In ordinary straight turning, the cutting tool moves along a line parallel to the axis of the work, causing the finished job to be the same diameter throughout. However, when cutting a taper, the tool moves at an angle to the axis of the work, producing a taper. Therefore, to turn a taper, the work must either be mounted in a lathe so that the axis upon which it turns is at an angle to the axis of the lathe, or cause the cutting tool to move at an angle to the axis of the lathe.
When the diameter of a piece changes uniformly, from one end to the other, the piece is said to be tapered. Taper turning as a machining operation is the gradual reduction in diameter from one part of a cylindrical workpiece to another part Tapers can be either external or internal. If a workpiece is tapered on the outside, it has anexternal taper; if it is tapered on the inside, it has an internal taper.
The method used for turning a taper depends on the degree, length, location of the taper (internal or external), and the number of pieces to be done. The three basic methods of turning a taper require the operator to use either a compound rest, offset the tailstock, or use the taper attachment. With any of these methods, the cutting tool must be set exactly on centre with the axis of the workpiece or the workpiece will not be truly conical, and the rate of taper will vary with each cut.