My computer mouse has a pretty interesting design. At a glance it is unclear how the device is assembled or where one might insert batteries.
Those of us in the know, find a small black button near the back of the mouse that pops the cover off. Underneath there are a myriad of interesting retaining features molded into the plastic. For this week's seek and geek I decided to reverse engineer the design of this portion of my mouse!
The cover is constrained to the mouse body by 8 mating features, denoted on the pictures above. For convenience of language, consider the front side of the mouse the positive Y direction and the right side of the mouse the positive X direction.
Points 1,2, and 8 both constrain and preload the mouse cover. These features disallow rotation about the X axis as well as upwards translation in the Z direction. However, because the cover is also used as a spring to transmit forces to the left and right mouse buttons, negative motion in the Z direction is allowed at points 1 and 2.
In order to prevent the negative Z translation from lowering too far and thus as a result putting the mouse buttons in a constant 'ON' position, features 3 (bump), 4, and 5 create a slight gap between the mouse body and the cover in the Z direction.
Lastly, features 3(slot), 4, and 5 constrain translation in X and Y, as well as rotation about Z. Feature 3 constrains X-translation. 4, and 5 work together to constrain both rotation and translation about Z. Since Features 4 and 5 are peg-hole constraints, they technically constrain all three planar DoF but a tighter tolerance in the slot in feature 3 supersedes constraints applied by features 4 and 5 in the X direction.
I learn a ton about design by reverse engineering projects like this. Not only can I learn from successful products, but with a little luck I'll be able to find a mistake or pitfall to avoid in my next project!