How do you identify workpiece reference points and systems of tolerance?
Reference points: Think about how the part will be made, and how
it will be inspected. Usually something will make sense. Try to
keep common references between drawing views, and if possible try
to base everything from points, planes, or edges that appear in
multiple views.
Tolerances: Consider how the part will be made- what minimum
tolerances can be held on the intended equipment and processes?
What is the maximum tolerance on the individual parts that will
allow the assembly to fit together and function properly? The right
answers lie between those extremes.