CNC Machining Center Research Articles

Effects of milling condition on the surface integrity of hot forged steel

This paper presents a study on the influence of milling condition on workpiece surface integrity focusing on hardness and roughness. The experimental work was carried out on a CNC machining center considering roughing and finishing operations. A 25 mm diameter endmill with two cemented carbide inserts coated with TiN layer were used for end milling operation. Low carbon alloyed steel Cr-Mo forged at 1200 oC was used as workpiece on the tests. Two kinds of workpiece conditions were considered, i.e. air cooled after hot forging and normalized at 950 oC for 2 h. The results showed that finishing operation was able to significantly decrease the roughness by at least 46% without changing the hardness. On the other hand, roughing operation caused an increase in hardness statistically significant by about 6%. The machined surface presented deformed regions within feed marks, which directly affected the roughness. Surface finish behavior seems to correlate to the chip ratio given the decrease of 25% for roughing condition, which damaged the chip formation. The material removal rate for finishing operation 41% greater than roughing condition demonstrated to be favorable to the heat dissipation and minimized the effect on material hardness.

EXPERIMENTAL STUDY AND LOGISTIC REGRESSION MODELING FOR MACHINE CONDITION MONITORING THROUGH MICROCONTROLLER-BASED DATA ACQUISITION SYSTEM

Machine condition monitoring plays an important role in machining performance. A machine condition monitoring system will provide significant economic benefits when applied to machine tools and machining processes. By applying Taguchi design method, real-time pilot experimental study was conducted on a CNC machining center for monitoring the end mill cutting operations through the vibration data collection via a microcontroller-based data acquisition system. Featured machining signals were identified through data analyses and regression models were established that incorporates different combinations of featured machining signals and machining parameters in using logistic regression modeling approach. The onsite tests show that the developed logistic models including the featured machining signals can correctly distinguish worn and new cutting tools. Therefore, they can help construct decision-making mechanism for machine condition monitoring.

Design and static performance of high speed machining centre with “direct drive”

“Direct drive” is an ideal method for speeding machine tools. In the structure of a CNC machining centre with “direct drive”, the linear motor’s primary and secondary parts are assembled into the worktable and machine bed respectively to directly drive the worktable. The built-in rotary motor is assembled in the spindle to realize the main transmission system of the machine tool “direct drive”. All mechanical transmission elements in machine tools are eliminated. High speed, efficiency, and accuracy are easily obtained. However, in this type of “direct drive” machining centre, the magnetic attraction force between the primary and the secondary linear motors and the dynamic impact at acceleration and deceleration are directly imposed on the machine tool, and influence the performance of the machining centre. This paper analyzes the special demands of “direct drive” on the machine centre, and introduces a new structure of the machining tool. The worktable and machine bed are optimized to meet the special demands of “direct drive”. Static performance simulation on the machining centre is done to reveal an ideal result.

Optimization in MAF operations using Taguchi parameter design for AISI304 stainless steel

The magnetic pole is an important finishing tool in magnetic abrasive finishing (MAF). This study used finite element method to analyze magnetic field characteristics for three different magnetic poles such as solid cylindrical pole, hollow cylindrical pole, and hollow cylindrical pole with grooves design. The results showed that the hollow cylindrical with grooves can generate the better surface roughness in MAF. The operations were demonstrated using a permanent magnetic polishing mechanism installed at a CNC machining center. The operations were performed using Taguchi experimental design, considering the effects of magnetic field, pole rotational speed, feed rate, working gap, abrasive, and lubrication. The optimal parameter conditions was obtained after experimental data analysis, the quality surface roughness (R max = 0.1 mm) which is similar to a mirror surface was obtained after confirmatory tests. The optimal parameter conditions for material removal weight were also obtained in MAF. The results showed that MAF technique can meet customer requirement and raise the value-added products simultaneously.

Machining Accuracy Improvement by Automatic Tool Setting and On Machine Verification

This paper proposes a methodology for improving the machining accuracy based on auto tool setting & work-piece measuring on the machine using laser tool setting system & inspection probe. In this study, laser tool setting systems were analyzed as considering principles, convenience and efficiency and auto tool setting method and operating macro software were developed. As compared with conventional manual tool setting and touch-type auto tool setting, the importance of automatic non-contact tool setting using laser tool setter has been discussed. Also correct tool-setting methods in accordance with different tool shapes and sizes were defined and pocket features were machined by each of setting tools and measured by the inspection probe system to verify machining accuracy. Lastly we tried to demonstrate the superiority of laser tool setting systems by analyzing the cutting results when the CNC machining center was fitted with laser tool setting system.

Precision Spindle Metrology

Professor Marsh is at the Penn State University Mechanical Engineering faculty working in the field of precision engineering.His book Precision Spindle Metrology (161 pages) starts with a case study of a CNC lathe demonstrating effects of spindle error motions to a turned workpiece. This is followed by a brief history of spindle testing, starting with Schlesinger and Tlusty and ending with ASME B89.3.4 standard Axis of Rotation which is the basis for definitions and expressions used in the book.Therefore the book opens the world of axial, radial, tilt and face error motion (instead of runout), influence of the structural loop, fixed and rotating sensitive direction, average and asynchronous error motion, and thermal drift. Excellent figures help in understanding these error motions and differentiations as well as their effects on a workpiece.Two chapters are dedicated to testing spindles and data analysis, which is mainly based on Lion Precision's spindle tester (Lion holds the copyright of the book together with Precision Instruments). Environmental effects, aliasing effects, analogue-to-digital conversion and filtering are clearly explained, again supported with instructive figures.As precision spindles are targeted in this book, reversal techniques (Donaldson reversal, face error motion reversal, multi-probe error separation, multi-step error separation, Grejda reversal, master axis reversal) are described and explained in detail in order to separate spindle errors from artifact errors in the sub-micrometer range.Case studies show the application of all the theory on a surface grinder, a CNC turning centre, a CNC machining centre, and a CNC diamond turning machine. The last chapter applies the theory on tapered roller bearing spindles, giving information on instrumentation for rolling element bearings, and the repeatability of such bearings and of ball bearings. Furthermore air bearing stiffness and pressure, and gas compositions and asynchronous error are discussed.157 references, starting in 1873 (Maxwell) and ending in 2006, covering axis of rotation measurement, roundness measurement, error separation technique, influence of temperature, measurement uncertainty, and spindle design, give a good overview of work presented in this field.Designers of precision spindles and people testing spindles, who want to apply the concept of spindle error motion which is also overtaken by ISO 230-7:2006, Test code for machine tools—Part 7: Accuracy of axes of rotation, will be supported by this book. The book also shows how to move measurement uncertainty to new levels by applying error separation techniques correctly, and gives insight into design of precision spindles.

A hybrid fuzzy MCDM approach to machine tool selection

The selection of the appropriate machine tools for a manufacturing company is one of the important points to achieving high competitiveness in the market. Besides, an appropriate choice of machine tools is very important as it helps to realize full production quickly. Today’s market offers many more choices for machine tool alternatives. There are also many factors one should consider as part of the appropriate machine tool selection process, including productivity, flexibility, compatibility, safety, cost, etc. Consequently evaluation procedures involve several objectives and it is often necessary to compromise among possibly conflicting tangible and intangible factors. For these reasons, multiple criteria decision making (MCDM) has been found to be a useful approach to solve this kind of problem. Most of the MCDM models are basically mathematical and ignore qualitative and often subjective considerations. The use of fuzzy set theory allows incorporating qualitative and partially known information into the decision model. This paper describes a fuzzy technique for order preference by similarity to ideal solution (TOPSIS) based methodology for evaluation and selection of vertical CNC machining centers for a manufacturing company in Istanbul, Turkey. The criteria weights are calculated by using the fuzzy AHP (analytical hierarchy process).

Scheduling of unrelated parallel manufacturing cells with limited human resources

In this paper the scheduling of batches of parts within a manufacturing system consisting of M unrelated parallel CNC manufacturing cells and employing a limited workforce capacity is investigated: the objective is to maintain the number R of available workers lower than the number M of the cells leaving unchanged the whole system output. Each cell of the system consists of a CNC machining centre which works batches of small mechanical parts, grouped together on a common pallet in accordance to the group technology principles. Batch splitting among CNC cells is not allowed and the setup time required by each batch is both sequence and cell dependent. The setup activities are fully performed by the workers employed within the manufacturing system; the working operations within the CNC cells are completely automated, thus the workers tasks are restricted only to the setup operations. The effect of reducing the human resource capacity on scheduling of parts within cells, and on several process performance measures, is investigated through an extensive design of experiment involving different process scenarios. The process activities and resources dynamics are simulated and optimized by means of an integrated simulation framework, consisting of an object-oriented simulator interfaced with a heuristic procedure, designed within the eM-Plant™ software environment. The obtained results show how the human resource can be maintained significantly lower than the number of cells without affecting the output of the manufacturing system.

Ultra-precision surface finish of NAK80 mould tool steel using sequential ball burnishing and ball polishing processes

Abstract The sequential ball burnishing and ball polishing processes have been applied successfully to the ultra-precision surface finish of the NAK80 mould tool steel on a CNC machining centre. The optimal flat surface polishing parameters of the developed system for the NAK80 mould tool steel have been determined by conducting the experiments of the Taguchi's L 18 orthogonal table, analysis of variation (ANOVA), and the full factorial experiments. The burnished surface roughness of the test specimens can be improved from about R a 0.06 μm to 0.016 μm in average using the determined optimal polishing parameters. Applying the optimal flat surface ball burnishing and ball polishing parameters sequentially to a fine milled spherical lens mould cavity of NAK80 mould tool steel, the surface roughness R a of the surface on the tested parts can be improved sequentially from about 1.0 μm in average to 0.020 μm in average.

A method for evaluating spindle rotation errors of machine tools using a laser interferometer

This paper presents a method for assessing radial and axial error motions of spindles. It uses the Hewlett Packard 5529A laser interferometer. The measurement is made using reflection directly from a high-precision sphere. Such object is used as the optical reflector. The sphere is affixed at the end of a wobble device, which is clamped in the spindle. The principle of measurement is similar to that of a linear interferometer, except that the high-precision sphere is used in place of the usual retroreflector. A convergent lens is utilized to focus the laser beam to a small spot on the sphere surface. This minimizes the dispersion of the beam due to the reflection on the spherical surface. A software package has been developed for data acquisition and presentation of the error motion polar plots of the spindle. Application of this spindle error calibrator on a CNC machining centre is undertaken. The results are presented and discussed.

Development of computer-aided maintenance resources planning (CAMRP): A case of multiple CNC machining centers

Total productive maintenance (TPM), total maintenance assurance, preventive maintenance, reliability-centered maintenance (RCM), and many other innovative approaches to maintenance problems all aim at enhancing the effectiveness of machines to ultimately improve productivity. Each of these concepts demands a unique decision support system for maintenance resources planning, and implementing each of them requires a radical restructuring of work. Introducing computer-aided maintenance resources planning (CAMRP) system is a major challenge because the maintenance operations environment is usually traditional and unfavorable to change. This paper presents a computer-aided planning system for a maintenance business unit that serves a number of manufacturing facilities, each consisting of a set of high-precision CNC machining centers. The workforce in the maintenance business unit is responsible for preventive as well as corrective maintenance activities of all CNC machines within the different manufacturing sites. The newly developed decision support tool embodies a set of structured heuristics methods for the coordination of tasks among maintenance crews which will help conduct different maintenance activities of manufacturing units in a more synchronized way. Through what-if-analyses, the tool generates well-founded decisions for capacity planning are; synchronizes maintenance activities to predict maintenance start and expected finishing times, thereby leads to lower costs.

Effect of drilling parameters on mechanical strength in drilling glass fibre reinforced plastic

The damage generated during the drilling of Glass Fibre Reinforced Plastics (GFRP) is detrimental for the mechanical behaviour of the composite structure. This work is focused on analysing the influence of drilling parameters (spindle speed and feed) on the strength of the GFR woven fabric laminates and further to study the residual stress distribution around the hole after drilling. Holes were drilled at the centre of the specimens in a CNC machining centre using 6 mm diameter micrograin carbide drill for various spindle speeds (1000 4000 rpm) and feed rates (0.02, 0.06, 0.10 and 0.20 rev/min). Degree of damage depends on the feed rate and spindle speed. Experimental results indicate that failure strength and stress concentration are related to the drilling parameters and a drilling parameter (3000 rpm and 0.02 mm/rev), which gives better mechanical strength.

Study of magnetic abrasive finishing in free-form surface operations using the Taguchi method

This study employed magnetic abrasive finishing (MAF) to conduct free-form surface abrasion of stainless SUS304 material operations. The operations were demonstrated using a permanent magnetic finishing mechanism installed at the CNC machining center. The operations were performed using the Taguchi experimental design, considering the effects of magnetic field, spindle revolution, feed rate, working gap, abrasive, and lubricant. Furthermore, the experimental data was collected using the Taguchi experimental design. The optimal parametric conditions for processing stainless SUS304 material were applied in a two-stage process comprised of rough finishing that involved MAF followed by a precise finishing of the surface. Prior to rough finishing, the Rmax value was 2.670 μm; after rough finishing, the value was 0.158 μm. Precise finishing yields an even lower value of 0.102 μm similar to that of the mirror surface. Therefore, the results revealed that MAF provides a highly efficient way of obtaining surface finish.

Integrated machining error compensation method using OMM data and modified PNN algorithm

This paper presents an integrated machining error compensation method based on polynomial neural network (PNN) approach and inspection database of on-machine-measurement (OMM) system. To improve the accuracy of the OMM system, geometric errors of the CNC machining center and probing errors are compensated. Machining error distributions of a specimen workpiece are measured to obtain error compensation parameters. To efficiently analyze the machining errors, two machining error parameters, W err and D err , are defined. Subsequently, these parameters can be modeled using the PNN approach, which is used to determine machining errors for the considered cutting conditions. Consequently, by using an iterative algorithm, tool path can be corrected to effectively reduce machining errors in the end-milling process. Required programs are developed using Ch language, and modified termination method are applied to reduce computation times. Experiments are carried out to validate the approaches proposed in this paper. The proposed integrated machining error compensation method can be effectively implemented in a real machining situation, producing much fewer errors.

Testing the diagonal measuring technique

Abstract This paper describes the results of a set of linear displacement accuracy measurements performed on two vertical CNC machining centers. The main scope of the work is to verify or disprove some of the recently claimed limitations of the conventional diagonal measurement method and of the “laser vector” or “sequential diagonal” method. Basically, we tested the effect of a large linear error deliberately introduced into one of the machine tool's axes. It is concluded that the laser vector method has not correctly identified this error and distributed the error into the remaining axes of the machine tool.

Grinding force control in an automatic surface finishing system

An automatic grinding system with grinding force control on a CNC machining center is developed to reduce surface roughness. This system includes an electric hand grinder mounted on a CNC machining center, a force sensor to measure the normal grinding force, and a force control sub-system to adjust the grinding depth. The CNC spindle vertical movement is produced by “external machine zero point shift”. The experiments performed in this study include: (1) Taguchi's parametric design to determine an ideal combination of the horizontal feed rate and the desired force value during force control and (2) grinding along an inclined surface. The results show that the force control technique may indeed reduce the average grinding force and grinding force variation.

Automated surface finishing of plastic injection mold steel with spherical grinding and ball burnishing processes

This study investigates the possibilities of automated spherical grinding and ball burnishing surface finishing processes in a freeform surface plastic injection mold steel PDS5 on a CNC machining center. The design and manufacture of a grinding tool holder has been accomplished in this study. The optimal surface grinding parameters were determined using Taguchi’s orthogonal array method for plastic injection molding steel PDS5 on a machining center. The optimal surface grinding parameters for the plastic injection mold steel PDS5 were the combination of an abrasive material of PA Al2O3, a grinding speed of 18000 rpm, a grinding depth of 20 μm, and a feed of 50 mm/min. The surface roughness Ra of the specimen can be improved from about 1.60 μm to 0.35 μm by using the optimal parameters for surface grinding. Surface roughness Ra can be further improved from about 0.343 μm to 0.06 μm by using the ball burnishing process with the optimal burnishing parameters. Applying the optimal surface grinding and burnishing parameters sequentially to a fine-milled freeform surface mold insert, the surface roughness Ra of freeform surface region on the tested part can be improved from about 2.15 μm to 0.07 μm.

Modelling the machining dynamics of peripheral milling

The machining dynamics involves the dynamic cutting forces, the structural modal analysis of a cutting system, the vibrations of the cutter and workpiece, and their correlation. This paper presents a new approach modelling and predicting the machining dynamics for peripheral milling. First, a machining dynamics model is developed based on the regenerative vibrations of the cutter and workpiece excited by the dynamic cutting forces, which are mathematically modelled and experimentally verified by the authors [Liu, X., Cheng, K., Webb, D., Luo, X.-C. Improved dynamic cutting force model in peripheral milling—Part 1: Theoretical model and simulation. Int. J. Adv Manufact Tech, 2002, 20, 631–638; Liu, X., Cheng, K., Webb, D., Longstaff, A. P., Widiyarto, H. M., Jiang, X.-Q., Blunt, L., Ford, D. Improved dynamic cutting force model in peripheral milling—Part 2: Experimental verification and prediction. Int. J. Adv Manufact Tech, 2004, 24, 794–805]. Then, the mechanism of surface generation is analysed and formulated based on the geometry and kinematics of the cutter. Thereafter a simulation model of the machining dynamics is implemented using Simulink. In order to verify the effectiveness of the approach, the transfer functions of a typical cutting system in a vertical CNC machine centre were measured in both normal and feed directions by an instrumented hammer and accelerometers. Then a set of well-designed cutting trials was carried out to record and analyse the dynamic cutting forces, the vibrations of the spindle head and workpiece, and the surface roughness and waviness. Corresponding simulations of the machining processes of these cutting trials based on the machining dynamics model are investigated and the simulation results are analysed and compared to the measurements. It is shown that the proposed machining dynamics model can well predict the dynamic cutting forces, the vibrations of the cutter and workpiece. There is a reasonable agreement between the measured and predicted roughness/waviness of the machined surface. Therefore the proposed approach is proven to be a feasible and practical approach analysing machining dynamics and surface roughness/waviness for shop floor applications.

Automation of fixture design using feature based modelling

Fixturing is an important manufacturing activity, but there are very few hard and fast engineering principles involved in the fixture design process. Even the available limited analytical rules are not so flexible enough for the correct fixture design. Further more, there is no exact definition as to what constitutes a good fixture design. A skilled tool designer learns his trade through apprenticeship training and many years of experience. With the current decline in the number of skilled machinists, tool and die workers, there is a clear need for an increase in the level of fixture automation. The present paper describes an interactive Computer Aided Fixture Design (CAFXD) system using the Gauss Elimination Method for the design of a fixture to hold prismatic components during machining on a CNC machining centre. The Feature Based Modeller (FBM), which consists of fixturing modules, allows the user to model the component in terms of meaningful manufacturing features and help in the selection of correct location points. The mechanical analysis of the component which has been carried out in the paper will suggest the clamping details.

MINIMIZING TOOL CHANGEOVERS IN THREE PARALLEL AND IDENTICAL CNC MACHINING CENTRES

This paper considers the problems of scheduling N jobs of the same family, each manufactured in a number of operations, in order, on three CNC machining centres which are equipped with automatic tool changers. A generalized procedure to minimize tool changeovers has been developed and tested on real industrial problems considering a set of possible constraints. The procedure suggested in this paper results in minimal changeovers. Validity of the heuristics suggested is borne out of the fact that when the same case-problem is considered as a Travelling Salesman problem and solved using Hungarian Method, identical result is attained. The procedure is simple and easy to implement in a CNC environment.