Which process involves shaping a workpiece with a gradually decreasing diameter?

The process that involves shaping a workpiece with a gradually decreasing diameter is tapering. Tapering is specifically designed to create a transition where one end of the workpiece is wider than the other, resulting in a smooth, conical shape. This technique is commonly employed in various engineering applications where components need to fit together accurately, such as in shafts or fittings.

Drilling, while it can produce cylindrical holes in a workpiece, does not create a tapering effect and maintains a constant diameter throughout the hole. Slot milling is used to create slots or grooves and does not focus on changing diameters, and threading involves cutting a helical ridge into a cylindrical object but is more concerned with creating a specific form of fastening rather than a gradual change in diameter.