Criteria for determining machinability and techniques of improving machinability of steel and alloys

It has been established that during of steel and other work materials there is a definite cutting speed at which chip-tool-contact changes abruptly from contact with built-up-edge (BUE) formation to contact without BUE formation. This cutting speed has been termed as critical cutting speed, Vc. Intensity of wear of a tungsten carbide tool is minimum at a speed slightly lower that Vc, when a soft BUE protects the tool from intensive wear. In case of with Tungsten-Titanium carbide tool there is a second cutting speed, slightly above Vc at which a second peak of tool life is observed. These speeds have been referred to as optimum cutting speeds, Vopt1, and Vopt2 respectively. Since both these speeds have definite relationship with the critical cutting speed, Vc which has a specific value for every pair of work and tool materials and conditions of cutting, so the speed Vc may be used on an important criteria for determining machinability of work materials. Three different methods of determining cutting speed Vc has been discussed in this paper. Two different methods of improving machinability of steel has been discussed in the paper. In the first method machinability has been improved by micro-alloying of steel with calcium, which leads to a considerable shift of V towards higher cutting speed (by approximately 140%). The reasons for this improved machinability have been explained in the paper. In the second method low machinability of Titanium alloys and heat resistant steel, associated with instability of chip formation leading to micro and macro chipping of the tool is overcome by preheating of these materials to optimum temperatures. Preheating considerably raises tool life due to lower amplitude and average value of cutting force and higher chiptool contact length resulting in lower stress, acting on the tool tip

Influence of the characteristics of Machine-Tool-Fixture-Work (MTFW) system on chatter

Experiments have been carried out to determine the influence, of different elements of MTFW system on the frequency and. amplitude of chatter and on the ranges of cutting speeds where chatter exists. It has been established that, at definite ranges of cutting speeds metal cutting process is accompanied with chatter. When either spindle work system or tool obit holder vibrates with their natural frequencies. Source of such chatter is the unstability of metal cutting process at all cutting speeds with frequencies which is directly, proportional to cutting speed. Dependence of the frequencies of chatter, when resonance occurs, on cutting speed is defined by straight tines, either parallel to or a bit inclined with the axis of cutting speed. Frequency level and amplitude of such chatter and extent of the ranges of cutting speed, where it exists depend mainly on the rigidity of spindle-work system for low frequency chatter and on the rigidity of tool-bit holder for medium frequency chatter

Influence of work material, cutting conditions and rigidity of tool holders on machine tool chatter

Experimental investigation have been carried out to determine the influence of job materials, cutting speed and stiffness of tool holder on chatter phenomenon which arises during metal cutting operation. Experiments were performed on Engine Lathe of model Celtic-14. Medium Carbon Steel Stainless Steel, Cast Iron and Brass were taken as work materials. Cemented Carbide containing 92% WC and 8% Co was taken as cutting tool material. Three different tool holders having different cross¬ sectional areas and, mechanical clamping arrangement for tool bits, were used. An experimental set-up, consisting of a function generator, amplifiers, vibrator head, pressure transducer, oscilloscope and camera were used for determining the natural frequency of the different elastic elements, including the tool holders. Frequency and amplitude of vibration and chatter were determined by two methods - by using the above mentioned experimental set-up and by measuring the average distance between two vibration marks on job surface or between two chip elements using an instrumental microscope. It has been established by the experimental investigation that single or multiple number of horizontal segments of the frequency versus cutting speed curve correspond to chatter, which appear within narrow range of cutting speeds during cutting of Brass and Cast-Iron; but within wide ranges of cutting speeds during cutting of Medium Carbon and stainless steel. Frequency of chatter corresponding to different horizontal segments of the frequency characteristic curve are almost-integer multiples (1, 2, 3, 4, etc.) of the natural frequency of tool holder. It has been also found that the amplitude of chatter and the range of cutting speed corresponding to the horizontal segments of the frequency characteristic curve decrease and these segments are shifted towards higher cutting speeds with an increase in the stiffness of the tool holder and vice versa. The lengths of the horizontal segments of the frequency characteristic curve decrease but their number increase with an increase in the "stiffness" of the instability of the chip formation process and brittleness of job materials

Surface roughness model for high speed end milling of soda lime glass using carbide coated tools with compressed air blowing

Glass materials play a vital role in advancement of science and technology. They have found wide spread application in the industry, in laboratory equipment and in micro-gas turbines. Due to their low fracture toughness they are very difficult to machine, moreover there are the chip depositions on the machined surface which affects surface finish under ductile mode cutting conditions. In this research, high speed end milling of soda lime glass is performed on CNC vertical milling machine to investigate the effects of machining parameters i.e. spindle speed, depth of cut, and feed rate on machined surface roughness. Design of experiments was performed following Central Composite Design (CCD) of Response Surface Methodology (RSM). Design Expert Software was used for generating the empirical mathematical model for average surface roughness. The model’s validity was tested to 95% confidence level by Analysis of Variance (ANOVA). Subsequent experimental results showed that the developed mathematical model could successfully describe the performance indicators, i.e. surface roughness, within the controlled limits of the factors that were considered

Multi-criteria optimization in end milling of AISI D2 hardened steel using coated carbide inserts

This paper proposes a multi-criteria optimization technique using the mathematical models developed by the response surface methodology (RSM) for the target responses combined with desirability indices for the determining the optimum cutting parameters in end milling of AISI D2 hardened steels. Different responses may require different targets either being maximized or minimized. Simultaneous achievement of the optimized (maximum or minimum) values of all the responses is very unlikely. In machining operations tool life and volume metal removed are targeted to be maximized whereas the machined surface roughness need to be at minimum level. Models showing the combined effect of the three control factors such as cutting speed, feed, and depth of cut are developed. However, a particular combination of parameter levels appears to be optimum for a particular response but not for all. Thus adoption of the method of consecutive searches with higher desirability values is found to be appropriate. In this study the desirability index reaches to a maximum value of 0.889 after five consecutive solution searching. At this stage, the optimum values of machining parameters - cutting speed, depth of cut and feed were determined as 44.27 m/min, 0.61 mm, 0.065 mm/tooth respectively. Under this set condition of machining operations a surface roughness of 0.348 μm and volume material removal of 7.45 cm3 were the best results compared to the rest four set conditions. However, the tool life would be required to compromise slightly from the optimum value

Cost Model for end-milling of AISI D2 tool steel

In this research paper, user-friendly and accurate mathematical model for estimating the cost of end-milling of AISI D2 tool steel using Polycrystalline Cubic Boron Nitride (PCBN) cutting tool inserts is developed. Initially, the different components of machining cost were identified, followed by establishment of equations to determine their values. Then, the required experimental and non-experimental data were collected and the bottom-up approach was adopted for evaluating the cost of machining corresponding to each of fifteen experimental runs. The Response Surface Methodology (RSM) was used to develop the model in which the cost of machining is given as a function of the machining parameters; cutting speed, feed per tooth, and depth of cut, and expressed in Ringgit Malaysia per cubic cm (RM per cm)3. Analysis of Variance (ANOVA) was utilized to check the adequacy of the developed model. The developed model was found to be statistically adequate

Preheating in end milling of AISI D2 hardened steel with coated carbide inserts

This study was conducted to investigate the effect of preheating through inductive heating mechanism in end milling of AISI D2 hardened steel (60-62 HRC) by using coated carbide tool inserts. Apart from preheating, two other machining parameters such as cutting speed and feed were varied while the depth of cut constant was kept constant. Tool wear phenomenon and machined surface finish were found to be significantly affected by preheating temperature and other two variables. End milling operation was performed on a Vertical Machining Centre (VMC). Preheating of the work material to a higher temperature range resulted in a noticeable reduction in tool wear rate leading to a longer tool life. In addition, improved surface finish was obtained with surface roughness values lower than 0.4 um, leaving a possibility of skipping the grinding and polishing operations for certain applications

Influence of work and tool materials on parameters of electrical discharge machining(EDM)

Influence of the properties of work and tool materials on material removal rate (MRR), tool wear ratio (TWR), thickness of the recast layer, surface roughness and accuracy of machining in EDM process has been investigated. Copper brass, stainless steel, mild steel and grey cast iron have been used in various combinations as work and tool materials. From the experimental results it is found that MRR slows down with machining time. Apart from that it has been found that MRR and TWR are inversely proportional to the melting points of the work and tool materials respectively. Electrical conductivity of the tool material also has appreciable influence on tool wear ratio. Wear ratio was found to be minimum in the case of the copper electrode having maximum electrical conductivity. It has been also observed that for all combinations of work and tool materials, a recast layer is formed on the machined surface. It has been observed that the micro cavities formed in the cases of lower melting point electrode materials like copper and brass having higher electrical conductivity are comparatively smaller in size (2-3 μm) as compared to the sizes of the micro cavities (8-20 μm) formed in the cases of high melting pointer electrodes having also lower electrical conductivity. Consequently the machined surface roughness produced in the latter cases is higher. It has been also observed that the debris concentration increases due to side sparking of the electrode. The tendency of debris concentration is the maximum at the middle of the tool¬job interface resulting in high bottom surface inaccuracy, specially when high melting point work materials are machined with electrodes like brass having low melting point and relatively lower electrical conductivity. From the point of view of MRR, brass electrodes have been found to be the most suitable tool, but from the point of view of machining accuracy and surface finish copper electrodes were found to yield the best result for the given set of job materials. So it was concluded that brass electrodes should be recommended for rough machining and copper electrodes for finish machining of the given work materials

Selection of cemented carbide turning tools using EMF and optimization criteria

Tool life and quality of surface finish of the workpiece influence production rate and production cost. Cemented carbide tools have found wide application in the production arena of Bangladesh. In the absence of any testing facility in the country for the selection of cost-effective tools, poor quality carbide tools are frequently imported. A testing method and a selection criteria were developed for testing carbide tools of different shapes and sizes and for selecting the most cost-effective tool. For each tool cutting tests were performed to generate tool life data under different cutting conditions. Mild steel served as the work material. The relationship between tool life and cutting speed was developed using a curve-fitting software on a PC. From these relationships the values of the constants of the Taylor’s equation were determined. The cost equation was then derived for each tool for a given amount of work in a cutting speed range. Finally, the most cost-effective tool was selected on the basis of the relative location of the cost curves of the tools tested