Effects of inclination angles on geometrical features of machined surface in five-axis milling

With the development of manufacturing technology, five-axis milling has been one of the most important solution strategies in machining field. To deepen the understanding of multi-axis processing and improve the application level of the technology, the current work was carried out. This paper investigated the effects of tilt and lead angle on the scallop height, surface roughness, surface topography, and surface damages in five-axis ball-end milling process. Both geometrical analysis and experiment research are conducted to investigate the scallop height after five-axis milling, and variation of the surface roughness and surface topography with tool inclination angle obtained from the experiments was analyzed. Surface damages under the different inclination angles were also observed and analyzed with optical profiler. Several conclusions are made as follows. The inclination angles of the ball-end mill have no effect on the scallop height when only the spherical part of the cutter participates in the cutting process according to the geometrical analysis. Surface roughness with regard to tilt angles presents symmetrical characteristic around 0°. Surface texture feature, especially the texture direction, is closely related with the tool posture. The surface concave pits, convex marks, microscopic cracks, and spot corrosions are mainly the damage forms of the machined surface. More surface blemishes appeared when small inclination angles are adopted in cutting. As a result, the recommendatory inclination angle values for inclination angle are proposed. A better understanding of the five-axis machining process would be given by the detailed analysis of generation reason of the machined surface features, and the results could provide support for process parameter optimization.     

MODELING OF 5-AXIS MILLING PROCESSES

5-axis milling operations are common in several industries such as aerospace, automotive and die/mold for machining of sculptured surfaces. In these operations, productivity, dimensional tolerance integrity and surface quality are of utmost importance. Part and tool deflections under high cutting forces may result in unacceptable part quality whereas using conservative cutting parameters results in decreased material removal rate. Process models can be used to determine the proper or optimal milling parameters for required quality with higher productivity. The majority of the existing milling models are for 3-axis operations, even the ones for ball-end mills. In this article, a complete geometry and force model are presented for 5-axis milling operations using ball-end mills. The effect of lead and tilt angles on the process geometry, cutter and workpiece engagement limits, scallop height, and milling forces are analyzed in detail. In addition, tool deflections and form errors are also formulated for 5-axis ball-end milling. The use of the model for selection of the process parameters such as lead and tilt angles that result in minimum cutting forces are also demonstrated. The model predictions for cutting forces and tool deflections are compared and verified by experimental results.     

MODELING OF 5-AXIS MILLING PROCESSES

5-axis milling operations are common in several industries such as aerospace, automotive and die/mold for machining of sculptured surfaces. In these operations, productivity, dimensional tolerance integrity and surface quality are of utmost importance. Part and tool deflections under high cutting forces may result in unacceptable part quality whereas using conservative cutting parameters results in decreased material removal rate. Process models can be used to determine the proper or optimal milling parameters for required quality with higher productivity. The majority of the existing milling models are for 3-axis operations, even the ones for ball-end mills. In this article, a complete geometry and force model are presented for 5-axis milling operations using ball-end mills. The effect of lead and tilt angles on the process geometry, cutter and workpiece engagement limits, scallop height, and milling forces are analyzed in detail. In addition, tool deflections and form errors are also formulated for 5-axis ball-end milling. The use of the model for selection of the process parameters such as lead and tilt angles that result in minimum cutting forces are also demonstrated. The model predictions for cutting forces and tool deflections are compared and verified by experimental results.     

Dispatching rules for unrelated parallel machine scheduling with release dates

A simplified procedure is proposed to predict the surface integrity of complex-shape parts generated by ball-end finishing milling. Along a complex cutting path, the tool inclination may vary within a large range. A geometrical study is performed to predict the effect of the tool inclination (lead angle) on the micro-geometry of the machined surface and on the effective cutting speed. This geometrical study brings out a range of values of the lead angle for which the machined surface is damaged by cutting pull-outs. This geometrical study also brings out a range of values of the lead angle for which the effective cutting speed is null. This case corresponds to extreme values of the cutting forces and to high compressive residual stresses. These predictions are verified for a selection of tool inclinations and other cutting parameters such as cutting speed, feed per tooth and cusp height. These machining tests are performed on a high-strength bainitic steel. The experimental campaign includes milling tests with cutting forces measurements, 2-D optical micro-geometry measurements and X-ray diffraction measurements.     

Improved dynamic cutting force model in ball-end milling. Part I: theoretical modelling and experimental calibration

An accurate cutting force model of ball-end milling is essential for precision prediction and compensation of tool deflection that dominantly determines the dimensional accuracy of the machined surface. This paper presents an improved theoretical dynamic cutting force model for ball-end milling. The three-dimensional instantaneous cutting forces acting on a single flute of a helical ball-end mill are integrated from the differential cutting force components on sliced elements of the flute along the cutter-axis direction. The size effect of undeformed chip thickness and the influence of the effective rake angle are considered in the formulation of the differential cutting forces based on the theory of oblique cutting. A set of half immersion slot milling tests is performed with a one-tooth solid carbide helical ball-end mill for the calibration of the cutting force coefficients. The recorded dynamic cutting forces are averaged to fit the theoretical model and yield the cutting force coefficients. The measured and simulated dynamic cutting forces are compared using the experimental calibrated cutting force coefficients, and there is a reasonable agreement. A further experimental verification of the dynamic cutting force model will be presented in a follow-up paper.     

Cutting force prediction for ball-end mills with non-horizontal and rotational cutting motions

Accurate cutting force prediction is essential to precision machining operations as cutting force is a process variable that directly relates to machining quality and efficiency. This paper presents an improved mechanistic cutting force model for multi-axis ball-end milling. Multi-axis ball-end milling is mainly used for sculptured surface machining where non-horizontal (upward and downward) and rotational cutting tool motions are common. Unlike the existing research studies, the present work attempts to explicitly consider the effect of the 3D cutting motions of the ball-end mill on the cutting forces. The main feature of the present work is thus the proposed generalized concept of characterizing the undeformed chip thickness for 3D cutter movements. The proposed concept evaluates the undeformed chip thickness of an engaged cutting element in the principal normal direction of its 3D trochoidal trajectory. This concept is unique and it leads to the first cutting force model that specifically applies to non-horizontal and rotational cutting tool motions. The resulting cutting force model has been validated experimentally with extensive verification test cuts consisting of horizontal, non-horizontal, and rotational cutting motions of a ball-end mill.     

Investigating the cutting phenomena in free-form milling using a ball-end cutting tool for die and mold manufacturing

This paper presents an investigation of nonplanar tool-workpiece interactions in free-form milling using a ball-end cutting tool, a technique that is widely applied in the manufacturing of dies and molds. The influence of the cutting speed on the cutting forces, surface quality of the workpiece, and chip formation was evaluated by considering the specific alterations of the contact between tool-surface along the cutting time. A trigonometric equation was developed for identifying the tool-workpiece contact along the toolpath and the point where the tool tip leaves the contact with the workpiece. The experimental validation was carried out in a machining center using a carbide ball-end cutting tool and a workpiece of AISI P20 steel. The experimental results demonstrated the negative effect of the engagement of the tool tip into the cut on machining performance. The length of this engagement depends on the tool and workpiece curvature radii and stock material. When the tool tip center is in the cut region, the material is removed by shearing together with plastic deformation. Such conditions increase the cutting force and surface roughness and lead to an unstable machining process, what was also confirmed by the chips collected.     

球头铣刀高速铣削斜面的三维数值模拟研究

用球头铣刀高速铣削斜面是在三轴加工中心上加工模具时的一种走刀方式。根据球头铣刀高速铣削斜面的特点,建立了在垂直向上和向下、水平向上和向下四种走刀方式下高速铣削45°斜面,以及在垂直向下走刀方式下高速铣削30°、60°、75°斜面的三维有限元模型,以分析不同走刀方式下铣削斜面以及铣削不同角度斜面时切削力和切削温度的变化规律。模拟结果表明,在铣削45°斜面时,采用向上走刀方式较向下走刀方式的切削力幅值小、波动大,且切削温度高;采用垂直向下走刀方式铣削大角度斜面时也出现类似情况。对切削力的实测结果验证了该模型的可靠性。     

Effects of cutting edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and forces in finish turning of hardened AISI H13 steel

In this study, the effects of cutting edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and resultant forces in the finish hard turning of AISI H13 steel were experimentally investigated. Cubic boron nitrite inserts with two distinct edge preparations and through-hardened AISI H13 steel bars were used. Four-factor (hardness, edge geometry, feed rate and cutting speed) two-level fractional experiments were conducted and statistical analysis of variance was performed. During hard turning experiments, three components of tool forces and roughness of the machined surface were measured. This study shows that the effects of workpiece hardness, cutting edge geometry, feed rate and cutting speed on surface roughness are statistically significant. The effects of two-factor interactions of the edge geometry and the workpiece hardness, the edge geometry and the feed rate, and the cutting speed and feed rate also appeared to be important. Especially honed edge geometry and lower workpiece surface hardness resulted in better surface roughness. Cutting-edge geometry, workpiece hardness and cutting speed are found to be affecting force components. The lower workpiece surface hardness and honed edge geometry resulted in lower tangential and radial forces.     

微铣金属表面微沟槽结构的粗糙度及形貌分析

为了提高和改善微沟槽表面质量,设计了高速微铣削实验,研究了微沟槽底面表面粗糙度和侧壁残留毛刺的变化规律。从理论角度引入了已加工表面的形成机理,建立了微观表面粗糙度理论模型,提出了刀具跳动对侧壁形貌变化影响的规律。利用三轴联动精密微细铣削机床加工微细直沟槽,并选取主轴转速、轴向切深、进给速度、刀具跳动量和材料组织结构为研究因素。采用多因素正交实验和极差分析法,对表面粗糙度值进行数值分析。铝合金,钢和钛合金三类微沟槽底面对应的最佳表面粗糙度值变化范围分别为1.073~1.481 μm,0.485~0.883 μm,0.235~0.267 μm;无刀具跳动钛合金微沟槽壁毛刺的最大高度为7.637 μm,而当刀具存在0.3 μm的径向综合跳动量时对应的微槽壁毛刺的最大高度为21.79 μm。铣削参数对表面粗糙度值的影响按从大到小依次为进给速度、主轴转速、轴向切深,且随着进给速度和轴向切深的增大,表面粗糙度值增大;随着主轴转速的增大,表面粗糙度值先减小后增大;在相同加工条件下,若微圆弧刀刃无磨损,微刀具的跳动量对微直沟槽侧壁表面质量有较大影响。同时,不同金属材料特性也是影响微沟槽表面质量的潜在因素。     

INDUCTION-HEATED TOOL MACHINING OF ELASTOMERS—PART 2: CHIP MORPHOLOGY, CUTTING FORCES, AND MACHINED SURFACES

The induction-heated tool and cryogenically cooled workpiece are investigated for end milling of elastomers to generate desirable shape and surface roughness. Elastomer end milling experiments are conducted to study effects of the cutting speed, tool heating, and workpiece cooling on the chip formation, cutting forces, groove width, and surface roughness. At high cutting speed, smoke is generated and becomes an environmental hazard. At low cutting speeds, induction heated tool, if properly utilized, has demonstrated to be beneficial for the precision machining of elastomer with better surface roughness and dimensional control. Frequency analysis of cutting forces shows that the soft elastomer workpiece has low frequency vibration, which can be correlated to the surface machining marks. The width of end-milled grooves is only 68 to 78% of the tool diameter. The correlation between the machined groove width and cutting force reveals the importance of the workpiece compliance to precision machining of elastomer. This study also explores the use of both contact profilometer and non-contact confocal microscope to measure the roughness of machined elastomer surfaces. The comparison of measurement results shows the advantages and limitations of both measurement methods.     

A MACHINING PERFORMANCE STUDY IN DRY CONTOUR TURNING OF ALUMINUM ALLOYS WITH FLAT-FACED AND GROOVED DIAMOND TOOLS

This paper presents the results from an experimental study of dry contour turning operations on aluminum alloys (6061B and 2011-T3) using PCD flat-faced and diamond coated grooved tools. The machining performance is assessed on the basis of cutting forces, chip flow, chip-form and surface roughness observed during contour turning operations. The constantly varying cutting conditions (especially effective depth of cut due to varying geometry of the contour surface) and effective tool geometry cause a wide fluctuation in cutting forces and the ensuing chip flow. The chip flow angle is measured along the contour geometry using high-speed filming techniques and these results are compared with predicted chip flow values from the measured experimental cutting forces (which are measured along the entire contour geometry). The resultant surface roughness at different locations along the contour profile is measured and correlated with the chip flow and chip-form variations. Machining performance issues specifically relevant to dry contour turning of aluminum (such as problems due to poor chip flow and the resultant poor surface roughness) are studied and the effectiveness of selective work-tool (both tool material and tool geometry) pairs is illustrated.     

Model-based identification of tool runout in end milling and estimation of surface roughness from measured cutting forces

This paper presents a model-based approach for the identification of tool runout and the estimation of surface roughness from measured cutting forces. In the first part of the paper, the effect of tool runout on variations in the cutting forces and the effect on surface roughness generation are studied. Thereby, several influencing parameters are identified and examined systematically. Based on theoretical considerations, systematic relationships between tool runout, resultant process force variations, and surface roughness characteristics are deduced. The sensitivity of process force variation is investigated for varying runout parameters by experimental tests. In the next part, the model-based runout identification method is developed, which identifies runout parameters accurately from the measured process forces. The approach has been tested extensively and was verified by measured runout parameters and the correlation of surface roughness characteristics of the machined workpiece. The performance of the developed approach is demonstrated in the final part by comparing the result of the model-based surface reconstruction with the measured surface topography.     

INDUCTION-HEATED TOOL MACHINING OF ELASTOMERS—PART 2: CHIP MORPHOLOGY,CUTTING FORCES,AND MACHINED SURFACES

ABSTRACT

The induction-heated tool and cryogenically cooled workpiece are investigated for end milling of elastomers to generate desirable shape and surface roughness. Elastomer end milling experiments are conducted to study effects of the cutting speed, tool heating, and workpiece cooling on the chip formation, cutting forces, groove width, and surface roughness. At high cutting speed, smoke is generated and becomes an environmental hazard. At low cutting speeds, induction heated tool, if properly utilized, has demonstrated to be beneficial for the precision machining of elastomer with better surface roughness and dimensional control. Frequency analysis of cutting forces shows that the soft elastomer workpiece has low frequency vibration, which can be correlated to the surface machining marks. The width of end-milled grooves is only 68 to 78% of the tool diameter. The correlation between the machined groove width and cutting force reveals the importance of the workpiece compliance to precision machining of elastomer. This study also explores the use of both contact profilometer and non-contact confocal microscope to measure the roughness of machined elastomer surfaces. The comparison of measurement results shows the advantages and limitations of both measurement methods.     

High-performance circular sawing of AISI 1045 steel with cermet and tungsten carbide inserts

This work investigated the influence of cutting speed and feed rate on cutting forces, surface roughness, and slot width circular sawing of AISI 1045 steel. The effects of tool material (cermet and tungsten carbide) and geometry (chip breaker flute and pre-cutting/post-cutting teeth) were also investigated. Thrust and radial forces generally tended to decrease as the cutting speed increased and tended to increase with the feed rate. The lowest values of thrust and radial forces were obtained using a tungsten carbide saw ground with precutting and post-cutting teeth. With regard to the quality of the machined wall, the lowest surface roughness was obtained by applying the highest cutting speed and lowest feed rate and employing a cermet brazed saw. Under this condition, roughness values comparable to face turning and parting off operations were obtained. The cermet brazed saw was responsible for producing the narrowest slot widths.     

Dynamic Force Modelling for a Ball-End Milling Cutter Based on the Merchant Oblique Cutting Theory

A new dynamic force model for a ball-end milling cutter is presented in this paper. Based on the principle of the power remaining constant in cuts, the Merchant oblique cutting theory has been successfully used for the differential cutting edge segment of a ball-end milling cutter. A concise method for characterising the relationship of the complex geometry of a ball-end milling cutter and the milling process variables is determined, so that the force coefficients can be decomposed. The geometric property of a ball-end milling cutter and the dynamics of the milling process are integrated into the general model to eliminate the need for the experimental calibration of each cutter geometry and milling process variable. The milling experiments prove that this model can predict accurately the cutting forces in three Cartesian directions.     

凸曲面拼接模具球头铣刀的瞬时铣削力预测

在曲面模具拼接区域球头铣刀铣削过程中,刀具载荷变化大,瞬态铣削力有突变现象,影响模具拼接区域的加工精度和表面质量。为了预测拼接区域球头铣刀的瞬态铣削力,首先,建立考虑冲击振动的球头铣刀三维次摆线轨迹方程,得到瞬时未变形切屑厚度模型;然后,基于铣削微元的思想,建立凸曲面双硬度拼接模具球头铣刀的瞬态铣削力模型,该模型能够综合考虑拼接区冲击振动、硬度变化、刀具工件切触角度变化对瞬态铣削力的影响;最后,进行凸曲面拼接区域球头铣刀铣削加工实验。实验结果表明,预报的瞬态铣削力和实验测量结果在幅值上和变化趋势上具有一致性,在平稳切削时最大铣削力预测误差值基本在15%以内,验证了该模型能有效地预报凸曲面模具拼接区域球头铣刀的瞬态铣削力。     

Machinability aspects concerning micro-turning of PA66-GF30-reinforced polyamide

This paper aims to study the behavior of machining forces and machined surface finish when micro-turning PA66-GF30-reinforced polyamide with various tool materials under distinct cutting conditions. The performance of polycrystalline diamond (PCD), CVD diamond coated carbide and plain cemented carbide tools (K15-KF and K15) were investigated in addition to the influence of feed rate on cutting forces, surface roughness and chip formation. The results indicated that the radial force was the highest force component because of the reduction in the effective cutting edge angle. Moreover, the cutting force increased almost linearly with feed, whereas the feed and radial forces remained unaltered. The cutting tools possessing lower edge radius promoted lower surface finish and turning forces, i.e., the best results were provided by the PCD tool, followed by the uncoated carbide inserts and finally by the CVD diamond-coated carbide tool.     

Process modeling and toolpath optimization for five-axis ball-end milling based on tool motion analysis

In free-form surface machining, the prediction of five-axis ball-end milling forces is quite a challenge due to difficulties of determining the underformed chip thickness and engaged cutting edge. Part and tool deflections under high cutting forces may result in poor part quality. To solve these concerns, this paper presents process modeling and optimization method for five-axis milling based on tool motion analysis. The method selected for geometric stock modeling is the dexel approach, and the extracted cutter workpiece engagements are used as input to a force prediction. The cutter entry?Cexit angles and depth of cuts are found and used to calculate the instantaneous cutting forces. The process is optimized by varying the feed as the tool?Cworkpiece engagements vary along the toolpath, and the unified model provides a powerful tool for analyzing five-axis milling. The new feedrate profiles are shown to considerably reduce the machining time while avoiding process faults.     

<Emphasis Type="Bold">Prediction of Surface Topomorphy and Roughness in Ball-End Milling</Emphasis>

Milling is today the most effective, productive and flexible-manufacturing method for machining complicated or sculptured surfaces. Ball-end tools are used for machining 3D freeform surfaces for dies, moulds, and various parts, such as aerospace components, etc. Milling data, such as surface topomorphy, surface roughness, non-deformed chip dimensions, cutting force components and dynamic cutting behaviour, are very helpful, especially if they can be accurately produced by means of a simulation program. This paper presents a novel simulation model, the so-called MSN-Milling Software Needle program, which is able to determine the surface produced and the resulting surface roughness, for ball-end milling. The model simulates precisely the tool kinematics and considers the effect of the cutting geometry on the resulting roughness. The accuracy of the simulation model has been thoroughly verified, with the aid of a wide variety of cutting experiments. Many roughness measurements were carried out on workpieces, which were cut using a 5-axis machining centre. The calculated roughness levels were found to be in agreement with the experimental ones. The proposed model has proved to be suitable for determining optimal cutting conditions, when finishing complex surfaces. The software can be easily integrated into various CAD-CAM systems.