If you read this poem aloud, you will hear its meter: a swinging, striking rhythm that emulates the work of a blacksmith. Consider the lines -- "What the hammer, what the chain/In what furnace was thy brain?"
Blake here likens the severe, ferocious beauty of a tiger to the work of a metals shop: burning, banging, crafting until a beautiful product is rendered.
Think also of the lines, "What immortal hand or eye/could frame thy fearful symmetry?" Here, Blake gives credit to a great cosmic creator for the lines and vicious appearance of the Tyger. That same creator is credited with the "work" of crafting the animal's nature, as previously mentioned. The meter contributes to answering Blake's own questions.