1、原文1Investigation of the torque characteristics in vibration tapping of hardened steel Baolin Yinand Rongdi Han Department of Mechanical Engineering and Automation, Harbin Institute of Technology, Harbin 150001, Peoples Republic of China Received 1 February 2005; accepted 7 July 2005. Available onlin
2、e 6 September 2005. Abstract Vibration tapping is presented in this paper to solve this problem, as high-speed steel tap is incapable of tapping small-hole (M3) in hardened steel (50HRC). Theoretical analysis with fracture mechanics indicates that the impact effect of the tap on the workpiece result
3、s in increased II-type stress intensity factor and extended micro cracks, leading to lower plastic deformation, reduced cutting forces and a much lower tapping torque, and the torsional rigidity of the tap is enhanced in vibration tapping as proved by dynamic analysis. The experimental results show
4、that with well chosen amplitudes, tapping torque decreases as vibration frequency increases, and tapping torque increases as net cutting time ratio increases, where net cutting time ratio influences the tapping torque more significantly. Vibration tapping is then proved to be a practical solution to
5、 the problem of small-hole tapping in hardened steel. Keywords: Vibration tapping; Hardened steel; Micro crack; Tapping torque; Torsional rigidity 1. Introduction Small-hole tapping (M3) in hardened steel (45HRC) is a very difficult task. Hardened 0.45%C steel possesses high strength (b1700MPa) and
6、the tapping torque (M3) in these materials is approximately 1.3Nm. Therefore, tap breakage appears to be one of the major problems in the process, possibly due to excessive torque. Hardened steel possesses high hardness, which is close to the hardness of high-speed steel (HSS) tap. It always causes
7、machining troubles such as tool wear and tipping. High tapping torque results in other problems associated with the tapping vibration chatter including thread dimensional accuracy and thread shape error. It is clear that conventional process could not fulfill the requirement of small-hole tapping fo
8、r hardened steel in view of tap strength, hardness and torsional rigidity. However, with the rapid development of modern manufacturing technologies, such as aircraft and space shuttle, some components need to be tapped after being quenched, taking geometrical accuracy and surface strength into accou
9、nt. Traditionally, the machining of hardened steel components is the domain of grinding operations. The technology of vibration cutting presented 50 years ago by Kumabe 1 has various effects, e.g. reducing cutting force, improving surface quality, restraining tool wear, and so on. Vibration tapping
10、has been applied to titanium alloys and other materials to increase the tapping efficiency and reduce the overall tapping torque. There have been many contributions to vibration tapping on different materials under different process conditions 2, 3, 4 and 5. Zhang 2 built a vibration-assisted tappin
11、g device, in which a piezoelectric actuator was used to generate vibration along the axis of the tap at a frequency of 501600Hz and an amplitude of 0.15m. It turned out that a torque reduction was always obtainable in vibration tapping of brass. Zhang 3 and Gou 5 carried out the experiments on vibra
12、tion tapping in titanium alloys. It was reported that the frictional torque was reduced and the tap life was prolonged. Patil 4 carried out the research on the influence of different process conditions on tapping torque and thrust during machining, and optimum conditions were found to lengthen the t
13、ap life. However, little information can be available on vibration tapping of hardened steel. A systematic investigation is therefore of great importance. This paper presents theoretical analyses on two major mechanisms for vibration tapping of hardened steel, i.e. reduction of tapping torque and en
14、hancement of torsional rigidity of the tap. The tapping experiments were also carried out to verify the theoretical analyses. 2. Theoretical analysis of vibration tapping process Fig. 1(a) shows the torsional vibration tapping process. In order to simplify analysis, the process is simplified to an o
15、rthogonal cutting model for a single cutting tooth as shown in Fig. 1(b). The motional locus of the tool edge in the operation of separative vibration cutting is shown in Fig. 1. In each vibration cycle, a layer of metal is cut ahead of the tooth in a distance lc. From point c to d, separation betwe
16、en the rake face and the chip occurs. The distance between c and d is specified by lg. Subsequently the tooth comes back to the workpiece, which moves from point e to f, with the contact between the tooth and the chip being re-established, so that the new vibration cycle is formed. The pulsating cutting force and the alternate motion due to vibration application are the two major mechanisms, which are different from conventional p
