PURPOSE:To prevent grinding fluid or the like from infiltrating into constitutional parts inside a collet chuck, by coating a collet and a tightening tapered ring with a collet cover which consists of elastic material extending and contracting in accordance with movement of the collet. CONSTITUTION:When a workpiece 10 is mounted and removed, compressed air is fed into an air hole 23. In this way, a piston 21 moves downward, and a chuck, depressing an outer ring 18 and moving a tightening tapered ring 16 to the bottom, opens a collet 13. And in order to chuck the workpiece 10, compressed air is fed into an air hole 24. This causes the piston 21 to move upward, and the chuck, lifting the tightening tapered ring 16 to the upper and urging the collet 13 to the inside, clamps the workpiece 10 by a clamping means 14. Here the chuck, in which the collet 13 and the tapered ring 16 or the like are coated with a collet cover 26, prevents grinding fluid and/or grinding sludge from infiltrating into a sliding part.
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