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外文翻譯
EFFECTS OF CUTTING EDGE GEOMETRY,WORKPIECE HARDNESS，F(xiàn)EED RATE AND CUTTING SPEED ON SURFACE ROUGHNESS AND FORCES IN FINISH TURNING OF HARDENED AISI H13 STEEL
切削刃幾何形狀、工件硬度、進(jìn)料率和切削速度對(duì)H13 鋼精加工表面粗糙度和力的影響
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EFFECTS OF CUTTING EDGE GEOMETRY,WORKPIECE HARDNESS, FEED RATE AND CUTTING SPEED ON SURFACE ROUGHNESS AND FORCES IN FINISH TURNING OF HARDENED AISI H13 STEEL
Tugrul ?zel, Tsu-Kong Hsu, Erol Zeren
Department of Industrial and Systems Engineering Rutgers, The State University of New Jersey, New Jersey 08854 USA
Abstract
In this study, effects of cutting edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and resultant forces in the finish hard turning of AISI H13 steel were experimentally investigated. Cubic boron nitrite inserts with two distinct edge preparations and through-hardened AISI H-13 steel bars were used. Four factor (hardness, edge geometry, feed rate and cutting speed)- two level fractional experiments were conducted and statistical analysis of variance was performed. During hard turning experiments, three components of tool forces and roughness of the machined surface were measured. This study shows that the effects of workpiece hardness, cutting edge geometry, feed rate and cutting speed on surface roughness are statistically significant. The effects of two-factor interactions of the edge geometry and the workpiece hardness, the edge geometry and the feed rate, and the cutting speed and feed rate are also appeared to be important. Especially, small edge radius and lower workpiece surface hardness resulted in better surface roughness. Cutting edge geometry, workpiece hardness and cutting speed are found to be affecting force components. The lower workpiece surface hardness and small edge radius resulted in lower tangential and radial forces.
1. INTRODUCTION
Hard turning, machining ferrous metal parts that are hardened usually between 45-70 HRC, can be performed dry using polycrystalline cubic boron nitride (PCBN, commonly CBN) cutting tools as extensively reported in literature [1-8]. Research results in the literature concerning mechanism of serrated chip formation in order to relate process characteristics and stability of cutting to the chip shapes during hard turning [9-19]. Other research concerning with composition, temperatures and wear characteristics of CBN cutting tools [1,8,20,21,22,28] and effects of work material properties, tool geometry and cutting conditions on surface integrity of the finish machined parts [23-28] indicate challenges in hard turning and identifies various process, equipment and tooling related factors affecting surface quality, tool life and productivity. After reviewing the literature, factors affecting forces, tool wear/failure and roughness and integrity of the finished surfaces in hard turning using CBN cutting tools and their influences on each other are illustrated with a chart shown in Fig. 1. In this chart, the parameters above the horizontal dashed lines are considered as factors or inputs to the hard turning process and they can only be selected in the beginning except tool vibration. All other parameters, that are located below these dashed lines, considered as performance measures or outputs of the hard turning process. Review of the literature reveals that almost all of the factors given in this chart affect performance of the hard turning process. Those factors can be classified as follows:
1.1 Cutting tool geometry and material properties
Hard turning with CBN cutting tools demands prudent design of tool geometry. CBN cutting tools have lower toughness than other common tool materials, thus chipping ismore likely [2]. Therefore, a nose radius and proper edge preparation are essential to increase the strength of cutting edge and attain favorable surface characteristics on finished metal components [23]. CBN cutting tools designed for hard turning feature negative rake geometry and edge preparation (a chamfer or a hone, or even both). Specifications of the edge preparation design are often finalized after extensive experimentation. Fig. 2 shows the types of edge preparations common for CBN cutting tools. According to recent studies, it is evident that effect of edge geometry on surface quality is significant [23-28].
Fig. 1. A flow chart illustrating relationships of factors in hard turning.
Theile et al. [24, 25], presented research results of an experimental investigation of effects of cutting edge geometry and workpiece hardness on residual stresses in finish hard turning of AISI 52100 steel. They indicated that both factors are significant for the surface integrity of finish hard turned components. Specifically, they showed that large hone radius tools produce more compressive stresses, but also leave “white-layers”. ?zel [26] investigated the influence of edge geometry in CBN tools with respect to stress and temperature development through finite element simulations in hard turning. Chou et al. [28] experimentally investigated the influence of CBN content on surface quality and tool wear in hardened AISI 52100 steel tool. This study concluded that low content CBN tools produce better surface roughness with respect to higher content CBN tools and depth of cut has minor effect on tool wear rate.
Fig. 2. Type of edge preparations for CBN cutting tools.
1.2 Workpiece hardness
Due to the changes in properties of hardened workpiece material, basic shearing process and formation of chips differ in hard turning [5]. Prior research showed that workpiece hardness has a profound effect on the performance of the CBN tools [1,2,8] and also integrity of finish machined surfaces [23,25]. Matsumoto et al. On the return trip the bottom coal is mined with the advantage of a free face and a smaller proportion of the leading drum cutting coal; consequently leading to less restrictions of the haulage speed due to the specific cutting energy of the material. The shearer sumps in mid seam near the head gate to the full web without invoking unproductive cycle time. [23] and Thiele et al. [25] studied the effect of workpiece hardness on residual stresses. In a recent study, Guo and Liu [27] investigated material properties of hardened AISI 52100 bearing steel using temperature controlled tensile tests and orthogonal cutting tests and demonstrated that hardness greatly influences the material properties accounting for high variation in flow stress properties.
1.3 Cutting speed, feed rate and depth of cut
Performance of CBN cutting tools is highly dependent on the cutting conditions i.e. cutting speed, feed, feed-rate, and depth of cut [7]. Especially cutting speed and depth of cut significantly influence tool life [22]. Increased cutting speed and depth of cut result in increased temperatures at the cutting zone. Since CBN is a ceramic material, at elevated temperatures chemical wear becomes a leading wear mechanism and often accelerates weakening of cutting edge, resulting in premature tool failure (chipping), namely edge breakage of the cutting tool. In addition, Thiele et al. [24] noticed that when feed rate is increased, residual stresses change from compressive to tensile.
1.4 Surface integrity, residual stresses and tool wear
In general, residual stresses become more compressive as workpiece hardness increases. The hardness and fracture toughness of CBN tools decrease with reduced CBN content [8]. Owing to ceramic binder phase, CBN-L tools have a lower thermal conductivity, which causes increasing temperatures of cutting edge during hard turning. Chou and Barash [9] reported that CBN-L tools are more suitable for finish turning of hardened steel. At low cutting speeds, tool life of CBN-L is superior to CBN-H, whereas at higher cutting speeds, the reverse is true, and also surface roughness is less favorable when using CBN-H tool [28]. Thiele et al. [24] reported that residual stresses generated by large edge hone tools are typically more compressive than stresses produced by small edge hone tools and they also leave white-layers. In addition, the effects of edge geometry play an important role in thermoplastic deformation of the workpiece. Koenig et al. [3] reported that an increase in feed rate raises the compressive residual stress maximal and deepens the affected zone. It was also suggested that the chamfer is unfavorable in terms of attainable surface finish when compared to honed or sharp edges.
1.5 Accuracy and rigidity of the machine tool
Another parameter that is often ignored is tool vibration. In order to reduce tool vibration it is necessary provide sufficiently rigid tool and workpiece fixtures. Assuring that there is minimal tool vibration is an easy way to improve surface roughness. It is also necessary that the tooling system be extremely rigid to withstand the immense cutting forces. It is well known that the radial force is the largest among force components during hard turning. Many researchers indicated that extremely rigid, high power, and high precision machine tools are required for hard turning because CBN tools are brittle and prone to chipping [3, 7, 8, 14, 23]. It is also suggested that having higher rigidity in machine tool-clamping-tooling system achieves better surface quality on the part. It is well known that vibration and chatter are important problems that degrade part quality and tool performance.
To improve the overall efficiency of finish hard turning, it is necessary to have a complete process understanding. To this end, a great deal of research has been performed in order to quantify the effect of various hard turning process parameters to surface quality. In order to gain a greater understanding of the hard turning process it is necessary to understand the impact of each of these variables, but also the interactions between them. It is impossible to find all of the variables that impact surface quality in finish hard turning. In addition, it is costly and time-consuming to discern the effect of every variable on the output.
2. EXPERIMENTAL PROCEDURE
2.1 Workpiece material
The workpiece material used in this study was AISI H13 hot work tool steel, which is used for high demand tooling. The cylindrical bar AISI H13 specimen that are utilized in this experiments had a diameter of 1.25 inches and length of 2 feet. The bar specimens were heat treated (through-hardened) at in-house heat treatment facility in order to obtain the desired hardness values of 50 and 55 HRC. However, the subsequent hardness tests by using Future Tech Rockwell type hardness tester revealed that the actual hardness of each specimen was 51.3±1.0 and 54.7±0.5 HRC. Henceforth, the hardness values are defined by the mean values of the measured workpiece hardness.
2.2 Tooling and edge geometry
CBN inserts with two distinct representative types of edge preparations were investigated in this study. These edge preparations include: a) “chamfered” (T-land) edges and b) “honed” edges as illustrated in Fig.2. Solid top CBN inserts (TNM-433 and GE Superabrasives BZN 8100 grade) inserts were used with a Kennametal DTGNR-124B right hand tool holder with 00 lead and –50 rake angles. Honed and chamfered insert edge geometry were measured in coordinated measurement machine with three replications using a high precision touch-trigger probe. For the honed inserts, an average radius of 10.5 ±4.0 μm was found. Chamfered insert edge geometry was found to have 200 chamfer angle and 0.1 ± 0.03 mm chamfer width using same instruments with three replications and was approximated to an equivalent hone radius of 101.6 ±5.1 μm.
A categorization of shearer loader cutting sequences is realised by four major parameters . Firstly, one can separate between mining methods, which mine coal in two directions – meaning from the head to the tailgate and on the return run as well – or in one direction only. Secondly, the way the mining sequence deals with the situation at the face ends, to advance face line after extract-ing the equivalent of a cutting web, is a characteristic parameter for each separate method. The nec-essary travel distance while sumping varies between the sequences, as does the time needed to per-form this task, too. Another aspect defining the sequences is the proportion of the web cutting coal per run. Whereas traditionally the full web was used, the introduction of modern AFC and roof sup-port automation control systems allows for efficient operations using half web methods. The forth parameter identifying state of the art shearer loader cutting sequences is the opening created per run.
2.3 Experimental design
A four factor – two level factorial design was used to determine the effects of the cutting edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and resultant forces in the finish hard turning of AISI H 13 steel. The factors and factor levels are summarized in Table 1. These factor levels results in a total of 16 unique factor level combinations. Sixteen replications of each factor level combinations were conducted resulting in a total of 256 tests. Each replication represents 25.4 mm cutting length in axial direction. The response variables are the workpiece surface roughness and the cutting forces.
Longitudinal turning was conducted on a rigid, high-precision CNC lathe (Romi Centur 35E) at a constant depth of cut at 0.254 mm. The bar workpieces were held in the machine with a collet to minimize run-out and maximize rigidity. The length of cut for each test was 25.4 mm in the axial direction. Due to availability constraints, each insert were used for one factor level combination, which consisted of 16 replications. (A total of three honed and three chamfer inserts were available). In this manner each edge preparation was subject to the same number of tests and the same axial length of cut. Finally, surface roughness and tool wear measurements were conducted when the cutting length reached 203.2 mm (8 inches) and 406.4 mm (16 inches) during each factor level combination. The surface roughness was measured with a Taylor-Habson Surtronic 3+ profilometer and Mitutoyo SJ-digital surface analyzer, using a trace length of 4.8 mm, a cut-off length of 0.8 mm. The surface roughness values were recorded at eight equally spaced locations around the circumference every 25.4 mm distance from the edge of the specimen to obtain statistically meaningful data for each factor level combination. CBN inserts were examined using a tool-maker microscope to measure flank wear depth and detect undesirable features on the edge of the cutting tool by interrupting finish hard turning process.
2.4 Cutting force measurements
The cutting forces were measured with a three-component force dynamometer (Kistler Type 9121) mount on the turret disk of the CNC lathe via a custom designed turret adapter (Kistler type 9121) for the toolholder creating a very rigid tooling fixture. The charge signal generated at the dynamometer was amplified using charge amplifiers (Kistler Type 5814B1). The amplified signal is acquired and sampled by using data acquisition PCMCIA card and Kistler DyanoWare software on a laptop computer at a sampling frequency of 2000 Hz per channel. Time-series profiles of the acquired force data reveal that the forces are relatively constant over the length of cut and factors such as vibration and spindle run-out were negligible. Three components of the resultant force are shown schematically in Fig. 3.
Fig. 3. Measured cutting-force components.
3. RESULTS AND DISCUSSION
An analysis of variance (ANOVA) was conducted to identify statistically significant trends in the measured surface roughness and cutting force data. Separate ANOVA analyses were conducted for Ra surface roughness values and for each component of the cutting force i.e. axial (feed), radial (thrust), and tangential (cutting) forces. Additionally, plots of significant factors corresponding to each ANOVA analysis were constructed. These plots provide a more in-depth analysis of the significant factors related to the surface roughness and cutting forces in finish hard turning of AISI H13 steel using chamfered and honed CBN inserts.
3.1 ANOVA results
ANOVA tables for Ra surface roughness parameters are given in Table 2. In addition to degree of freedom (DF), mean square (MS) and F values (F) the table shows the P-values (P2) associate with each factor level and interaction. A low P-value indicates an indication of statistical significange for the source on the response. Table 2 show that the main effects of edge geometry, cutting speed and feed rate except hardness, interactions between edge geometry and hardness, feed rate, and cutting speed, the interactions between cutting speed and feed rate are significant to surface roughness. Feed rate is the dominant parameter associated with the surface roughness. This is expected because it is well known that the theoretical surface roughness is primarily a function of the feed for a given nose radius and varies as the square of the feed rate [8].
The advantage of the half- or, more precisely, partial- opening cutting sequence is the fact that the face is extracted in two passes. Figure 2b shows that the upper and middle part of the seam is cut during the pass towards the tailgate. Whereas the last part of this trip for the equivalent of a ma-chine length the leading drum is raised to cut the roof to allow the roof support to be advanced. On the return trip the bottom coal is mined with the advantage of a free face and a smaller proportion of the leading drum cutting coal; consequently leading to less restrictions of the haulage speed due to the specific cutting energy of the material. The shearer sumps in mid seam near the head gate to the full web without invoking unproductive cycle time. Like for the trip the tailgate the leading drum has to be lowered a machine length ahead of the main gate.
The radial force is usually the largest, tangential force is the middle and the axial (feed) force is the smallest in finish hard turning. In general, cutting force components are influences by cutting speed, edge geometry and feed rate. Tables 3-5 are ANOVA tables corresponding to the radial, axial (feed force) and tangential components of the cutting force, respectively. These tables show that the main effects of workpiece hardness, the edge geometry, cutting speed and feed rate (except for axial force) are all significant with respect to the forces in the radial, axial and tangential directions.
Table 3 shows that the main effects of the edge geometry, cutting speed, hardness and the interactions between edge geometry and hardness, cutting speed, feed rate are significant with respect to the forces in the axial (feed) direction. Axial (feed) force is not much influence by the change in feed rate.
Table 4 shows that the main effects of the edge geometry, cutting speed, hardness and only the interactions between edge geometry and cutting speed, feed rate are significant with respect to the forces in the radial direction.
Table 5 shows that the main effects of the edge geometry, cutting speed, hardness, feed and only the interactions between edge geometry and hardness, cutting speed, feed rate are significant with respect to the forces in the tangential direction.
3.2 Effect of feed rate and edge preparation on surface roughness
Graphs of Ra surface roughness parameters are shown in Figures 4 and 5. These figures have been constructed to illustrate the main effects of edge geometry and feed rate parameters on the surface roughness. Based on the previous analysis, the main effect of the interaction between edge geometry and feed