There is a famous statement which says ‘ein binyan b’keilim’ – there is no prohibition to construct items. Does this hold true in all circumstances?

Although the gemora in Beitza 10a says that ‘ein binyan b’keilim’ – there is no prohibition to construct items, the Rishonim explain that this statement is referring to some very specific cases. For example, Tosefos [1] says that assembling various parts of a k’li are permitted unless force and craftsmanship are required, in which case the assembly would be a Torah transgression on making a k'li. One cannot deduce from this that every assembly of keilim without force and craftsmanship should be permitted, because there are many restrictions, as we will be”H see.

The Ramban, in the beginning of the 12^th perek of Shabbos says that ein binyan b’keilim refers to assembling keilim that have come apart, but making a k’li from scratch is definitely Boneh. [2] He adds, that according to this opinion, if a k’li came apart in such a way that it requires a craftsman to reassemble it, the re-assembly would be classified as Boneh, because by coming apart it lost its status as a k’li, and its re-assembly is considered making a k’li. The Maggid Mishne explains the Rambam (10:13) in this fashion as well.

Is one permitted to set up a baby’s crib on Shabbos?

We must realize that the poskim regard tightening nuts and screws as a full-blown prohibition. [3] Therefore the answer is that it depends on the type of crib. If it is a folding crib and its assembly does not require any screwing or force in order to attach the pieces together, it is permitted. If, however, the pieces are screwed or fastened tightly to each other, it is forbidden to assemble the crib. [4]

In addition, it is forbidden to raise or lower the base of the crib, if this is accomplished by firmly inserting a rod into a hole in the frame. If however, the rod is made to slip easily into the hole in the frame it is permitted.

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 [5] 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. [6]

If the base of a thermos flask came loose, am I permitted to tighten it on Shabbos?

A thermos flask is made in such a way that the inner, insulated bottle is inserted within an outer shell and screwed into place. This bottle is not intended to be unscrewed and therefore if it became loose, tightening it would be forbidden.

Conversely, the screw cap on the top of the flask, even though it is shut tightly so as not to let the hot water escape, it is made to open and close and tightening it cannot be called ‘making a k’li’ and therefore it is permitted. [7]

What else is compared to the base of a thermos flask?

Tightening the leg of a cake stand; tightening a pan or pot handle; these items are not meant to detach and tightening the screw is Boneh.

Am I allowed to raise and lower a stender on Shabbos?

When a screw is used to fasten items together on a permanent basis, it is categorized as tikun manna – creating or repairing a k’li and is forbidden on Shabbos. [8] However, when a screw is tightened as part of its day-to-day function, as above, it is permitted because one does not see the fastening of the screw as creating a k’li rather as using a k’li.

Accordingly, the screws attached to a stender frame which are used to tighten the frame after lowering or raising the stender, may be unscrewed and tightened according to one’s needs, because the screws are not making the k’li, they are merely being used as part of its normal usage. [9]

What about raising and lowering a fan head which involves opening and tightening a screw?

The same rule applies to the screw on the side of the fan. Since that screw is tightened and loosened as an integral part of the fan’s use it is permitted. If, for example, the front panel of the fan would fall out of its place (or any other unit for that matter) it would be forbidden to insert a screw and screw it back into place, because it is not intended to become loose as part of its regular use and screwing it back into place would be tikun manna.