What’s the big deal about making cubes? It’s about fundamentals. Onshape is chock full of tools. It’s important to be familiar with all of them. If all you have is a nail, everything looks like a hammer. This tutorial is for you, neophyte Onshape user. Everything here is only 6 steps long at most.

At the end your cube should look like this:

Extrude

On the top plane sketch a square with dimensions 20mm by 20mm.

Extrude 20mm. Easy enough?

Sweep

On the top plane sketch a square with dimensions 20mm by 20mm.

On the Front plane, sketch a line that is 20mm high.

Use the sweep command: select Sketch 1 as the face, and Sketch 2 as the direction.

Loft

On the top plane sketch a square with dimensions 20mm by 20mm.

Create an offset plane from the top plane, 20mm away.

Sketch an identical square on the offset plane.

Use the loft command by selecting both faces.

Thicken

On the top plane sketch a square with dimensions 20mm by 20mm.

Use the thicken command by 20mm.

Enclose

On the top plane sketch two perpendicular lines with dimensions 20mm by 20mm.

Sweep a new surface. Select one line to sweep, and the other line as the direction.

On the front plane sketch a line 20mm high.

Sweep a new surface using that new line, and the perimeter of the square as the direction.

Sweep the top face by selecting your favorite line, and your 2nd favorite perpendicular line.

Use the enclose command by selecting all 6 faces.

Here is a link to my beautiful cubes. The nice thing about Onshape is that you can view what I did step by step.