Effect of coefficient of friction at the sliding zone of chip-tool interface on chip curl diameter and secondary shear zone thickness during tapping process

During the tapping process, chip snarling at high cutting speeds is a major obstacle in the improvement of machining efficiency. A cut tap tool coated with nickel–phosphorus (Ni–P)/abrasive particle composite film reduced the chip curl diameter and prevented chip snarling under high cutting speed conditions (50 m/min). In this study, the local coefficient of friction at the sliding zone of the chip–tool interface was estimated on the basis of the sticking–sliding friction model and using the cutting torque and thrust force measured during the tapping process. In addition, the effects of the local coefficient of friction at the sliding zone on chip curl diameter and secondary shear zone thickness were analyzed to investigate the mechanism of preventing chip snarling when using tapping tools coated with composite film. Tapping tests were conducted using four tapping tools that were steam treated, TiCN film coated, Ni–P/cBN film coated, or Ni–P/SiC film coated. The workpiece material was a rolled structure steel. The cutting speeds were 10, 30, and 50 m/min. The results demonstrated that chip curl diameter decreased with an increase in the local coefficient of friction at the sliding zone. The local coefficients of friction for tapping tools coated with composite films (1.51–1.91) were higher than those for tapping tools with conventional surface treatments (1.35–1.58). The secondary shear zone thickness also increased with an increase in the local coefficient of friction at the sliding zone. These results indicated that the tapping tool coated with Ni–P/abrasive particle composite film provided a high local coefficient of friction at the sliding zone, which increased the secondary shear zone thickness and reduced the chip curl diameter.