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1. Brief Paper: A Preliminary Study on the Dynamics of Modulated Turning (MT) With Spindle Speed Variation (SSV)

   https://doi.org/10.1115/MSEC2024-131699  

   Ozdemir, Mert; Sencer, Burak (June 2024, American Society of Mechanical Engineers)

   This paper proposes a novel strategy to enhance dynamic stability of turning processes. It presents a novel assistive strategy, which combines Spindle Speed Variation (SSV) and Sinusoidal Tool Modulations to dramatically improve chatter stability of high-speed turning. Chatter vibrations are a type of self-exiting vibrations that originate due to the dynamic flexibilities in the machine/workpiece/tool. Once chatter is triggered, it rapidly grows to destroy the surface finish, harms the tool and even the machine tool components. Chatter is the most limiting factor restricting productivity and attenable material removal rates (MRR) in most high-speed turning operations. A well-known strategy to improve chatter stability in turning is to use SSV. Continuously varying the spindle speed helps disturb and weaken the regenerative effect (regenerations) and thus improve chatter stability. Most recently, it is also reported that adding sinusoidal tool modulations also help improve chatter stability. This process is called the modulated turning (MT), and sinusoidal tool modulations cause the tool to disengage from the workpiece (cut) repeatedly introducing time for the regeneration effect to die out. This paper, for the first time, proposes to utilize sinusoidal tool modulations and SSV at the same time to assist and improve chatter stability of turning even further. The semi-discrete time domain approach is utilized to analyze chatter stability of this newly created turning process. It is observed, that jointly using tool modulations and SSV provides greater asymptotic chatter stability margins enabling average 10∼20% greater material removal rates to be achieved. Furthermore, it modifies existing stability lobes and helps create additional lobes, which may be utilized to maximize material removal rate at other desired target spindle speeds. Overall, joint application of SSV and tool modulations provide greater stability in turning. 

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2. Trochoidal milling: investigation of dynamic stability and time domain simulation in an alternative path planning strategy

   https://doi.org/10.1007/s00170-018-03280-y  

   Akhavan Niaki, F; Pleta, A; Mears, L; Potthoff, N; Bergmann, Jim A; Wiederkehr, P (July 2019, International journal, advanced manufacturing technology)

   Trochoidal milling is an alternative path planning strategy with the potential of increasing material removal rate per unit of tool wear and therefore productivity cost while reducing cutting energy and improving tool performance. These characteristics in addition to low radial immersion of the tool make trochoidal milling a desirable tool path in machining difficult-to-cut alloys such as nickel-based superalloys. The objective of this work is to study the dynamic stability of trochoidal milling and investigate the interaction of tool path parameters with stability behavior when machining IN718 superalloy. While there exist a few published works on dynamics of circular milling (an approximated tool path for trochoidal milling), this work addresses the dynamics of the actual trochoidal tool path. First, the chip geometry quantification strategy is explained, then the chatter characteristic equation in trochoidal milling is formulated, and chatter stability lobes are generated. It is shown that unlike a conventional end-milling operation where the geometry of chips remains constant during the cut (resulting in a single chatter diagram representing the stability region), trochoidal milling chatter diagrams evolve in time with the change in geometry (plus cutter entering and exiting angles) of each chip. The limit of the critical depth of cut is compared with conventional end milling and shown that the depth of cut can be increased up to ten times while preserving stability. Finally, the displacement response of the cutting tool is simulated in the time domain for stable and unstable cutting regions; numerical simulation and theoretical results are compared. 

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3. Effect of Directional Relations on Milling Chatter Stability and Development of a Stability Index

   https://doi.org/https://doi.org/10.1016/j.promfg.2018.07.045  

   Maulimov, Mukhtar; Sencer, Burak (July 2018, Procedia manufacturing)

   This paper analyzes effect of directional relationships on chatter vibrations experienced in peripheral milling process. Based on the directional relationships, a geometry-based chatter stability index (CSI) is proposed to improve chatter stability of the process. It is well-known that chatter stability depends on cutting conditions and tool geometry; whereas it is less known that it also depends strongly on the directional relations between the machining process and the flexible directions of the machine. In this research, these directional factors affecting chatter stability are extracted from process kinematics and dynamically compliant directions of the structure. Three cases are considered and analyzed; namely, 1) if the machine tool/workpiece structure is flexible only in single direction, 2) if it is flexible in two orthogonal directions and finally 3) when those flexible directions are not orthogonal. Tool feed direction is considered to be the optimization parameter to maximize process stability. Overall, this research aims to present new knowledge on the effect of directional relationships for chatter stability and how they can be utilized in a practical manner based on a chatter stability index (CSI) that can be computed from geometry, process kinematics and limited knowledge of machine dynamics. 

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4. Effect of Directional Relations on Milling Chatter Stability and Development of a Stability Index

   Maulimov, Mukhtar; Sencer, Burak (January 2018, Procedia manufacturing)

   This paper analyzes effect of directional relationships on chatter vibrations experienced in peripheral milling process. Based on the directional relationships, a geometry-based chatter stability index (CSI) is proposed to improve chatter stability of the process. It is well-known that chatter stability depends on cutting conditions and tool geometry; whereas it is less known that it also depends strongly on the directional relations between the machining process and the flexible directions of the machine. In this research, these directional factors affecting chatter stability are extracted from process kinematics and dynamically compliant directions of the structure. Three cases are considered and analyzed; namely, 1) if the machine tool/workpiece structure is flexible only in single direction, 2) if it is flexible in two orthogonal directions and finally 3) when those flexible directions are not orthogonal. Tool feed direction is considered to be the optimization parameter to maximize process stability. Overall, this research aims to present new knowledge on the effect of directional relationships for chatter stability and how they can be utilized in a practical manner based on a chatter stability index (CSI) that can be computed from geometry, process kinematics and limited knowledge of machine dynamics. 

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5. Analysis of burr formation in finish machining of nickel-based superalloy with worn tools using micro-scale in-situ techniques

   https://doi.org/10.1016/j.ijmachtools.2023.104030  

   Zannoun, Hamzah; Schoop, Julius (June 2023, International Journal of Machine Tools and Manufacture)

   The formation of burrs is among the most significant factors affecting quality and productivity in machining. Burrs are a negative byproduct of machining processes that are difficult to avoid because of a limited understanding of the complex burr formation mechanisms in relation to cutting conditions, including both process parameters and tool condition. Thus, the objective of this work was to characterize burr formation under finish machining conditions via a high-speed, high-resolution in-situ experimental method. Various parameters pertaining to burr geometry such as height, thickness, and initial negative shear angle were measured both during and after cutting. Results showed that varying the conditions of uncut chip thickness, tool-wear, and cutting speed all have a significant effect on burr formation, although certain burr metrics were found to be insensitive with respect to different process conditions because the difference was statistically insignificant. This study provides new insights into the relationships between the workpiece material’s microstructure, machining parameters, and tool condition on both crack formation and propagation/plasticity during burr formation. Using digital image correlation (DIC) and a physics-based process model not previously utilized for burr formation analysis, the displacement and corresponding flow stress were calculated at the exit burr root location. This novel semi-analytical approach revealed that the normalized stress at the exit burr root was approximately equal to the flow stress for a variety of different conditions, indicating the potential for model-based prediction of burr formation mechanics. Finally, this study investigates factors that influence fracture evolution during exit burr formation. It was found that negative exit burrs are a direct result of high strain rate and high uncut chip thickness, which was expected, but also a microstructural size effect and a tool-wear effect, neither of which have been previously reported. By harnessing ultra-high-speed imaging and advanced optical microscopy techniques, this manuscript deals with the fundamentals of burr formation, including new insights into material response at the grain-scale to the loads imposed with both sharp and worn tools. 

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