by Rabbi Ari Enkin
Although many things can be done by machine, tying tzitzit strings is still not one of them. In order to render a tallit usable, the tzitzit strings must be hand tied by a God-fearing individual. What is amusing, however, is the largely mistaken belief that it is forbidden to tie the tzitzit strings onto a tallit at night. 
The source for this misunderstanding is a concern that doing so may be a violation of the Talmudic principle of ta’ase v’lo min ha’asui. This principle teaches that a tzitzit-bearing garment must be functional upon completion. When one ties tzitzit onto a tallit at night, the tallit has no halachic function and is essentially unusable until the next morning since the mitzva of tzitzit is one of those mitzvahs which apply only during the day and not at night.
This delay between the assembly of the tallit and one’s first opportunity of being able to use it only arriving the next day, is deemed by some authorities as a possible violation of ta’ase v’lo min ha’asui.
Tying Tzitzit at Night
In deference to this view, there a number of authorities who advise against tying tzitzit at night. While the majority of halachic authorities don’t disqualify tzitzit which were tied at night, several of them recommend not doing so if at all possible.
Nevertheless, normative halacha is not in accordance with these views and tying tzitzit at night is permitted without reservation. As is the case with all stringencies, those who choose not to tie tzitzit at night are conducting themselves in a manner not required by normative halacha.
Related to this issue is the writing of the tefillin parchments, which all authorities permit one to write at night although the mitzvah of tefillin is one which is only performed during the day. Accordingly, one should not hesitate to wear tzitzit that were tied at night.
The Chazon Ish’s Tzitzit
The Chazon Ish was known to permit tying tzitzit at night and specifically requested that his own tallit be prepared at night. One should only use tzitzit strings that were hand spun, though one should not criticize those who use the machine-made variety.
Many tzitzit professionals have the custom never to cut tzitzit strings with scissors or any other metal object. Instead, any necessary cutting or shortening is done with one’s teeth. This idea of not using metal in the process of making tzitzit mirrors the building of the Beit Hamikdash. We are told that it was forbidden to use any metal utensils when hewing the stones which were to be used in the construction of the Altar. As such, some suggest that the use of metal should be forbidden in the process of making tzitzit as well. This common denominator between the Altar and tzitzit is their role in lengthening human life,  while metal is a material used in weaponry whose primary purpose is to shorten it.
It is interesting to note that a person has thirty-two teeth, which is the number of tzitzit strings on the tallit. One who is unable to cut tzitzit strings with his teeth may use a knife made from silver, bronze, or plastic. It is said that stringent adherence to the mitzvah of tzitzit is a segula for preventing tooth aches.
Another application of the principle of ta’ase v’lo min ha’asui requires that the Tzitzit strings be placed upon the tallit and not the other way around. For example, if one tore through the corner of one’s tallit causing the tzitzit strings to fall from the garment, it would not be permissible to simply replace the tzitzit strings onto the corner and then sew up the tear. Rather, the tear must first be sown up and then the tzitzit strings re-tied on to the garment anew.
Similarly, in the event that one’s tallit had ripped in half, even if the strings remained fastened in the corners, it would not be permitted to simply sew together the two halves of the tallit. Rather, the tzitzit must be removed, the tallit sewn together and then the tzitzit strings tied anew. A blessing is not recited when tying tzitzit because neither the tzitzit strings nor even the tying process are inherent requirements relating to a tallit.
Tzitzit strings are among those ritual items, that while not inherently a mitzvah, they are necessary component for discharging one, a standing known as hechsher mitzva. As such, it is perfectly permissible to own a four cornered garment which one does not intend to attach tzitzit to, or even ever use. It is, however, forbidden to wear such a garment until the tzitzit are tied to the corners. Authorities cite this concept as additional grounds to permit tying tzitzit strings at night.
The principle of ta’ase v’lo min ha’asui is highly applicable with regards to the preparations for Sukkot. Among the rules in building a Sukka is the requirement to first build the frame and walls for one’s Sukka and only then to place the sechach on top. It would not be permitted to first prepare the sechach and only afterwards to erect the walls of the Sukka. 
This article was used with permission, courtesy of Rabbi Enkin and Hirhurim.
 Pri Megadim 18:1  Menachot 43a  Afikei Maginim 11; Biurim 13, Shulchan Hatahor ch. 14 and 18  Shulchan Hatahor 18:1  Silmat Chaim 28  Rivevot Efraim, O.C. 3:27; Teshuvot V’hanhagot 2:13, Yabia Omer 8:3  Aruch Hashulchan O.C. 14:7, Yabia Omer 8:3  The Butchacher Gaon, cited by Da’at Torah of Maharsham O.C. 10:10  Neot Deshe 62, Nachalat Binyamin 12, Halacha L’moshe 39, Lev Chaim 3:29, Chachmat Shlomo 18, Maharshag 2:74, Ben Ish Chai Lech Lecha, all cited in “Chokrei Minhagim” by Rabbi Eliyhau Yochanan Gurari  Dinim V’hanhagot Mimaran Ba’al Hachazon Ish 2:11  Piskei Teshuvot 11:2, Divrei Chaim 2:1  Eretz Tzvi 1:5, Shevet Halevi 1:6, Tzitz Eliezer 6:15  Mishna Berura 11:61  Shabbat 32b  Shulchan Aruch Harav 11:24, Middot 3:4  Machatzit Hashekel 11  Piskei Teshuvot 11:29  Kaf Hachaim 11:17  Menachot 41a, Yevamot 90b  See Mishna Berura 17:5  Chochmat Shlomo O.C. 14:1, 18:1  Rema O.C. 735:1
See also Rabbi Mois Navon’s summary of the topic.