What is another term used to describe slab mills?

Slab mills are indeed often referred to as plain mills. This term highlights the function of these milling tools, which utilize flat cutters to perform surface machining operations. Slab mills are designed to remove material from flat surfaces and are characterized by their long, narrow cutting surfaces that can produce wide, flat areas on a workpiece. The designation "plain" signifies their straightforward design and use, primarily focusing on generating flat surfaces rather than more complex profiles or shapes.

On the other hand, face mills are designed for machining broader, flat surfaces but can also have angled edges for producing grooves. Planer mills are a different category altogether, used in a milling machine that can accommodate larger pieces of material. End mills, while versatile cutting tools, are normally used for detailed work and can cut in various directions, including sideways, which is not the primary function of slab or plain mills. Understanding these distinctions is crucial in selecting the appropriate milling tool based on the machining requirements.