There is no intrinsic fraction for MM. If something is measured as a whole number of MM then it is an integer value, not a fraction.
mm is not a number and so, in itself, it cannot have a fraction form.
7/2 mm
The fractional part of 1.5 mm is 0.5 mm. However, if you want 1.5 mm as a fraction of some other quantity then you need to specify that second quantity.
It is 5/2 mm as an improper fraction
A mm is 1/1000 of a meter
1 7/20 mm
3.5 mm as an improper fraction is 7/2 mm
1 inch = 25.4 mm so 8 mm = 8/25.4 inches = 40/127 inches.
7/1 mm or 14/2 mm etc
It is 30 mm/1 m = 30 mm/1000 mm = 3/100
0.059055"
1/1000 mm