АКУСТИЧЕСКИЙ ЖУРНАЛ, 2011, том 57, № 1, с. 127-134
^ ФИЗИЧЕСКИЕ ОСНОВЫ ^^^^^^^^^^^^
ТЕХНИЧЕСКОЙ АКУСТИКИ
УДК 534.83:621.992.4
INFLUENCES OF VARIOUS CUTTING PARAMETERS ON THE SURFACE ROUGHNESS DURING TURNING STAINLESS STEEL
© 2011 Zhou Zhimin, Zhang Yuanliang, Li Xiaoyan*, Zhou Huiyuan**, Sun Baoyuan
Dalian University of Technology, Dalian 116023, P.R China
e-mail: zzmlxy888@yahoo.cn Dalian University of Technology, Dalian 116023, P.R China e-mail: zylgzh@dlut.edu.cn *DaLian University, DaLian 116010, China e-mail: chinalndlzzm@163.com **Honors School, Harbin Institue of Technology, Harbin 150001 e-mail: eminem_zhou@163.com Dalian University of Technology, Dalian 116023, P.R China Received March 3, 2010
Abstract — This paper presents an investigation of the process factors affecting the surface roughness in ultra-precision diamond turning with ultrasonic vibration. Stainless steel was turned by diamond tools with ultrasonic vibration applied in the feed direction with an auto-resonant control system. Surface roughness was measured and compared along with the change of the cutting parameters. The relation curves between the cutting parameters and surface roughness were achieved by comparing the experimental results with different cutting speeds, feed rates, cutting depths. Experimental results indicate that cutting parameters have an obvious effect on the surface roughness. The conclusions are draw in given conditions, the smaller amplitude of the vibration, the worse the surface quality and the higher vibrating frequency, the better surface quality, and the deeper the cutting depth and the more the feed rate, the worse the surface quality. Among these parameters, the feed rate was the most important factor on surface quality.
Keywords: Turning; Diamond Tool; Stainless Steel; Ultrasonic Vibration; Surface Roughness.
1. INTRODUCTION
Stainless steel is one of the metal materials because of a favorable combination of mechanical properties, corrosion resistance and cost effectiveness. It has been widely used in industry, especially in aerospace and military fields in which there are more and more strict demands on machining surface quality of the stainless steel workpiece [1]. However, stainless steel is very difficult to machine with conventional tools. Even the cubic boron nitride and precise ceramic tools can not meet the higher manufacturing requirements because of their mechanical and physical characteristics [2]. But natural single crystal diamond can be made into sharp knife-edge as to cut down such thin sheer scraps, and it can be used to manufacture a mirror with high accuracy and perfect surface quality. Whereas the tools are worn out very fast when being used to machine stainless steel workpiece in conventional methods, by which the operations can't carried on smoothly, and the surface quality of the workpiece can not be ensured [3]. In this paper, ultrasonic vibration was applied in turning stainless steel experiments by natural single crystal diamond tool. Therefore, the ultrasonic vibration turning of these materials is feasible in modern
manufacturing environment [4]. The results of ultrasonic vibration machining are significantly affected by the inherent vibration and accuracy of the machine tool. Special ultra-precision machines and arrangements were used for the ultrasonic applications [5]. However, when the ordinary machine tools were used, in most cases an improvement of surface finish would be reported. Achieving improvements on surface finished is relevant to the industrial applications, which can simplify or even eliminate some additional manufacturing operations.
In machining parts, surface quality is one of the most specified requirements by customers. The surface roughness is the main indication of the surface quality of machined parts. Surface roughness is an important parameter representing the surface quality of the ultra-precision machining workpiece [6]. Numerous theoretical and experimental studies on surface roughness of machined products have been reported. There are many factors influencing the surface roughness of stainless steel workpiece in ultrasonic turning, such as machining specification, machining data, tool material, tool geometry parameters and its abrasion, the material structure and characteristic and the turning space between the workpiece and tools, lubricating
Fig. 1. The gas-fluid atomization device in turning experiment.
gas-CO2
liquid-oil
i_i—, mist
The cutting force is measured by Vertical Parallel Octagon and the cutting temperature is measured by tool-workpiece thermocouple method. Surface roughness Ra is measured by comparison with standard surface roughness mass.
The good separation between tools and the cutting scraps in ultrasonic vibration turning, reduces sticking phenomena between them, and breaks the forming condition of the chip build-up and scale-stab [7], diminishes friction force. Meanwhile, the ultrasonic vibration can improve the rigidity and stability of the system. Surface roughness is decreased and geometrical precision is improved for small cutting force, low cutting temperature [8]. When vibration turning, though the tool edge vibrates, the position of the tool edge stays invariable in all moment when tool edge and workpiece contact and produce scraps. Machining precision is improved because of invariability of position with the time when turning the workpiece [9].
The supersonic vibration experiment system of turning stainless steel by natural single crystal diamond is composed of turning lathe, supersonic transducer, and amplitude changing pole, bracket and diamond turning tools. Fig. 2 shows the theoretical sketch of turning stainless steel under the state of supersonic vibration by natural single crystal diamond tools.
Fig. 2. The nozzle composition and atomization mechanism.
and cooling condition for tools, and rigidity of the machine fixture, tools, and workpiece system. It is important for the turning efficiency and surface roughness to determine proper cutting quantity, ensuring machining quality and reducing machining cost [6]. There are various cutting parameters having effects on the surface roughness, but those effects have not been adequately quantified. In order for manufacturers to maximize their gains from utilizing finish hard turning, accurate predictive models for surface roughness and tool wear must be constructed. The aim of the present research was to create an ultrasonic turning facility to explore the effects of cutting parameters on surface roughness when turning stainless steel by diamond. These results show that the cutting conditions (such as cutting speed, feed rate, cutting depth, tool geometry, and the material properties of both the tool and workpiece) can influence the surface roughness significantly.
2. EXPERIMENTAL DESIGN AND STATISTICAL ANALYSIS
Fig. 1 shows the photo of processing sites of the ultrasonic vibration turning system used in these experiments. Fig. 2 shows the schematic of vibration turning stainless steel.
2.1. Evaluation of the surface quality and the surface roughness
The surface quality was evaluated by measuring surface roughness along the axial direction of the workpiece [10]. The value of the centre line average (Ra) was used to analyze the surface roughness of machined workpiece. However, other major roughness parameters are also available. The perimeter of the workpiece was divided into five equal parts and five surface roughness measurements were performed on the cylindrical surface. It is a well-established fact that the surface finish of a machined workpiece is extremely sensitive to any changes in the machining process. Hence it is logical to assume that measurement of the surface finish could be used to identify special features of a special manufacturing process and to control the same cases by controlling the identified features [11].
The difference ARth between the machining surface roughness Ramax and the geometrical surface roughness Ra is expressed by
ARth = Ramax - Ra
Rth -
1 S_
8 R
A Rth = 1 SàS +1A, 4 R 2
(2.1) (2.2)
(2.3)
where s is the feed rate, AS is the feeds variation, At is the cutting depth variation, R is the radius of the tool cutting point. According to insensitivity vibration
turning mechanism, At « 0, AS ~ 0, so above equations, ARth ~ 0. The roughness of machining surface is approximate to Rth. It means that the vibration machining surface roughness Rmax is almost equal to the geometrical surface roughness Rth.
Therefore, the measurement of the surface roughness in terms of Ra is used to identify the characteristics of the ultrasonic machining process compared with the conventional machining [12].
2.2. Descriptions of the mechanical and control arrangements
Lathe: S1-222 precise diskette lathe Workpiece material: stainless steel (1Cr18Ni9Ti) Workpiece diameter: 30mm Tools: natural single crystal diamond tools The auto-resonant system maintains the resonant mode of vibration during the dynamic changes of the load. The maximal level of vibration of the cutting tip was about 12 ^m peak-to-peak at 19.7 kHz, and decreased during the turning up to 30—40% depending on the cutting conditions. All surfaces of the work-piece, cylindrical and faces were machined prior to the experiments. After fixing workpiece in the three-jaw chuck, a finish cut with a very small depth was performed using the same insert to be used in the test in order to eliminate any remaining eccentricity. This also allows the insert to get a stable tool wear region before starting each test. The first cut was made with ultrasonic vibration, and as soon as the tool had traversed 15 mm the vibration was switched off, meanwhile the second cut was proceed under the same cutting conditions but without ultrasonic vibration. After changing the cutting speed by changing the spindle rotational speed the next two cuts were performed with and without application of ul
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