Milling Tests Research Articles

Analysis of the coefficient of friction at the workpiece-tool interface in milling of high strength compacted graphite cast irons

The coefficient of friction at the workpiece-tool interface is one of the main influencing parameters in machining. It can be tribologically investigated using conventional and open tribometers, however, there is no consensus as to which type of test has the best correlation with the real machining process. This work aims at investigating the use of three different methods to measure the coefficient of friction in the machining of three types of high strength compacted graphite cast irons, with variations in the size of the free graphite phase and the presence or absence of molybdenum carbides in the matrix. The coefficient of friction of the process was measured using two progressive load conventional tribological tests, the progressive load single sliding test with load ranging from 0.5 to 40 N and the progressive load reciprocate sliding test in both dry and starved lubrication regimen, with load ranging from 48 to 83 N. The coefficient of friction was also measured in a conventional machining center used as an open tribometer, where ramp milling tests were performed in workpieces fixed on a piezoelectric dynamometer, with a cutting depth varying from 100 µm to 0, to simulate a regressive load. The following machining parameters were varied: cutting speed, feed rate, tool geometry, tool coating and the use of minimum quantity of lubrication in contrast to dry machining. As the main results, the predominance of lubrication, even in small quantities, was observed as the most influential parameter in the coefficient of friction. An inverse relationship was also observed between the hardness of the materials and the coefficient of friction independent of the lubrication condition used. The results of the coefficient of friction obtained in progressive load reciprocate sliding and open tribometer tests showed the best correlation with those found in the literature for this kind of materials.

Time-Varying Chatter Frequency Characteristics in Thin-Walled Workpiece Milling With B-Spline Wavelet on Interval Finite Element Method

As the most significant material removal method, milling plays a very important role in the manufacturing industry. However, chatter occurs frequently in milling, which will seriously affect the production efficiency. The accurate prediction of chatter frequency can contribute to chatter monitoring and the design of the controller for chatter mitigation. During thin-walled workpiece milling under chatter, a new phenomenon of time-varying chatter frequency is discovered and explained in this paper. This phenomenon can be explained as follows, with the workpiece material removal, the modal parameters change during thin-walled milling, which can cause the continuous change of chatter frequency. In order to predict the varying modal parameters, this paper provided an efficient tool, the B-spline wavelet on interval finite element method (BSWIFEM), which can possess the material removal problem more accurately and more rapidly. Based on the calculated modal parameters, the time-varying chatter frequency can be obtained with the chatter frequency calculation formulas. To verify the calculated results, a number of milling tests are implemented on thin-walled parts. The experimental results show that the calculated chatter frequency is in good agreement with the measured chatter frequency, which validates the effectiveness of the proposed method.

Material properties and partial factors for resistance of low yield point steels in China

Abstract As promising structural materials in seismic isolation and energy dissipation systems, low yield point (LYP) steels have been developed and have begun to be adopted in seismic design. This article reports statistical analyses of the material properties of LYP steels and calibration of their partial factors for resistance. An extensive range of mill test data collected from major steel manufacturers in China was analyzed using a mathematical statistical method. On the basis of the statistical analyses and existing research results, reliability analysis was performed for LYP steels using probabilistic theories and structural reliability theories. Finally, suggested partial factors for the resistance of LYP steels were provided. As an important part for codifying the Technical specification for application of low yield point steel in China, this paper may provide valuable reference for further research and promote the engineering applications of LYP steels in China.

CFD and experimental analysis of the coolant flow in cryogenic milling

Abstract Cryogenic cooling could improve the machining performance for hard-to-cut materials. Deployment of this modern technology for milling applications, specifically because of their severe working conditions and complicated physical phenomena inside cryogenic fluids, requires further research to become credible for industrial applications. Obtaining accurate models for coolant behavior is essential for optimum design, prediction of operational limits, and safe control of the cooling process. In this paper, Computational Fluid Dynamics (CFD) were used to determine the behavior of the liquid nitrogen (LN2) inside the coolant delivery system and the interaction of the coolant jet with the cutting area. The role of working conditions as well as nozzle geometry, formation of cavitation inside the tool, coolant pressure, and wall temperature on the efficiency of the coolant delivery were investigated in this study. Initial experimental flow measurements were used to predict the simulation setup and evaluate the results. Experimental milling tests were performed to verify the numerical findings. Cutting forces, coolant mass flow rate, temperature, and pressure were measured during the tests. A mechanistic cutting force model and the morphology of the chips were used to interpret the effect of liquid nitrogen on the cutting mechanisms. Results of this study offer three main suggestions for reliable industrial cryogenic milling: using liquid nitrogen in the range of 2–4 bar, improving the insulation of the feeding line and finding a technical solution for non-insulated parts, and increasing the quality of the liquid in the coolant mixture close to the milling head. Outcomes of this study could help to improve the cooling performance and implement a reliable industrial solution for cryogenic milling.

A Method of Cutting Power Monitoring for Feed Axes in Milling by Power Measurement Device

This study shows a method to monitor the cutting power by a power measurement device installed on the feed-drive motors of a machining center. The load information, which is one of the critical process variables, is highly demanded in monitoring/analyzing machining process. Although force/torque sensors can directly measure the load information of the cutting process, their investments costs are high and they reduce the stiffness of the machine-workpiece systems. Therefore, this study utilizes a power measurement device, which is composed of voltage and current sensors. The cutting power is the multiplication of cutting force (the load information) and the speed information. However, in this study, the cutting power is monitored by estimating the induced voltage of the drive motor and correcting the motor current. The method is experimentally validated in a two-dimensional milling test and compared to a dynamometer-based signal.

High Sensitivity C-Reactive Protein and Tread Mill Test Responses in Patients with Type 2 Diabetes Mellitus

High Sensitivity C-Reactive Protein and Tread Mill Test Responses in Patients with Type 2 Diabetes Mellitus

Modeling of flange-mounted force sensor frequency response function for inverse filtering of forces in robotic milling

Abstract Force/torque sensors have great potential for use in industrial robotic applications. However, the Frequency Response Function (FRF) of the robot flange-mounted sensor is influenced by the dynamics of the robot arm, which distorts the force sensor measurements. The sensor FRF is required for developing an inverse filter to compensate for the distortion. However, the sensor FRF changes as a function of the robot end effector position, and therefore must be accurately modeled. This paper proposes a Gaussian Process Regression (GPR) model-based approach to predict the FRF of a force sensor mounted on the end effector of a six degree-of-freedom (6-dof) industrial robot. The force sensor FRF as a function of the robot position in the workspace is characterized using impact hammer experiments. GPR models that predict the magnitude of a specific frequency bin are developed using the impact hammer results. The GPR modeling approach is evaluated using cross evaluation metrics. The predicted sensor FRFs are used for inverse filtering the milling forces measured by the flange force sensor during robotic milling tests. The filtered forces are shown to be in good agreement with forces measured by a ground-mounted quartz-based piezoelectric force dynamometer; specifically, the inverse filter lowers the maximum peak-to-peak error in the force measured by the flange sensor from 146 N to 25.4 N. Hence, the proposed methodology is shown to be valid for improving the accuracy of robot end effector-mounted force/torque sensors.

Feature extraction using dominant frequency bands and time-frequency image analysis for chatter detection in milling

Chatter is a cause of low surface quality and productivity in milling and crucial features need to be extracted for accurate chatter detection and suppression. This paper introduces a novel feature extraction approach for chatter detection by using image analysis of dominant frequency bands from the short-time Fourier transform (STFT) spectrograms. In order to remove the environmental noises and highlight chatter related characteristics, dominant frequency bands with high energy are identified by applying the squared energy operator to the synthesized fast Fourier transform (FFT) spectrum. The time-frequency spectrogram of the vibration signal is divided into a set of grayscale sub-images according to the dominant frequency bands. Statistical image features are extracted from those sub-images to describe the machining condition and assessed in terms of their separability capabilities. The proposed feature extraction method is verified by using dry milling tests of titanium alloy Ti6Al4V and compared with two existing feature extraction techniques. The results indicate the efficiency of the time-frequency image features from dominant frequency bands for chatter detection and their better performance than the time domain features and wavelet-based features in terms of their separability capabilities.

Robust active control based milling chatter suppression with perturbation model via piezoelectric stack actuators

Abstract Milling, as the most widely used processing method, plays an important role in advanced manufacturing. However, milling chatter has severely restricted the development of advanced manufacturing industry. Therefore, it is of great theoretical significance and application value to study the active control of milling chatter. This paper focuses on the design of robust controller for chatter suppression. At first, the milling dynamic equations with active control force are established and simplified with the approximate linear time-invariant model for robust controller design. Then, the perturbation modeling of modal parameters and milling parameters are built and analyzed. Based on the perturbation models, the control algorithm is designed. The numerical simulation in milling process verifies the effectiveness of the designed controller. In order to validate the practical effect, the flank milling and end milling tests are implemented, respectively. The experimental results show that the designed algorithm is able to suppress chatter greatly and lead to the better workpiece surface, which prove the effectiveness and robustness of the designed controller.

On modelling of laser assisted machining: Forward and inverse problems for heat placement control

Abstract Laser assisted machining (LAM) is one of the most efficient ways to improve the machinability of difficult-to-cut materials (e.g. Nickel-based superalloys). In the conventional LAM process, the laser beam is focused ahead of the cutting area at a fixed location, which leads to a series of restrictions, e.g. small heating area and non-uniform heat distribution due to the limitation of beam size and energy distribution. In this paper, a novel spatially and temporally (S&T) controlled laser heating method was proposed, in which a large area can be heated up with a small laser spot by controlling the beam scanning, i.e., laser power, path and speed of scanning. The laser configuration for the prescribed HAZ (heat affected zone) was achieved by solving the inverse heat conduction problem where the laser power together with either laser path or laser speed were optimised to achieve a particular temperature distribution in the chip to be removed by the following milling cutter. The proposed S&T laser heating method was thoroughly validated both for the direct and, the more important, inverse heating models by performing extensive temperature experiments by both infrared thermal camera and thermocouple array and further verified by laser assisted milling (LAMill) tests of Inconel 718 for large widths of cuts. The results showed that by applying path-optimised LAMill based on the inverse solution of the thermal problem, the peak and mean principal cutting forces were reduced by 55% and 47.8% respectively compared with the conventional dry milling process while the surface roughness improved by at least 14%. Moreover, after controlling the HAZ using the inverse thermal problem, a microstructure analysis of the machined surface showed that the proposed laser heating method avoids overheating of the workpiece below the planned depth of cut for the milling operation.

Modelling of elliptical dimples generated by five-axis milling for surface texturing

Abstract Surface texturing processes that improve workpiece surface properties such as the friction of textured surfaces, the lubrication of sliding surfaces and the adhesion of workpiece surfaces are of increasing interest in industry. The most frequently employed processes for surface texturing are based on laser methods and on chemical etching processes. A widespread application of surface texturing is the improvement of tribological properties in terms of friction and load capacity. In these applications, surface texturing usually consists of generating dimples that are uniformly distributed on the workpiece surface. Five-axis milling with a ball-end mill can be an effective and productive option for these processes in these applications, as it is able to generate surface texturing that consists of periodic elliptical dimples if the tool geometry and the cutting conditions (feed, depth of cut and yaw and tilt angles defining tool axis orientation) are appropriately selected. In this paper, a model that predicts the geometry (shape, dimensions and orientation) of elliptical dimples machined by five-axis milling on flat surfaces for given cutting conditions is developed. In order to achieve this, equations expressing the trajectory followed by the cutting edges in five-axis milling are derived. The model takes into account the effect of tool parallel axis offset on dimple geometry. Next, the influence of cutting conditions on dimple geometry is analysed in order to define the cutting conditions that generate a given dimple geometry. From this analysis, a significant influence of the tool's tilt angle on the elliptical shape of the dimples and a linear dependence of the yaw angle on dimple orientation are observed. In order to mill elliptical-shaped dimples, tilt angles larger than 30° and feed, step over and depth of cut values that avoid interference between the dimples generated by different cutting edges of the ball-end mill are required. Finally, a series of five-axis milling tests was carried out in order to validate model predictions. A low dispersion in the dimensions and area of milled dimples was obtained. The shape and dimensions of predicted and measured dimples are compared and good correspondence between them is observed, the largest error being 7%.

Numerical and experimental optimizations of nozzle distance in minimum quantity lubrication (MQL) milling process

Minimum quantity lubrication (MQL) is the efficient and environmentally friendly technology, which is desirable to achieve sustainability during machining process. The nozzle distance has its significance in dominating the MQL spray and the related droplet transportation and penetration. In this paper, the nozzle distance has been optimized for MQL milling process through numerical and experimental methods. A two-way computational method has been employed to solve for the comprehensive flow field and particle trajectories, with the wall condition established on the spray impingement theory. The interactions of air flow rates and spindle rotational speeds on droplet penetration are investigated in details. The optimal ranges of nozzle distance setup obtained by simulation for different conditions were verified via slot milling tests, in which the nozzle distances were selected according to the key points obtained in the numerical process. The cutting force and surface roughness were recorded for the verification of adhesion ability and the related cutting performance. The comparison between numerical simulations and milling experiments has shown great consistency. This paper has achieved better understanding of the nozzle orientation setup and device development in MQL milling process, especially for external MQL. The theoretical basis and scientific instruction have been provided for the optimization of MQL operating parameters in industrial applications.

On-Line Energy-Based Milling Chatter Detection

Milling exhibits forced vibrations at tooth passing frequency and its harmonics, as well as chatter vibrations close to one of the natural modes. In addition, there are sidebands, which are spread at the multiples of tooth passing frequency above and below the chatter frequency, and make the robust chatter detection difficult. This paper presents a novel on-line chatter detection method by monitoring the vibration energy. Forced vibrations are removed from the measurements in discrete time domain using a Kalman filter. After removing all periodic components, the amplitude and frequency of chatter are searched in between the two consecutive tooth passing frequency harmonics using a nonlinear energy operator (NEO). When the energy of any chatter component grows relative to the energy of forced vibrations, the presence of chatter is detected. The proposed method works in discrete real time intervals, and can detect the chatter earlier than frequency domain-based methods, which rely on fast Fourier Transforms. The method has been experimentally validated in several milling tests using both microphone and accelerometer measurements, as well as using spindle speed and current signals.

On improving chatter stability of thin-wall milling by prestressing

Some long strip-like thin-walled workpieces such as blades usually need to be manufactured by milling with two ends being clamped on the machine table. This paper presents a stability improvement method in this kind of thin-wall milling by applying tensile prestress to the workpiece. The methodology is described by developing theories to establish the relationship between the needed tensile prestress and the expected critical axial depth of cut. Influence of the workpiece natural frequency on the critical axial depth of cut is theoretically explained for the first time. It is found that increasing the workpiece natural frequency can locally improve the stability in milling, and based on this conclusion, quantitative equations relating the expected critical axial depth of cut to the workpiece natural frequency are theoretically derived and solved in detail. With the aid of modal analysis from finite element simulation, problem on how much tensile prestress should be used to increase the workpiece natural frequency to the expected level is explained. In short, the stability lobe of a thin-wall milling system is shifted to the expected stable zone by manipulating tensile prestress to the clamping areas of long strip-like part. Besides, a prototype device, which has the capacity of fixing and prestressing the workpiece, is originally invented to check the effectiveness of the proposed methodology. The proposed method together with the invented prestressing device is experimentally validated by carrying out a series of milling tests with and without prestress.

Tribological behavior and cutting performance of monolayer, bilayer and multilayer diamond coated milling tools in machining of zirconia ceramics

In this work, a comparative study of diamond films consisting of alternate microcrystalline (MCD) and nanocrystalline diamond (NCD) layers is conducted. Diamond films including monolayer, bilayer and multilayer diamond films are coated on cemented tungsten carbide (WC-Co) substrates by adopting a bias-enhanced hot filament chemical vapor deposition (HFCVD) technique. Tribological properties of the diamond films are evaluated by using a reciprocal tribometer without lubrication. Further milling tests are carried out to examine the cutting performances with sintered zirconia ceramics as workpiece comparatively. In friction test against zirconia ceramics, the monolayer NCD film shows the lowest friction coefficient (0.128) because of its smooth surface. Also, the bilayer diamond film with surface coating of NCD layer (MNCD) and both of the multilayer diamond films exhibit good friction property while the monolayer MCD film has the largest friction coefficient (0.292). The milling test demonstrates that the monolayer diamond (MCD and NCD) coated milling tools show poor tool life due to abrasive action of hard workpiece. Working life of all the bilayer and multilayer diamond coated tools is enhanced, but large area shedding of coatings appears after some milling passes except the multilayer diamond film with surface coating of NCD layer (MNMN-CD). The multilayer film (MNMN-CD) presents superior machining performance and its working life increases by 3–7.5 times compared with the monolayer diamond coated ones.

Cutting parameter optimization in shoulder milling of commercially pure molybdenum

Due to its superior properties such as high melting point (2617 °C), high thermal conductivity and low thermal expansion at higher temperatures, molybdenum is a refractory metal and is used for making critical parts in defense, space, electronics and nuclear industries. High cost of molybdenum makes the selection of suitable cutting conditions imperative in machining operations in order to obtain the required surface quality and dimensional accuracy. In this study, suitable cutting tools and cutting parameters were aimed to be determined in terms of cutting forces (Fc) and average surface roughness (Ra) when shoulder milling commercially pure molybdenum. The milling tests were carried out at 0.05, 0.1, 0.15, 0.2 mm/tooth feed rates and 75, 100, 125, 150 m/min cutting speeds using four different cutting tools. The Taguchi’s experimental design (L16 orthogonal array) technique was implemented. Analysis of variance was used to determine the effects of cutting tools and cutting parameters on Ra and Fc. The results showed that the feed rate is the most influential parameter for Fc while the cutting speed is for Ra.

Prediction model to estimate the specific energy and product particle size distribution of semi-autogenous mill

The optimal use of energy for grinding of ores in mills is one of the key objectives of this study. An attempt has been made to present an experimental model for the specific energy and product particle size distribution of semi-autogenous mills for the simulating of real conditions of the particles collision. A novel semi-autogenous mill test has been suggested in order to measure the required energy for grinding. The effect of the mill’s operation time and various energy levels on the particle size distribution and feed ore hardness has been investigated with samples collected from the Sarcheshmeh Copper Complex, Kerman, Iran. The obtained results demonstrated that there is a high congruence between the experimental product particle size distribution and energy, which are predicted by the proposed novel method and ball-drop test. Using the proposed analytic relationships, the required comminution energy and semi-autogenous mill operating time to achieve the specific product particle size distribution can be estimated with high accuracy ( R2 = 0.96). Finally, the accuracy of the results was evaluated by a ball drop-weight test.

Mineralogy and texture of the Storforshei iron formation, and their effect on grindability

Investigating how ore mineralogy and texture affect the recovery from the processing plant is important for any mining operation. The results will assist in production planning and optimising the utilisation of a deposit. Easily available validated tests are desirable and useful.The Storforshei iron formation (IF) consists of several iron oxide deposits with mineralogical and textural differences. Although the Fe grades of the ores are similar, mineralogical and textural characteristics of the deposits affect the individual recoveries from the magnetic separation. For this paper three of the ore deposits were sampled, and important mineralogical and textural properties were investigated and tested. The investigations included geological mapping and optical microscopy, and the test work involved surface hardness measurements by Schmidt hammer and Equotip, and autogenous milling tests (i.e., grindability). The aim of the study was to investigate whether ore mineralogy and textures can be correlated to surface hardness measurements, and whether these three parameters can be used to evaluate grindability. The ores were classified into six ore types based on mineralogy and textures. The results show that the ore mineralogy and texture influence the surface hardness. Fine-grained ore types with irregular-to-no visible grain boundaries have higher surface hardness than coarser-grained ore types with straight grain boundaries. Furthermore, surface hardness measurements and grindability evaluations (using throughput (kg/h) and specific energy consumption (kWh/tonne)) of samples from three of the iron oxide deposits indicate that grindability decreases with increasing surface hardness. The relationship found between the parameters ore mineralogy, texture, surface hardness, and grindability suggests that geological mapping and surface hardness measurements can be used to evaluate grindability, and thus assess ore processing performance.

A novel triaxial optoelectronic based dynamometer for machining processes

A compact triaxial dynamometer for detecting the cutting forces in machining is developed. Force measurements are performed using photo-interrupters. Structural parameters of a monolithic flexural component are chosen through parametric analysis for coupling the exerted forces with the optical sensors. A prototype of the dynamometer is manufactured, and the calibration tests are conducted in three orthogonal directions to determine the linearity, hysteresis, repeatability and resolution. Force measurements are also compared with a reference dynamometer (Kistler 9256C1). Modal analysis and milling tests are performed to observe the dynamic properties and operability of the force sensor for machining applications. Results of the experimental studies validate that the proposed sensor is a feasible low-cost solution for force measurement in machining without compromising reliability and accuracy.

A mineralogical perspective on the recovery of platinum group elements from Merensky Reef and UG2 at the Two Rivers mine on the Eastern limb of the Bushveld Complex in South Africa

Published studies dealing with the process mineralogy of Pt mines on the Bushveld Complex is generally limited to the Western Bushveld. The recognition by mine management that another resource, in addition to the Upper Group 2 (UG2) reef currently being mined at the Two Rivers platinum mine (TRP), is urgently required in order to extend the life of mine, presented an opportunity to conduct such a study on the Eastern Limb of the Bushveld Complex. A process mineralogical investigation was undertaken on ore from the Merensky Reef (MR) and the UG2 at TRP. This was conducted on a suite of geological samples (channel samples) collected from the underground workings, as well as metallurgical samples obtained from the rougher circuits at the concentrator plant during the processing of MR and UG2 ore. The geological and metallurgical samples were analysed for bulk composition and quantitative mineralogy, while the geological samples were also subjected to laboratory-scale milling and flotation tests. This study shows that, although mineralogically distinct, the MR and UG2 behave similarly in terms of metallurgical performance. This holds promise for the proposed blending of MR and UG2 ores at TRP. An evaluation of the bulk rock (ore) Pt/Pd ratio as a possible indicator of the level of hydrothermal alteration of the ore, demonstrates that this may be of use in predicting recovery plant performance.