Is the saw a symbolic object in Robert Frost's "Out, Out—" and if so, what does it symbolize?

The "buzz saw" in this poem can indeed be interpreted as a symbol. Certainly the saw seems to have greater agency in the poem than the boy does. At the beginning of the poem, it is the saw itself which "snarled and rattled in the yard." Later, the poet repeats this: "the saw snarled and rattled, rattled and snarled." The chiasmus in the line—the two phrases almost perfect mirrors of each other—emphasizes the extent to which the saw just continues on with what it is doing, moving from snarling to rattling, to rattling, to snarling, without any input from anyone else. We may interpret the saw, then, as symbolizing fate, which simply rattles on unperturbed by human protests against what it is doing.

This analysis is also supported by the allusion in the title, which recalls Macbeth: "Out, out, brief candle." After his wife has been killed, Macbeth notes that life is "full of sound and fury, signifying nothing." The "sound and fury" element of life is represented in Frost's poem by the snarling and rattling of the saw. Meanwhile, the extent to which a life can be snuffed out only for everyone else to simply continue on their way is implied in the final lines of the poem, wherein the others "turned on to their affairs" despite the death of the boy.

It is the saw in this poem which "leaped out at the boy's hand," as if "to prove it knew what suppers meant." The boy has little influence over whether he lives or dies, or even what happens to his hand, although he tries to influence it: "the hand was gone already." The saw, then, seems to symbolize the unexpected ways in which fate can affect and end our lives, without our having any control over it.

There is the symbolic nature of the title 'Out, Out -', in that it is taken from the Shakespeare play Macbeth. An answer to a previous question on the poem summarises the effect that Frost was trying to create with this choice of title

http://www.enotes.com/out-out/q-and-a/how-name-out-out-poem-by-robert-frost-relevant-27231

Both writers are exploring the unpredictability and futility of life and death.

How does the saw appear sinister in Robert Frost's "Out, Out—?"

The saw definitely seems sinister because of Frost's repeated description of how it "snarled and rattled," but Frost is also careful to present a certain level of indifference within the saw, which adds some insidiousness to the sinister nature of the object. Line 7 repeats the "snarled and rattled" description, and then Frost adds, "As it ran light, or had to bear a load," in line 8. This shows the consistency of the saw. No matter the level of cutting it does, it maintains the same behavior. It does not change due to any external factors; it is not deeply affected by anything it comes in contact with. Frost cleverly connects the indifference of the saw to the indifference of those who see the boy's eventual plight. The people at the end, who see what is happening to the young boy, maintain a level of indifference: "since they / Were not the one dead, turned to their affairs." Through the connection between the severing of the hand and the actions of those who saw the aftermath, Frost presents the sinister nature of an inanimate object, which cannot care, but more importantly presents the sinister nature of people who should care and don't.

The language Frost uses in his poems is always carefully chosen to convey the feeling or meaning he wants to create. Certainly, the words he uses to talk about the saw, from the very beginning of the poem, contribute to the impression of potential danger associated with the saw.

The poem repeats the phrase three times - "The buzz saw snarled and rattled in the yard...snarled and rattled, snarled and rattled." Calling the tool a buzz saw somehow seems more threatening than calling it a chain saw or a power saw. The word snarled immediately reinforces the feeling that this is not a friendly saw. To say that it rattled could remind the reader of other things that rattle, as in this foreshadowing  of the rattling skeleton of death's approach.

When the snarling, rattling saw "leaped out at the boy's hand," the reader instinctively knows that this is a dangerous turn of events.