Fundamental studies of driven and self-propelled rotary tool cutting processes—II. Experimental investigation

An extensive experimental investigation has been carried out to verify the developed mechanics of cutting analyses for the fundamental driven and self-propelled rotary tool cutting processes. This involved testing the dynamic or perfect equivalence between the rotary tool and equivalent classical processes over a wide range of inclination angles, cut thickness and rake angles using statistical processing techniques. The collinearity conditions at the shear plane and rake face have also been tested as part of the model verification. It has been shown that all the force components, deformation and basic cutting parameter trends and quantities required for perfect equivalence have been satisfied as were the necessary collinearity conditions. The verified models provide a deeper understanding of the cutting mechanics and characteristics of these ingenious material removal operations and form the basis for the development of predictive cutting models for the fundamental and complex practical rotary tool operations.     

Modeling and verification of cutting tool temperatures in rotary tool turning of hardened steel

This paper addresses modeling of the tool temperature distribution in self-propelled rotary tool (SPRT) machining of hardened steels. Since tool life is significantly influenced by cutting temperatures, a model is developed to analyze the heat transfer and temperature distribution in rotary tool turning of hardened 52100 steel (58 HRC). The model is based on the moving heat source theory of conduction and employs the finite element method (FEM) for its solution. The model is experimentally verified through measurements of the cutting tool temperature distribution using an infrared camera under different cutting conditions. Finally, both rotary and equivalent fixed tool cutting processes are compared in terms of cutting tool temperatures generated.     

Prediction of chip flow direction during machining with self-propelled rotary tools

This paper presents a model for chip flow prediction during tube-end machining process using self-propelled rotary tools. The analysis performed is based on the transformation of the relative kinematic relationships in rotary cutting to that of the conventional cutting with large nose radius. Both relative and absolute chip flow angles are investigated. Tests were performed to measure the absolute chip flow angle, insert self propelled motion, and the deformed chip thickness during machining with self-propelled rotary tool under different cutting conditions. The predicted values of the absolute chip flow angle were in good agreement with that measured experimentally.     

Prediction of cutting force for self-propelled rotary tool using artificial neural networks

In this paper, a cutting force model for self-propelled rotary tool (SPRT) cutting force prediction using artificial neural networks (ANN) has been introduced. The basis of this approach is to train and test the ANN model with cutting force samples of SPRT, from which their neurons relations are gradually extracted out. Then, ANN cutting force model is achieved by obtaining all weights for each layer. The inputs to the model consist of cutting velocity V, feed rate f, depth of cut a_p and tool inclination angle λ, while the outputs are composed of thrust force F_x, radial force F_y and main cutting force F_z. It significantly reduces the complexity of modeling for SPRT cutting force, and employs non-structure operator parameters more conveniently. Considering the disadvantages of back propagation (BP) such as the convergence to local minima in the error space, developments have been achieved by applying hybrid of genetic algorithm (GA) and BP algorithm hence improve the performance of the ANN model. Validity and efficiency of the model were verified through a variety of SPRT cutting samples from our experiment tested in the cutting force model. The performance of the hybrid of GA–BP cutting force model is fairly satisfactory.     

Identification of the specific cutting force for geometrically defined cutting edges and varying cutting conditions

Cutting force modeling is a major discipline in the research of cutting processes. The exact prediction of cutting forces is crucial for process characterization and optimization. Semi-empirical and mechanistic force models have been established, but the identification of the specific cutting force for a pair of tool and workpiece material is still challenging. Existing approaches are depending on geometrical idealizations and on an extensive calibration process, which make practical and industrial application difficult. For nonstandard tools and five axis kinematics there does not exist a reasonable solution for the identification problem.In this paper a co-operative force model for the identification of the specific cutting forces and prediction of integral forces is presented. The model is coupled bidirectionally with a multi-dexel based material removal model that provides geometrical contact zone information. The nonlinear specific forces are modeled as polynomials of uncut chip thickness. The presented force model is not subjected to principal restrictions on tool shape or kinematics, the specific force and phase shift are identified with help of least square minimization. The benefit of this technique is that no special calibration experiments are needed anymore, which qualifies the method to determine the specific forces simultaneously during the machining process. In this paper, experiments with different cutting conditions are analyzed and systematically rated. Finally, the method is validated by experiments using different cutting conditions.     

Tool wear and chip formation during hard turning with self-propelled rotary tools

This paper presents a performance assessment of rotary tool during machining hardened steel. The investigation includes an analysis of chip morphology and modes of tool wear. The effect of tool geometry and type of cutting tool material on the tool self-propelled motion are also investigated. Several tool materials were tested for wear resistance including carbide, coated carbide, and ceramics. The self-propelled coated carbide tools showed superior wear resistance. This was demonstrated by evenly distributed flank wear with no evidence of crater wear. The characteristics of temperature generated during machining with the rotary tool are studied. It was shown that reduced tool temperature eliminates the diffusion wear and dominates the abrasion wear. Also, increasing the tool rotational speed shifted the maximum temperature at the chip–tool interface towards the cutting edge.     

A worn tool force model for three-dimensional cutting operations

Previous studies have shown that there is a region on the flank of a worn cutting tool where plastic flow of the workpiece material occurs. This paper presents experimental data which shows that in three-dimensional cutting operations in which the nose of the tool is engaged, the region of plastic flow grows linearly with increases in total wearland width. A piecewise linear model is developed for modeling the growth of the plastic flow region, and the model is shown to be independent of cutting conditions. A worn tool force model for three-dimensional cutting operations that uses this concept is presented. The model requires a minimal number of sharp tool tests and only one worn tool test. An integral part of the worn tool force model is a contact model that is used to obtain the magnitude of the stresses on the flank of the tool. The force model is validated through comparison to data obtained from wear tests conducted over a range of cutting conditions and workpiece materials. It is also shown that for a given tool and workpiece material combination, the incremental increases in the cutting forces due to tool flank wear are solely a function of the amount and nature of the wear and are independent of the cutting condition in which the tool wear was produced.     

Real-time estimation of cutting forces via physics-inspired data-driven model

A method is presented to estimate the cutting forces in real time within machine tools for any spindle speed, force profile, tool type, and cutting conditions. Before cutting, a metrology suite and instrumented tool holder are used to induce magnetic forces during spindle rotation, while on-machine vibrations, magnetic forces, and error motions are measured for various combinations of speeds and forces. A physics-inspired data-driven model then relates the measured accelerations to the magnetic forces, such that during cutting, on-machine measured vibrations are used in the model to estimate the cutting forces in real time.     

Modeling of turning process cutting forces for grooved tools

A mechanistic modeling approach to predicting cutting forces for grooved tools in turning has been developed. The model assumes the existence of an equivalent orthogonal cutting operation for any oblique operation. The effects of tool nose radius and chip flow have been incorporated by defining a set of equivalent groove parameters. Two calibration methods have been presented for the model. A variety of commercial grooved inserts were chosen to validate the model. The workpiece material used was AISI 1018 steel. The force predictions from the model were found in good agreement with the measured forces. The effects of cutting conditions and groove parameters on the cutting forces and their implication in designing grooved tools were also determined.     

Improved modelling of cutting force coefficients in peripheral milling

Titanium is one of the most widely used metals in the aircraft and turbine manufacturing industries. Accurate prediction of cutting forces is important in controlling the dimensional accuracy of thin walled aerospace components. In this paper, a general three-dimensional mechanistic model for peripheral milling processes is presented. The effects of chip thickness, rake angle and cutting geometry on chip flow, rake face friction and pressure, and cutting forces are analyzed. A set of closed form expressions with experimentally estimated cutting force factors are presented for the prediction of cutting forces. The model is verified experimentally in the peripheral milling of a titanium alloy. For a given set of cutting conditions and tool geometry, the model predicts the cutting forces accurately for the chip thickness and rake angle ranges tested.     

A cutting force model for finishing processes using helical end mills with significant runout

Analytical cutting force models play an important role in a wide array of simulation approaches of milling processes. The accuracy of the simulated processes directly depends on the predictive power of the applied cutting force model, which may vary under specific circumstances. End milling processes with small radial cutting depths, e.g. finishing processes, are particularly problematic. In this case, the tool runout, which is usually neglected in established cutting force models, can become quite significant. Within this article, well-known cutting force models are implemented for runout-prone finishing processes and modified by integrating additional parameters. A method is presented for how these additional runout parameters can be efficiently determined alongside commonly used cutting coefficients. For this purpose, a large number of milling experiments have been performed where the cutting forces were directly measured using a stationary dynamometer. The measured cutting forces were compared with the simulated cutting forces to verify and assess the modified model. By using the presented model and calibration method, cutting forces can be accurately predicted even for small radial cutting depths and significant tool runout.     

Estimation of cutting forces in micromilling through the determination of specific cutting pressures

In this paper a new model for the estimation of cutting forces in micromilling based on specific cutting pressure is presented. The proposed model includes three parameters which allow to control the entry of the cutter in the workpiece and which consider also the errors in the radial position of the cutting edges of the tool.

Due to the difficulties presented in the manufacturing of the micromilling tools, manufacturing errors frequently appear. These are errors in the radial and angular position of the cutting edge and have significant influence in the estimation of the instantaneous cutting force in micromilling.

The accuracy of the estimated parameters of the cutting force expression plays a major role in the resulting cutting force. For this reason, the influence of the fitting of the specific cutting pressure is analyzed.

The new mechanistic force model determines the instantaneous cutting force coefficients using experimental data processed for one cutter revolution. The model has been validated through experimental tests over a wide range of cutting conditions. The results obtained show good agreement between the predicted and measured cutting forces.     

Feed rate scheduling model considering transverse rupture strength of a tool for 3D ball-end milling

This paper presents an analytical model of off-line feed rate scheduling to determine desired feed rates for 3D ball-end milling. Off-line feed rate scheduling is presented as the advanced technology to regulate cutting forces through change of feed per tooth, which directly affects variation of uncut chip thickness. In this paper, the uncut chip thickness is calculated by following the movement of the position of the cutter center, which is determined by runout and cutter deflection. Also, since the developed cutting force model uses the cutting-condition-independent coefficients, off-line feed rate scheduling can be effectively performed regardless of continuous change of cutting conditions. Transverse rupture strength of the tool is used to determine the reference cutting force at which resultant cutting forces are regulated through feed rate scheduling. Experiments validated that the presented feed rate scheduling model reduced machining time drastically and regulated cutting forces at the reference cutting force.     

Improving Cycle Time in Sculptured Surface Machining Through Force Modeling

In this paper, an enhanced mathematical model is presented for the prediction of cutting force system in ball end milling of sculptured surfaces. This force model is also used as the basis for off-line feed rate scheduling along the tool path in order to decrease the cycle time in sculptured surface machining. As an alternative for setting a constant feed rate all along the tool path in rough machining of sculptured surfaces, resultant cutting forces are aimed to be kept under a pre-set threshold value along the tool path by off-line scheduled piecewise variable feed rates. In this paper, it is shown that machining time, depending on complexity of sculptured surfaces, can be decreased significantly by scheduling feed rate along the tool path. The model is tested under various cutting conditions and some of the results are also presented and discussed in the paper.     

Practical implementation of cutting force model for step drill using 3D CAD data

As an extension of the cutting force model, which was developed by analyzing the forces on rake and relief face, a practical method of cutting force model is suggested. By a calibration test, cutting coefficients are determined, including the information on tool geometry, work material and cutting conditions such as cutting speed and feed rate. As inputs for the force model, normal rake angle and normal relief angle were obtained by a new method from the measured angles using a 3D CAD machine. This practical force model can be applied for machining CFRP successfully, which was proved from the experiments.     

Experimental studies of cutting force variation in face milling

The purpose of this paper is to present a developed cutting force model for multi-toothed cutting processes, including a complete set of parameters influencing the cutting force variation that has been shown to occur in face milling, and to analyse to what extent these parameters influence the total cutting force variation for a selected tool geometry. The scope is to model and analyse the cutting forces for each individual tooth on the tool, to be able to draw conclusions about how the cutting action for an individual tooth is affected by its neighbours.A previously developed cutting force model for multi-toothed cutting processes is supplemented with three new parameters; eccentricity of the spindle, continuous cutting edge deterioration and load inflicted tool deflection influencing the cutting force variation. A previously developed milling force sensor is used to experimentally analyse the cutting force variation, and to give input to the cutting force simulation performed with the developed cutting force model.The experimental results from the case studied in this paper show that there are mainly three factors influencing the cutting force variation for a tool with new inserts. Radial and axial cutting edge position causes approximately 50% of the force variation for the case studied in this paper. Approximately 40% arises from eccentricity and the remaining 10% is the result of spindle deflection during machining. The experimental results presented in this paper show a new type of cutting force diagrams where the force variation for each individual tooth when two cutting edges are engaged in the workpiece at the same time. The wear studies performed shows a redistribution of the individual main cutting forces dependent on the wear propagation for each tooth.     

Effects of micro-milling conditions on the cutting forces and process stability

Determining stable cutting conditions for corresponding cutting tools with specific geometries is essential for achieving precision micro-milling with high surface quality. Therefore, this paper investigates the influence of the tool rake angle, tool wear and workpiece preheating on the cutting forces and process stability. An advanced micro-milling cutting force model considering the tool wear is proposed. The micro-milling cutting forces are predicted and compared with experimentally obtained results for two cutting conditions and four edge radii measured at different stages of the tool wear. It is found that the cutting forces increase by increasing the edge radius. It is also observed that the cutting forces are higher at a rake angle of 0° compared with a rake angle of 8°. The increase of the cutting forces is mainly associated with the change of the friction conditions between the tool and workpiece contact. Stability lobes are obtained for different edge radii, rake angles of 0° and 8°, initial workpiece temperature and different measured static run-outs. The predicted stability lobes are compared with the micro-milling force signals transformed into the frequency domain. It is observed that the predicted stability limits result in good correlation with the experimentally obtained chatter free conditions. Also, the stability limits are higher at smaller edge radii, higher preheating workpiece temperature and positive rake angles.     

Prediction of cutting forces in milling of circular corner profiles

This paper proposes an approach to predict the cutting forces in peripheral milling of circular corner profiles in which varying radial depth of cut is encountered. The geometric relationship between an end mill and the corner profile is investigated and a mathematical model is presented to describe the different phases of the cutter/workpiece contact. The milling process for circular corner is discretized into a series of steady-state cutting processes, each with different radial depth of cut determined by the instantaneous position of the end mill relative to the workpiece. A time domain analytical model of cutting forces for the steady-state machining conditions is introduced to each segmented process for the cutting force prediction. The predicted cutting forces can be calculated in terms of tool/workpiece geometry, cutting parameters and workpirece material property, as well as the relative position of the tool to workpiece. Experiments are conducted and the measured forces are compared to the predictions for the verification of the proposed method.     

A multi-sensor strategy for tool failure detection in milling

A multi-sensor monitoring strategy for detecting tool failure during the milling process is presented. In this strategy, both cutting forces and acoustic emission signals are used to monitor the tool condition. A feature extracting algorithm is developed based on a first order auto-regressive (AR) model for the cutting force signals. This AR(1) model is obtained by using average tooth period and revolution difference methods. Acoustic emission (AE) monitoring indices are developed and used in determining the setting threshold level on-line. This approach was beneficial in minimizing false alarms due to tool runout, cutting transients and variations of cutting conditions. The proposed monitoring system has been verified experimentally by end milling Inconel 718 with whisker reinforced ceramic tools at spindle speeds up to 3000 rpm.     

The effect of the multi-layer surface coating of carbide inserts on the cutting forces in turning operations

In this paper, the effects of the multi-layer hard surface coating of cutting tools on the cutting forces in steel turning are presented and discussed, based on an experimental investigation with different commercially available carbide inserts and tool geometries over a range of cutting conditions. The cutting forces when turning with surface coated carbide inserts are assessed and compared qualitatively and quantitatively with those for uncoated tools. It is shown that hard surface coatings reduce the cutting forces, although the reduction is marginal under lighter cutting conditions. The cutting force characteristics for surface coated tools are also discussed and shown to have similar trends to those of uncoated tools.