Advice on lockback folder design.

W

WiseDuck

Joined
Dec 9, 2022
Messages
22
Hello again folks!

While I'm waiting for an opportunity to shape my fixed blades, I recently started designing a simple folding lockback knife. I've had to design this within the confines of the material I ordered, for the stainless handles this is 30 x 150 x 3mm and the blade material is RWL34 with some rather generous proportions, enough for the blade and lock bar to be cut out. A friend of mine has offered to plasma-cut everything for me after I submit a final design as DXF. The lock bar will also be made of the same material hardened (professionally by a company I got in touch with thanks to the folks here at Bladeforums!) to the same level. I will reuse the spring from a different folding knife that broke, it fits nicely in the slot but it needs to be shortened down slightly. I design these things in Fusion360 and rotate the blade and lock bar to see exactly how these parts interact with one another and how much the lock bar needs to move to clear the blade and also where the spring needs to be placed. So what I have now is a fully functional plastic folding knife, the lock snaps in place when opened and it also snaps shut. I realize the design allows the blade to come into contact with the backspacer/spring holder if you raise the lock bar and push on the blade. I have a stop pin that'll be right where the sharpening choil is when the blade is closed, this should prevent the blade from hitting anything. I think the tricky part is going to get the geometry right by hand for the blade pivot area, there are a lot of precise angles and such that make it all work. Here's the design. The next picture shows you my V1 print (well, actually more like v6 or so...) while V2 was printed yesterday, since then I have a V3 with some slight adjustments to the pivot area for a better stop against the lock bar when closed. The blade is 111mm from tip to the rear of the tang. The handle is approx 120mm from tip to butt. Is there anything I need to know, or adjustments that should be made? Everything I've made here, has been done through trial and error, and some glances at other knives in this category.

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I personally like folding knives that have a high blade to handle length ratio, unless they're designed specifically for a shorter blade
 
Bill DeShivs said:
You lockback spring is too short. There will be too much pressure on the spine.
Click to expand...
I mean, it depends on the spring, doesn't it? I have a spring from a crappy Bear Grylls knife I dissected and threw out. The spring can be adapted to fit this and it'll be slightly longer than the one shown in the photos above. I'll see if I can make a V3 of the lock-bar and shorten it so it fits this spring I have better, since it's longer the lock-bars pivot may have to be moved towards the blade a bit. I'll post the new design here later once it's done. I may be able to print the lock-bar with 100% infill to get a better feel for whether the spring I have would work or not in the V3 design. I also intend to put together a working knife with some scrap stainless steel I got from work, before I move on to the expensive stuff.
 
Quite simply- make the spring longer. You shouldn't have to change anything, unless it's making the lock bar a little thinner.
 
Bill DeShivs said:
Quite simply- make the spring longer. You shouldn't have to change anything, unless it's making the lock bar a little thinner.
Click to expand...
Here is the spring I have. As you can see, it's a fair bit longer. But I think it's mainly the part past the bend in the middle that does all the springing. So I'm thinking I should adapt the channel for the width of this spring, move the center pivot of the lock bar forward a bit so the leverage from the V2 design doesn't change too much. I do want a very securely locked up blade, not to mention a blade that snaps shut and stays shut with minimal risk of the edge getting exposed from knocks and such. I've had folders before with such weak lockup that the tip of the blade was easily exposed with minimal effort... I don't want that. The current position of the spring channel also has another important function, it severs as the "stop" for the back of the lock-bar, so it can't be pushed too deep and nick the blade.
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Just a comment on your design order. Designing a folder around an existing spring is backwards. Make a new spring to fit the new knife.
 
Stacy E. Apelt - Bladesmith said:
Just a comment on your design order. Designing a folder around an existing spring is backwards. Make a new spring to fit the new knife.
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Oh I know that. But it was the easiest way to go about things since I know very little about springs and what lengths, thicknesses and such are good for what. (And it's unlikely that this spring will stay at this size, I may sand it down, reduce the length and so on to better fit this knife) This spring came out of a knife with roughly the same dimensions (at least in terms of length of the handle and blade), which also led to a design that fit within the confines of my material mentioned in the first post. So it wasn't just the spring, it was a combination of things. I'd love to make a proper spring tho. Once I finish a prototype with the scrap sheet of stainless I have, I can then cut out a spring from RWL34 stainless and try some ideas. I don't know where to start right now though, surely there's got to be some good ratios out there that people stick to.
 
The active part of your spring is about 40% shorter on your model, also consider that where it is touching the lock-bar is further from the pivot and you will therefore be bending the spring further which will increase the stress considerably.

If the spring is of uniform cross section, the point of maximum bending/stress will be where it enters the backspacer, this is where it would most likely fail.

Best to design and make a dedicated spring. For simplicity you could use some grade 5 titanium or some piano wire - neither would need any heat treatment.

If you must use that spring, make it as long as possible and possibly taper it which will spread the stress more evenly and also reduce the spring force.
 
Mike Read said:
The active part of your spring is about 40% shorter on your model, also consider that where it is touching the lock-bar is further from the pivot and you will therefore be bending the spring further which will increase the stress considerably.

If the spring is of uniform cross section, the point of maximum bending/stress will be where it enters the backspacer, this is where it would most likely fail.

Best to design and make a dedicated spring. For simplicity you could use some grade 5 titanium or some piano wire - neither would need any heat treatment.

If you must use that spring, make it as long as possible and possibly taper it which will spread the stress more evenly and also reduce the spring force.
Click to expand...
Some solid advice there! Over the weekend I'll make a V3 that takes the current springs length into consideration. I'll also check the previous knife again to see where the spring bends and how far to see what mods are needed. (such as pivot location, spring length etc) I also happen to have received a length of thin titanium the other day meant to be shaped into a pocket clip. But it seems like I can just buy a pre-made one instead and adapt the piece of titanium for this purpose instead!
 
Hello again folks!

So, I have a few more questions, but first an update. I've been working on a prototype using some scrap stainless from work. It turns out this stainless steel was extremely hard and chewed up a good number of drills and such while working on it. However, I ended up with an extremely hard template that's difficult to screw up once its done. I've begun the process of using this prototype to transfer the finished shapes and hole placements to the actual material that will be used in the finished knife. 4 out of 6 parts have been made so far. I'm also using the prototype to figure out the lock-bar tolerances, how to best shape the locking interfaces, spring placement, length and many other things. It turns out that both the spring from the other knife I butchered AND the small length of 1.15mm titanium I had works as the spring. The springs are both quite strong which is exactly what I wanted. And the blade will be thinner and have less material in the finished knife and also run on phosphor-bronze bushings so the spring should have even more snap. All that's missing in the prototype is a blade stop which will go where the sharpening choil is, this will be the very last addition I make to the final handles as this very much depends on how I grind the blade pivot area and such...

Video of the current prototype:

With that said. Once it's time to send the blade and lock-bar off for heat-treatment, (I will make one set in X50CrMov15 and one in RWL34) how much material should I take off? Will I have to get the lock-bar and knife interface 100% done and calibrated before HT, or leave 0.1-0.2 mm and do the final fine-tuning once I have the heat-treated parts? Also, as I understand it, stainless steels are heat-treated through and through unlike carbon steels which only get hard on the surface and a short distance into the material. So I can't end up with an area that is not hardened if I grind a bit extra to say, get the lock-bar right and so on?
 
Low carbon steel is sometimes case hardened-which is a surface hardening treatment.
I suspect your springs are high carbon steel.
We have been making hardened carbon steel knives for over 1500 years.
 
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