I have noticed that it usually takes 5 tries to get a design right. Sometimes these can be total rewrites of your software library, or like this time around, it took me 5 tries to design a pair of scissors.
Here's my first three attempts.
The first design was total disaster. For some reason I designed the scissor blades the wrong way around! Silly me. In my very first attempt I did not realize that both blades are actually printed laying on the same side, that is, the second blade is not mirrored.
In the second design I also tried to address what happens when you press the handles together. I made the handles twice as think as the blades. This worked well, but the handles were too close to each other, there needs to be gap.
In the third version I separated the sides a bit and added a small nob there so that the handles would stop at right angle. Still not working, though! The handles are too close to the joint. That prevents the blades to open enough and there feeling is not quite right. It needs more leverage.
Here's the final design. I moved the handles a bit to the back. It turns out that the secret to make intuitive to use scissors is that distance from the joint to your fingers should be the same as the distance from the joint to the point which you usually use to cut.
I used the same joint mechanism as I used in my Strandbeest prototype. There are two washers which are fastened together with an 12mm M3 screw and everything is slightly loose fit so that the blades rotate even if the whole package is fatened tight.
So this is how the design process usually goes. You make a desing, make it, analyse the problem and improve it. It is super rare that you get something right the first time you try it. You usually find the most glaring problems initially, sometimes it takes few months of use to find the final tweaks.
There is one more thing I would like to improve about the scissor design. For right handed scissors the topmost blade should be on left side. This allows you to apply some force to push the blades more tightly together and make more accurate cuts. That would be the 5th, perfect, design :)
You can find the Toddle Scissor Tinkercad project here. I challenge you to improve the design!