Traditional manufacturing is based on process-planning by experienced engineers, who iteratively try their methods in the shop until the results are satisfactory. This trial-and-error-based approach increases the product costs due to high scrap rates and conservative manufacturing strategies. The trend is to digitally simulate, optimize, and plan manufacturing before costly physical trials. The quality of virtual manufacturing depends on the accuracy of process models based on scientific principles. The aim of this special issue is to show how the manufacturing process physics can be incorporated into digital engineering steps to achieve virtual manufacturing.

This special issue has four articles illustrating sample virtual machining strategies. Process plans are automatically generated by considering the part features; machining of flexible thin walls is planned by considering their static stiffness; the environmental impact is considered in simulating machining costs of parts and chatter free conditions are predicted in milling metallic parts with robots.

I thank the authors and reviewers for their valuable contributions to the special issue. We hope to trigger digital modeling of various, challenging machining operations in the future.

A unique machining knowledge has led to several different perspectives between planners and operators as regards in designing a machining process plan. All precedents have shown the need to maintain a suitable machining process plan. Commercial Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) systems have facilitated the manipulation of 3D models to generate a machining process plan. The open Advanced Programming Interfaces (APIs) are also helpful in tailoring decision support systems to determine process plans. This study proposes an emergent system to generate flexible machining process plans. The proposed system considers the integration between design and manufacturing perspective to produce relevant machining process plan. The generation of process plans begins by considering the total removal volume of the raw material, estimating the removal features, thus analyzing and ordering several candidates of machining process plans. The total machining time and number of setups from each machining process plan candidate is analyzed and evaluated. Eventually, the proposed system is tested using several prismatic 3D models of a workpiece to show the outcomes.

An evaluation system for calculating equivalent CO₂ emissions and machining costs is developed using an activity-based model. The system can evaluate a machining process from an NC program, workpiece information, and cutting tool information, and it can then calculate accurate equivalent CO₂ emissions and the machining cost. The cutting speed of an end mill operation is evaluated in terms of the equivalent CO₂ emission and the machining cost. Based on the results, optimal cutting conditions are determined to minimize the equivalent CO₂ emissions and the machining cost to the extent possible.

This paper focuses on the deformation of a thin wall during the milling process. Cutting experiments were performed to investigate the influence of the workpiece thickness on its deformation and the cutting force. An FEM-based model was developed to simulate the deformation of a thin-wall workpiece during the milling process. With a tool’s rotation, the cutting force is distributed along the helical cutting edge, and the workpiece deformation can be calculated for a given time interval. The simulated results were compared with those of a simpler model where a constant cutting force is uniformly distributed along an oblique line representing the material removal by a cylindrical tool. Finally, the application of these results to the design of ribs for thin-wall parts during machining was considered.

Because of the high flexibility and low investment costs, industrial robots are increasingly being employed for machining processes. However, milling robots can only be used for applications requiring low accuracy and minor cutting forces. The main reason for this is the low static and dynamic stiffness of the robot structure, which lead to huge deflections of the tool and heavy chatter oscillations, especially when steel is being machined. To extend the areas in which milling robots are applied, a model-based controller to compensate for path deviation has been developed at the Institute of Machine Tools and Industrial Management of TU Munich (iwb). In addition, process-based strategies to reduce chatter have been analyzed. This paper focuses on the dynamic behavior of robots to increase the stability of the cutting process, but it also gives an overview of the design of the controller for static deviation compensation.

The purpose of the study described in this paper was to develop an energy-saving strategy for machining of multi-functional machine tools by pairing various turning and milling processes with various cutting conditions. The amounts of electric energy consumed during turning, facing, end milling, and drilling were measured and analyzed. Based on the experimental results, the most efficient machining processes and methods for reducing electric energy were identified. It was found to be important to employ severe cutting conditions as much as possible and to reduce the electric energy associated with machining of multi-functional machine tools during standby periods.

Recent research on hydraulic systems has mainly focused on energy saving. This is because the efficiency of hydraulic systems is very low even though they have large power-to-size ratios. In mobile hydraulic equipment, conventional hydraulic spool valves with pressure compensators have already been replaced by valve assemblies with four-valve independent metering with electronically controlled pressure compensation. The independent metering concept and microprocessor control have much more potential to save more energy than conventional proportional valve control because of the increased controllability of the system. The primary focus of this study is to reduce the number of Independent Metering Valves (IMV) by introducing one directional control valve. This new model offers two degrees of freedom, i.e., controlling velocity and pressure, just as in conventional IMVs. In the system described here, two of the three independent valves are active during metering. In this paper, the theory behind a new method of flow control based upon load feedback is presented for two of the five distinct metering modes, and its performance is investigated and compared to that of a conventional IMV configuration.

Grinding is usually applied to the peripheral finishing of hardened steel since the high specific cutting force and low stiffness of slender end mills often causes chatter vibration. On the other hand, varied-helix end mills have suppressed regenerative chatter vibration successfully in the rough machining of flexible workpieces. In this research, varied-helix end mills are applied to the extremely low radial immersion finishing of hardened steel, and the validity of this application is discussed and verified experimentally in terms of suppression of regenerative chatter vibration. A special varied-helix end mill with an extremely large helix angle difference is developed for this new application, and its performance is compared to that of an ordinarily-varied-helix end mill for the low-radial-immersion peripheral finishing of hard materials.

Micro deep drilling of hard materials requires introducing of step feed in process that increases machining time. To decrease the machining time by increasing the step feed, we apply low-frequency vibration (∼ 10 μm, 150 – 250 Hz) to the drilling process by oscillating the workpiece. To cope with the low-frequency vibration-assisted drilling of a curved surface, we have developed a fixing system for 2-dimensional vibration. The Fixing System for 2-Dimensional Vibration (FS2DV) consists of horizontal, and vertical actuators plus spring systems with variable rigidities along the directions of the actuators. A thrust force of 6 – 10 N from the drilling process may induce an unintended displacement of the workpiece. If the rigidities of the horizontal and the vertical spring systems are not balanced, unintended displacement may create diameter error during the drilling process. In this study, a method is proposed for configuring of the FS2DV with balanced spring systems to minimize the effects of the unintended displacement on diameter error. Frequency response function analysis of the vertical and horizontal spring systems is done for successful use of the FS2DV during the low-frequency vibration-assisted 2-axis drilling. Based on this analysis, setting requirements for the FS2DV are proposed for a particular vibration frequency. The behavior of the resultant vibration is evaluated while force is loaded along the intended angle of the drilling process. As a result, the effects of unintended displacement at the FS2DVare decreased for use within the vibration frequency range of 150 – 250 Hz with the vibration amplitude of 10 μm. The system can be used properly with a thrust force of up to 10 N and any angle from 0 to 90^◦ by selecting appropriate rigidities for the spring systems.

Form standards with different profile geometries were measured using 3- and 4-axis scanning on a 3D CMM prototype with a precision rotary table. Physical boundary conditions, including probing force, probe diameter, probe geometry, probe material, and scanning speed, were varied, and then the results were analyzed. Some results were found to be equivalent to those obtained using form measuring machines. Limitations of the current implementation of this technique are discussed.

In mass production cylindrical grinding operations, the ground surface finish has never been measured in each grinding cycles because existing surface roughness testers take a relatively long time to measure surface finishes, and the introduction of a surface roughness tester in each grinding cycle would affect mass production processes. Therefore, the surface finish of all parts manufactured in a production lot is generally evaluated based on the measured results of only few parts by sampling checks. We have proposed a unique quick technique for measuring surface roughness. It uses electromotive force based on the slight frictional heat to evaluate the surface finish of cylindrical workpieces rotating on cylindrical grinding machines, and it can do this in a split second just after each grinding cycle, so it does not hamper mass production grinding processes. The proposed quick measurement system can capture the variations in surface finish in a set of repeated cylindrical grinding cycles without dressing. The possibility of accurate judging dressing time by using the quick measurement system in mass production grinding processes is shown experimentally.

The application of a servo die cushion to the back-pressure forging process improves the shape accuracy of forged parts. Servo die cushions have excellent performance in precise motion control and high responsiveness to set loads. To use a servo die cushion to obtain these features, back pressure is applied to the bottom outer punch during forward extrusion-type forging. Without back pressure, material flow delay around the central counter punch corner results in an unfilled corner at the bottom outer punch. Applying back pressure to the outer punch reduces the area of the unfilled corner. However, extensive back pressure at the beginning of the forming process causes burrs at the bottom because of the clearance between the counter punch and the outer punch; variable back-pressure settings along the punch stroke effectively remove burrs while also providing a smaller unfilled area by allowing for low back pressure at the beginning of the forming process and high back pressure during the forming process. Furthermore, using the flexible slide motion of the servo press to vary the punch motion leads to even further reduction in the unfilled area.