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If you find this model useful, feel free to buy me a coffee. 

Watch this video for a demonstration and assembly instructions: 

UPDATED March 28, 2022: Some users were reporting the threads were too tight, so I investigated and fixed the issue. Everything should print smoothly now without having to adjust horizontal expansion.

This locking Egg will be the highlight of your next Easter Egg hunt. There are three different lock shapes with three corresponding keys. Place keys and candy in the various eggs, lock them up, and hide them. Then give the hunters only one key. The idea is that the hunters will have to find an egg that matches their key in order to get the keys for the other eggs.

This is also a fun way to hide a geocache, though you may want to place the egg in a plastic bag to avoid the elements.

The locking mechanism uses a hidden barrel cam to move a pin up and down. I tried 10 or so various locking methods before I settled on the barrel cam.

Instructions:

1. Slice and print the top and bottom of the egg, titled 1of7_LockingEggTopHalf and 2of7_LockingEggBottomHalf.

Print as oriented. Supports are not required. I recommend a layer height of 0.2mm or smaller, wall thickness of 1.2mm, a bottom thickness of 0.6mm, and a top thickness of 0.8mm. Infill density of 12 percent or higher. I tried to allow for horizontal expansion in the design, so first try printing it without adjusting horizontal expansion, slicing tolerance, or x/y compensation. If it prints too tight or too loose, then you can use one of those slicing features to compensate. Threads are notorious for being the hardest item to print, so please be patient. If you want to save time while testing, you can print the included calibration test instead of the full model, but that is optional.

Once the egg halves are printed, screw the threads in and out repeatedly to remove any stray particles. It may feel too tight on the first threading but should screw easily after ten seconds of doing this.

2. Slice and print 3of7_LockingEggPin, 4of7_LockingEggLid, and 5of7_LockingEggClip using the same settings as above.

3. Now you need to decide which key shape you want. There are three options: Square, Triangle, and Pentagon. If you only print one style, I recommend the Square Key.

Print 6of7_LockingEggKey... and 7of7_LockingEggLock... in the shape you choose, using the same settings as above. The lock file is a barrel cam that moves the pin up and down. I had no trouble printing the lock without supports, but if you run into trouble, you can add supports to that file only.

4. Make sure the Pin end can slide freely in the Lock (barrel cam) groove. The pin should pass over the bump on each end of the groove without much resistance. If it doesn't, sand the groove until you get an easy slide. Refer to the video to see what I mean.

5. Follow this assembly video to see how to put together. You'll need some small needle nose pliers. https://youtu.be/xxG6Ntnun-I

If you find that the parts are too loose or too tight, you can compensate with Cura's Horizontal Expansion or PrusaSlicer's XY Size Compensation. For example, if the locking pin is not catching or if the lid is popping out, you need a positive value in those settings. I found that most printers will print the parts at the right snugness, but ultra-precise printers such as my Prusa Mini+ actually require a positive XY Size Compensation value. (In my case, I used 0.1mm for a snug fit.) 

On the other hand, if the lid won't fit or the locking pin doesn't move freely, then you need a negative value in Cura's Horizontal Expansion or PrusaSlicer's XY Size Compensation. Start with -0.1mm and go from there.

I am also including the original Sketchup files. Feel free to remix this with surface skins and whatever you can dream up!

Happy Easter!