:

"What happens with the old Strider lock bar interface (radius tang grind) is when the lock bar works itself into the "cup" on the tang over time and use, the lockbar is too short by an extremely small amount but enough to translate into the vertical play you get with lock rock. A flat angle grind on the tang and sometimes the lock bar face too, prevents this from happening as much or at all. That's why the Strider fix is a larger stop pin. That places the lock bar face back to the left and outside of the "cup" again. In effect, the larger stop pin lengthens the lock bar again by creating an earlier lock up by virtue of the blade stopping earlier.

From my experience, only the Spyderco Military manages to do the radius tang grind effectively and without a reputation for eventual lock rock. This is probably because it uses a steel lock bar that is thin rather than thick like frame locks can be. The thin, steel lock bar seats itself much better inside the "cup" and stays within a tight fit. The steel lock bar doesn't wear as fast either. Other than that, what I see is many makers and MFG's are abandoning the radius tang grind in favor of the straight angle now."

"Many are moving away from the radial grind but I also believe the problem is exacerbated with thicker, titanium lock bars such as on many frame locks. That is just my own observation. I have seen it mostly on frame locks but some liner locks too. If the lockbar effectively becomes shorter when it works its way into the cup of the radius (50% lockup, give or take) then there is nothing you can do except a repair. I have seen older Striders where the lock bar sat in the cup of the radius and didn't have enough grip or lock bar spring tension to grab the steeper angle of the tang on the right side of the grind if you look at the lock up with the lock bar facing up. In some bad cases like this, the lock bar can just float in there. Climbing or gripping that steep grind on the right side of the radial tang grind requires a lot more lock bar tension than is typical. You can try bending the lock bar but that requires disassembly and will probably be temporary in some cases. The reason "I" blame the thicker titanium lock bars is because Ti wears faster than steel lock bars and they wear more on the inside part of the lock bar face, so as the lock bar face is working its way rightward with use, material is wearing away as well, making the contact point effectively shorter. By the time the lock bar face is in the cup of the radius there is less material left and you have a shorter lock bar that also mimics the radius of the tang grind in geometry now. That's just a personal, uneducated assumption of mine

I'm sure the geometry of individual MFG's/maker's grinds has a lot to do with this happening or not too.

If you study the Spyderco Military, where I personally feel the radial tang grind works as it was intended, you will see a thinner, steel lock bar and a lock bar with a LOT of spring tension. I believe this is why Spyderco is succesful with this tang grind method on the Military. I have never had a Military out of a dozen or more, that wasn't rock solid for as long as I owned them.

Degree of angle means a lot too. Even on straight angle tang grinds. I have seen various proclamations of what that angle should be but I have seen 8 degrees mentioned more than others although most I have read are in the 7-10 degree range. You aren't just chasing solid lock up, you are also trying to achieve a balance between lockup and smooth lock bar release.

As simple as a frame lock or liner lock may seem, there is a lot going on and the slightest of missed tolerances translate into things knife lovers don't like . . . It's a fine balancing act to properly set up one of these locks and make them last too."

 

[Lycosa]

Great post, man. ^

 

[Gareth Bull]

Great post Reeek.

I feel it mainly comes down to simplicity at the end of the day. It's a whole lot easier to set a flat grinding machine like a disc sander to the right zone (I believe in 7.5 - 8.5 degrees) than it is to set up a machine to give a radius. Unless the system has changed CRK employs flat locking faces. To the best of my knowlege, they have a jig set up on a reciprocating surface grinder where they pop the blade into place using the pivot hole as the locator, clamping it in place and the blade runs back and forth under a high-rev grinding wheel.

As you pointed out, a pivoting bar, especially in framelocks where the "hinge" is more prominent (in linerlocks the whole spring bends) the length of the lock dictates the radius required. As a result, if I were to offer various sized models of the same knife it stands to reason that I would need to calculate the new radius size for each lock and get the corresponding wheels. From there you'd have to set the machine up in such a way that the radii DIDN'T actually match, if they did, the slightest bit of wear would have the lockbar touching the opposite side of the frame very soon. There needs to be some extra meat to compensate for wear and tear.

This problem is probably easier for big companies like Spyderco to deal with as they can afford to set a machine just so and do large runs. Smaller companies need the freedom with tools and machinery. Any engineer will tell you, set-up time's the real killer.

In Bob Terzuola's book he outlines using the radiused lockface and actually exagerating it thereby creating a cup that will all but negate accidental disengagement of the lock. As Reek's post noted, it's more effective with thinner locks such as linerlocks than framelocks.

One thing to note however is that due to the nature of a flat lockface, the lock naturally gets "tighter". If you do draw it up you'll see the effect. Basically because it is a segment of that arc created by the rotating bar, it takes a LOT of wear for a good framelock to wear out naturally. When I say "good" I mean properly finished locking faces with the sharp edges knocked off so that the hardened blade doesn't eat into the lockbar. If all those boxes have been ticked, a flat lock should give just as good service as it's curved counterpart or at least be close enough that the vast majority would be hard pressed to tell the difference.

Gazza

 

[Lycosa]

Gazza- Thanks for the add-on.

 

[Reeek]

Thanks, Gazza You too , Rolf

Great post Reeek.

I feel it mainly comes down to simplicity at the end of the day. It's a whole lot easier to set a flat grinding machine like a disc sander to the right zone (I believe in 7.5 - 8.5 degrees) than it is to set up a machine to give a radius. Unless the system has changed CRK employs flat locking faces. To the best of my knowlege, they have a jig set up on a reciprocating surface grinder where they pop the blade into place using the pivot hole as the locator, clamping it in place and the blade runs back and forth under a high-rev grinding wheel.

As you pointed out, a pivoting bar, especially in framelocks where the "hinge" is more prominent (in linerlocks the whole spring bends) the length of the lock dictates the radius required. As a result, if I were to offer various sized models of the same knife it stands to reason that I would need to calculate the new radius size for each lock and get the corresponding wheels. From there you'd have to set the machine up in such a way that the radii DIDN'T actually match, if they did, the slightest bit of wear would have the lockbar touching the opposite side of the frame very soon. There needs to be some extra meat to compensate for wear and tear.

This problem is probably easier for big companies like Spyderco to deal with as they can afford to set a machine just so and do large runs. Smaller companies need the freedom with tools and machinery. Any engineer will tell you, set-up time's the real killer.

In Bob Terzuola's book he outlines using the radiused lockface and actually exagerating it thereby creating a cup that will all but negate accidental disengagement of the lock. As Reek's post noted, it's more effective with thinner locks such as linerlocks than framelocks.

One thing to note however is that due to the nature of a flat lockface, the lock naturally gets "tighter". If you do draw it up you'll see the effect. Basically because it is a segment of that arc created by the rotating bar, it takes a LOT of wear for a good framelock to wear out naturally. When I say "good" I mean properly finished locking faces with the sharp edges knocked off so that the hardened blade doesn't eat into the lockbar. If all those boxes have been ticked, a flat lock should give just as good service as it's curved counterpart or at least be close enough that the vast majority would be hard pressed to tell the difference.

Gazza