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Using a knife to cut Tzitzis strings: 
It is proper to be aware not to cut Tzitzis using a knife. Rather one is to cut the strings using one’s teeth. [Thus, if one’s Tzitzis strings are long and one desires to shorten them, one is not to cut them using a knife.]
Is one to avoid using a knife even prior to entering the Tzitzis into the hole?
Is one to avoid using a knife even after the Tzitzis is already braided?
Is one to avoid using a knife even if the Tzitzis are invalid, or one is putting on a new pair?
Seemingly one is to avoid doing so. Vetzaruch Iyun.
May one use a non metal knife, such as a plastic knife?
One may do so in a time of need.
 11/24; M”A 11/18; Mateh Moshe 13; Shelah Chulin; M”B 11/61
 The reason: This is based on the verse “Lo Sanif Aleihem Barzel etc” [ibid] This verse states regarding the Mizbeiach that metal is not to be used on it, being that metal shortens the life of man, while the Mizbeiach lengthens the life of man. The same applies to Tzitzis through which one is blessed with a long life. [See Piskeiy Teshuvos 11/29]
 The reason: There are 32 teeth which correspond to the 32 strings. [Kitzur Shelah; Ben Ish Chaiy Lech Lecha 10; Kaf Hachaim 11/17]
 Implication of Admur ibid which writes this Halacha immediately after concluding in 11/23 that one is to cut the Tzitzis prior to entering it into the hole.
 Implication of Admur ibid; Kaf Hachaim 11/17
 Piskeiy Teshuvos 11/29