Programmed Feed Rate Research Articles

Finding dimensional stability considering deflection effects in cylindrical plunge grinding

In spite of the existence of several models to effectively design grinding cycles, they are not frequently adopted in production lines, due to the lack of knowledge, difficulties in determining some input parameters and discrepancies observed in the obtained results during the production of a batch of parts. This paper proposes a contribution to the modeling and simulation of cylindrical plunge grinding cycles in terms of dimensional stability of the process, prediction of the workpiece diameter along the grinding cycle and the minimum required spark-out time, incorporating into the model grinding tolerance limits according to the chosen capability scenario. The prediction of the part diameter and the spark-out time in a cylindrical plunge grinding operation is the key parameter to design optimized grinding cycles. The proposed model covers typical fast grinding cycles in industry, in which the spark-in time is not long enough to guarantee that at its end, the real workpiece radius reduction speed is very close to the programmed feed rate. In the proposed model, the remaining radial stock to be removed during the spark-out stage is designed to be equal to or lower than the tolerance limit for the grinding operation, but considering different scenarios of capability index: barely capable process (Cp= 1); capable (Cp= 1.3); and six-sigma quality level (Cp= 2.0). The minimum spark-out time necessary to reduce the total radial stock to the specified capability limit is calculated by the simulation. Experimental plunge cylindrical grinding tests were performed in which the reduction in the workpiece diameter was measured using an in-process diameter gauge, varying the in-feed velocity and the spark-out time. As a result, a reasonable compromise between the proposed theoretical model and the experimental data was achieved. The dependency of the “time constant” and the in-feed speed was demonstrated for fast spark-in cycles. It was also observed that longer spark-out times will be needed to achieve tighter capability indexes for a given in-feed speed and “time constant” value.

Online Workpiece Height Estimation for Reciprocated Traveling Wire EDM Based on Support Vector Machine

The reciprocated traveling wire EDM is a unique type of wire EDM machine, in which the wire electrode is not discarded after passing the discharge gap but being re-winded on the wire drum instead. It is of specific features such as ultra-thick workpiece cutting and reuse of wire electrode. While cutting variable height workpieces with constant machining parameters, this type of WEDM faces the same challenges as western style WEDM, either the risk of wire breakage or the low cutting speed. Due to the intermittent discharge process and the varying machining characteristics, online estimation of workpiece height becomes even more difficult for this type of WEDM.This paper proposes a method of online workpiece height estimation based on support vector machine (SVM), an effective machine learning method. The inputs of the SVM model are effective discharge frequency, pulse interval, programmed feed rate and actual feed rate and the output is the estimated height. The algorithm is integrated in a newly developed computer numerical control (CNC) system for reciprocated traveling WEDM, with a sampling circuit collecting current and voltage signals from the discharge gap and an adaptive control unit which adjusts the machining parameters according to the workpiece height estimation.The data for training the SVM were produced by cutting stair-shaped workpieces to build the SVM model. Then verification was carried out by cutting through workpieces with variable height cross sections. The results demonstrated the effectiveness of the proposed method. The estimation error was less than 2mm and the machining time was reduced by more than 30%.

Material Removal Simulation for Abrasive Water Jet Milling

Abrasive water jet (AWJ) milling, often referred to as abrasive water jet milling, has proven to be one of the most flexible non-conventional production techniques for difficult-to-machine materials. Currently no suitable material removal simulation for the water jet milling process is available. A modularized material removal simulation for the AWJ controlled depth machining process has been developed. Because the volume of material removal strongly depends on energy input per unit length of programmed toolpath, undesired deviations of the programmed feed rate like feed rate drops at sharp corners introduced by the water jet machine tool must be considered.

Cutting time in pocket machining for different tool-path approximation segments

Despite of the importance of cutting time in pocket machining for manufacturing cost prediction and process planning, its estimation is usually neglected and conducted dividing the total tool path length by the programmed feed rate. Nevertheless, driving the tool with a constant feed rate is, in general, not possible. We study the cutting time dependency with the tool-path curvature, type of interpolation (linear G01 or circular G02) and the distance between interpolation points (segment length). The study, conducted in two different milling machines, shows how shorter segments increase the actual cutting time, which is lower for G01 than for G02. According to the experimental results, we define a piecewise equation that estimates the real cutting time respect to the segment length and type (G1 or G2).

Powder Flux Regulation in the Laser Material Deposition Process

In the present research work a powder flux regulation system has been designed, developed and validated with the aim of improving the Laser Material Deposition (LMD) process. In this process, the amount of deposited material per substrate surface unit area depends on the real feed rate of the nozzle. Therefore, a regulation system based on a solenoid valve has been installed at the nozzle entrance in order to control the powder flux. The powder flux control has been performed based on the machine real feed rate, which is compared with the programmed feed rate. An instantaneous velocity error is calculated and the powder flow is controlled as a function of this variation using Pulse Width Modulation (PWM) signals. Thereby, in zones where the Laser Material Deposition machine reduces the feed rate due to a trajectory change, powder accumulation can be avoided and the generated clads would present a homogeneous shape.

Application of Siemens 840Di System in the Reconstruction of KISC Roll Collar Grinder

This work studies the reconstruction of high-speed roll grinder for sculptured surfaces with high feed rates. Experiments and predictions were focused on representative surfaces of dies and molds, whose geometric complexity and complexity distribution were modified parametrically. CNC programs for machining these surfaces were executed in a HERKULES HSF2CNC machining center with a SIEMENS 840D controller, with different levels of programmed feed rate. Discrepancies between programmed and actual feed rates were evaluated. A mechanistic approach for roll grinder in high-speed milling of sculptured surfaces is proposed. The mechanistic model construction is based on: (a) the frequency distribution of linear interpolation path lengths in the CNC program and (b) a characterization of the machine tool for brisk and smooth movements. Two case studies were used to demonstrate the effectiveness of the proposed approach. The reconstruction model is capable of the manufacturing.

Kinematic Behaviour Modeling of the Axes of a Machining Center in High Speed Milling

In the context of high speed milling HSM, the feed rate does not always reach the programmed value during the machining process which implies an increase of machining time and non compliance with the programmed feed rate. This phenomenon leads to productivity issues and an underestimation of the cost of machining for the industry. The aim of this study is to identify the kinematic behaviour of the machine tool during any type of discontinuity between linear and circular contours in different combination by taking into account the specific machining tolerances. In order to achieve this, a model of the law of the axes motion and the actual trajectory at discontinuities is necessary. This method is based on the subdividing of the trajectory into elementary geometries according to the type of interpolation (circular or linear). The proposed method can estimate the cycle time with a maximum error of 5% between the actual and the prediction cycle time. Finally, an experimental study was carried out on a high speed machine. It is based on elementary tests in order to analyze the axes behavior during any type of discontinuity and to validate the developed models.

Evaluation of Dynamic Behavior of Machine Tools for Sculptured Surface Manufacturing

The production sector of molds and dies is under a continuous pressure for process optimization due the high lead time required for the production of complex tools. In recent years, to improve this sector, numerous studies have been aimed at optimization of the CAD/CAM/CNC chain. However, despite recent technological innovations such as development of cutting tools, machine tools, and process simulation systems, the machining step still represents a major bottleneck in this process. The current generation of Numerical Control (NC) programs that represents the tool path do not consider the dynamic characteristics of the machine tool in the decision-making process to choose the most appropriate cutting strategies, resulting in NC programs that do not reach levels of excellence. The main objective of this paper is to present the development of a reference index based on the dynamic characteristics of the Machine Tool to support the NC programming with a commercial CAM system. This index considers the Machine Response Time (MRT) of NC controller, the acceleration and deceleration on the axes of the machine tool and the tool path segment (size and inclination). The index makes possible an experimental evaluation of relationships between the programmed feed rate for the tool path and dynamic performance due to movements of the machine axes, thus becoming an important decision-making factor in choosing the best machining strategy and consequently a better plan of the production process in die and mold industries.

Evaluation of servo, geometric and dynamic error sources on five-axis high-speed machine tool

Many sources of errors exist in the manufacturing process of complex shapes. Some approximations occur at each step from the design geometry to the machined part. The aim of the paper is to present a method to evaluate the effect of high-speed and high dynamic load on volumetric errors at the tool center point. The interpolator output signals and the machine encoder signals are recorded and compared to evaluate the contouring errors resulting from each axis follow-up error. The machine encoder signals are also compared to the actual tool center point position as recorded with a non-contact measuring instrument called CapBall to evaluate the total geometric errors. The novelty of the work lies in the method that is proposed to decompose the geometric errors into two categories: the quasi-static geometric errors independent from the speed of the trajectory and the dynamic geometric errors, dependent on the programmed feed rate and resulting from the machine structure deflection during the acceleration of its axes. The evolution of the respective contributions for contouring errors, quasi-static geometric errors and dynamic geometric errors is experimentally evaluated and a relation between programmed feed rate and dynamic errors is highlighted.

Optimisation of spiral tool path for five-axis milling of freeform surface blade

This paper deals with optimisation of five-axis tool paths in the context of milling of turbine blade. The purpose is to generate tool paths that respecting kinematical performance of machine tool while ensuring the geometrical conformity of the machined part. To achieve this object, outer transverse curves are created at the leading edge and trailing edge of the blade while generating spiral tool path, it allows the machine tool to maintain the programmed feed rate while respecting the kinematical limits of the machine tool. As for the leading and trailing edges of the blade, they are machined along height direction of the blade. Machining experiments showed that machine tool moved smoothly and no overcut or dot marks would occur with the proposed method, thereby demonstrating the feasibility of the presented approach.

Mechanistic approach to predict real machining time for milling free-form geometries applying high feed rate

An accurate estimate of machining time is very important for predicting delivery time, manufacturing costs, and also to help production process planning. Most commercial CAM software systems estimate the machining time in milling operations simply by dividing the entire tool path length by the programmed feed rate. This time estimate differs drastically from the real process time because the feed rate is not always constant due to machine and computer numerical controlled (CNC) limitations. This study presents a practical mechanistic method for milling time estimation when machining free-form geometries. The method considers a variable called machine response time (MRT) which characterizes the real CNC machine’s capacity to move in high feed rates in free-form geometries. MRT is a global performance feature which can be obtained for any type of CNC machine configuration by carrying out a simple test. For validating the methodology, a workpiece was used to generate NC programs for five different types of CNC machines. A practical industrial case study was also carried out to validate the method. The results indicated that MRT, and consequently, the real machining time, depends on the CNC machine’s potential: furthermore, the greater MRT, the larger the difference between predicted milling time and real milling time. The proposed method achieved an error range from 0.3% to 12% of the real machining time, whereas the CAM estimation achieved from 211% to 1244% error. The MRT-based process is also suggested as an instrument for helping in machine tool benchmarking.

Evaluation and modeling of productivity and dynamic capability in high-speed machining centers

The current standards for machine tool performance evaluation do not consider productivity and dynamic capability at high feed rates. Therefore, industry does not have a basis for machine tool comparison. Testing was conducted with the Heidenhain KGM 181 grid encoder to record the actual path of the machine at high feed rates (up to 16 m/min). This paper shows the significance of linear vs. circular interpolation, acceleration and rate of change in acceleration (jerk) on the productivity and dynamic capability of this type of machine. Under high feed rate interpolations, the actual cycle time can differ significantly from the ideal cycle time (calculated with the traveled distance and programmed feed rate). A modeling approach with simple testing protocols is presented in order to predict the cycle time during machining with high feed rates. The proposed modeling approach provides less than 6% error in estimating cycle time.

Experimental investigation of feed rate limitations on high speed milling aimed at industrial applications

This paper presents an experimental investigation on feed rate [mm/min] limitations when milling free form surfaces, commonly found in dies and moulds, applying the high speed cutting technology-HSC. The results appoint feed rate as the bottleneck to achieve the real benefits of HSC in terms of machining time, surface quality and process stability. That is due to unwanted large variations on the initially programmed feed rate, mainly when milling free form surfaces. These “ups” and “downs” on feed rate result in several setbacks for the machining process itself and it can be found even when using a suitable HSC milling machine available in the market. This paper addresses some of the causes for feed rate variations and evaluates an alternative approach to describe a tool path using spline polynomial technique. In order to focus industrial applications, all equipment, materials and software used are accessible in the market. The milling experiments were accomplished on a high-speed milling machine controlled by an open architecture CNC and a high-end CAD/CAM software was used. A 3D free form workpiece was designed and a real-time monitoring system was developed to investigate the feed rate variations during milling operation. The surface quality after milling and the machine tool/CNC performance were also assessed.

Cycle time prediction in high-speed milling operations for sculptured surface finishing

This work studies the cycle time prediction of high-speed milling for sculptured surfaces with high feed rates. Experiments and predictions were focused on representative surfaces of dies and molds, whose geometric complexity and complexity distribution were modified parametrically. CNC programs for machining these surfaces were executed in a HURON KX-10 machining center with a SIEMENS 840D controller, with different levels of programmed feed rate. Discrepancies between programmed and actual feed rates were evaluated. A mechanistic approach for cycle time evaluation in high-speed milling of sculptured surfaces is proposed. The mechanistic model construction is based on: (a) the frequency distribution (histogram) of linear interpolation path lengths in the CNC program and (b) a characterization of the machine tool for brisk (large changes in tool path direction) and smooth movements (small changes in tool path direction). Two case studies were used to demonstrate the effectiveness of the proposed approach (a set of representative sculptured surfaces with spherical caps and a forging die surface). Comparing the actual cycle time versus ideal cycle time under programmed feed rates up to 16 m/min, discrepancies of 300–800% were found. The proposed model is capable of predicting cycle time with a maximum error of 5–22%.

Influence of tool path strategy on the cycle time of high-speed milling

This work at first look discusses the influence of the tool path strategy on the cycle time of high-speed milling operations. Experiments and predictions were focused on pocketing operations with a zig-zag tool path, quantifying the significant discrepancy between the programmed feed rate and the actual average feed rate. A mechanistic approach for cycle time evaluation is proposed. The mechanistic model construction is based on the experimental measurement of the machine tool acceleration and specific geometric assumptions regarding tool motion. For high feed rates, the proposed approach is capable of capturing the influence of the zig-zag tool path orientation on the machining cycle time.

Control of feed rate to a fed-batch culture using a heat-flux sensor

Substrate inhibition in batch fermentations can be avoided by employing the fed-batch technique in which substrate concentration is kept at low levels by a programmed feed rate. This research demonstrates the use of a heat-flux sensor to control substrate addition by continuously monitoring evolving heat which is proportional to fermentation rate. Batch fermentation with 240 g/L glucose in the medium was compared with a fed-batch starting with 20 g/L glucose in the medium and increased, with 500 g/L glucose, to a final equivalent glucose concentration of 240 g/L. The batch fermentation produced 106 g/L ethanol in 39 hr at 2.72 g/L/h, while the best fed-batch produced 114 g/L ethanol in 34 hr at 3.35 g/L/h with the same nutrients.