You may remember this little boy's monster trap which was really a Rube Goldberg machine. I discovered two really nifty websites that you might want to use with your students to spark even more interest in making their own machine.
The first is by FOSS, a science company. The students can make adjustments to make the machine work.
The second is by PBS, called ZOOM, and again allows for student's to make adjustments to make the machine work.
One very important skill for children to learn is that most great inventions didn't work the first time. You have to study it and make revisions. If all things worked easily, we wouldn't have those old sayings like..."try, try again", and "practice makes perfect"...
Perserverence is a value that many of this generation have a hard time with, because they have grown up in a fast paced, instant answer time. It is a value that is going to have to be taught. This would be a great way to do it.