Most consumer-grade locks aren’t that secure. To wit, if you ever need to open a basic padlock or combination lock without a key, all you need is a soda can, a boxcutter, and a little precise cutting.

Careful what you leave in your lockers, high school students and gym-goers. An invasion of 3-D–printed robots may be coming, capable of popping one of the world’s most ubiquitous brands of combination ...