Which tool is primarily used for machining keyways in hubs or bores?

A milling machine is primarily used for machining keyways in hubs or bores due to its ability to move the cutting tool or workpiece in multiple directions, allowing for precise and efficient cutting operations. Keyways are typically created using flat end mills or slotting cutters that are mounted on the milling machine's spindle.

The milling machine's versatility enables it to handle various shapes and sizes of components, making it ideal for the creation of keyways, which are often necessary for securing gears and pulleys to shafts in mechanical assemblies. The process not only allows for accurate depth and width control of the keyway but also enables machining on both the interior and exterior of a component.

In contrast, while other tools such as lathes, drilling machines, and grinders serve important purposes in machining operations, they are not specifically tailored for creating keyways. A lathe is primarily used for turning operations and shaping cylindrical pieces, drilling machines are focused on creating holes, and grinders are typically used for finishing operations to achieve a smooth surface. Thus, the milling machine stands out as the most appropriate choice for keyway machining tasks.