Cutting Tzitzit with Iron

Is it permissible to cut the tzitzit strings with regular scissors?

Permissible. 

The Mishnah Berurah in Siman 11, Se'if Katan 61 writes that one should cut the tzitzit strings with teeth and not with a knife.

There is a question whether when one wants to shorten the tzitzit strings when they are too long (Siman 11, Se'if 4), and also when one wants to completely remove the tzitzit from the garment, whether in these cases one should also avoid cutting with a knife.

The source of the Mishnah Berurah's words is from the words of the Mateh Moshe (Laws of Tzitzit, Letter 13) and the Shlah (Tractate Chullin), and their words are regarding the long string from which (nowadays in factories) four strings are made for the making of tzitzit. It is explained there that the reason not to cut with iron is similar to what we find regarding the altar, about which it is said 'do not lift iron upon them', and since the altar prolongs a person's days and iron shortens a person's days (as the sword is made of iron), it is not fitting that the shortening should be lifted upon the prolonging. And since tzitzit also prolongs a person's life (see Gemara Shabbat 32:), it is not fitting that iron, which shortens a person's life, should be lifted upon it.

It seems that all the stringency is only in the making of the tzitzit, like in the making of the altar, but after the strings are made and one wants to shorten them or when removing the tzitzit completely - there is no need to be concerned about this.

There is a question whether in factories where the long string is cut into four strings, it is permissible initially to cut with a stone or plastic knife.

It is explained in the Mateh Moshe and the Shlah (there) that one should cut with teeth because the number of strings is 32, like the number of teeth. But if it is difficult to cut with teeth, one should at least avoid cutting with an iron knife as mentioned above, and cut with a stone or plastic knife or by fire.

Azmera L'Shimcha (Issue 168)