Sliced using Cura 2.3.1 and printed on both an Ultimaker 2+ Extended and an Ultimaker 3 Extended at .1mm resolution. The "Spring.stl", "Gear Spring.stl" and all axles were printed with 100% infill, the remaining parts at 20% infill. Prior to assembly, test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding may be required. Study "Assembly.stl", carefully noting the locations and positions of the various components as assembly proceeds. I assembled Table Top Tri-Mode Spring Motor Rolling Chassis as follows: 1) Press both "Gear Axle Rear.stl" onto "Axle Rear.stl". 2) Assemble "Spring.stl", "Gear Spring.stl" and "Cap Gear Spring.stl" onto "Axle Spring.stl". 3) Press "Axle Gear Idler Small.stl" into "Gear Idler Small.stl" for both gear idler small assemblies. 4) Press "Axle Gear Idler Large.stl" into "Chassis Right.stl". 5) Press the axle spring assembly into "Chassis Right.stl". 6) Position one of the gear idler small assemblies into "Chassis Right.stl", then press "Chassis Center.stl" into "Chassis Right.stl". Note the gear idler small assembly must freely spin and slide in the guide slot. This is the "Drive Pinion". 7) Position one "Gear Idler Large.stl" onto "Axle Gear Idler Large.stl". 8) Position "Spacer Axle Gear Idler Large.stl" onto "Axle Gear Idler Large.stl". 9) Position the remaining "Gear Idler Large.stl" onto "Axle Gear Idler Large.stl". 10) Position the remaining gear idler small assembly into "Chassis Center.stl". This is the "Wind Pinion". 11) Position the rear axle assembly into the chassis right assembly. 12) Press "Chassis Left.stl" onto "Chassis Right.stl". Note that all gears must move freely. 13) Press "Axle Front.stl" into one "Wheel Front.stl". Slide the assembly into the chassis assembly, then press the remaining "Wheel Front.stl" onto the remaining end of "Axle Front.stl". 14) Press both "Wheel Rear.stl" onto each end of the rear axle assembly. 15) At this point, the chassis should easily roll forward when pushed forward. When the chassis is rapidly moved in reverse, the winding floating pinion (the pinion adjacent to "Chassis Right.stl") should engage and cause a winding of the spring. I do not recommend exceeding a 30" total rollback distance in order to avoid spring separation. Hope you like it!