How can we make the free float zero ???

It depends on how you would use it.



You have to be careful and limit the use of constraints in your programme as this can influence the real computation of free and total floats (critical path).



Putting zero float is (albeit free float is better than total float) compels an activity to be planned as late as possible. So everytime programme is time analysed this activity always on the critical path which should not be the case.



Assigning early constraint date is less poison because the constraint effect vanished once the data date overuns it.

you could apply a "zero free float" constraint



If you are going to do this, it would be good practice to include some consealed float (lag) in the relationship between procurement and construction activities.