Analytical model to determine the critical conditions for the modes of material removal in the milling process of brittle material

Brittle materials are prone to cleavage-based fracture during machining. In conventional scale machining of brittle material, crack-propagation is the dominant mechanism of material removal which results in a degraded machined surface. The challenge is to perform machining of brittle material such that the material removal occurs predominantly by chip formation rather than the characteristic brittle fracture. In this case, a high quality finish is achieved on the machined surface. Ductile-mode machining has emerged as a promising technique to finish a crack-free machined surface on macroscopically brittle materials. In the past, ductile-mode machining has mostly been performed by single-edge cutting process. This paper outlines an analytical model to determine the critical conditions for finishing a crack-free surface on brittle material by milling process. Four distinct modes of machining have been identified in the milling process of brittle material. In this model, the critical conditions for different modes of machining have been determined with respect to the relationship between the radial depth of cut and the depth of subsurface damage caused by the brittle fracture during machining. Verification tests were performed on tungsten carbide workpiece and the experimental results have validated the proposed machining model. It has been established that if the radial depth of cut is greater than the subsurface-damage depth in the milling process of brittle material, it is possible to finish a crack-free machined surface by removal of material through a combination of plastic deformation and brittle fracture. However, if the radial depth of cut is less than the subsurface damage depth, brittle fracture must be prevented in ductile-mode milling to finish a crack-free machined surface.     

Analytical model to determine the critical feed per edge for ductile-brittle transition in milling process of brittle materials

Brittle materials like glass are considered difficult-to-machine because of their high tendency towards brittle fracture during machining. The technological challenge in machining such brittle materials is to achieve material removal by plastic deformation rather than characteristic brittle fracture. In ductile mode machining, the material is removed predominantly by plastic deformation and any cracks produced due to possible fracture in the cutting zone are prevented from extending into the machined surface. This is achieved by selecting an appropriate cutting tool and suitable machining parameters. In ductile machining by milling process, fracture induced cracks are diverted away from final machined surface by selecting a suitable feed per edge less than a critical threshold value. Hence determination of critical feed per edge is of paramount importance to achieve ductile mode machining by milling process. This paper presents an analytical model based on fracture mechanics principles to predict the critical feed per edge in milling process of glass. The size and orientation of cracks originating from brittle fracture during machining have been quantified by using indentation test results and the critical value of feed per edge has been determined analytically as a function of intrinsic materials properties governing brittle fracture and plastic deformation. Furthermore, an equivalent tool included angle has been suggested for machining operation as against the indenter included angle to correlate the indentation and machining test results with improved degree of accuracy. Experimental results validated the proposed model fairly accurately. It has been established that if the longest cracks oriented in radial direction to the cutting edge trajectory are prevented from reaching the final machined surface by selecting a feed per edge less than or equal to a critical value, a crack-free machined surface can be achieved.     

Effect of tilt angle on cutting regime transition in glass micromilling

Recent development in mechanical micromachining technology has increased the realization of micromachining as a feasible manufacturing process of micro-scale components including glass-based devices. It has been found that glass can be machined in a ductile regime under certain controlled cutting configurations. However, favorable ductile regime machining instead of brittle regime machining in micromilling of brittle glass is still not fully understood as a function of cutting configuration. In this study, the effect of tilt angle along the feed direction on cutting regime transition has been studied in micromilling crown glass with a micro-ball end mill. Straight glass grooves were machined in water bath by varying the tool tilt angle and the feed rate, and the resulting surface was characterized using the scanning electron microscope and the profilometer to investigate the glass cutting regime transition. In characterizing the cutting regimes in glass micromilling, rubbing, ductile machining, and brittle machining regimes are hypothesized according to the undeformed chip thickness. It is found that a crack-free glass surface can be better machined in the ductile mode using a 45° tilt angle and feed rates up to 0.32 mm/min. During each milling pass, surface roughness was found to decrease from the entry zone to the groove bottom and then increase to the exit zone regardless of the cutting regime.     

A study of the surface scallop generating mechanism in the ball-end milling process

This paper presents the model, simulation and experimental verification of the scallop formation on the machined surface in the ball-end milling process. In the milling process, the cutting edges of the cutter are in a motion of combined translation and rotation. The periodical variation of the cutting edge orientation during spindle rotation results in two kinds of scallops generated on the machined surface: the pick-interval scallop and the feed-interval scallop. Because of the low feed and comparably large pick used in the conventional ball-end milling process, the emphasis of previous works has been placed on studying the geometric generating mechanism of the pick-interval scallop while the feed-interval scallop has been largely ignored. Trend of the high-speed and high efficiency machining, however, has pushed the feed reaching the same level of the pick. For the high-speed machining where the high feed/pick ratio is used, the feed-interval scallop must be taken into account. This paper presents a new model that describes the path-interval and feed-interval scallops generating mechanism in the ball-end milling processes. Parameters such as the tool radius, feed/pick ratio, initial cutting edge entrance angle, and tool-axis inclination angles have been studied and experimental verified. It was found that the feed-interval scallop height was 3–4 times large than the path-interval scallop height at the high-speed machining case. The scallop height was continuously reduced by increasing the tool-axis inclination angle. An inclination angle up to 10° is, however, good enough for most tool diameters from the surface roughness viewpoint.     

Statistical analysis of the effects of machining parameters and workpiece hardness on the surface finish of machined medium carbon steel

The objective of this study is to ascertain the effect of machining parameters and workpiece hardness on surface roughness of machined components and to develop a better understanding of the effect of process parameters on the machined surface. Such an understanding can provide insight into the problems of controlling the finish of machined surfaces when the process parameters are adjusted to obtain a certain surface finish. The collected data were analyzed using parametric analyses of variance (ANOVA) with surface finish as the dependent variable and hardness of the workpiece material, cutting tool position from the surface of the clamping device (chuck), depth of cut, cutting velocity, and cutting feed as independent variables. The results showed that surface roughness is significantly affected by the workpiece hardness, cutting feed, cutting speed, depth of cut, cutting tool position from the chuck, and their interactions with each other. The results suggest that feed rate and cutting speed can be adjusted to produce a certain surface finish when the position of the cutting tool from the surface of a clamping device or the hardness of the workpiece is changed.     

Study on the adaptability of thick film diamond tool to cutting composites

In order to explore the adaptability of a thick film diamond tool to the finish machining of composites, tool wear and its effect factors as well the machined surface roughness are investigated in this paper. The experimental results show that the thick film diamond tool has a low wear rate and the machined surface cut with the tool has a fine finish for the cutting of composites. The negative rake is beneficial for the tool standing wear and collision.     

A study of back cutting surface finish from tool errors and machine tool deviations during face milling

The surface finish of mechanical components produced by face milling is given by factors such as cutting conditions, workpiece material, cutting geometry, tool errors and machine tool deviations. The contribution of the different tool teeth to imperfections in the machined surface is strongly influenced by tool errors such as radial and axial runouts. The surface profile of milled parts is not only affected by chip removal due to front cutting, but also by back cutting, which must be taken into account when predicting surface roughness. In the present work, the influence of back cutting on the surface finish obtained by face milling operations is studied. Final part surface roughness is modelled from tool runouts and height deviations that affect the surface marks provoked by back cutting. Round insert cutting tools and surface positions defined by cutter axis trajectory are considered, and milling experiments are developed for a spindle speed of 750 rpm, depth of cut of 0.5 mm and feeds from 0.4 to 1.0 mm/rev. Experimental observations are compared with the theoretical predictions provided by the surface roughness model, and good agreement is found between both results. Surface imperfections caused by front and back cutting are analysed, and discrepancies between experiments and numerical predictions are explained by undeformed chip thickness variations along the tool tooth cutting edge, the tearing of the workpiece material, and fluctuations in the feedrate and height deviation during machine tool axis displacement.     

Application of experimental design in ball burnishing

The ball burnishing process is used in place of other traditional methods to finish ASSAB XW-5 tool steel on a vertical machining centre. From an initial roughness of about R_tm 4 μm, the specimen could be finished to a roughness of 0.27 μm. The response surface methodology (RSM) was used to establish mathematical models correlating three process parameters: depth of penetration; feed; and burnishing speed. The experimental design and method of analysis for fitting polynomials of the first and second degrees (first-and second-order designs) are discussed in this paper.     

Roughness and texture generation on end milled surfaces

Plane surface generation mechanism in flat end milling is studied in this research. The bottom of a flat end mill has an end cutting edge angle that plays an important role in surface texture. Surface texture is produced by superposition of conical surfaces generated by the end cutting edge rotation. The machined surface is cut once again by the trailing cutting edge. This back cutting phenomenon is frequently observed on surfaces after finishing. Tool run-out and tool setting error including tool tilting and eccentricity between tool center and spindle rotation center are considered together with tool deflection caused by cutting forces. Tool deflection affects magnitude of back cutting and the surface form accuracy. As a result, the finished surface possesses peaks and valleys with form waviness. Surface topography parameters such as RMS deviation, skewness and kurtosis are used for evaluating the generated surface texture characteristics. Through a set of cutting tests, it is confirmed that the presented model predicts the surface texture and roughness parameters precisely including back cutting effect.     

The preparation of cutting edges using a marking laser

During the machining process, high mechanical and thermal loads occur at the cutting edge. Such loads can cause tool failure. Specifically non-uniform and sharp cutting edges that have a low cutting edge stability lead to such failures. In order to enhance the tool performance, the cutting edges are prepared by manufacturing both a pre-defined cutting edge geometry, and an appropriate cutting edge roughness. This paper describes the use of a low-cost marking laser for the preparation of cutting edges as an alternative to conventional preparation techniques, such as brushing or blasting. Cutting edge radii of 9?C47 ??m can be prepared with a machining accuracy of 1.5 ??m. The maximum preparation time for an individual cutting edge is approximately 10 s. Uncoated indexable inserts manufactured in this way were tested in a face milling operation. The results of these investigations (using prepared cutting edges) show both an increase in tool life and an improved surface roughness of the machined workpieces compared to those using non-prepared cutting edges.     

Investigation in orthogonal turn-milling towards better surface finish

Turn-milling is a newly emerging machining process, which tends to make use of the advantages of both turning and milling, wherein both the work piece and the cutting tool are given rotary motion simultaneously. The objective of the present experimental work is to understand the phenomenon of orthogonal turn-milling especially in relation to the effects of work piece revolution, cutter diameter and depth of cut. Surface finish of the machined surface and the optimum work speed at which the surface roughness was minimum has been studied. It has been shown that surface quality obtained by turn-milling process is better than that of conventional milling process. The experiments have been conducted for orthogonal turn-milling of mild-steel work piece with high speed steel milling cutters using planning of experiments technique to study the surface finish achieved.     

Mechanism of Micro Chip Formation in Diamond Turning of Al-Mg Alloy

This paper deals with diamond turning experiments of Al-Mg alloy. Variation in tool setting angles using straight tools with sharp cutting edges was performed to study the difference between the machined surface roughness and the theoretical surface roughness estimated with a steady vibration model. Tears with 0.1 μm depth which are generated on the side cutting edge deteriorates the machined surface roughness. The tears are cut off by the trailing end cutting edge at a negative tool setting angle. Burrs generated at the tool setting angle less than -0.1° are another cause for the deteriorating the machined surface roughness.     

Tool wear and machining performance of cBN–TiN coated carbide inserts and PCBN compact inserts in turning AISI 4340 hardened steel

PCBN is the dominant tool material for hard turning applications due to its high hardness, high wear resistance, and high thermal stability. However, the inflexibility of fabricating PCBN inserts with complex tool geometries and the prohibitive cost of PCBN inserts are some of the concerns in furthering the implementation of CBN based materials for hard turning. In this paper, we present the results of a thorough investigation of cBN plus TiN (cBN–TiN) composite-coated, commercial grade, carbide inserts (CNMA 432, WC–Co (6% Co)) for hard turning applications in an effort to address these concerns. The effect of cutting speed and feed rate on tool wear (tool life), surface roughness, and cutting forces of the cBN–TiN coated carbide inserts was experimented and analyzed using analysis of variance (ANOVA) technique, and the cutting conditions for their maximum tool life were evaluated. The tool wear, surface roughness, and cutting forces of the cBN–TiN coated and commercially available PCBN tipped inserts were compared under similar cutting conditions. Both flank wear and crater wear were observed. The flank wear is mainly due to abrasive actions of the martensite present in the hardened AISI 4340 alloy. The crater wear of the cBN–TiN coated inserts is less than that of the PCBN inserts because of the lubricity of TiN capping layer on the cBN–TiN coating. The coated CNMA 432 inserts produce a good surface finish (<1.6 μm) and yield a tool life of about 18 min per cutting edge. In addition, cost analysis based on total machining cost per part was performed for the comparison of the economic viability between the cBN–TiN coated and PCBN inserts.     

Predictive modeling of surface roughness and tool wear in hard turning using regression and neural networks

In machining of parts, surface quality is one of the most specified customer requirements. Major indication of surface quality on machined parts is surface roughness. Finish hard turning using Cubic Boron Nitride (CBN) tools allows manufacturers to simplify their processes and still achieve the desired surface roughness. There are various machining parameters have an effect 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. This paper utilizes neural network modeling to predict surface roughness and tool flank wear over the machining time for variety of cutting conditions in finish hard turning. Regression models are also developed in order to capture process specific parameters. A set of sparse experimental data for finish turning of hardened AISI 52100 steel obtained from literature and the experimental data obtained from performed experiments in finish turning of hardened AISI H-13 steel have been utilized. The data sets from measured surface roughness and tool flank wear were employed to train the neural network models. Trained neural network models were used in predicting surface roughness and tool flank wear for other cutting conditions. A comparison of neural network models with regression models is also carried out. Predictive neural network models are found to be capable of better predictions for surface roughness and tool flank wear within the range that they had been trained.Predictive neural network modeling is also extended to predict tool wear and surface roughness patterns seen in finish hard turning processes. Decrease in the feed rate resulted in better surface roughness but slightly faster tool wear development, and increasing cutting speed resulted in significant increase in tool wear development but resulted in better surface roughness. Increase in the workpiece hardness resulted in better surface roughness but higher tool wear. Overall, CBN inserts with honed edge geometry performed better both in terms of surface roughness and tool wear development.     

Dynamic variable depth of cut machining using piezoelectric actuators

This paper describes the design of a piezoelectric actuated cutting tool and implementation of digital servo controls for machining surfaces with dynamically varying depth of cut. Through a flexure hinge, the tool holder could generate 50 μm travel at the tip of the cutting insert. Tool motion errors of less than 0.5 μm were achieved in tracking cyclic waveforms by employing a digital repetitive servo control. When applied to turning aluminum and steel workpieces with variable depth of cut using carbide tools, less than 5 μm machined surface errors were measured.     

Experimental investigation on hard milling of high strength steel using PVD-AlTiN coated cemented carbide tool

High strength steel 30Cr3SiNiMoVA (30Cr3) is usually used to manufacture the key parts in aviation industry owing to its outstanding mechanical properties. However, 30Cr3 has poor machinability due to its high strength and high hardness. Hard milling is an efficient way in machining high strength steels. This paper investigated hard milling of 30Cr3 using a PVD-AlTiN coated cemented carbide tool with regard to cutting forces, surface roughness, chip formation and tool wear, respectively. The experimental results indicated that the increase of cutting speed from 70 to 110 m/min leads to direct reduction of cutting forces and improvement of surface finish, while both feed rate and depth of cut have negative effect on surface finish. The occurrence of oxidation on chip surfaces under high cutting temperature makes the chips show different colors which are strongly influenced by cutting speed. Saw-toothed chips were observed with the occurrence of the thermo-plastic instability within the primary shear zone. Micro-chipping and coating peeling were confirmed to be the primary tool failure modes. Serious abrasion wear and adhesive wear with some oxidative wear were confimed to be the main wear mode in hard milling of 30Cr3.     

The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel

The affected layer is generated within the machined surface layer through the cutting process. Cutting conditions such as the nose radius of the tool, feed rate and shape of cutting edge at the finishing operation affect the residual stress, surface hardness, and surface roughness. In this paper, it is shown that such machined surface property could be controlled by the setting of the cutting conditions to some extent. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test using the specimen finished under various cutting conditions. It was shown that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. Such a situation was realized when the generated residual stress was small and the induced surface hardness was high. A longer fatigue life for the machined components can be obtained by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool.     

Freeform surface finish of plastic injection mold by using ball-burnishing process

The objective of this study is to introduce the possible ball-burnishing surface finish process of a freeform surface plastic injection mold on a machining center. The design and manufacture of a burnishing tool was first accomplished in this study. The optimal plane ball-burnishing parameters were determined by utilizing the Taguchi’s orthogonal array method for plastic injection molding steel PDS5 on a machining center. Four burnishing parameters, namely the ball material, burnishing speed, burnishing force, and feed, were selected as the experimental factors of Taguchi’s design of experiment to determine the optimal burnishing parameters, which have the dominant influence on surface roughness. The optimal burnishing parameters were found out after conducting the experiments of the Taguchi’s L18 orthogonal table, analysis of variation (ANOVA), and the full factorial experiment. The optimal plane burnishing parameters for the plastic injection mold steel PDS5 were the combination of the tungsten carbide ball, the burnishing speed of 200 mm/min, the burnishing force of 300 N, and the feed of 40 μm. The surface roughness R_a of the specimen could be improved from about 1 to 0.07 μm by using the optimal burnishing parameters for plane burnishing. Applying the optimal burnishing parameters for plane burnishing to freeform surface plastic injection mold, the surface roughness R_a of freeform surface region on the tested plastic injection part could be improved from about 0.842 to 0.187 μm, through a comparison between using the fine milled and using the ball-burnished mold cavity.     

Surface integrity of dry machined titanium alloys

The study is focused on the machined surface integrity of titanium alloy under the dry milling process. Roughness, lay, defects, microhardness and microstructure alterations are investigated. The result of surface roughness shows that the CVD-coated carbide tool fails to produce better Ra value compared to the uncoated tool. Lay is found to be dependent on cutting speed and feed speed directions. Microhardness is altered down to 350 μm beneath the machined surface. The first 50 μm is the soft sub-surface caused by thermal softening in the ageing process. Down to 200 μm is the hard sub-surface caused by the cyclic internal work hardening and then it gradually decreased to the bulk material hardness. It was concluded that for titanium alloys, dry machining can be carried out with uncoated carbide tools as far as cutting condition is limited to finish and/or semi-finish operations.     

Ultrasonic elliptical vibration transducer driven by single actuator and its application in precision cutting

The ultrasonic elliptical vibration cutting has been successfully applied to precision cutting due to its superior performances, such as low cutting force, high quality surface finish and long tool life. This paper presented an asymmetrical structural model of the ultrasonic elliptical vibration transducer with only the longitudinal excitation. Based on the modal and static analysis with finite element method, various parameters of the model were modified to meet the needs of the vibration modality and the inherent frequency. A cutting system of the ultrasonic elliptical vibration driven by single longitudinal actuator was developed and the effect of the transducer amplitude and cutting depth on the cutting force was studied in detail. A part with surface roughness of R_a 0.08 μm was achieved. The results showed that the ultrasonic elliptical vibration transducer can be designed rationally with finite element method and single driven ultrasonic elliptical vibration machining can be used in precision cutting.