If screwing pieces together is forbidden, how is it that we screw shut a baby’s bottle?
There is a vast difference between the crib and a baby’s bottle. A baby’s bottle is continually opened and closed as part of its use, whereas the crib does not need to be dismantled as part of its use.
The gemora [Shabbos 47a.] says that if one were to assemble on Shabbos a traveling salesman’s bed he would be liable to bring a korban, i.e. he would have transgressed a biblical prohibition. The explanation being that although the bed is not assembled for permanency, but since it does not need to be dismantled as part of its use, it is deemed as making and completing the k’li. A baby’s bottle is not seen to be 'complete' when the top is screwed on and ‘broken’ when it is not, because it is made to open and close continually.
The same would apply to a saltshaker (salt cellar) and to all screw cap lids. [M”B simon 313:45, Rav Shlomo Zalman Auerbach in the Minchas Shlomo page 70]