I use a lot of 3mm (diameter) x 3mm (height) cylindrical rare earth magnets to hold 3d-printed puzzles and models together. The magnets can be hard to work with-- they are too small to comfortably grip, and it often takes some force to get them to fit into the holes on the model.

These tools solve the problem:

• include a 3.2 mm (dia) x 3.2 mm (height) cylindrical recess in your CAD model
• load the + or - tool with a magnet (the correct orientation will stick)
• add a little JB Weld epoxy to keep the magnets in place for the long run
• insert magnet into your model using the comfy handle
• done!

This tool was developed as part of the project `Motivating First-Year Calculus with Robotics' supported by the National Science Foundation under DUE-12-45540. For more from this project, see my webpage.