1. Workpieces: External cylindrical grinders are primarily used for grinding the outer surfaces of workpieces, including shaft-type parts, rollers, gears, etc. Surface grinders are mainly used for grinding the flat surfaces of workpieces, such as flat parts and flat mating surfaces.

2. Operating Principles: External cylindrical grinders clamp the workpiece onto a headstock and use a rotating grinding wheel mounted on a spindle to achieve precise machining of the workpiece's outer cylindrical surface. Surface grinders secure the workpiece to the grinding machine's table and then move the grinding wheel over the workpiece to achieve precise machining of the workpiece's flat surface.

3. Structural Design: Due to the differences in workpiece types, external cylindrical grinders and surface grinders also have some structural differences. External cylindrical grinders typically feature a rotating spindle, sliding guideways, and a feed system to enable workpiece rotation and feed. Surface grinders generally have a reciprocating grinding wheel and a worktable to facilitate relative motion between the wheel and the workpiece.

4. Precision Requirements: Because of the differences in the shapes they process, external cylindrical grinders and surface grinders have varying precision requirements. External cylindrical grinders typically require high roundness and parallelism to ensure the accuracy of the outer cylindrical surface. Surface grinders, on the other hand, demand high parallelism and surface finish to ensure the flatness and smoothness of the ground surface.

In summary, external cylindrical grinders and surface grinders differ in terms of the workpieces they process, operating principles, structural design, and precision requirements. They each find application in different machining needs and industries.