The previous educator answer has identified the primary type of metaphor used in this poem: personification. While, of course, not all metaphor is personification, personification is a particular type of metaphor in which an inanimate object is described as if it were human, and ascribed human characteristics and motivations. In this poem, the saw is personified throughout: the speaker imagines it behaving as if it "knew what supper meant," reacting to the words of the boy's sister.
Another example of metaphor derives from this prevailing type, then, when we see the connection between the saw and the boy's hand described as a "meeting." This is an interesting choice of words: it suggests that there is, in the moment, nothing violent about the connection; it is seemingly voluntary. This helps us to understand how it looked to an observer, the strange inevitability of the hand, tragically, encountering the saw without resistance.
Afterwards, the boy holds up his hand as if to "keep the life from spilling." Here, "blood" is implied, but the word "life" is used to represent it, as if the boy could prevent his life from pouring out of his wounded hand in the form of blood.
Finally, the boy descends into the "dark of ether," not a literal darkness, but a metaphorical twilight world which precedes the darkness of death itself.