Multi-axis Milling Research Articles

Influence of Cutter Orientations on Cutting Force in Ball End Milling of TC17

In order to improve the ball end milling multi-axis milling titanium alloy workpiece machining method, the influence of cutter orientations on cutting force is studied in this paper by experiment. This paper uses the tool rotation angle and the tool inclination angle to describe the cutter orientations. Analyse the cutting force under different milling posture. The results prove that, when the tool inclination angle is constant, the change of tool rotation angle will cause changes in the value and direction of the cutting force. Though maintain the tool rotation angle, the change of the tool inclination angle will change the cutting speed, thus make the cutting force decrease first then increase.

Selecting industrial robots for milling applications using AHP

Industrial robots are usually used for pick-and-place applications, which require only point-to-point motion control. However, the recent developments, both in robot technology and in Computer Automated Machining (CAM) software, allow the use of these equipment in applications which require continuous path control, such as multi-axis milling processes. However, the producers do not offer robots specifically developed for this kind of application, thus the user has to choose the most appropriate robot for this goal from a wide range of general purpose robot types. This research work proposes a method based upon Analytic Hierarchy Process (AHP) for selecting the industrial robot for milling applications. Several attributes of the robots are proposed as selection criteria, and a simulation study of the milling capabilities of the robotic structure is also taken into consideration.

Process optimization and typical application based on geometrical analysis and response surface method for high-speed five-axis ball-end milling operation

High-speed five-axis ball-end milling operations could achieve high efficiency, good surface integrity, and high accuracy in machining of complex components for many manufacture fields. This work concentrated on the machined surface properties and cutting performance produced by the multi-axis ball-end milling process in order to enhance the high performance application of this technology. Variations of the potential tool–workpiece contact zone; the cutting section area, and perimeter corresponding to eight types of tool posture; and the influence of the different tool postures on the machining characteristics were analyzed by geometrical modeling method. The tool postures with negative tilt angles, positive lead angles, or compound inclination angles with negative tilt angle and positive lead angle are beneficial to improve cutting effects with larger effective cutting speed. Discussions on the response analysis and optimization of the machined surface roughness, surface hardness, and the average and maximum cutting forces were conducted under up milling condition. Furthermore, the multi-objective coupling optimization for process performance and surface integrity and typical application in practical machining were carried out for high-speed multi-axis ball-end milling, and the optimization plans are validated by detection and analysis of the machined surface. The research results would promote the high performance application of the high-speed multi-axis ball-end milling operation.

Generalized model for dynamics and stability of multi-axis milling with complex tool geometries

Multi-axis milling offers increased accessibility in milling of parts having geometrical constraints or free form surfaces, where variety of cutting tools and edge geometries are utilized to improve stability and productivity of the processes. In order to machine the desired geometries effectively, in conjunction with multi-axis orientations, special tools ranging from taper end mills to process specific form tools are utilized. For such cases, the cutter workpiece engagement boundaries (CWEB) and directional force vector definitions are very complex and cannot be defined analytically. Furthermore, irregular cutting edge geometries, such as variable helix, pitch and serrations introduce multiple time delays between successive cuts where the conventional analytical frequency domain stability solution cannot be used. Prediction of stability diagrams for such variety of tool forms and edge geometries requires both the process and the tool to be defined in a generalized manner. In this paper, a numerical frequency domain milling stability solution method is proposed. The CWEB for complex multi-axis cases are calculated by the general projective geometry approach, where the cutting tool envelope and the cutting edges are represented as organized point clouds. Zeroth-Order approximation (ZOA) frequency domain method is adapted by introducing a speed average time delay term to encompass regular and irregular tool geometries. The stability limits are predicted by iterative solution of the eigenvalue problem using the ZOA frequency domain with the proposed revision. The effect of the process damping is also introduced into the generalized stability solution in order to predict the increase in the stability limits at low cutting speeds. A previously proposed approach is used to calculate the average process damping coefficients, which are introduced as modifiers to the modal parameters. The proposed generalized methodology is applied on several cases and it is shown that stability limits can be predicted within a reasonable accuracy for wide variety of cutting tools and milling operations.

Accessible regions of tool orientations in multi-axis milling of blisks with a ball-end mill

Noninterference tool orientations are hard to be determined in multi-axis milling of blisks, because the integrated structure of blisks introduces more geometrical constraints. To address this problem, an original approach without interference detection is proposed to solve accessible regions of tool orientations in milling of blisks. Based on the visibility of checking surfaces, only critical points on surface profiles are searched and processed to construct the accessible regions. In this approach, the start point of each profile is sought on boundaries of the main surface with a constant step length. From this point, a new critical point is first searched along a specified direction and then adjusted iteratively until it locates on a profile. When all critical points on profiles are searched like this, the searched points on each checking surface are reordered to form one or more than one closed curve to present the accessible region related to this checking surface. These curves are then mapped onto a unit sphere and divided into nonintersecting segments. After that, a concise approach is proposed to combine accessible regions of all checking surfaces into simply connected regions. This algorithm is finally verified with two kinds of blisks and compared with a referenced method. The results show that the proposed method can efficiently solve accessible regions in multi-axis milling of blisks with an expected accuracy.

Instantaneous Dynamics of Multi-Axis Milling Thin-Walled Workpiece with Complex Curved Surface

The first step to predict the milling stability is to identify the dynamic characteristics of cutting process. And the mass loading effects of removal material play an important role on the dynamic characteristics of milling process for thin-walled parts, such as impeller, turbine blades and automobile components, which is changing with cutting time or tool position. Therefore, how to identify the instantaneous dynamic characteristics of milling process is one of the most significant problems. In the paper, a structural dynamic modification method with variable mass to predict the instantaneous dynamic characteristics of multi-axis milling thin-walled workpiece with complex curved surface is proposed. The proposed method takes into account the variations of dynamics characteristics of workpiece with the tool position and material removal. And the material cutting process is regarded as the structural dynamic modifications of cutting system, the instantaneous dynamic characteristics of which can be estimated by the extended Sherman-Morrison-Woodbury formula to obtain the corrected frequency response function (FRF). Experiments were carried out to obtain the instantaneous dynamics of a thin-walled workpiece and the results were verified by finite element method (FEM).

Cutter-workpiece engagement determination for general milling using triangle mesh modeling

Abstract Cutter-workpiece engagement (CWE) is the instantaneous contact geometry between the cutter and the in-process workpiece during machining. It plays an important role in machining process simulation and directly affects the calculation of the predicted cutting forces and torques. The difficulty and challenge of CWE determination come from the complexity due to the changing geometry of in-process workpiece and the curved tool path of cutter movement, especially for multi-axis milling. This paper presents a new method to determine the CWE for general milling processes. To fulfill the requirement of generality, which means for any cutter type, any in-process workpiece shape, and any tool path even with self-intersections, all the associated geometries are to be modeled as triangle meshes. The involved triangle-to-triangle intersection calculations are carried out by an effective method in order to realize the multiple subtraction Boolean operations between the tool and the workpiece mesh models and to determine the CWE. The presented method has been validated by a series of case studies of increasing machining complexity to demonstrate its applicability to general milling processes. Highlights A new method to determine cutter-workpiece engagement geometry in milling. Applicable to general milling processes: all cutter types, in-process workpiece shapes and tool paths containing even self-intersections. Results validated by a series of case studies of increasing machining complexity.

Multi-axis NC spiral milling of thin-walled aluminium 7075/T651 blade with spiral tool path generation algorithm

Aiming at low machining accuracy, low efficiency, cumbersome programming process, strong empirism and great programming difficulty of thin-walled blade machining, a new NC spiral milling method of high quality and high efficiency is put forward to process thin-walled blades. The spiral tool path to process a blade on a machine centre DMU 60 monoBLOCK is generated, simulated and post-processed on NX platform. Through a series of machining experiments, the method is validated.

Design of five-axis ultrasonic assistant compound machine tool

A compound machine tool was designed, which combined rotary ultrasonic assisted grinding, electrical discharge machining (EDM) and multi-axis milling. Experimental results indicated that its positioning accuracy was less than 5.6 µm and its repetitive positioning accuracy was less than 1.8 µm; the vibration amplitude of ultrasonic grinding system was uniform and stable, and the EDM system worked well and stably. A smooth surface of K9 optical glass component was achieved by the grinding method.

Adaptive location of repaired blade for multi-axis milling

Abstract Free-form blades are widely used in different industries, such as aero-engine and steam turbine. Blades that are damaged during service or have production deficiencies are usually replaced with new ones. This leads to the waste of expensive material and is not sustainable. However, material and costs can be saved by repairing of locally damaged blades or blades with localized production deficiencies. The blade needs to be further machined after welding process to reach the aerodynamic performance requirements. This paper outlines an adaptive location approach of repaired blade for model reconstruction and NC machining. Firstly, a mathematical model is established to describe the localization problem under constraints. Secondly, by solving the mathematical model, localization of repaired blade for NC machining can be obtained. Furthermore, a more flexible method based on the proposed mathematical model and the continuity of the deformation process is developed to realize a better localization. Thirdly, by rebuilding the model of the repaired blade and extracting repair error, optimized tool paths for NC machining is generated adaptively for each individual part. Finally, three examples are given to validate the proposed method. Highlights A mathematical model is established to describe the localization problem. Detailed solution steps are given to solve the problem. Global location method and cross section location method are introduced. Repaired blade model is built for allowance extraction and tool path generation. Material and cost savings can be achieved by repairing of locally damaged blades.

Mechanics-based feedrate scheduling for multi-axis plunge milling

Multi-axis plunge milling has an increasing application in the manufacturing industry to rough machine complex parts, such as blisks and impellers. In order to prevent excessive cutting forces and unexpected tool breakage, cutting parameters of multi-axis plunge milling are usually assigned to be conservative according to the maximum cutting load in one feeding phase. This is an obstacle to improve the productivity of rough machining. To address this challenge, an original approach is proposed to schedule the feedrate of multi-axis plunge milling based on the mechanistic model. The cross section of the removed material is used to calculate the complicated engagement of multi-axis plunge milling. First, an accurate mathematic model of the cross section is established by its defining parameters. Second, a specific mechanistic model for multi-axis plunge milling is deduced based on the identified cross section. Then, a mechanics-based feedrate scheduling strategy is proposed to regulate the cutting forces and torque in multi-axis plunge milling. Finally, three feedrate scheduling methods with different references are conducted and compared in experimental tests to verify this new approach. The method proposed in this paper can reduce the cutting time and smooth the variances of cutting forces and torque.

Integrated Simulation System for 5-axis Milling Cycles

Abstract Multi axis milling is a promising technology for machining of complex geometries and surfaces. Thanks to the additional rotary axis, it provides flexibility to overcome the accessibility issues. However, the process becomes more complicated in terms of geometry, mechanics and dynamics due to the additional degrees of freedom and ball nose end mills. In this study, mechanics and stability of 5-axis ball end milling operations is simulated throughout a given toolpath. The cutting tool-workpiece engagement boundary is calculated and workpiece is represented using distance field approach. Distance field approach features very low computational time, close to real-time simulation. The cutting forces are modeled using orthogonal-to-oblique transformation. Stability limits are estimated in frequency domain.

Axis Position Dependent Dynamics of Multi-axis Milling Machines

Numerous analytical methods are available to predict the stability of milling processes. Most of these methods base on the assumption, that the dynamics of the machine tool are time invariant. This assumption seems to be valid in many cases. However, in case of huge translational or rotatory axes movements or process-induced changes in the workpiece's mass and elasticity a time variant dynamic model might be needed. This paper presents a method to model the axis position dependent dynamics of a multi-axis milling machine. According to this method, the modal parameters of the machine tool are predetermined in different discrete axis positions. An interpolation strategy allows calculatingthe modal parameters in arbitrary resolution along arbitrary toolpaths. Here, an exemplary 2.5-dimensional milling process serves as an example. The conventional step-by-step time domain simulation procedure is complemented by the modal interpolation strategy to account for changing machine dynamics. The effect of changing dynamics on the process is determined and a comparison to a cutting test is performed.

Modeling and on-line simulation of surface topography considering tool wear in multi-axis milling process

The understanding and prediction of the generated surface topography are very important in milling process. This paper presents a surface topography model and an on-line simulation method of surface topography considering tool wear in a multi-axis milling process. First, the cutting edge equation is established by dividing the cutting edge into a series of cutting edge elements considering tool wear influence and tilt angle of tool axis on each discrete cutting edge element, and an algorithm is proposed to determine the range of divided position angle. Then, the sculptured surface workpiece is divided into a series of elements along the feed direction with the same or less than discrete precision of cutting edge. The intersection points between the discrete cutting edge elements and the planes of both sides of each divided element on workpiece can be obtained based on the established cutting edge equation in the cutting process. The left cutting trajectory points on workpiece are got by reserving the points with the minimum coordinate value in the Z-axis direction under the same coordinate value in the X-axis and Y-axis direction among the intersection points and points on workpiece. Thus, surface topography model can be constructed based on the established cutting edge equation, the determined range of position angle, discrete cutting edge elements, and divided element on workpiece surface. Finally, an on-line simulation algorithm is proposed to simulate the generation process of surface topography in multi-axis milling operation. Experimental work and validation of the established model are performed on a five-axis milling center by using stainless steel 0Cr17Ni12Mo2 and cemented carbides milling cutter. The results about the topography generation of the constructed B-spline surface show that the predictions of the established surface topography model correlate well with the experimental results.

Generalized cutting force model in multi-axis milling using a new engagement boundary determination approach

Simulation of cutting processes provides valuable insight into machining applications which have complex mechanics. In this paper, a generalized cutting force model is proposed for multi-axis milling operations. In the proposed model, the cutting tool envelope is defined either as revolution of a multi-segmented curve or using seven-parameter milling tool definition. The engagement between the cutting tool envelope and workpiece is calculated using a new, robust, and fast approach based on projective geometry. Exact chip thickness expression is used to simulate cutting kinematics for all types of edge geometries, such as serrated, variable pitch, and variable helix cutting flutes. The performance of the method proposed for determination of engagement boundaries is discussed through calculation time studies under several conditions. The predictions are verified and discussed through cutting experiments, conducted at multi-axis machining conditions using various cutting tool geometries.

A Localization Model for Snubber Machining of Precision Forging Blade

The compressor blades of aero-engines are usually produced with precision forging technology to get net-shape free-form surface of the blades. Due to the complex shape of blade snubber, it’s very difficult to get net-shape for it. While, there are still lots of raw material around it, further multi-axis machining is needed. However, without raw material left on blade surfaces, it is very difficult and skill-dependent to design special fixture to localize the blade with its net-shape free-form surface for multi-axis milling. In order to mill blade snubber with high accuracy, a mathematical model for localization precision forging blade with snubber is proposed in this paper. According to the processing characteristic of precision forging blades, a point set selection method based on the CAD model is proposed. In this method, localization transform parameter is obtained by using SVD (Singular Value Decomlocalizing), and solving process is completed by employing iterative nearest point (ICP) algorithm. Finally, a machining simulation experiment is carried out to validate the effectiveness of the proposed method.

Toolpath dependent chatter suppression in multi-axis milling of hollow fan blades with ball-end cutter

Aiming at the issue of toolpath dependent machining vibration in multi-axis milling of hollow fan blades, this paper presents an optimal selection method of cutting parameters based on single-line toolpath to suppress cutting chatter. Firstly, the impact of hollow structure on the blade structural modal was analyzed by using the modal analysis method. And the unstable regions of hollow blade surface have been predicted, which were prone to induce machining deformation and vibration. Secondly, the relationship between the hollow structure and the dynamic characteristics was revealed by analyzing the dynamic responses to the different cutting positions of blade surface. Thirdly, the optimization of cutting parameters based on single-line toolpath was proposed by establishing the 3D stability lobe diagram. Finally, the feasibility and effectiveness about the analysis of dynamic characteristics and the suppression method of cutting chatter were verified by a milling experiment of hollow blade.

A method for off-line compensation of cutting force-induced errors in robotic machining by tool path modification

Industrial robots are promising cost-effective and flexible alternatives for multi-axis milling applications in machining of complex parts of light materials with lower tolerances, having freeform surfaces. As it is well-known, the poor accuracy, stiffness, and the complexity of programming are the most important limiting factors for wider adoption of robotic machining in machine shops. The paper presents the developed method for off-line compensation of machining robot tool tip static displacements as a dominant part of cutting force-induced errors. The developed method is based on modification of programmed trajectory in G-code. Off-line modification of programmed trajectory is performed according to the predicted static tool tip displacements calculated based on developed robot compliance model and cutting forces predicted by mechanistic model. The obtained experimental results show the relevance of developed method since the machining errors could be significantly reduced. This allows the desired accuracy of robot machining to converge towards nominal specifications.

Extraction of Cutter-Workpiece Engagement for Multi-Axis Milling

Cutter-workpiece engagement (CWE) is the contact zone where the cutting flutes enter and exit the workpiece as a function of axial depth of cut. Determination of CWE is the basis for predictions of cutting forces, machining error and chatter stability in multi-axis milling. In this paper, a solid modeling approach is developed to extract CWE for multi-axis milling by means of removal volume, which is obtained by subtracting analytic tool swept volume from in-process workpiece. Meanwhile, the feasible contact surfaces are used to trim the removal volume in order to determine CWE surfaces. Simulations are also performed to confirm the validity of the proposed method.

Advanced CNC Programming Methods for Multi-Axis Precision Machining

This paper presents a method to simultaneously increase the accuracy and decrease the calculation time for complex tool path programming in multi-axis machining centers. Examples of complex parts requiring such complex tool paths include various kinds of turbine blades, pump-forcing augers, teeth surfaces, etc. It explains the creation of topological structures on the basis of analytical spline curves with floating range definitions. The method for tool path calculation accommodates the specific requirements for multi-axis milling. The algorithms developed are the foundation for the CAD/CAM software that allows for NC programming and machining on 5-axis centers employing any design model. Industrial tests reveal a 70-80% reduction of NC programming time of parts with complex surfaces, reduced machining time of approximately 40-50% using basic high-speed cutting methods and custom-made tools. The advanced methods of NC programming result in substantially increasing machining accuracy.