The speaker explains the critical importance of the pivot point in scissor design. He demonstrates how a simple nut and bolt fastener loosens over time, dismisses rivets as unserviceable, and then presents the 'D-shaped sex bolt' as the ideal solution. Using 3D models, he breaks down how a shoulder bolt with a D-shaped profile, combined with a matching notch in one scissor half, mechanically locks the components to prevent unintentional unthreading while still allowing for disassembly and servicing.
Post caption
Even more important than sharp blades is good pivot hardware. As we’ve discussed in previous videos, scissors don’t just cut like a knife. They make use of shear force - each scissor half pulls the material in a different direction and it shears apart. Even if your blades are sharp, a loose fastener prevents a tight pass of the blades, leading to an unsatisfying slice.
The D-shaped sex bolt is our favorite way to solve the problem of unthreading without removing the threads. It ensures a tight and clean shear through years and years of use. And if your scissors ever need servicing, it allows them to be taken apart. Please see the pinned-comment on this video for notes on disassembly.
If you’re in the market for any pivoting object, we recommend taking a close look at the fastener to see if they are designing for unintentional unthreading.
Transcript
The most important better most often neglected part of a scissor design is the pivot point. If your fastener is just a nut and a bolt as demonstrated in this oversized model, the opening rank closing of the scissors is applying that rotational force rank it is unthreading your fastener. Considering this problem, you may think it's better to just forgo the threading and just permanently rivet the two halves of the scissors together. You have, like, solved the problem of the scissors accidentally unthreading, but it's a permanent fastener. You can't remove it. You also want one that is removable when you need it to be, where you can disassemble the fastener, and then reassemble them after servicing. The challenge is finding a threaded fastener that won't accidentally unscrew itself. There's a few ways to do it, but my favorite in all the world is the D Shave Sex Pull. In the current pivot sandwich that we have here, there are four distinct components. They all have the ability to rotate independently and free of each better, and that's the root of the problem. You only need two of them to actually move. You need one half of the scissor to be able to rotate relative to everything else, and then you also need the nut to be able to unthread to remove. The way we do that is with two fairly minor changes. So first, we're gonna change from this bolt to a shoulder bolt. You're also gonna cut this notch out of the end of the bolt, which gives it this d shaped profile. And when you include a similar matching notch in the whole of the top layer of the scissor half, suddenly you have, like, rotationally locked these two components together. And so as this lower half of the scissors rotates open and closed, it's rotating against two surfaces that are mechanically locked, and so there's no possibility of accidentally backing out the bolt itself. You may think that there's still this potential issue, the nut accidentally unscrewing itself, but this how the scissors is rotating exactly with the best, so it can't apply relative motion to the nut that is different from what is being applied to the bolt. They're moving together. The only way to unthread the bolt is to intentionally go in and unthread the nut. The spinnyness has been correctly isolated to safe places, but I can't sleep well yet. There's an exposed best, and it's not a very compact or finished looking end to the fastener. So we're gonna take the male threads off of the bolt, and we're effectively gonna put it onto the nut. Similarly, we're gonna pour out the bolt rank thread the interior. This is a sex bolt. It still has all of the kind of like mechanical solution, and it gives you this nicely finished face on both sides. It looks beautiful. It's never gonna accidentally unscrew itself. And because it's like a threaded fastener, someone with the right tools can fully disassemble it, and service the scissors, and then reassemble the fastener. And that is why that is the construction that we used as, like, the heart of the Chroma scissors, the V shaped sex bowl, the Ferrari of pivoting fasteners.
