Hello! This Instructable will be showing how to add a secret compartment to 3D print files using Tinkercad. I will demonstrate how I added secret compartments to my designs so that you can apply the same techniques to prints you'd like to add secret compartments to!

Supplies

Tinkercad
3D printer
Sand Paper
Paint

Step 1: Tomato and Pumpkin Compartment

The first step is to choose the model you want to add a compartment to. I started with a model of a tomato and a pumpkin I made. From there, Import the model into Tinkercad.

Step 2: Cylinders

I'm going to demonstrate the method on the tomato, however it will work for the pumpkin as well as many other prints. Duplicate the object, I'll refer to the front tomato as tomato 1 and the back tomato as tomato 2. Tomato 1 will become the bottom piece and tomato 2 will become the top piece.

Add a Hole Cylinder to tomato 1, which will create the compartment space. Make sure it doesn't go through the bottom of the object, and that it does go through the top. Next, add a Hole Cylinder with a diameter 4-6 millimetres longer than the first cylinder. It should only cut a few mm into the object. This Hole Cylinder creates the lip which the top will sit on.

Finally, add a Hole Cylinder to tomato 2 that has a diameter between the size of the two Cylinder Holes on tomato 1. It should go the same depth into the object as the Hole Cylinder on tomato 1 that creates the lip. It also must be stretched vertically over the top of the object (see image).

Step 3: The Top

To make the top, add a Solid Box covering tomato 2. Combine the Cylinder and the Box. Change the Box Cylinder Combination from a Solid to a Hole. Finally, Combine the Holes with their corresponding copy to achieve the second image!

Step 4: Cupcake Compartment

To add a compartment to this project, a cupcake, I used a different approach than for the tomato and the pumpkin. Adding a lip which is larger than the base of the frosting isn't possible because it would cut through the object as seen in the third image. This means a different approach is needed. The second method is to remove the lip and add an attachment to the base of the top piece which inserts into the compartment space. By removing the lip, compartment space is maximized, but precise measurement and supports are required.

Step 5: Separate the Parts

We have to choose how the object will be divided to have a top and a bottom. In this case it's obvious, the wrapper should be the bottom and the frosting and cherry should be the top. Duplicate the object. For one copy remove the bottom with a Hole Box and for the other copy remove the top with a Hole Box. This left the second image.

Step 6: Compartment Space

Now we will create the compartment in the wrapper bottom of the cupcake and an attachment to steady the frosting top in the compartment. Add a Hole Cylinder in the wrapper to create the compartment space. Then add a Solid Cylinder on the bottom of the frosting. I made it 1.5 millimetres smaller than the compartment diameter.

I determined the 1.5 mm. difference between the compartment space and the attachment by using a caliper to measure the actual size of prototypes and compared it to what it should've been based on the Tinkercad model. Results will likely vary based on filament type, printer, and slicer. It's important to measure this accurately because otherwise the top might not fit, or it might slide around.

Step 7: Printing

For the actual print you'll need to add supports to the top piece since it has a 90 degree overhang. Once you've printed the part and removed the supports you may need to sand the bottom of the ledge if the supports don't come off well, and you also may need to sand the rim of the cylinder add on if it's too tight to fit.

Step 8: Finding the Files

I've uploaded my designs to Thingiverse here if you'd like to print them. I encourage you to try to add a compartment to a print of your own! Good luck!