Orbital sanders create a scratch free finish by moving the sanding pad in a random pattern. They are best used for doing fine sanding passes.
The pad of a random orbit sander not only rotates, but also moves side to side slightly. The two motions combine to create a "random" pattern. This pattern eliminates deep scratches that can appear when using the other types of sanders in a single direction for too long.
Larger orbits and higher speeds increase the sanding speed of the sander. Disk size significantly increase the productivity of the tool, since the outer edges of the pad move significantly faster as the diameter increases.
Compared to Belt Sanders
Orbital sanders are not good at rapid stock removal. Use a belt sander for this feature.
Compared to Palm Sanders
Sanding performance falls of near the rim of the sanding disk due to imperfect downward pressure in that area. It is very difficult to sanding up to edges and into corners with an orbital sander and produce a consistent finish. The palm sander with its square sanding pad is the better choice of tool for this task.