Geeky question about framelocks

[hank_rearden]

which is the ideal side for milling the channel on the lockbar, the outer side or the inner? my own WAG would be 'it doesn't matter.' i'm not an expert on steels (obviously.) i'm just making an analogy with wooden self bows. the 'bent' side consists of heartwood which resists compression while the 'bow' side has the sapwood which resists tension. we know of course that removing excess wood (tillering) is done on the bent side, leaving the sapwood untouched.

still comparing the lockbar to a bow, shouldn't the more ideal machining method entail tapering the lockbar thickness gradually from the base all the way to the bearing surface which engages the blade tang? thanks.

[singularity35]

If I had to make a guess, I'd say the outer part leaving a straight line from point of contact to the thinnest part.

[MikeC]

I've talked to mechinical and structural engineers I work with and they both said it doesn't matter

[whateatsrabbits]

I like to looks of the inside cut out, but I think the outside cutout works better.

[RevDevil]

I've talked to mechinical and structural engineers I work with and they both said it doesn't matter

I've also heard this from some MEs. I have no real preference myself, so long as the lockup is solid.

[65535]

If milled to an appropriate thickness it shouldn't matter.

Milling the outside gives you a linear compression force on the cross section of the thinnest part of the lock where milling the inside will give you a more angular compressive force and could cause it to bend out and fail.

Doubtful any but the most abusive users (the ones that shouldn't be using edged tools in the first place) would ever have an issue either way.

Also consider the design of the knife if the milled area will throw off your design.

[marthinus]

I've talked to mechinical and structural engineers I work with and they both said it doesn't matter

Same here.

[bdmicarta]

I've talked to mechinical and structural engineers I work with and they both said it doesn't matter

I'm a structural engineer and I believe that in theory it does matter. In practice maybe the difference is not significant, I don't have any way to test it. In terms of looks I prefer the cut to be on the inside and I have a Bradley Alias I clipped in my pocket right now, with its cut on the inside. Among other popular knives, the ZT 0560 is cut on the outside, Spyderco Military and Sage Ti are cut on the inside, Hinderer is cut on the outside, Strider is cut on the outside, Sebenza on the inside, so there is no consensus among these guys.

Milling the outside gives you a linear compression force on the cross section of the thinnest part of the lock where milling the inside will give you a more angular compressive force and could cause it to bend out and fail.

This isn't a very technical explanation but I think you've got the idea. If you cut the outside of the frame then the material remaining is closer to the line of force from the blade, if you cut the outside then the material remaining is farther from the line of force, in other words it has more eccentricity. The only photo I've seen of a failed frame lock had the lock failing in a sidesway mode, and a compression member with higher eccentricity involved will have more lateral force to go with it which would make it more prone to a sidesway failure. I don't think there is much difference in a knife application and there might not be enough to matter, but theoretically cutting the outside would be stronger. There are a lot of other parameters of the geometry of the blade, pivot and frame lock that would probably matter as much or more than whether the lock is milled on the inside or outside.

[hank_rearden]

...but theoretically cutting the outside would be stronger. There are a lot of other parameters of the geometry of the blade, pivot and frame lock that would probably matter as much or more than whether the lock is milled on the inside or outside.

i suppose my proposal for a gradual reduction in thickness throughout the lockbar is impractical to manufacture?

[brandonc]

The consensus seems to be that either it doesn't matter or that it is better on the outside. The main makers of heavy duty framelocks, ZT, Strider, and Hinderer all do it on the outside and claim it is stronger. I carry a ZT and like that it is theoretically stronger.

[pialia]

i suppose my proposal for a gradual reduction in thickness throughout the lockbar is impractical to manufacture?

I've seen(and started one myself) several threads on exactly this topic, but never did the thought occur to me that you could use the same mechanics of a bow. Interesting. Would love to hear an engineers view on this, ignoring of course the manufacturing difficulties it might bring

[Haze]

On a folding knife used by human strength hands it doesn't matter in reality, theory can be and is often quite different of course. In reality when it reaches the point of failure it will fail regardless of it being cut from either side or both sides and met in the middle, the load is the same and the materials ability to hold that load is not changed and I would say they will all fail similarly. You can mull and kick the theory of it about all you want, I'm happy in the reality based assessment. I personally prefer the spring cut on the inside as it makes for a cleaner looking knife, FWIW.

i suppose my proposal for a gradual reduction in thickness throughout the lockbar is impractical to manufacture?

It would take more work but this is how I have thought about doing it. I have a frame lock proto on the bench just now that will have that done to the lock bar.

[Socom Elite]

Doubt it really matters. Framelocks really aren't that strong of a lock anyways and are usually made wrong.

But I prefer the CRK style on the inside as it looks cleaner like Haze said and its a lot nicer on pockets.

[Chris "Anagarika"]

As with anything mechanical, the easiest way is to make the case extreme.

Imagine a lock bar that is 10mm thick for a normal 5" folder. If the cutout is 7mm on the outside, how would it look like, an where the forces are going to be distributed when the engagement with blade tang is 2mm, and when it is fully engaged at 10mm (assuming thick blade, 0.5"). Remember to include the angle of the bent. Compare to the cutout on the inside.

As negligible as it is, cutout on outside should be stronger (haven't done all the math, simply looking at the structure & direction of forces) The same approach can be done to understand micro bevel, convex grind, etc.