Which type of screws are primarily used for fastening metal parts together?

Machine screws are specifically designed for fastening metal parts together due to their uniform threads, which allow them to fit smoothly into pre-drilled holes in metal components. These screws can be used with various types of nuts and washers to create a secure connection, making them ideal for applications in machinery, automotive parts, and appliance assembly, where strength and precision are crucial.

While wood screws are optimized for fastening wood, with a tapered body and coarse threads to grip wood fibers, and lag screws are designed for heavy-duty fastening in wood or masonry with a hex head, they aren't intended for metal connections. Self-tapping screws can be used in metal if they have a sharp tip designed to create their own hole in the material, but their primary function is for lighter applications and not strictly for fastening dissimilar metal parts together. Thus, machine screws are the appropriate choice for fastening metal parts effectively.