Modio is a new and free iPad app that anyone can use to design model robots for 3D printing. The app features a large collection of robot parts that you snap together to design your 3D robot model. You can mix and match parts to heart’s content. Modio provides a visualization screen that will reveal the dimensions of your model before you print it. Included with those dimensions is an estimate of the time it will take to print the model.
I tried Modio this morning and found the model robot design interface to be intuitive and easy to use. The downloading and printing options were not quite as easy to figure out. The download and print options are found by tapping the little model robot icon in the bottom-left corner of the screen that you see while constructing a model. You will probably have to reserve some time for teaching students the intricacies of printing options.