Time Finite Element Chatter Stability Characterization of a Three Tooth Plastic End-Milling Cnc Machine (original) (raw)
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Chatter Stability Characterization of a Plastic End-Milling CNC machine
The desire to carry out this work arose from an observation during a practical work on a typical plastic end milling CNC machine. It was noticed that under certain conditions of cutting, operation of the machine became noisy with increasing depth of cut and eventual perforation of workpiece therefore the basic aim is to generate stability characterization of the machine in the form of a chart on the plane of cutting parameters on which stable operation is demarcated from the unstable operation . In modelling this machine, a slot creating mode of operation is used since the machine is mainly used for creating logos which are basically collection of slots. The significance of the resulting stability chart lies in the result that the cause of the aforementioned noisy operation is due to unstable parameter combination. For example a laboratory operation at spindle speed of 1500rpm and depth of cut of 1.5mm was noisy while that at spindle speed of 1500rpm and depth of cut of 1mm was serene. The stability chart generated for the system thus shows close agreement with both practice and theory. A unique impact this work will have on the reading community will be in the area of validity of the resulting stability chart on the basis of MATLAB dde23 numerical simulation. The parameters of the end milling process are; tool mass tool natural frequency damping factor and workpiece cutting coefficient .
4 Time Domain Chatter Stability Comparison Turning and Milling Processes
The delay differential equations describing the turning and milling processes are solved using MATLAB and results compared. The same set of parameter combinations are used for turning tool, one tooth, three tooth and six tooth milling tools in generating graphical trajectories of cutting process. This resulted in the comparison that gave rise to the conclusion that under full-immersion conditions milling stability characteristics get closer to that of turning as the number of teeth of milling tool increases.
Time Domain Chatter Stability Comparison of Turning and Milling Processes
2012
The delay differential equations describing the turning and milling processes are solved using MATLAB and results compared. The same set of parameter combinations are used for turning tool, one tooth, three tooth and six tooth milling tools in generating graphical trajectories of cutting process. This resulted in the comparison that gave rise to the conclusion that under full-immersion conditions milling stability characteristics get closer to that of turning as the number of teeth of milling tool increases.
Comparing Chatter Stability of End-Milling Processes of different number of teeth
The cutting forces and dynamic stability of end milling of full radial immersion are compared for end-millers of one to ten teeth. The parameters; tool mass tool natural frequency , tool damping ratio and feed speed are considered fixed for the millers. An end-milling tooth normally has positive rake angle and small cutting edge radius thus workpiece material cutting coefficient is also considered fixed since ploughing effect is not expected to be noticeably affected by change in number of teeth. It is seen that periodic cutting force reduces as the number of teeth increases. A method of milling stability analysis as proposed by Ding et al known as full-discretization is utilized in generating the stability charts. Use is made of the Simpson's rule in establishing for the studied system that chatter stability decreases as the number of teeth increases in the low spindle speed range and that lowest chatter stability at high spindle speed range occurs for the five tooth miller. Recommendations are made for the machinist based on these findings. The critical characteristic multipliers at single minimum point of each secondary Hopf bifurcation lobe (SHBL) are postulated to leave the unit circle along imaginary axis when number of teeth of slotting miller is greater than two. This phenomenon is noticed for the one and two tooth millers at two turning points that are not necessarily local minima of each SHBL.
Comparison of Analytical Milling Stability Analyses with Time Domain Simulation
2016
INTRODUCTION In the past few decades, machine and cutting tool technology has increased in both capability and complexity. Despite the significant advancements, however, self-excited vibrations (or chatter) remain a limiting factor for material removal rates and part quality. In 1965, Tobias showed that chatter is selfexcited vibration which results from regeneration effects on instantaneous chip thickness [1]. The cutting edge removes a chip that was produced by in the previous pass (the prior revolution in turning or tooth in milling). The chip thickness, which affects the force and therefore the vibration response, depends on the phase between the previously cut surface and the current vibration. This understanding led to the development of analytical algorithms which are used to generate a stability map of the limiting axial depth of cut, blim, versus spindle speed, Ω. This map is known as a stability lobe diagram. As an alternative, the governing equations of motion may be solv...
Effects of Radial Immersion and Cutting Direction on Chatter Instability In End-Milling
Proceedings …, 2002
Low radial immersion end-milling involves intermittent cutting. If the tool is flexible, its motion in both the x-and ydirections affects the chip load and cutting forces, leading to chatter instability under certain conditions. Interrupted cutting complicates stability analysis by imposing sharp periodic variations in the dynamic model. Stability predictions for the 2-DOF model differ significantly from prior 1-DOF models of interrupted cutting. In this paper stability boundaries of the 2-DOF milling process are determined by three techniques and compared: (1) a frequency-domain technique developed by Altintas and Budak (1995); (2) a method based on time finite element analysis; and (3) the statistical variance of periodic 1/tooth samples in a time-marching simulation. Each method has advantages in different situations. The frequency-domain technique is fastest, and is accurate except at very low radial immersions. The temporal FEA method is significantly more efficient than time-marching simulation, and provides accurate stability predictions at small radial immersions. The variance estimate is a robust and versatile measure of stability for experimental tests as well as simulation. Experimental upmilling and down-milling tests, in a simple model with varying cutting directions, agree well with theory.
Prediction of stability limit for multi-regenerative chatter in high performance milling
International Journal of Dynamics and Control, 2014
Chatter is one of the most important factors that inhibit the improvement of productivity and deteriorate the machined surface quality in milling process. In order to obtain good surface quality, classical machining process usually has to take conservative milling parameters. Based on the authors' previous work, this paper presented a new thirdorder discretization method to compute the stability lobes considering multi-regenerative chatter effect. A mathematical model, which is suitable for the dynamic system with non-uniform pitch cutter or cutter run-out, is first established for multi-regenerative chatter. Then, three examples are performed to test the validity of the proposed method. The first example is for the case that the system takes a non-uniform pitch cutter. In the second example, after the modal parameters, run-out parameters and cutting force parameters are gained from experiments, the stability lobes are predicted using the proposed method and subsequently testified by a series of experiments. The third example is for the case of existing cutter run-out. The final computation and experiment results indicate the effectiveness and validity of the proposed method. It is applicable in high performance machining for achieving a good parameter combination.
Journal of Advanced Mechanical Design Systems and Manufacturing, 2016
Chatter is one of the main limitations to milling performances. Prediction of such unstable phenomenon via stability lobe diagrams requires the measurement of the Frequency Response Functions (FRFs) for each tool and machine tool setup. This paper presents a hybrid FE-experimental approach to identify tool-tip FRFs with only one set of measurements, taking into account tool change without any other experimental test. Machine tool dynamics is modeled using a Finite Element (FE) approach. Machine, spindle and tool-holder are described by a lumped model characterized by frequency-dependent stiffness, while the tool is FE modeled. Lumped model and tool are connected by means of stiffness matrices extracted using the Craig-Bampton dynamic reduction method. The obtained simplified model of machine tool enables chatter prediction by means of stability lobe diagram for different tool without the need for extensive experimentation. Once a new tool is clamped no other measurements are needed, just the new tool FE model. Experimental validation under different conditions is provided, showing accuracy and reliability of proposed approach.
Comparing up and Down Milling Modes of End-Milling Using Temporal Finite Element Analysis
Two types of end milling at partial radial immersion are distinguished in this work, namely; up and down end-milling. They are theoretically given comparative study for a three tooth end miller operating at 0.5, 0.75 and 0.8 radial immersions. 0.5 and 0.8 radial immersion conditions are chosen so that analysis covers situations in which repeat and continuous tool engagements occur wh ile 0.75 radial immersion just precludes tool free flight. It results fro m analysis that the down end-milling mode is better favoured for workshop application than the up end-milling mode fro m both standpoints of cutting force and chatter stability. Th is superiority in chatter stability is quantified by making use of the Simpson's rule to establish that switching fro m up end-milling mode to down end-milling mode at 0.5 rad ial immersion almost doubles the possibility of chatter free milling while at 0.75 and 0.8 radial immersions this possibility almost triples. This result conforms to the age l...