Machining vibrations and dynamic instability of machine tools is an important consideration in machining systems. Common approaches for improving their dynamic performance target either the process, or intelligent, yet complex control systems with actuators. Given that machine tools' dynamic characteristics are largely defined by the characteristics of the joints, this article proposes a novel concept, attempting to create a new paradigm for improving the dynamic behaviour of machine tools-introducing modular machine tools components (Joint Interface Modules-JIMs) with joints deliberately designed for increasing dynamic stiffness and enhancing damping with the use of viscoelastic materials. Through a systematic model-based design process, a prototype replicating a reference tool holder was constructed exploiting viscoelastic materials and the dynamic response of the machining system was improved as a result of its introduction; in machining experiments, the stability limit was increased from around 2 mm depth of cut to 4 mm depth of cut, without compromising the rigidity of the system or changing the process parameters. The article also includes the results of investigations regarding the introduction of such prototypes in a machine tool and discusses the shortcomings of the stability lobe diagrams as a method for evaluating the performance of machine tool components with viscoelastically treated joints.

Titanium alloys are known to produce shear-localized chips during machining, resulting in cyclic variations in cutting forces which in turn could cause severe problems with vibrations. However, at low cutting speeds and feed rates, continuous chips are formed, with an increase in both parameters favoring the transition to shear-localized chips. This transition is affected by work material microstructure, where a coarse microstructure gives anisotropic effects, e.g., when the size of alpha colonies is on the same order of magnitude as the primary cutting zone. The change in chip morphology with an increase in cutting parameters will then be dependent on the orientation of alpha colonies within the cutting zone. The microstructure of work material can show large variations depending on product form, e.g., cast, wrought, or sheet material, thus affecting whether the chip formation is isotropic or anisotropic. Other sources of variations also exist that can be found within the same component, such as segregation of alloying elements and differences in thermo-mechanical history during processing due to geometry. In this study, the interaction between work material microstructure, process parameters, and the machining system’s structural characteristics is studied. The aim is to further increase the knowledge about vibrations during machining of titanium and the role of microstructure and machining system properties. Different microstructures were produced by adding boron to cast Ti-6Al-4V material, where the resulting colony sizes gave both isotropic and anisotropic chip formation within the chosen cutting data range. The machining systems dynamic properties were varied by using different tool overhangs, thereby simulating different configurations of natural frequencies and stiffness. The results show the influence of both microstructure and machining system’s structural characteristics on the dynamic response of the system for different process parameters. This information can be used to increase robustness of machining operations taking into consideration this three-way relationship.

Machine tool joints have significant influence on the dynamic characteristics of the machine tool and therefore on the response of the machining system to excitations from the cutting process. In cases of unstable response, generally described as chatter, surface quality of a machined workpiece and tool life deteriorate significantly. This paper presents a novel way of exploiting joints in order to control the dynamic response of the system, by integrating a mechatronic tool holder (Joint Interface Module - JIM) in the machine tool. This system has a purposely designed joint interface with controllable natural characteristics (stiffness and damping). These characteristics are controlled by altering the applied pre-load on the internal joint interface of the tool holder. The preload on the joint interface is controlled by pneumatic means. In doing so, a milling process during which the stability limit was exceeded became stable during the machining process, without alteration of the process parameters.

The increasing demands for precision and efficiency in machining call for effective control strategies based on the identification of static and dynamic characteristics under operational conditions. The capability of a machining system is significantly determined by its static and dynamic stiffness. The aim of this paper is to introduce novel concepts and methods regarding identification and control of a machining system’s dynamics. After discussing the limitations in current methods and technologies of machining systems’ identification and control, the paper introduces a new paradigm for controlling the machining system dynamics based on design of controllable structural Joint Interface Modules, JIMs, whose interface characteristics can be tuned using embedded actuators. Results from the laboratory and industrial implementation demonstrate the effectiveness of the control strategy with a high degree of repeatability.

The precision of machining systems is ever increasing in order to keep up with components’ accuracy requirements. At the same time product variants areincreasing and order quantities are decreasing, which introduces high demands on the capability of machining systems. The machining system is an interaction between the machine tool structure, the process and the control system and is defined in terms of capability by the positional, static, dynamic and thermal accuracy. So far, the control of the machining system, in terms of static and dynamic stability is process based which is often translated into sub-optimum process parameters and therefore low productivity.This thesis proposes a new approach for control of the machining systemwhich is based on the capability to control the structural properties of themachine tool and as a result, controlling the outcome of the machining process.The control of the structural properties is realized by carefully designed Joint Interface Modules (JIMS). These modules allow for control of the stiffness and damping of the structure, as a result of tuning the contact conditions on the interface of the JIM; this is performed by control of the pre-load on the interface,by treatment of the interface with damping enhancing materials, or both. The thesis consists of a presentation of the motivation behind this work, the theoretical basis on which the proposed concept is based and a part describing the experimental investigations carried out. Two prototype JIMs, one for a milling process and one for a turning process were used in the experimental investigations that constitute the case studies for examining the validity of the proposed concept and demonstrating its applicability in a real production environment.

A main consideration in the operation of machine tools is vibrations occurring during the cutting process.Whether they are forced vibrations or self-excited ones, they have pronounced effects on surface quality, tool lifeand material removal rate. This work is an experimental study of interactions between natural characteristics,control parameters and process parameters of a machining system designed with adaptive dynamic stiffness. Inorder to comprehend these interactions, the effect of changes in dynamic stiffness on the system’s response isexamined. The system under study consists of an end-milling tool, a steel workpiece and a work holding devicewith controllable stiffness. Natural dynamic characteristics of the system components are determined throughmodal impact testing. Then the behaviour of the whole machining system is examined under both high and lowcutting speed conditions by analysing vibration levels using acceleration signals acquired through a tri-axialsensor mounted on the workpiece. Cutting is performed in both directions of the horizontal plane of a CNCmilling machine. In both cases the results are presented for two extremes of stiffness and damping in the workholding device. The effect of control parameters on the system’s natural characteristics could be identifiedtogether with a relation between these parameters and the system’s response in high and low cutting speedconditions. The high-damping configuration reduces the vibration amplitudes significantly, while the increaseof pre-stress has a different effect depending on the cutting conditions.

Vibration phenomena are a main consideration during the material removal operation, as it has prominent effects on the product quality, cutting tool life, and productivity of that machining operation. Within the context of machining performance, role of enhanced stiffness and damping on the dynamic behaviour of machining systems such as turning and milling is well established. In this experimental analysis, investigations have been conducted for identifying the natural characteristics and dynamic responses of a milling process with the application of a novel carbon based (CNx) nano-composite damping material. TheCNx material has been applied into the joint interface of a workholding device with adaptive dynamic stiffness. Prior investigations of this material, produced by theplasma enhanced chemical vapor (PECVD) process, showed inherent damping capacity via interfacial frictional losses of its micro-columnar structures. For thisstudy, natural characteristics of the workholding system have been characterized bythe modal impact testing method. Dynamic responses during the machining processhave been measured through the vibration acceleration signals. The ultimate objective of this study is to comprehend the potentiality of CNx coating material forimproving machining process performance by analyzing the frequency response functions and measured vibration signals of the investigated milling process with varying stiffness and damping levels.

Vibration in metal cutting processes has been studied to a great extent resulting in for instance stability lobe diagrams under which stable machining parameters can be selected. One limitation of accurately estimated stability diagrams is the change in process and dynamic characteristics of the machine tool under operation. The machine tool dynamic response is often analysed with experimental modal analysis under off operational conditions. One drawback with this approach is the large number of measurements required to fully describe a machine tool and workpiece in different positions and time of machining. Another drawback is that the change of dynamic characteristics under operation is excluded. Operational modal analysis has been applied in machining under different conditions resulting in successfully improved stability lobe prediction. This research includes operational modal analysis of the workpiece, fixed on a stiffness controllable joint and stability prediction to stress the importance of various machining conditions.

Reliability is a collective term covering several abilities of the technical system: to deliver required functions, to uphold quality of products and services, to assure that the safety requirements associated with the system are properly fulfilled with regards both to the users and the environment and finally to uphold the durability of the technical system during its whole life cycle. All this has to be performed at acceptable risks, optimal cost, and correspond to operational needs of the business. Even though there is an advanced, well thought-out concept for this purpose - reliability centred maintenance (RCM) - that correctly applied might result in very good quality maintenance programs, it is not broadly used in the industry due to the vast efforts required for its implementation. An appropriate methodology supporting systematic functional break down of a studied systems, and guidelines how to couple functional failures to failure modes, integrated with RCM, would greatly speed up generating of effective maintenance programs. In this paper we present our research towards development of such a methodology, and show a pilot implementation to analysis of machine tool spindle. The methodology is based on Hubka's theory of design and AFD/TRIZ.