The Shulhan Aruch (Orah Haim 180) records a custom to cover the knife on the table before reciting Birkat Ha’mazon. This applies both on Shabbat and on weekdays.
Numerous different approaches have been taken to explain this practice.
Some explain that our tables resemble the altar in the Bet Ha’mikdash, and the Torah forbids hewing the stones for the altar with a metal instrument (Debarim 27:5). Metal weapons are used to shorten human life, Heaven forbid, and they therefore may not be involved in preparing the altar, which serves to prolong human life. We thus similarly remove metal instruments from the table, at least for the recitation of Birkat Ha’mazon.
Others claim that this custom arose from an unfortunate incident where a person reciting Birkat Ha’mazon reached the prayer for the rebuilding of Jerusalem ("Ubneh Yerushalayim…"), and this prayer brought to mind the absence of the Bet Ha’mikdash. He was so distraught that he took hold of the knife on the table and stabbed himself. From that point on, it became customary for people to cover the knives on the table so as to prevent this from ever happening again.
Yet another theory is proposed by Rav Yaakov Haim Sofer (Baghdad-Israel, 1870-1939), in his Kaf Ha’haim. Yaakob Abinu was given the blessing of "the dew of the heavens and the fat of the land" (Bereshit 27:28) – the blessing of agricultural prosperity, one of the themes of Birkat Ha’mazon, in which we thank God for the land and its produce. Esav, by contrast, received the blessing of "Ve’al Harbecha Tihye" – that he would "live by the sword" (Bereshit 27:40). We therefore cover or remove the knives from our table during the recitation of Birkat Ha’mazon, as it would be inappropriate to have a symbol of Esav’s blessing present while we speak of Yaakob’s blessing.
The Kaf Ha’haim also adds a different explanation, noting that throughout Birkat Ha’mazon there is no "Peit Sofit" (final Peit). This letter does not appear anywhere in the text of Birkat Ha’mazon. The reason, the Kaf Ha’haim explains, is that the names of harmful angels – such as Shesef, Kesef, Za’af, Negef and Reshef – end with this letter. We avoid the letter "Peit Sofit" to indicate that through the Misva of Birkat Ha’mazon we avoid the risks posed by these destructive forces. By the same token, then, we cover the knives to demonstrate that we are free from harm as a result of Birkat Ha’mazon.
Others explain that in Birkat Ha’mazon we pray for the rebuilding of the Mikdash, at which time, as the prophet Yeshayahu (2:4) promises, there will be no more warfare or hostility between nations: "No nation shall raise a sword against another nation." We thus cover the knives to allude to our hope that this prophetic promise be fulfilled.
The Kabbalists added that normally, when one eats a meal, he has on the table four primary items: Sakin (knife), Melah (salt), Ochel (food), and Lehem (bread). The first letters of these words spell the name Samael, the name of the Satan. Before Birkat Ha’mazon, one removes the Sakin and brings in its place a Kos (cup). The first letters then spell the name Michael, one of God’s special angels.
The Arizal (Rabbi Yishak Luria, 1534-1572) held that a person whose soul originates from the soul of Kayin must not merely cover the knife during Birkat Ha’mazon, but rather remove it from the table. Since we cannot know the origin of our soul, it is proper to remove the knives from the table, rather than simply covering it. This is also the position of the Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909).
The work Bet Abi raises the question of whether one must remove plastic knives from the table for Birkat Ha’mazon, and notes that this will likely depend on the reasons for the custom. If we associate this practice with the prohibition against using metal instruments in hewing stones for the altar, then clearly it is restricted to metal knives. If, however, this custom relates to the danger of somebody stabbing himself during Birkat Ha’mazon, then it would likely include plastic knives, as they, too, could be used for harm. It is therefore proper to remove even plastic knives from the table for Birkat Ha’mazon.
Certainly, however, if one did not cover or remove the knives from the table for Birkat Ha’mazon, he has nevertheless fulfilled his obligation of Birkat Ha’mazon, though it is proper to follow this custom and remove the knives from the table.
Summary: It is proper to remove all knives from the table – including plastic knives – for Birkat Ha’mazon, both on Shabbat and during the week.
