As students build their machines, provide grapes for them to test. On the last day have a student demonstration of their machines. They should be able to identify the simple machines and how energy is transferred from on to the other. If you want to make it a contest, see who can crush the most grapes in a minute. Class winners could compete with one another or with other schools.
Experiment is logical and clear.
Data is recorded and thoroughly analyzed. Diagrams are present. Conclusions thorough and thoughtful. 6 Competent Completes all steps. Experimental procedures lack thoroughness.