Everyone here has their own reasons for making/buying/using a 3d printer. Me? I needed more control over my halloween props.
https://lh3.googleusercontent.com/TcHmwzCiHp-SKDQPJlm-PU_I5_GFirXLlMWGAvmdGSPpbsWdUBxFlufHgBj8r-A_0b8GV_rskZLXQ6c=m22
well done mike 
Wow that is really cool.
looks good but needs more books moving
@Gary_Wilson there’s a trade off between the friction of the non-moving books, the moving books, and believe it or not, books with a dust cover on them. You need to have few enough moving books to create a stable system, otherwise the complexity goes way up with guides and lubrication and noise.
Honestly, Halloween props are an interesting balance between cheap as possible, and engineered well enough to run for 4 or 5 hours without interruption.
Nice to see a fellow haunter. This is my second year of doing a garage haunt. Last year was completely automated with micro controllers (Picaxe/arduino) monitoring PIRs to trigger props. Some props were home built pneumatics. Haven’t got to the point of incorporating the 3D printer, but it was on my mind as I’ve been slaving away outside.
Yeah, we’re in full crunch time mode. Automation using pic-a-boo controllers. Might have to do a pneumatic next year. 
good thinking