I heard we were doing homemades.
Hi, I’m dual_sport_dork. You may know me from such projects as, A Bullshit Plastic Knife, and This Other Bullshit Plastic Knife.
Now that I think back on it, it turns out I’ve been at this rather a long time.
I no longer own this… object… so I had to dig the pictures out of my digital reliquary. The reason this looks like it was photographed with a potato is because it was. (Actually, it was my wizened and venerable Cannon PowerShot A40.) I had to check the file date — I made this in December of 2002. This son of a bitch is old enough to vote.
I’m pretty sure the impetus for this was in response to people on the internet back then moaning about my locality’s recently passed knee-jerk ban on nonmetallic knives. So this is a fully functional, for acceptably small values of “functional,” frame locking folder made out of Plexiglass and nylon screws. The edges of it look mildly burned because I hogged most of it out by hand with a Dremel. I planned nothing and measured nothing; I just threw this together by the seat of my pants and it more-or-less worked.
It opened, closed, and locked. It also sported something approaching an edge. I don’t know if you’ve ever tried to sharpen a chunk of 0.100" Plexiglass, but the long and short of it is that it would keep an edge on it just about suitable for use as a letter opener. Hey, it was the principle of the thing.
I’m tickled pink to rediscover that even back then I apparently had a penchant for taking pictures of stuff on a white sheet of paper. It seems old habits die hard.
You must log in or # to comment.
I mean, as you said this would function perfectly as a foldable letter opener,
How did you make the frame lock? I see the cutout, but did you heat that tab and bend it inward toward the center or something?
Yep, exactly that. I just lightly cooked it with a heat gun and bent the lock part up a little bit.
Neat! Thanks for sharing
Wow, this is really neat. Sounds like it’s time to 3D print an updated version! 😁
I’ve applied some thought to this, actually. Not necessarily this thing in particular but liner locks in general.
It turns out liner locks are really a bitch to do with FDM printing for a couple of reasons. I’m pretty much decided they’re a no-go altogether, in fact.
Problem one is that it’s basically impossible to orient a liner lock mechanism for printing. The inherent shape is the definition of the kiss of death for filament printing: You need a long gently curved section that’s separated from the main body by an equally long slot. To avoid layer separation issues, you really want to orient this such that the slot is parallel to your layer lines, but that requires printing either the lock or the liner body with a significant portion of itself hanging in midair and unsupported on one end. This is impossible unless you use a ton of supports, which results in an ugly final product. The other option is to print the liner flat on the bed with the lock bar rising upwards, which results in a really shallow overhanging angle which is likewise impossible to print without supports.
But the main nail in the coffin is that PLA, the most common material, is famous for “cold creeping,” i.e. exhibiting permanent deflection if it’s bent and remains under load, even at room temperature. A liner lock mechanism is under load all the time whenever the blade is closed, with the lock bar straining against the heel of the blade all the time. This is a textbook example of a cold creep pitfall. Even if you managed to print such a thing the lock bar would turn into a wet noodle and after leaving it stored for probably even only a few hours it’d take a permanent set and wouldn’t lock anymore.
Acrylic (plexiglass) takes a lot more of a bend and/or exposure to way higher temperatures before that becomes an issue. You can mitigate it somewhat by printing in something other than PLA, but the more complex your part is the more of a pain in the ass that becomes. I believe ABS/ASA exhibit the lowest amount of permanent deflection under load out of the commonly available printable materials, which is why I make my Rockhopper liners out of it. But even it takes a noticeable set after a few days which is a surprisingly short amount of time. With how little I bend the spring bar in a Rockhopper it doesn’t take enough of a set to cause the spring action to quit working, but the difference is still noticeable.
A printed liner locker might work in ABS but given all of the above I haven’t been arsed to try.
