Chiyuv to Check the Knife
14. There is a chiyuv to check the knife before shechita (שו"ע סי' י"ח ס"ג ). The simple reason is because the average knife is assumed to not have been checked for shechita. Nicks are common, as a knife can get nicked very easily (דברי סופרים סקי"ט ). Some say the chiyuv to check the knife is part of the halacha l’Moshe miSinai (שלחן גבוה סק"ז ).
15. Before shechita. L’chatchila one must check the knife before shechita. One should not shecht without checking, and it is forbidden for one to shecht relying on the fact that he will check it after the shechita (שו"ע סי' י"ח ס"ג ). This is because he might forget to check it; or due to a concern for a bracha l’vatala if, after the shechita, the knife turns out to have a nick; or due to the potential issur of bal tashchis if it turns out to be a neveila because of a nicked knife (שמלה חדשה ).
16. Also after shechita. Even though one checked the knife before shechita, he must check it again after shechita, and if it is nicked, the animal is a neveila (שו"ע שם סי"ב ), as sometime the knife can get nicked while cutting the skin before the simanim. If that happened, the knife was already nicked when the simanim were shechted.
17. Only after shechita. If one forgot and did not check the knife before shechita, he must check it after the shechita. If it is fine, the shechita is kosher (שו"ע שם, תבואות שור ).
