Method for cutting a thread on a rotating workpiece

A method for cutting a thread on a workpiece (W) having a workpiece rotational axis (B) by means of a cutting tool (20) comprising a cutting edge (22), the method comprising:

• rotating the workpiece (W) about the workpiece rotational axis (B);
• and
• sequentially performing a total of N oscillating passes (Pm), where N is an integer greater or equal to three, m is a variable integer such that 1≤m≤N and Pm designates the mth oscillating pass, the N oscillating passes including an initial oscillating pass (P1) and a final oscillating pass (PN), each pair of sequential oscillating passes (Pm−1, Pm, for m=2, 3, . . . N), comprising a current oscillating pass (Pm) and an immediately preceding oscillating pass (Pm−1) associated therewith, each current oscillating pass (Pm) sequentially following its immediately preceding oscillating pass (Pm−1);
• wherein:
• each oscillating pass (Pm) comprises the steps of:
• moving the cutting tool (20) and/or the workpiece (W) relative to each other along a feed axis (F) defining a feed direction (FD);
• and
• applying an oscillation to the cutting tool (20) in opposite vibration inward and outward directions (VO, VI) towards and away from the workpiece rotational axis (B), respectively, such that:
• a trajectory of the cutting edge (22) defines an oscillating trajectory contour (OC) with respect to a horizontal axis (H) extending parallel to the feed axis (F) and a vertical axis (V) extending perpendicular to the feed axis (F), the oscillating trajectory contour (OC) having a cyclic waveform comprising a plurality of crests (C) and a plurality of troughs (T), each crest (C) comprising a furthest crest portion (CP) which is a furthest distance from the horizontal axis (H) and spaced apart therefrom in a direction of the vertical axis (V) by a crest distance (CD) and each trough (T) comprising a closest trough portion (TP) which is a closest distance to the horizontal axis (H) and spaced apart therefrom in a direction of the vertical axis (V) by a trough distance (TD);
• wherein:
• the oscillating trajectory contour (OC) of each current oscillating pass (Pm, m=2, 3, . . . , N) has a 180° phase shift from the oscillating trajectory contour (OC) of its immediately preceding oscillating pass (Pm−1);
• the oscillating trajectory contour (OC) of each current oscillating pass (Pm, m=2, 3, . . . , N) intersects the oscillating trajectory contour (OC) of its immediately preceding oscillating pass (Pm−1) once in every one of the former's complete cycles to form a plurality of chip separation sections (SS) and a plurality of chip formation sections (FS) alternating with the plurality of chip separation sections (SS) in the feed direction (FD), each chip separation section (SS) having a chip separation length (LS) and each chip formation section (FS) having a chip formation length (LF), both measured in the direction of the horizontal axis (H);
• and
• the chip formation length (LF) of each chip formation section (FS) of a final pair of sequential oscillating passes (PN−1, PN) is shorter than the chip formation length (LF) of each chip formation section (FS) of any other pair of sequential oscillating passes (Pm−1, Pm, m=2, 3, . . . , N−1).