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1. 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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2. 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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3. Chatter Diagnosis in Milling Using Supervised Learning and Topological Features Vector

   https://doi.org/10.1109/ICMLA.2019.00200  

   Yesilli, Melih C.; Tymochko, Sarah; Khasawneh, Firas A.; Munch, Elizabeth (December 2019, 2019 18th IEEE International Conference On Machine Learning And Applications (ICMLA))

   Chatter detection has become a prominent subject of interest due to its effect on cutting tool life, surface finish and spindle of machine tool. Most of the existing methods in chatter detection literature are based on signal processing and signal decomposition. In this study, we use topological features of data simulating cutting tool vibrations, combined with four supervised machine learning algorithms to diagnose chatter in the milling process. Persistence diagrams, a method of representing topological features, are not easily used in the context of machine learning, so they must be transformed into a form that is more amenable. Specifically, we will focus on two different methods for featurizing persistence diagrams, Carlsson coordinates and template functions. In this paper, we provide classification results for simulated data from various cutting configurations, including upmilling and downmilling, in addition to the same data with some added noise. Our results show that Carlsson Coordinates and Template Functions yield accuracies as high as 96% and 95%, respectively. We also provide evidence that these topological methods are noise robust descriptors for chatter detection. 

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4. Intermethod Comparison and Evaluation of Measured Near Surface Residual Stress in Milled Aluminum

   https://doi.org/10.1007/s11340-021-00734-5  

   Chighizola, C. R.; D’Elia, C. R.; Weber, D.; Kirsch, B.; Aurich, J. C.; Linke, B. S.; Hill, M. R. (October 2021, Experimental Mechanics)

   Abstract Background While near surface residual stress (NSRS) from milling is a driver for distortion in aluminum parts there are few studies that directly compare available techniques for NSRS measurement. Objective We report application and assessment of four different techniques for evaluating residual stress versus depth in milled aluminum parts. Methods The four techniques are: hole-drilling, slotting, cos(α) x-ray diffraction (XRD), and sin 2 (ψ) XRD, all including incremental material removal to produce a stress versus depth profile. The milled aluminum parts are cut from stress-relieved plate, AA7050-T7451, with a range of table and tool speeds used to mill a large flat surface in several samples. NSRS measurements are made at specified locations on each sample. Results Resulting data show that NSRS from three techniques are in general agreement: hole-drilling, slotting, and sin 2 (ψ) XRD. At shallow depths (< 0.03 mm), sin 2 (ψ) XRD data have the best repeatability (< 15 MPa), but at larger depths (> 0.04 mm) hole-drilling and slotting have the best repeatability (< 10 MPa). NSRS data from cos(α) XRD differ from data provided by other techniques and the data are less repeatable. NSRS data for different milling parameters show that the depth of NSRS increases with feed per tooth and is unaffected by cutting speed. Conclusion Hole-drilling, slotting, and sin 2 (ψ) XRD provided comparable results when assessing milling-induced near surface residual stress in aluminum. Combining a simple distortion test, comprising removal of a 1 mm thick wafer at the milled surface, with a companion stress analysis showed that NSRS data from hole-drilling are most consistent with milling-induced distortion. 

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5. 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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