Showing papers in "International Journal of Machine Tools & Manufacture in 2000"

Error compensation in machine tools — a review: Part I: geometric, cutting-force induced and fixture-dependent errors

Abstract: Accuracy of machined components is one of the most critical considerations for any manufacturer. Many key factors like cutting tools and machining conditions, resolution of the machine tool, the type of workpiece etc., play an important role. However, once these are decided upon, the consistent performance of the machine tool depends upon its ability to accurately position the tool tip vis-a-vis the required workpiece dimension. This task is greatly constrained by errors either built into the machine or occurring on a periodic basis on account of temperature changes or variation in cutting forces. The three major types of error are geometric, thermal and cutting-force induced errors. Geometric errors make up the major part of the inaccuracy of a machine tool, the error caused by cutting forces depending on the type of tool and workpiece and the cutting conditions adopted. This part of the paper attempts to review the work done in analysing the various sources of geometric errors that are usually encountered on machine tools and the methods of elimination or compensation employed in these machines. A brief study of cutting-force induced errors and other errors is also made towards the end of this paper.

Sensor signals for tool-wear monitoring in metal cutting operations—a review of methods

Abstract: The state of a cutting tool is an important factor in any metal cutting process as additional costs in terms of scrapped components, machine tool breakage and unscheduled downtime result from worn tool usage. Several methods to develop monitoring devices for observing the wear levels on the cutting tool on-line while engaged in cutting have been attempted. This paper presents a review of some of the methods that have been employed in tool condition monitoring. Particular attention is paid to the manner in which sensor signals from the cutting process have been harnessed and used in the development of tool condition monitoring systems (TCMSs).

Error compensation in machine tools — a review: Part II: thermal errors

Abstract: One of the major errors in machine tools namely geometric/kinematic errors was discussed at length in Part I of this paper. Here, in Part II, another major source of inaccuracy, namely thermal error that occurs due to extended usage of the machine is analysed. Continuous usage of a machine tool causes heat generation at the moving elements and this heat causes expansion of the various structural elements of the machine tool. It is this expansion of the structural linkages of the machine that leads to inaccuracy in the positioning of the tool. Such errors are called thermal errors and constitute a significant portion of the total error in a machine tool. Thus the overall volumetric error of a machine tool is not only dependent on errors due to the assembly and the specific kinematic structure of the machine but also on the thermal errors. In Part II of this paper, an attempt is made to review the work carried out over the last decade in the estimation and compensation of temperature dependent errors.

Derivation of machine tool error models and error compensation procedure for three axes vertical machining center using rigid body kinematics

Abstract: Volumetric positional accuracy constitutes a large portion of the total machine tool error during machining. In order to improve machine tool accuracy cost-effectively, machine tool geometric errors as well as thermally induced errors have to be characterized and predicted for error compensation. This paper presents the development of kinematic error models accounting for geometric and thermal errors in the Vertical Machining Center (VMC). The machine tool investigated is a Cincinnati Milacron Sabre 750 3 axes CNC Vertical Machining Center with open architecture controller. Using Rigid Body Kinematics and small angle approximation of the errors, each slide of the three axes vertical machining center is modeled using homogeneous coordinate transformation. By synthesizing the machine's parametric errors such as linear positioning errors, roll, pitch and yaw etc., an expression for the volumetric errors in the multi-axis machine tool is developed. The developed mathematical model is used to calculate and predict the resultant error vector at the tool–workpiece interface for error compensation.

On-line metal cutting tool condition monitoring.: I: force and vibration analyses

Abstract: Excessive wear on cutting tools give rise to distortions in dimension of manufactured components, sometimes increasing scrapped levels thereby incurring additional costs. Methods for detecting and monitoring the wear on a cutting tool is therefore crucial in most metal cutting processes and several research efforts have striven to develop on-line tool condition monitoring systems. This paper describes an experimental and analytical method for one such technique involving the use of three mutually perpendicular components of the cutting forces (static and dynamic) and vibration signature measurements. The ensuing analyses in time and frequency domains showed some components of the measured signals to correlate well to the accrued tool wear.

Modeling micro-end-milling operations. Part I: analytical cutting force model

Abstract: A new analytical cutting force model is proposed for micro-end-milling operations. The model calculates the chip thickness by considering the trajectory of the tool tip while the tool rotates and moves ahead continuously. The proposed approach allows the calculation of the cutting forces to be done accurately in typical micro-end-milling operations with very aggressively selected feed per tooth to tool radius ( ft/r) ratio. The difference of the simulated cutting forces between the proposed and conventional models can be experienced when ft/r is larger than 0.1. The estimated cutting force profile of the proposed model had good agreement with the experimental data. © 2000 Elsevier Science Ltd. All rights reserved.

Non-conventional machining of particle reinforced metal matrix composite

Abstract: Particle Reinforced Metal Matrix Composites (PRMMC's) have proved to be extremely difficult to machine using conventional manufacturing processes due to heavy tool wear caused by the presence of the hard reinforcement. This paper presents details and results of an investigation into the machinability of SiC particle reinforced aluminium matrix composites using non-conventional machining processes such as Electro Discharge Machining (EDM), laser cutting and Abrasive Water Jet (AWJ). The surface integrity of the composite material for these different machining processes are examined and compared. The influence of the ceramic particle reinforcement on the machining process was analysed by tests performed on samples of the non-reinforced matrix material.

Parametric process optimization to improve the accuracy of rapid prototyped stereolithography parts

Abstract: The functional requirements of a rapid prototyping system are speed and accuracy, and they are both functions of vendor defaulted and user selected manufacturing parameters. Accuracy is evaluated by dimensional errors, form errors and surface roughness of manufactured parts. A specially designed specimen with 20 dimensional, geometrical, and surface roughness features has been used in the inspection of RP manufacturing processes. In terms of Taguchi experimental design techniques, an orthogonal array of experiments has been developed which has the least number of experimental runs and desired process parameter settings. Using a 3-D coordinate measuring machine and surface profilometer, a series of measurements in evaluating the SLA parts quality has been conducted to find the functional relationships between the output part quality and input manufacturing process parameters. Two analysis tools, response surface methodology and Analysis of Variance (ANOVA), have been used to evaluate the SLA RP process and to perform the product optimization. The optimal setups of SLA manufacturing parameters for both individual features and a general part with various features have been concluded from this study.

Determination of workpiece flow stress and friction at the chip-tool contact for high-speed cutting

Abstract: This paper presents a methodology to determine simultaneously (a) the flow stress at high deformation rates and temperatures that are encountered in the cutting zone, and (b) the friction at the chip–tool interface. This information is necessary to simulate high-speed machining using FEM based programs. A flow stress model based on process dependent parameters such as strain, strain-rate and temperature was used together with a friction model based on shear flow stress of the workpiece at the chip–tool interface. High-speed cutting experiments and process simulations were utilized to determine the unknown parameters in flow stress and friction models. This technique was applied to obtain flow stress for P20 mold steel at hardness of 30 HRC and friction data when using uncoated carbide tooling at high-speed cutting conditions. The average strain, strain-rates and temperatures were computed both in primary (shear plane) and secondary (chip–tool contact) deformation zones. The friction conditions in sticking and sliding regions at the chip–tool interface are estimated using Zorev's stress distribution model. The shear flow stress ( k chip ) was also determined using computed average strain, strain-rate, and temperatures in secondary deformation zone, while the friction coefficient ( μ ) was estimated by minimizing the difference between predicted and measured thrust forces. By matching the measured values of the cutting forces with the predicted results from FEM simulations, an expression for workpiece flow stress and the unknown friction parameters at the chip–tool contact were determined.

Process simulation using finite element method — prediction of cutting forces, tool stresses and temperatures in high-speed flat end milling

Abstract: End milling of die/mold steels is a highly demanding operation because of the temperatures and stresses generated on the cutting tool due to high workpiece hardness. Modeling and simulation of cutting processes have the potential for improving cutting tool designs and selecting optimum conditions, especially in advanced applications such as high-speed milling. The main objective of this study was to develop a methodology for simulating the cutting process in flat end milling operation and predicting chip flow, cutting forces, tool stresses and temperatures using finite element analysis (FEA). As an application, machining of P-20 mold steel at 30 HRC hardness using uncoated carbide tooling was investigated. Using the commercially available software DEFORM-2D™, previously developed flow stress data of the workpiece material and friction at the chip–tool contact at high deformation rates and temperatures were used. A modular representation of undeformed chip geometry was used by utilizing plane strain and axisymmetric workpiece deformation models in order to predict chip formation at the primary and secondary cutting edges of the flat end milling insert. Dry machining experiments for slot milling were conducted using single insert flat end mills with a straight cutting edge (i.e. null helix angle). Comparisons of predicted cutting forces with the measured forces showed reasonable agreement and indicate that the tool stresses and temperatures are also predicted with acceptable accuracy. The highest tool temperatures were predicted at the primary cutting edge of the flat end mill insert regardless of cutting conditions. These temperatures increase wear development at the primary cutting edge. However, the highest tool stresses were predicted at the secondary (around corner radius) cutting edge.

Experimental investigations into abrasive flow machining (AFM)

Abstract: A new non-traditional finishing process known as abrasive flow machining (AFM) is used to deburr, radius, polish and remove recast layer of components in a wide range of applications. The process is relatively new, although around 2000 machines are in use worldwide. Material is removed from the workpiece by flowing a semisolid visco-elastic/visco-plastic abrasive-laden medium across the surface to be finished. Areas inaccessible to traditional methods, and complex passages, can be finished to high quality by this process. The process embraces a wide range of feasible applications including aerospace, dies and moulds, automotive parts, medical components, etc. In the present work, the effects of different process parameters, such as number of cycles, concentration of abrasive, abrasive mesh size and media flow speed, on material removal and surface finish are studied. The dominant process parameter found is concentration of abrasive, followed by abrasive mesh size, number of cycles, and media flow speed. Experiments are performed with brass and aluminum as work materials. Experimental and theoretical results are compared. The machined surface texture is studied using scanning electron microscopy.

Nongeometric error identification and compensation for robotic system by inverse calibration

Abstract: In order to achieve the stringent accuracy requirement of some robotic applications such as robotic measurement systems, it is critical to compensate for nongeometric errors such as compliance errors and thermal errors in addition to geometric errors. This paper investigates the effect of geometric errors, link compliance and temperature variation on robot positioning accuracy. A comprehensive error model is derived for combining geometric errors, position–dependent compliance errors and time–variant thermal errors. A general methodology is developed to identify these errors simultaneously. A laser tracker is applied to calibrate these errors by an inverse calibration method. Robot geometric errors and compliance errors are calibrated at room temperature while robot parameter thermal errors are calibrated at different temperatures when the robot warms up and cools down. Empirical thermal error models are established using orthogonal regression methods to correlate robot parameter thermal errors with the corresponding temperature field. These models can be built into the controller and used to compensate for quasi-static thermal errors due to internal and external heat sources. Experimental results show that the robot accuracy is improved by an order of magnitude after calibration.

Bearing induced vibration in precision high speed routing spindles

Abstract: This paper presents a detailed model of bearing vibration, including the effect of contact spring non-linearity in balls-to-raceways' contacts. The model incorporates the effect of surface waviness of rolling elements and off-sized balls upon the dynamic internal radial clearance of the bearing. The vibration forces and moments generated are formulated and the significant principal and secondary side-band contributions are highlighted. This model is employed successfully in the recognition of complex real-time vibration spectra of a precision routing spindle, obtained by accurate non-contact sensors.

The effects of cutting fluid application methods on the grinding process

Abstract: It is well known that a boundary layer of air is entrained around a rotating grinding wheel. The effects of the boundary layer have been under some scrutiny in recent years with most research being based on trying to overcome the boundary layer. The current investigation aims to show through experiment and modelling, the effects of the boundary layer on cutting fluid application and how it can be used to aid delivery by increasing flow rate beneath the wheel. Results from three experiments with different quantities of cutting fluid passing through the grinding zone are presented.

An Investigation into Roller Burnishing

Abstract: Burnishing, a plastic deformation process, is becoming more popular as a finishing process: thus, how to select the burnishing parameters to reduce the surface roughness and to increase the surface microhardness is especially crucial This paper reports the results of an experimental program to study the influence of different burnishing conditions on both surface microhardness and roughness: namely, burnishing speed, force, feed, and number of passes Also, it reports the relationship between residual stress and both burnishing speed and force The residual stress distribution in the surface region that is orthogonally burnished is determined using a deflection etching technique Mathematical models are presented for predicting the surface microhardness and roughness of St-37 caused by roller burnishing under lubricated conditions Variance analysis is conducted to determine the prominent parameters and the adequacy of the models From an initial roughness of about Ra 45 μm, the specimen could be finished to a roughness of 05 μm It is shown that the spindle speed, burnishing force, burnishing feed and number of passes have the most significant effect on both surface microhardness and surface roughness and there are many interactions between these parameters The maximum residual stress changes from tensile to compressive with an increase in burnishing force from 5 to 25 kgf With a further increase in burnishing force from 25 to 45 kgf, the maximum residual stress increases in compression

Modeling micro-end-milling operations. Part II: tool run-out

Abstract: The effect of run-out is clearly noticed in micro-end-milling operations, while the same run-out creates negligible change at the cutting force profile of conventional end-milling operations. In this paper, the cutting force characteristics of micro-end-milling operations with tool run-out are investigated. An analytical cutting force model is developed for micro-end-milling operations with tool run-out. The proposed model has a compact set of expressions to be able to estimate the cutting force characteristics very quickly compared to the numerical approaches. The cutting forces of micro-end-milling operations simulated by the proposed model had good agreement with the experimental data.

A new method of optimising material removal rate using EDM with copper–tungsten electrodes

Abstract: Electrical discharge machining (EDM) is widely used in the production of dies. This paper describes an investigation into the optimisation of the process which uses the effect of carbon which has migrated from the dielectric to tungsten–copper electrodes. This work has led to the development of a two-stage EDM machining process where different EDM settings are used for the two stages of the process giving a significantly improved material removal rate for a given tool wear ratio.

Cutting forces and surface finish when machining medium hardness steel using CBN tools

Abstract: Cutting forces generated using CBN tools have been evaluated when cutting steel being hardened to 45–55 HRC. Radial thrust cutting force was the largest among the three cutting force components and was most sensitive to the changes of cutting edge geometry and tool wear. The surface finish produced by CBN tools was compatible with the results of grinding and was affected by cutting speed, tool wear and plastic behaviour of the workpiece material.

Machining fixture layout optimization using the genetic algorithm

Abstract: Dimensional and form accuracy of a workpiece are influenced by the fixture layout selected for the machining operation. Hence, optimization of fixture layout is a critical aspect of machining fixture design. This paper presents a fixture layout optimization technique that uses the genetic algorithm (GA) to find the fixture layout that minimizes the deformation of the machined surface due to clamping and machining forces over the entire tool path. The advantages of the GA-based method over previously reported non-linear programming methods for fixture layout optimization are discussed. Two GA-based fixture layout optimization approaches are implemented and compared by applying them to several two-dimensional example problems.

Tool wear estimation in micro-machining.: Part I: tool usage–cutting force relationship

Abstract: The relationship between the cutting force characteristics and tool usage (wear) in a micro-end-milling operation was studied for two different metals. Neural-network-based usage estimation methods are proposed that use force-variation- and segmental-averaging-based encoding techniques.

Cutting force prediction of sculptured surface ball-end milling using Z-map

Abstract: The cutting force in ball-end milling of sculptured surfaces is calculated. In sculptured surface machining, a simple method to determine the cutter contact area is necessary since cutting geometry is complicated and cutter contact area changes continuously. In this study, the cutter contact area is determined from the Z-map of the surface geometry and current cutter location. To determine cutting edge element engagement, the cutting edge elements are projected onto the cutter plane normal to the Z-axis and compared with the cutter contact area obtained from the Z-map. Cutting forces acting on the engaged cutting edge elements are calculated using an empirical method. Empirical cutting mechanism parameters are set as functions of cutting edge element position angle in order to consider the cutting action variation along the cutting edge. The relationship between undeformed chip geometry and the cutter feed inclination angle is also analyzed. The resultant cutting force is calculated by numerical integration of cutting forces acting on the engaged cutting edge elements. A series of experiments were performed to verify the proposed cutting force estimation model. It is shown that the proposed method predicts cutting force effectively for any geometry including sculptured surfaces with cusp marks and a hole.

A theoretical and experimental investigation of surface roughness formation in ultra-precision diamond turning

Abstract: In this paper, a model-based simulation system is presented for the analysis of surface roughness generation in ultra-precision diamond turning. The system is based on a surface roughness model which takes into account the effect of tool geometry, process parameters and relative tool-work vibration. It is evaluated through a series of cutting experiments. The results indicate that the system can predict well the surface roughness profile and the roughness parameters of a diamond turned surface under various cutting conditions. With the use of the spectrum analysis techniques, the system can also help to analyze the effect of vibration on the surface quality of workpiece and to diagnose the machine faults. The potential application of the system in process optimization is also discussed in the text.

Modeling, measurement and error compensation of multi-axis machine tools. Part I: theory

Abstract: Multi-axis machine tools are important units in modern production systems. The complex structures of the machine tools produce an inaccuracy at the tool tip caused by kinematics parameter deviation resulting from manufacturing errors, assembly errors or quasi-static errors. To find out the error origins of these machine tools it is always necessary to have good mathematical models of the machine tools. These models can be used for measuring and analyzing the measured data. The diagnosed errors could be taken under consideration only by the precise description of the actual kinematics of the machine tools. By counting the new axes values by numerical iteration technique of the defective axes from the cutter location data (CL-data) or from the ideal NC program it may be possible to position the tool tip to the desired position. This paper describes two ways to modify NC programs (implemented in a postprocessor for CL-data or by extra NC program processor for ideal NC program) including modeling and measurement of real machine tools. Practice and application will be described in part two.

Selection of optimal conditions in multi-pass face-milling using a genetic algorithm

Abstract: In a Computer-Aided Process Planning (CAPP) system, one of the important steps is the selection of machining parameters which yield optimum results. In this paper, a face-milling operation has been considered. The machining parameters such as number of passes, depth of cut in each pass, speed and feed are obtained using a genetic algorithm, to yield minimum total production cost while considering technological constraints such as allowable speed and feed, dimensional accuracy, surface finish, tool wear and machine tool capabilities.

On-line metal cutting tool condition monitoring.: II: tool-state classification using multi-layer perceptron neural networks

Abstract: This paper outlines a neural networks based modular tool condition monitoring system for cutting tool-state classification. Test cuts were conducted on EN24 alloy steel using P15 and P25 coated cemented carbide inserts and on-line cutting forces and vibration data acquired. Simultaneously the wear lengths on the cutting edges were measured, and these together with the processed data were fed to a neural network trained to distinguish tool-state. The first part of the investigation concentrated on tool-state classification using a single wear indicator and progressing to two wear indicators. The developed system was tested for a variety of cutting conditions and its ability to distinguish changes in tooling material and cutting conditions from those arising from tool wear was assessed. The system was found to be capable of accurate tool state classification in excess of 90% accuracy but deteriorated when the cutting conditions were significantly changed.

Suppression of chatter vibration of boring tools using impact dampers

Abstract: In this study, improvement of the damping capability of boring tools and suppression of chatter vibration were attempted using impact dampers. Bending tests, impact tests and cutting tests were carried out, whilst widely varying the method of applying an impact damper to a boring tool. The effects of the amount of free mass and clearance, the overhang length of boring tools and their cutting condition were investigated. As a result, the following points were clarified. (i) The damping capability of boring tools is considerably improved using impact dampers. (ii) All three types of impact dampers used in the experiment can considerably suppress the vibration of boring tools in the vertical direction (principal force direction), but hardly suppress it in the horizontal direction (thrust force direction) where the amplitude is extremely small. (iii) In practical use, the method of equipping an impact damper on the flank face of a boring tool is desirable. (iv) Using an impact damper, it is possible to bore deeper holes in comparison with boring tools now on the market and to improve the efficiency of boring operations.

Machining parameters for an intelligent machining system for composite laminates

Abstract: Composite laminates exhibit very high in-plane strengths but are plagued by delamination damage when subjected to machining. This is due to their poor transverse strengths and low delamination fracture toughness. In drilling, delamination is initiated when the thrust force exceeds a threshold value, particularly at the critical entry and exit locations of the drill bit. To minimize damage, therefore, it is important to monitor process variables such as the machining forces and the position of the tool relative to the workpiece. The availability of a suitable model coupled with an intelligent control scheme would be a large advancement in the machining of composite laminates. This paper explores the development of such models for machining of composites and for coupling the models to intelligent control strategies. Using a machining center, a series of drilling experiments were conducted on carbon fiber-reinforced composite laminates to determine key process parameters for various cutting conditions. An intelligent machining scheme is proposed as the basis for the design of a new machine tool.

Calibration of a hexapod machine tool using a redundant leg

Abstract: Parallel configurations are recently being applied to the machine tool with the hopes of greater rigidity, stability, and accuracy than conventional multi-axis structures allow. However, the many calibration methods presently available for serial machine tools are not applicable to hexapod type structures. A calibration method is presented that uses a ball–bar or other simple length measuring device to act as an ‘extra leg,’ allowing calibration of the hexapod's true kinematic parameters. This method utilizes a total least squares minimization, does not require any special hexapod configuration or difficult six degree of freedom pose measurements, and is effective with as few as one additional length sensor. Selection of calibration pose sets is briefly discussed, as well as the influence of measurement noise on calibration accuracy. Simulations show the potential for this algorithm to significantly reduce errors to the point where machining errors are within 5–10 times the measurement errors.

Kinematic and geometric error compensation of a coordinate measuring machine

Abstract: In this paper, kinematic modelling of a Coordinate Measuring Machine (CMM) is carried out and the methodology followed in modelling is explained in detail. The model is simplified by certain assumptions which may result in over-simplification of the model. Consequently, the model is investigated and enhanced by adding the relevant and suitable geometric error terms. Different approaches are employed to evaluate the model coefficients. In the first approach, a commercial ring gauge is measured in a structured lattice in the work volume of the CMM. Resulting errors in these measurements are used in conjunction with some statistical methods to arrive at sets of model coefficients values. The second approach is based on measurement of the individual 21 error terms in the CMM by means of laser interferometry. These measurements are used to evaluate another set of model coefficients. A compensation strategy is proposed and tested using the model and the sets of coefficients obtained. Volumetric Performance of the CMM is evaluated according to ASME standards, before and after compensation. Improvement in the CMM volumetric performance is demonstrated and compared.

The use of FEA and design of experiments to establish design guidelines for simple hydroformed parts

Abstract: We present models to predict the protrusion height of “Tee-shaped” hydroformed parts, both because this information is of direct relevance to engineers attempting to build such parts and also to illustrate an advantageous process for developing design guidelines for tube hydroforming (THF) in general. A newly proposed design of experiments technique, Low Cost Response Surface Method (LCRSM), was utilized to facilitate the economical prediction and optimization of this height as a function of geometrical parameters subject to thinning of the wall thickness at the protrusion region. The same methodology is also proposed for the economical investigation of other geometries and conditions. As a result of this investigation, not only were known and expected trends of effect of parameters verified, but also numerical values within a practical range of parameters at certain conditions were obtained. In addition, interactions between factors were also revealed as predicted. Moreover, this information was gained from a substantially reduced number of finite element analysis (FEA) simulations via LCRSM compared to standard response surface method (RSM) or factorial techniques, avoiding costly physical experimentation.