Motorized Marble Run Kinetic Sculpture - Archimedes Screw Lift, Flip-Flop, Helix, Zig-Zag, Xylophone

A desk-size marble machine that keeps running on its own. A small N20 gear motor in the walnut base turns a brass Archimedes screw inside a teal tower. The screw lifts one marble per turn to the top, where a flip-flop switch sends the marbles alternately down two different routes: a 4-leg zig-zag on one side, or a spiral helix tower and a ringing xylophone ramp on the other. Both routes end in a collector that feeds the marbles back into the bottom of the screw, and the cycle starts again. Every part prints without supports and fits a 250 x 250 mm bed.

**How it works** - Lift: the marble sits in a vertical groove in the tower wall, so it cannot turn with the screw. The screw flight slides under it and carries it up (one marble per turn, about 18 mm per second with a 100 rpm motor). At the top a 45 degree deflector built into the tower pushes the marble out onto the spout - Drive: a 12-tooth brass pinion on the N20 motor shaft drives a 36-tooth gear printed on the foot of the screw (module 1.5, 3:1). The screw spins on an M3 bolt standing up out of the base - Flip-flop switch: a brass rocker with a centre fin. The fin leans to one side, so the next marble lands on the raised arm, tips the rocker over and rolls out that side. The rocker stays tipped, so the following marble goes the other way. Stop posts limit the swing to 18 degrees each side - Route A: 4 straight ramps in a zig-zag at 4.2 degrees. At the end of each leg the marble hits a stop wall and drops through the open floor onto the leg below. A short feeder chute takes it into the collector - Route B: a 2.75-turn teal helix tower with a raised outer lip, then a ramp with 6 graded brass chime bars as its floor, so the marble plays a short run of clicks on its way to the collector - Collector: a sloping basin with side mouths for both routes. It runs straight into the bottom of the screw, so several marbles can queue there

**Modular keyed tower** - The tower has a grid of diamond slots on all four sides. The zig-zag legs and the switch are held on with 14 printed diamond keys (press fit in the tower, slide fit in the parts), so they can be taken off and moved - The helix tower and the support posts stand on the base. The plinth ring and the helix are screwed down from underneath

**What is in the kit (29 STL files)** - Walnut base (230 x 230 mm, hollow, with room for the motor and the battery box underneath), gear plinth ring and 3 support posts - Teal lift tower (flange, gothic windows that show the turning screw, front sight slit, diamond key slots) and teal helix tower - Brass lift screw with its 36T gear, 12T motor pinion, tower crown with finial, flip-flop rocker and 6 chime bars - Cream exit spout, switch body and windowed cover, 4 zig-zag legs, feeder chute, xylophone ramp frame and collector basin - A set of 14 diamond keys and a tolerance test coupon

**Tolerance test coupon (prints in about 25 minutes)** - Print it first. It has the tower key slot (press fit), the part key hole (slide fit), the pinion D-bore for the N20 shaft, the screw's 3.6 mm axle bore and the N20 gearbox pocket. If a key or the motor is too tight or too loose in the coupon, adjust your printer's flow or horizontal expansion before you print the big parts

**Hardware you need (not included)** - 1 x N20 gear motor, 6 V, about 100 rpm, 3 mm D-shaft (12 x 10 mm gearbox). A slower 30-60 rpm motor also works and runs calmer - 1 x 4 x AA battery holder with on/off switch (or a 5 V USB lead for a 6 V motor), plus a little wire - 1 x M3 x 30 bolt and 1 x M3 washer (screw axle), 1 x M3 x 25 bolt (rocker pivot), 5 x M3 x 16 screws (plinth ring and helix tower) - 4 to 6 marbles, 16 mm (14-16 mm steel or glass)

**Designed and checked** - Every marble path was checked in CAD with a 16 mm marble: at least 1.5 mm clearance to the walls along all ramps, drops, the helix, the switch and the screw lift. Slopes are at least 4 degrees (zig-zag 4.2, switch ramps 5, collector 5, spout and xylophone 6, helix about 14) - The lift screw was turned through a full revolution in 72 steps and checked against the tower, crown, base and basin: no collisions, at least 0.38 mm running clearance. The gear pair was checked tooth by tooth (0.37 mm flank clearance, contact ratio about 1.4), and the rocker through its full swing (0.35 mm clearance) - Fits used: press fits 0.12 mm per side, slide fits 0.2 mm per side, running clearances 0.3-0.6 mm

**Size** - About 23 x 23 x 32 cm assembled

**Colours** - The 3MF has four colours: brass screw, gears, crown, rocker and chime bars; teal towers; cream tracks and switch; walnut base, plinth and posts

**Printing** - No supports needed. Designed and slice-checked in PrusaSlicer (0.4 mm nozzle, 0.2 mm layers, 3 walls, 15 percent infill): all 29 files slice clean without supports. Use a 5 mm brim for the small diamond keys - The towers, screw, crown, plinth ring and posts print upright; the base prints upside down on its top face; the tracks print on their floors; the switch body prints on its back plate; the cover, rocker, gears and chime bars print flat - About 830 g of filament in total (cream about 146 g, teal about 361 g, walnut about 230 g, brass about 97 g) and about 63 hours of printing. The tallest part is the lift screw at 245 mm - PLA or PETG

**Assembly** - Screw the M3 x 30 axle bolt up through the centre of the base, press the N20 gearbox into its pocket from below so the shaft comes up through the top, and lay the battery holder in the hollow base - Press the pinion onto the motor shaft. Put the washer on the axle and stand the screw on it, so the 36T gear meshes with the pinion - Lower the plinth ring over the screw and screw it down from below (3 x M3 x 16). Slide the tower down over the screw onto the plinth pegs, and set the crown on top so the screw's top pin runs in the crown - Press the diamond keys into the tower slots, then push on the switch (with the rocker on its M3 x 25 pivot and the cover clipped on) and the four zig-zag legs. Hook the spout into the top mouth of the tower - Screw the helix tower down (2 x M3 x 16), glue the three posts in place and set the collector basin, feeder chute and xylophone ramp on them, with the basin tongue in the bottom mouth of the tower. Push the chime bars into their slots - Wire the motor to the battery holder, drop in the marbles and switch on. If the screw turns the wrong way (the marbles do not rise), swap the two motor wires

Note: original design built from scratch in Blender; designed and slice-checked in PrusaSlicer, mechanism checked for collisions in CAD; this exact version has not been test-printed yet.