Variable Pitch Tools Research Articles

Stability for multiple delays machining system with variable pitch tools considering nonlinear cutting force coefficients

Stability for multiple delays machining system with variable pitch tools considering nonlinear cutting force coefficients

Optimal pitch angles determination of ball-end cutter for improving five-axis milling stability

Regenerative chatter is one of the major limitations in five-axis ball-end milling of complex shaped parts, resulting in reduced productivity and poor quality. Although a good solution can be provided for the chatter avoidance and the stability enhancement by using variable pitch cutters, it depends on the optimal selection of pitch angles, and few studies focus on determining the optimal pitch angles combination for the five-axis ball-end milling process in a continuous spindle speed range. In this paper, the pitch angle optimization is based on the fast and accurate stability prediction of five-axis ball-end milling with variable pitch tools. Considering that the search space of the stability border should be reduced as much as possible during the optimization process, an approach combining an improved hybrid multi-step method and an optimization strategy of stable area maximum is presented for efficiently finding the optimal pitch angles. For selected spindle speed ranges and given lead/tilt angles, the optimal pitch angles of variable pitch ball-end cutter are determined with the proposed optimization approach. The experimental results show the average stability of the customized optimal variable pitch ball-end cutter is significantly higher than that of the regular ball-end cutter, validating the proposed optimal pitch angles determination method. Compared with the semi-discretization method, the use of improved hybrid multi-step method for predicting the stability border reduces the simulation time by around 84 %. In contrast to the optimization strategy based on the average characteristic multiplier, the proposed optimization strategy is more efficient because it does not require a large number of characteristic multipliers.

Investigation of variable pitch tool design for chatter suppression in milling of Ti-6Al-4 V: a comparison of simulation and experimental results

Investigation of variable pitch tool design for chatter suppression in milling of Ti-6Al-4 V: a comparison of simulation and experimental results

Chatter Suppression during Milling of Ti-6Al-4V Based on Variable Pitch Tool and Process Damping Effect

Regenerative chatter is a major limitation in the milling of Ti-6Al-4V, resulting in undesirable surface and reducing cutting efficiency. It is well-acknowledged that variable pitch cutter and the process damping effect are effective methods to suppress chatter, and that these two methods are compatible. In this paper, a novel anti-vibration milling tool with combined variable pitch (pitch angles of 85°-95°-85°-95°) and wear edges (60 μm on the flank face), which helps to increase the process damping effect, is presented. The milling model of variable pitch tool considering process damping effect is developed and the dynamic milling stability is analyzed by using the semi-discretization method. To modify time delay, an analytical formula for optimal tuning variable pitch angles is presented. The effectiveness of the proposed anti-vibration tool is verified by stability diagrams and milling experiments. Industrial experimental results show that compared with the common regular pitch tool and onefold variable pitch tool, when the proposed anti-vibration tool is used, the stable axial depth of cut increases by 97.4% and 26.2%, respectively.

Dynamics and chatter stability of crest-cut end mills

Chatter is a major problem reducing part quality and productivity in milling operations. In order to improve productivity with standard end mills, higher cutting speeds must be employed to reach deeper stability lobes. However, due to machine tool and machinability limitations in cutting hard-to-cut materials, higher cutting speeds are not feasible. For those cases, special milling tools such as variable pitch and crest-cut cutters offer an alternative method to increase productivity. Due to their special geometry crest-cut tools introduce large stable regions at low-speed ranges where variable pitch tools provide narrow stability lobes. This advantage is caused by the wavy edge shape along the cutting edges of the crest-cut tools. In this study, stability of crest-cut tools is studied using the semi-discretization method by implementing a novel geometrical modeling approach enabling accurate cutting force predictions verified experimentally for the first time in literature. Based on the proposed model, the effect of edge wave geometry on stability is analyzed and compared with standard and variable pitch tools. Results show that the crest-cut tools introduce significant advantages in terms of stability limit and stable zones compared to other types of end mills. This superiority results in higher productivity as well as higher robustness at a different cutting speed zones. Finally, a guideline is presented for selection of the best crest-cut tool geometry for desired operating conditions.

State dependent regenerative stability and surface location error in peripheral milling of thin-walled parts

Chatter avoidance and error control are the core issues accompanying milling processes of thin-walled parts. Owing to the high flexibility of workpiece, the cutting vibrations can easily reach dozens of micrometers and become comparable to the nominal chip thickness. However, the classic dynamic models usually neglect the influence of system state on regenerative stability and surface location error (SLE), which consequently reduces the prediction accuracy. In this paper, the coupling relationships between cutter-workpiece engagement, time delay and system state are modeled by means of analyzing the true teeth trajectories that are composed of tool rotation, feed movement and cutting vibrations. The resultant dynamic model is a state dependent delay differential equation, which is capable of accounting for the fly-over and multi-regeneration nonlinear phenomena too. Afterwards, an efficient numerical algorithm is proposed to accurately compute the dynamic responses. By contrast to classic time domain simulation techniques, the proposed algorithm uses discrete root finding scheme to determine the state dependent time delay and the state dependent cutter-workpiece engagement, which ensures its high computational efficiency. Based on the proposed dynamic model and numerical algorithm, the stability limits and SLE are finally obtained, and experimentally validated on two thin-walled plates using two variable pitch tools. Theoretical comparison and experimental results reveal that the proposed state dependent dynamic model successfully predicts the milling stability and SLE while the classic model fails. The state dependency of regenerative stability and SLE is further investigated and explained in detail.

Generation Mechanism and Quality of Milling Surface Profile for Variable Pitch Tools Considering Runout

Abstract Surface profile is one of the foremost aspects to evaluate milling performance. Its generation mechanism is affected by a variety of factors such as tool geometry, runout values, and process parameters and thus still deserves further investigation. This paper aims to propose a unified method to study the surface generation mechanism and to predict the machining quality for variable pitch tools considering runout. First, a Floquet theory based algorithm is extended to analyze the machining stability and output the dynamic responses of the machining system. The resultant trajectories of cutting edges are obtained by kinematic synthesis of system vibrations, tool rotations, and machining feed. Next, both the surface location error (SLE) and the surface roughness are simultaneously extracted from the edge trajectories. A series of cutting tests are performed to validate the prediction results. Some new characteristics of the machined surface profile in terms of form errors and teeth marks are discovered and theoretically explained. Finally, the joint influences of tool geometry, runout values, and process parameters on the surface generation mechanism and quality are analyzed in detail with the proposed method.

Stability Analysis for Milling Process with Variable Pitch and Variable Helix Tools by High-Order Full-Discretization Methods

Chatter is one of the significant limitations in the milling process, which may cause poor surface quality, reduced productivity, and accelerated tool wear. Variable pitch and variable helix tools can be used to suppress regenerative chatter. This study extends the high-order full-discretization methods (FDMs) to predict the stability of milling with variable pitch and variable helix tools. The time-periodic delay-differential equation (DDE) with multiple delays is used to model the milling process using variable pitch and variable helix tools. Then, the DDE with multiple delays is reexpressed by the state-space equation. Meanwhile, the spindle rotational period is divided into many small-time intervals, and the state space equation is integrated on the small-time interval. Then, the high-order interpolation polynomials are used to approximate the state term, and the weights related to the time delay are employed to approximate the time-delay term. The second-order, third-order, and fourth-order extended FDMs (2nd EFDM, 3rd EFDM, and 4th EFDM) are compared with the benchmark in terms of the rate of convergence. It is found that the 2nd EFDM, 3rd EFDM, and 4th EFDM converge faster than the benchmark method. The difference between the curves obtained by different EFDMs and the reference curve is very small. There is no need to extend hypersecond FDMs to analyze the stability of milling with variable pitch and variable helix tools.

Dynamics modeling and stability analysis of five-axis ball-end milling system with variable pitch tools

Chatter has become a major obstacle in realizing high efficiency milling under the premise of ensuring structure integrity, as it creates violent vibration to the milling system and in turn results in poor surface finish and accelerated tool wear. Although some promising methods have been applied to the avoidance and suppression of chatter, and among them variable pitch tools play a critically important role, few studies focus on the stability prediction of five-axis ball-end milling with variable pitch tools. In this paper, a comprehensive dynamic model characterized by multiple delays is first built for five-axis ball-end milling system with variable pitch tools, and the milling stability lobe diagrams (SLDs) are predicted with an extended second-order semi-discretization method (2nd SDM) by taking the multiple modes of cutter into account. Then, the effects of tool orientation on the stability prediction of five-axis ball-end milling with variable pitch tools and regular tools are analyzed. The optimal cutter pitch angle can be obtained by comparing the SLDs for the stability improvement. Finally, the effectiveness of the proposed method is verified by experiments with a variable pitch ball-end cutter in five-axis machining center.

Optimum Selection of Variable Pitch for Chatter Suppression in Face Milling Operations.

Cutting capacity can be seriously limited in heavy duty face milling processes due to self-excited structural vibrations. Special geometry tools and, specifically, variable pitch milling tools have been extensively used in aeronautic applications with the purpose of removing these detrimental chatter vibrations, where high frequency chatter related to slender tools or thin walls limits productivity. However, the application of this technique in heavy duty face milling operations has not been thoroughly explored. In this paper, a method for the definition of the optimum angles between inserts is presented, based on the optimum pitch angle and the stabilizability diagrams. These diagrams are obtained through the brute force (BF) iterative method, which basically consists of an iterative maximization of the stability by using the semidiscretization method. From the observed results, hints for the selection of the optimum pitch pattern and the optimum values of the angles between inserts are presented. A practical application is implemented and the cutting performance when using an optimized variable pitch tool is assessed. It is concluded that with an optimum selection of the pitch, the material removal rate can be improved up to three times. Finally, the existence of two more different stability lobe families related to the saddle-node and flip type stability losses is demonstrated.

Effects of irregular tool geometry and machining process parameters on the wavelength performance of process damping in machining titanium alloy at low cutting speed

In machining titanium alloy as difficult-to-cut or hard materials, productivity is limited because of low heat conductivity at high spindle speed. Process damping is a damping mechanism during the machining process that can be exploited to improve the limited productivity in machining titanium alloy at low cutting speed. In the present study, experiments are performed to evaluate the wavelength performance of process-damped milling under irregular tool geometries such as uniform, variable helix, variable pitch, and variable helix/variable pitch. The effects of radial immersion, feed rate, depth of cut, and surface velocity as process parameters are also studied. By using flexible workpiece conditions, all tools are tested in machining titanium alloy at low speed to encourage the process damping phenomena. The wavelength performances are calculated on the basis of the chatter frequency domain analysis extracted from the acceleration signal in the time domain during the cutting process. The effect of irregular tool geometries shows that variable helix and variable pitch tools have the best wavelength performance compared with regular and other irregular tools. The effect of process parameters reveals that the radial depth of cut can also improve the process damping wavelength compared with regular tools. Irregular milling tool geometries with spindle speed and feed rate parameters can be used to suppress the chatter by exploiting the process damping behavior, thus improving machining productivity.

Study on performance in dry milling aeronautical titanium alloy thin-wall components with two types of tools

Abstract Cutting fluid may be considered an accessory which is frequently applied in order to increase material remove rate especially in cutting aerospace alloys where cutting temperature is very high. However the use of cutting fluids is nowadays limited due to the adverse effects such as health of the operator, environmental and economic reasons. In order to overcome these negative effects and meet the demand for environment-friendly cutting techniques, in this work, dry cutting is associated to compare the machinability of the two types of tools such as variable and uniform pitch tools in milling aeronautical thin-wall material Ti6Al4V. The experimental results have shown that the tool vibration is decreased obviously with variable pitch tool compared with uniform pitch tool during milling process of titanium alloy thin-part components. The vibration minimizing characteristics of variable pitch tool is analyzed using fast Fourier transform and wavelet analysis in the whole milling process.

Analytical Chatter Stability Prediction for Irregular Types of Milling Tools

High productivity, low cost and high profits in machining processes is important aspects in aerospace, automotive and tool/die metal manufacturing industries. However, the dynamic deflection of tool and work piece systems generates unstable cutting forces when machining with high material removal rate. The main focus of this paper is the study of variable tool geometries effect on chatter stability. A semi discretization method (SDM) was utilized to predict numerical chatter using variable helix, pitch and variable helix and pitch tools effect at high radial immersion. The results showed that variable helix and variable pitch milling tools exhibit better unstable behavior at 25 percent radial immersion.

Mechanism of Variable Helix Tools in Suppressing Chatter Using Process Damping for Machining Titanium Alloys at Low Cutting Speed

Titanium is difficult-to-cut materials due to its poor machinability and thermal conductivity when machining at high cutting speed. To overcome this machining titanium alloy problem, this study in interaction between machining structural system and the cutting process are very important. One of the main problems in the cutting process is chatter vibration. Due to chatter problem, the mechanism to suppress chatter named, process damping is a useful method can be manipulated to improve the limited productivity of titanium machining at low speed machining in milling process. In the present study, experiment are conducted to evaluate and study the process damping mechanism in milling using different types of variable tools geometries. These tools are variable he-lix/uniform pitch, variable pitch/uniform helix and variable helix and pitch and uniform helix/pitch. The result showed that the variable helix and pitch tools is very significantly improve process damping performance in machining titanium alloy compare to traditional of regular tools and other irregular tools.

Experimental Validation of Chatter Stability for Variable Helix Milling Tools

The occurrence of self-excited vibrations during machining is known as regenerative chatter, and this phenomenon can severely limit the machining productivity. By modifying and optimising the tool's pitch and helix geometry, this regenerative chatter can be suppressed to increase material removal rate. In this paper, experimental verification of an optimised variable helix and variable pitch tools is presented. The geometry was optimised using a Differential Evolution (DE) algorithm. Based on stability diagrams of original and optimised milling tools, the experiment was conducted and the results were compared when chatter occurred. The optimised cutter significantly outperformed the original cutter in term of chatter suppression.

Optimisation of variable helix tool geometry for regenerative chatter mitigation

It is well known that regenerative chatter can result in excessive tool wear, poor surface finish, and hence limited productivity during metal machining. Various mitigation methods can be applied to suppress chatter; however, the current paper focuses on applying optimal variable helix tool geometry. A semi-discretisation method is combined with Differential Evolution to optimise variable helix end milling tools so as to avoid chatter by modifying the variable helix and variable pitch tool geometry. The semi-discretisation method is first validated experimentally. The numerical optimisation procedure is then used to optimise tool geometry for a machining problem involving a flexible workpiece. The analysis predicted total mitigation of chatter using the optimised variable helix milling tool at a low radial immersion. However, in practice a five fold increase in chatter stability was obtained, compared to the traditional milling tool.

Analytical prediction of chatter stability for variable pitch and variable helix milling tools

Regenerative chatter is a self-excited vibration that can occur during milling and other machining processes. It leads to a poor surface finish, premature tool wear, and potential damage to the machine or tool. Variable pitch and variable helix milling tools have been previously proposed to avoid the onset of regenerative chatter. Although variable pitch tools have been considered in some detail in previous research, this has generally focussed on behaviour at high radial immersions. In contrast there has been very little work focussed on predicting the stability of variable helix tools. In the present study, three solution processes are proposed for predicting the stability of variable pitch or helix milling tools. The first is a semi-discretisation formulation that performs spatial and temporal discretisation of the tool. Unlike previously published methods this can predict the stability of variable pitch or variable helix tools, at low or high radial immersions. The second is a time-averaged semi-discretisation formulation that assumes time-averaged cutting force coefficients. Unlike previous work, this can predict stability of variable helix tools at high radial immersion. The third is a temporal-finite element formulation that can predict the stability of variable pitch tools with a constant uniform helix angle, at low radial immersion. The model predictions are compared to previously published work on variable pitch tools, along with time-domain model simulations. Good agreement is found with both previously published results and the time-domain model. Furthermore, cyclic-fold bifurcations were found to exist for both variable pitch and variable helix tools at lower radial immersions.

Modelling of the stability of variable helix end mills

Chatter in milling is still the main obstacle in achieving high-performance machining operations in industry. In this paper, an analytical model is presented to predict the chatter stability of the variable helix end mills. Owing to the lack of accurate and rapid modelling, variable helix tools are used simply on a trial and error basis, in the hope that they will improve the stability of the process. This work provides a comparative study of the performance of variable helix and variable pitch end mills. Time domain chatter recognition techniques and analytical models are explored and tested against experimental results. For the experimental validation, aluminium test pieces were used to enable a broad range of spindle speeds to be covered. The linearity of the machine tool dynamics is explored through validation of standard stability lobes. A comparison of the predicted and observed performance of variable helix against constant helix, variable pitch end mills are presented. The stability lobes are validated for each of the variable helix, variable pitch and standard tools. The analytical model assumes the variable helix tools to behave as variable pitch tools, calculating the average pitch angle for each flute. This approximation is proven to be accurate for some of the cases. For certain combinations of pitch and helix angle greatly enhanced stability is demonstrated empirically with up to a 20-fold increase in depth of cut for the variable helix over the equivalent variable pitch tool. This enhanced stability is neither predicted by the analytical nor time domain solutions. The time domain chatter recognition criterion is investigated and found to have little influence on the predicted stability plots. It is concluded that the enhanced stability is a result of some mechanism not represented in the well-established time domain model. It is possible that this is a result of the disturbance of regeneration in the manner of an alternating spindle speed or due to a non-linear process damping effect.