Dual-action sanders are a cross between a belt sander and a disk sander, but they should not be confused with either one. A dual-action sander is a very versatile power tool that can perform some of the same functions as the other two sanders, but it is not intended as a substitute for either one. A comparison of the three can clearly show the differences.
Belt sanders run a belt of sandpaper at high speed. These belts remove a lot of material very quickly. Belt sanders are used primarily on flat surfaces to remove high spots, but they are “brute force” tools that can overdo the job and remove too much if you're not careful. They can also leave scratches on wood if they get turned at an angle to the grain.
Disk sanders spin a disk of sandpaper in a high-speed circle, allowing them to dig in and sand uneven objects by using an edge of the disk. Some disk sanders can also make a “random orbital” motion, which adds an eccentric “wobble” to the spinning motion; this helps keep the sander from digging into the sanding surface quite so much. Still, disk sanders often leave scratch marks across the grain when used on wood. Disk sanders are also frequently used as grinders or polishers.
Dual-action sanders attempt to combine the best aspects of both types of sander. The sandpaper pad moves straight forward and backward, mimicking the belt sander’s linear action, but is also moves in a slightly rotary motion–sometimes with a random orbital motion rather than a purely rotary one. Unlike the other two sanders, however, this dual-action does not always look the same from one sander to another; the sandpaper can be circular and spin like a disk sander, or it can be rectangular and merely moved in a slightly rotary motion without turning the paper at all–similar to the way you might hold sandpaper in your hand and make a rotary stroke without twisting your wrist.
Why Two Are Better than One
The dual-action sander is a different animal than either of the other two sanders because it is more of a “finesse” tool. It is not designed to remove the large amounts of material the other two will, which gives the user more control over the results. In addition, the combination of movements combined with its reduced power eliminates much of the scratching common with the other tools. Dual-action sanders are better suited to the final finishing stages of a project than either of the other sanders.
How Usage Affects Design
Dual-action sanders designed for woodwork tend to use rectangular-shaped sandpaper pads, as this minimizes the possibility of scratch marks across the wood grain, and are often palm-sized. The disk-shaped dual-action sanders are very popular for use on other “grainless” materials and are generally larger; for example, they are used frequently in auto body repair for shaping putty repair patches and smoothing metal for painting.