Which milling cutter is often used to mill flat surfaces and is mounted on an arbor?

The correct choice is the slab mill. A slab mill is specifically designed for milling flat surfaces and is efficiently mounted on an arbor, which allows for stable and precise operations. This type of mill has a cylindrical shape and wide cutting edges that make it ideal for creating flat surfaces on the workpiece, making it a common choice in machining processes where flatness is critical.

While face mills are also used for milling flat surfaces, they typically feature multiple teeth and are designed to cut at the face, which differs from the primary function of slab mills that emphasize thickness and are commonly used for wider surfaces. Shell mills share similarities to face mills but have a more hollow structure; they can operate in a manner comparable to face mills but are not specifically tailored to perform the same tasks as slab mills. Side mills are primarily employed for milling vertical surfaces and shoulders, thus serving a different purpose than what slab mills are intended for.

Understanding the design and intended use of slab mills provides clarity on why they are the optimal choice for the milling of flat surfaces in certain applications.