Sander
Also called: belt sander, orbital sander, random orbit sander, disc sander
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A sander is a power tool that smooths wood by rubbing abrasive paper against the surface, wearing away a thin layer of fibres rather than slicing them off the way a blade would.
What it is for
Audels (1923) put the sander's job plainly: some shaping can be done on one, but smoothing or polishing a surface is its real purpose. It follows the tools that give a piece its form, a plane, chisel or saw, and comes in afterwards to clear away their marks and bring the surface to a level, even texture ready for polish or paint.
The main kinds
Several distinct machines share the name. The belt sander drives a continuous loop of abrasive paper round two rollers; a bench-mounted version handles wide panels, while a smaller portable version does the same job by hand and can flatten a glued-up board or true a badly cambered piece of lumber. The disc sander spins a flat abrasive disc and is prized for accuracy: Jones (1920) rated it highly for smoothing and finishing flat work such as drawers and small boxes, finding it would true up certain kinds of work to an unusually exact standard, and could even finish timber straight from the saw without any planing beforehand. Because the disc always turns the same way, the work must be fed against the side where the spin presses it down onto the table rather than lifting it away. A thickness sander, or drum sander, feeds a whole board through under a rotating abrasive drum and is the tool of choice for bringing glued-up panels to a uniform thickness. Smaller hand-held orbital sanders, moved in tight circles or short straight strokes, do the final finishing pass.
Grain direction and the finish
Finish depends on which way the abrasive travels. An orbital sander moves in small circles, so on any one pass some of its scratches run across the grain rather than along it, and those cross-grain marks hold slightly more stain than a surface worked only along the fibres. Hayward (1950) pointed to a practical consequence of this: on a job where part of the surface is hand sanded along the grain and another part goes through the orbital machine, the two areas take a stain unevenly, and the patch that went through the orbital sander ends up visibly darker once the stain is applied. The fix is consistency: finish a whole piece by the same method rather than mixing the two.
The sander's limit
Not every finish is better for power. Bird (2004) argued that no sander matches a well-tuned bench plane for a smooth final surface, since a sharp blade shears wood fibres cleanly where an abrasive, however fine, still tears them on a small scale. Sanding remains the faster and more forgiving route for most work, but for a final surface some joiners still reach for the plane last.