The strings of the tzitzit must be both spun and twined by a Jew who is obligated in mitzvot, and he must explicitly declare that he is doing so “for the sake of the mitzvah of tzitzit.” If they are produced without this intent (lo lishmah), the tzitzit are invalid.
One who wraps himself in a tallit with such tzitzit actually transgresses the positive commandment of tzitzit, since he is wearing a four-cornered garment without valid tzitzit. In addition, when he recites a blessing on it, he transgresses the prohibition of a blessing in vain. Thus, one who possesses only such a tallit is better off not wearing it at all.
Hand-made tzitzit strings with reliable certification are available. Yet, machine-made strings are the vast majority sold, and the tallitot sold “ready-made” almost always use machine-made strings (except by special order).
Some leading halachic authorities of our generation have permitted these machine-made strings—on condition that each time the production begins (or whenever the machine stops due to a technical malfunction and is restarted, something very common), a Jew operates it lishmah. However, without proper supervision of such machine production, it is quite possible that these strings are entirely invalid.
Anyone whose tallit was purchased “ready-made,” or who had machine-made strings tied on it, must ask his Rabbi, as it is likely he will need to have valid hand-made strings under proper certification tied on, so he may fulfill the mitzvah of tzitzit properly and recite its blessing according to halacha.
translated and adapted from Sichat HaShevua
