Tissue box insert

You'll need: 7x M2 screws for the rollers 8x M4 screws for assembling the frame Braided fishing line or some other thin thread A weak tension spring, length to be determined experimentally. I used a stainless steel spring, 0.4 mm wire dimeter, 3.5 mm outer dimeter. Oil to lubricate the rollers You don't need any threaded inserts. All the screws screw straight into the plastic. The roller holes need to be drilled to size with a 2 mm drill bit. The blue dashed line is a tension spring. The side I'm showing is the more complicated side. The wires in orange and magenta are routed to the opposite side, where there are two rollers to turn each of them downwards to support the corners on that side. I already had a certain length of that spring left from a longer coil, and my intuition was that it had a suitable amount of stiffness so that there exists a length of it that would provide the right amount of force and fits in the frame I printed. Specifically, in the spring formula f = k.x, I can control k by the (neutral) length of the spring however I want, provided that it fits in the frame. I also wanted a little bit of bias force when the platform is at the very top, so that requires an offset to x. This means there are two variables to play with: the (neutral) length of spring (which controls k), and how much stretch there should be when the platform is at the top position (which controls the x offset). The first step is to make sure that the 4 corners of the platform are always level. I did this by tugging on the wires until the platform is firmly resting against the top, then tying the wires together as a single bunch. I then experimentally found out how much of the spring to use (which controls k) and not worry about the x offset. I had the bunch of wires tied to one end of the spring, and I pinched the spring with my fingers at a certain length at the other end. While pinching the spring, I moved the pinched end of the spring to different positions to find one where the platform is lightly biased to the top and descends to the bottom when fully loaded with paper. I then tried pinching the spring at a different length and repeated the exercise until I found the optimal spring length. That fixes k. I then snipped off the excess length of the spring and hooked it up to the frame, and cut off the previous knot. I determined the tying position by looping all 4 wires through the eye of the free end of the spring, then pulling them until I got the right amount of bias when the platform is at the top position. I then made a mark on the wires where the knots need to be. This fixes the x offset. I then tied them to the free end of the spring.