Introduction: Impossible Object With Fusion 360 - Box and Ball

About: Retired engineer that now has time to play with all the toys I use to get paid to design.

This instructable assumes that you have a basic knowledge about creating and modifying objects in Fusion 360. These steps will show you how to create an impossible object: a 25mm cube with a 28mm ball inside it as shown in the ISO and Top views.

Step 1: Create the Cube

Use Create > Box to make a 25mm Cube sitting on the XY plane with a corner at the origin (0,0,0).

Step 2: Create a Cutting Sphere

Use Create > Sphere to make a 32mm diameter sphere at the origin, selecting the XY plane for it, as a New Body.

Step 3: Position Sphere for Cut

Select Body 2 (the Sphere) and move it X: -12.50mm, Y: 12.50mm, Z: 12.50mm to place it in the center of the Cube.

Step 4: Cut Openings in the Box

Use Modify > Combine and select the Box (Body 1) as the Target. Select the Sphere (Body 2) as the Tool. Select Cut as the Operation. Select Keep Tools also. This will save some steps later. Select OK to cut the Box.

Step 5: Create a Copy of the Sphere and Reduce It to 28mm

If we resize the sphere now, it will also change the size of the cut area because Fusion 360 has linked the Body 2 to the Cut operation applied to Body 1. So, we make a Copy of the Sphere and resize the copy. This way we don't have to go through the positioning process of Step 3.

RMC Body 2 and select Copy. Then RMC in an open area and select Paste. Select OK since the body is positioned is the center of the Cube the same as Body 2.

RMC Body 2 and select Remove.

Use Modify > Press Pull to resize Body 3 to 28mm diameter. (reduce by 2.00mm radius)

Step 6: Connecting the Two Bodies: Raise the Sphere

This step is used to trick the slicing software into thinking that the Box and Sphere are one object.

We do this by adding a 0.1mm x 0.1mm x 12.5mm bar where the objects will be flat on the printer's base plate. But first, we need to raise the sphere up 1.5mm so that its contact point is the same as the base of the Box.

RMC Body 3 and raise it up 1.5mm in the Z direction.

Step 7: Make a Connecting Bar: Part 1

First, hide Body 3 by clicking OFF its yellow light bulb.

Use Create > Box and select the XY plane and Select Top view on the view gizmo.This will allow us to use a center snap that will appear in the middle of the right edge of the Box.

Step 8: Make a Connecting Bar: Part 2

With this as the first corner, draw a 12.5mm x 0.1mm base that extends to the middle of the base of the Box and the give a 0.1mm height. Use the Join Operation to have it connected to the Box.

Step 9: Join Everything Together

Unhide the Sphere (Body 3) and use Modify > Combine with Body 1 as the Target and Body 3 as the Tool. Unselect Keep Tools if it is still selected. Use the Join Operation. Select OK.

Step 10: Form Final Object

RMC Bodies and select Create Components from Bodies to generate Component1:1.

Slowly LMC the name to change it to BoxBall or RMC the name and use Properties to Rename it.

Save the File and Save BoxBall:1 as a STL to use with your favorite slicer to print it.

NOTE: This prints fine at a 0.2mm layer. If you have sticking problems, try putting a few base supports on the sphere about 5mm from the base contact point.