Influence of the number of inserts for tool life evaluation in face milling of steels

Tool life tests are often employed to verify the behaviour of one or more inserts in a cutter in order to optimise machining productivity and minimise cost. In milling process, such tests are expensive and require many of tools and a lot of work material to achieve any of the stipulated tool rejection criterion in any of the inserts. In practice, tool life tests are usually carried out using only one or few edges in a face milling cutter in order to minimise cost. The aim of this study is to investigate the effect of the number of tools used in face milling operation and how they relate to the establishment of tool life under specified cutting conditions. Flank wear curves were evaluated for AISI 1045 and 8640 steels using 1, 2, 3 and 6 inserts in a face milling cutter. Test results show that reduction in the number of inserts in the milling cutter led to a reduction in the amount of material removed and also tend to increase tool life when machining at the same feed per tooth. Results obtained using reduced number of inserts in a milling cutter should only be used for comparison between two or more conditions and should not be used to establish tool life.     

硬质合金涂层刀片铣削AISI410不锈钢刀具寿命研究

AISI410不锈钢广泛应用于各种工业设备的制造,如汽轮机叶片,各种泵的机械零件和蒸汽设备等。加工过程中存在着导热率低、加工硬化严重、切削力大等问题,属于典型的难加工材料。本文通过硬质合金涂层刀片铣削AISI410不锈钢正交试验,研究了硬质合金涂层刀片铣削AISI410不锈钢时刀具寿命的变化规律,归纳了相应的刀具寿命模型,并对其中的各影响因素的独立作用效果进行了分析。试验结果表明:硬质合金涂层刀片加工AISI410不锈钢时,铣削方式、每齿进给量、切削速度、轴向切削深度对刀具寿命的影响依次减小;切削速度越低,每齿进给量对于刀具寿命的影响越大;切削效率一定时,降低切削速度增大每齿进给量有利于延长刀具寿命。     

Mechanics and dynamics of general milling cutters. Part II: inserted cutters

Inserted cutters are widely used in roughing and finishing of parts. The insert geometry and distribution of inserts on the cutter body vary significantly in industry depending on the application. This paper presents a generalized mathematical model of inserted cutters for the purpose of predicting cutting forces, vibrations, dimensional surface finish and stability lobes in milling. In this paper, the edge geometry is defined in the local coordinate system of each insert, and placed and oriented on the cutter body using the cutter's global coordinate system. The cutting edge locations are defined mathematically, and used in predicting the chip thickness distribution along the cutting zone. Each insert may have a different geometry, such as rectangular, convex triangular or a mathematically definable edge. Each insert can be placed on the cutter body mathematically by providing the coordinates of the insert center with respect to the cutter body center. The inserts can be oriented by rotating them around the cutter body, thus each insert may be assigned to have different lead and axial rake angles. By solving the mechanics and dynamics of cutting at each edge point, and integrating them over the contact zone, it is shown that the milling process can be predicted for any inserted cutter. A sample of inserted cutter modeling and analysis examples are provided with experimental verifications.     

Tool flank wear prediction in CNC turning of 7075 AL alloy SiC composite

Flank wear occurs on the relief face of the tool and the life of a tool used in a machining process depends upon the amount of flank wear; so predicting of flank wear is an important requirement for higher productivity and product quality. In the present work, the effects of feed, depth of cut and cutting speed on flank wear of tungsten carbide and polycrystalline diamond (PCD) inserts in CNC turning of 7075 AL alloy with 10 wt% SiC composite are studied; also artificial neural network (ANN) and co-active neuro fuzzy inference system (CANFIS) are used to predict the flank wear of tungsten carbide and PCD inserts. The feed, depth of cut and cutting speed are selected as the input variables and artificial neural network and co-active neuro fuzzy inference system model are designed with two output variables. The comparison between the results of the presented models shows that the artificial neural network with the average relative prediction error of 1.03% for flank wear values of tungsten carbide inserts and 1.7% for flank wear values of PCD inserts is more accurate and can be utilized effectively for the prediction of flank wear in CNC turning of 7075 AL alloy SiC composite. It is also found that the tungsten carbide insert flank wear can be predicted with less error than PCD flank wear insert using ANN. With Regard to the effect of the cutting parameters on the flank wear, it is found that the increase of the feed, depth of cut and cutting speed increases the flank wear. Also the feed and depth of cut are the most effective parameters on the flank wear and the cutting speed has lesser effect.     

方肩铣刀高速铣削TC4钛合金试验研究及参数优化

利用单因素试验方法,采用镶齿硬质合金涂层方肩铣刀进行钛合金高速铣削试验,研究每齿进给量、铣削宽度、铣削深度、铣削速度对于铣削力的影响。通过对铣削力进行分析,建立方肩铣刀高速铣削的铣削力模型,并采用MATLAB遗传算法以进给方向铣削力Fy最小为目标,对铣削参数进行了优化并给出了最优解集。实验结果表明,使用优化后的参数加工薄壁件可有效减小切削力。     

纳米改性金属陶瓷刀具切削塑性金属的影响规律

纳米改性金属陶瓷刀具是一种新型的刀具材料,本文通过对其力学性能的分析确定其应用的范围,并通过具体的切削实验,验证了其在加工塑性材料时具有优越的切削性能,并分别研究了进给量、切削速度及切削速度对刀具寿命的影响,推导出了切削用量三要素与刀具使用寿命之间关系的广义Taylor公式,从该公式的指数可以看出,进给量对刀具使用寿命的影响也很大,几乎和切削速度对刀具使用寿命的影响相当,实验所得出的结论将有助于合理正确地使用这种新型刀具。     

Cutting force prediction in ball end milling of sculptured surface with Z-level contouring tool path

This paper presents an approach to predict cutting force in 3-axis ball end milling of sculptured surface with Z-level contouring tool path. The variable feed turning angle is proposed to denote the angular position of feed direction within tool axis perpendicular plane. In order to precisely describe the variation of feed turning angle and cutter engagement, the whole process of sculptured surface milling is discretized at intervals of feed per tooth along tool path. Each segmented process is considered as a small steady-state cutting. For each segmented cutting, the feed turning angle is determined according to the position of its start/end points, and the cutter engagement is obtained using a new efficient Z-map method. Both the chip thickness model and cutting force model for steady-state machining are improved for involving the effect of varying feed turning angle and cutter engagement in sculptured surface machining. In validation experiment, a practical 3-axis ball end milling of sculptured surface with Z-level contouring tool path is operated. Comparisons of the predicted cutting forces and the measurements show the reliability of the proposed approach.     

Feed rate scheduling model considering transverse rupture strength of a tool for 3D ball-end milling

This paper presents an analytical model of off-line feed rate scheduling to determine desired feed rates for 3D ball-end milling. Off-line feed rate scheduling is presented as the advanced technology to regulate cutting forces through change of feed per tooth, which directly affects variation of uncut chip thickness. In this paper, the uncut chip thickness is calculated by following the movement of the position of the cutter center, which is determined by runout and cutter deflection. Also, since the developed cutting force model uses the cutting-condition-independent coefficients, off-line feed rate scheduling can be effectively performed regardless of continuous change of cutting conditions. Transverse rupture strength of the tool is used to determine the reference cutting force at which resultant cutting forces are regulated through feed rate scheduling. Experiments validated that the presented feed rate scheduling model reduced machining time drastically and regulated cutting forces at the reference cutting force.     

Tool wear and machining performance of cBN–TiN coated carbide inserts and PCBN compact inserts in turning AISI 4340 hardened steel

PCBN is the dominant tool material for hard turning applications due to its high hardness, high wear resistance, and high thermal stability. However, the inflexibility of fabricating PCBN inserts with complex tool geometries and the prohibitive cost of PCBN inserts are some of the concerns in furthering the implementation of CBN based materials for hard turning. In this paper, we present the results of a thorough investigation of cBN plus TiN (cBN–TiN) composite-coated, commercial grade, carbide inserts (CNMA 432, WC–Co (6% Co)) for hard turning applications in an effort to address these concerns. The effect of cutting speed and feed rate on tool wear (tool life), surface roughness, and cutting forces of the cBN–TiN coated carbide inserts was experimented and analyzed using analysis of variance (ANOVA) technique, and the cutting conditions for their maximum tool life were evaluated. The tool wear, surface roughness, and cutting forces of the cBN–TiN coated and commercially available PCBN tipped inserts were compared under similar cutting conditions. Both flank wear and crater wear were observed. The flank wear is mainly due to abrasive actions of the martensite present in the hardened AISI 4340 alloy. The crater wear of the cBN–TiN coated inserts is less than that of the PCBN inserts because of the lubricity of TiN capping layer on the cBN–TiN coating. The coated CNMA 432 inserts produce a good surface finish (<1.6 μm) and yield a tool life of about 18 min per cutting edge. In addition, cost analysis based on total machining cost per part was performed for the comparison of the economic viability between the cBN–TiN coated and PCBN inserts.     

Temperature of internally-cooled diamond-coated tools for dry-cutting titanium

The role of cutting fluids is well known for the importance of removing heat from the cutting edge, lubricating the sliding chip contact and transporting the metal chips away from the cutting zone. Dry machining leads to increased cutting temperatures and higher wear rates resulting in shorter tool life; this is particularly evident in the cutting of high strength materials. Diamond coated cutting inserts are not usually considered for machining titanium due to rapid oxidation of the coating at the temperatures typical of titanium machining. This paper examines the formation of hot-spots on the rake face during dry and near-dry turning of titanium using conventional cemented carbide inserts. Machining performance is assessed by measurement of tool wear and tool life. Trials with an internally cooled tool with a specially designed, diamond coated insert have shown that the heat from the cutting operation can be rapidly diffused over the entire surface of the insert and thus successfully drawn away from the tool via closed loop recirculation of coolant through the tool holder. This enables wear to be inhibited by management of rake face temperature to keep it below the critical temperatures at which these prominent wear mechanisms operate. Measurements of change in coolant temperature before and after circulation are used to quantify the heat removed from the cutting process. The low friction coefficient and high thermal conductivity of diamond, assisted by the indirect cooling, results in longer tool life whilst maintaining high standards of surface finish.     

亚微观倒棱切削刃对硬质合金刀片切削性能的影响特性研究

为了研究硬质合金刀片亚微观切削刃口形式对其切削性能的作用机制,针对断续切削过程中硬质合金刀具存在容易崩刃破损和脆性断裂等问题,选用两种不同牌号的硬质合金可转位刀片分别在多种亚微观刃口结构条件下进行断续切削实验,应用三维受力分析仪结合高速摄影机对切削过程中的冲击和切削力进行数据监测与收集,结合图形统计分析方法研究亚微观倒棱刃口对硬质合金刀片的强化作用机理,从而揭示不同切削刃口几何参数对硬质合金刀具使用寿命的影响规律,提出倒棱刃口参数与进给量的曲线化关系。研究结果表明:大进给量f条件下应选取较大的临界夹角β_C,同时,倒棱夹角β要大,倒棱宽度L要小;反之,在小进给量f条件下,临界夹角β_C和倒棱夹角β要小,倒棱宽度L要大。研究可为硬质合金可转位刀片的强化设计提供数据和技术支撑。      

立铣刀断屑槽对切削性能影响规律试验研究

采用析因试验法研究断屑槽结构的制备对整体式立铣刀切削性能的影响。选取2把断屑槽分布不同的刀具,以铣刀主轴转速、径向切削深度、每齿进给量为主要因素,建立正交铣削试验。结果表明：断屑槽分布密集的铣刀适合用于加工精度要求不高的零件;断屑槽分布稀释的铣刀,适合用于半精加工要求的工件;对比分析切屑形貌和表面质量,得到用断屑槽分布稀疏的刀具铣削工件时可获得较好的表面质量。     

Influence of grinding parameters on the quality of high content PCBN cutting inserts

Plunge-face grinding is commonly used to finish PCBN cutting inserts. In order to reach an adequate process design, an investigation of the influence of the grinding parameters on the quality of high content PCBN inserts is carried out in this work. For this, the inserts are ground with different grinding wheels (including a variation of grain size and bonding), dressing feed rates, feed and cutting speeds and the edge chipping and flank face roughness are measured. It was found that a reduction of the abrasive grain size as well as an increase of the dressing feed rate lead to an improvement of the insert edge and surface quality. Moreover, a variation of the cutting and feed speeds has only a small influence on the PCBN insert quality.     

Roughing,semi-finishing and finishing of laser surface modified nickel bonded NbC and WC inserts for grey cast iron (GCI) face-milling

An attempt to improve the machining performance of NbC-Ni cutting inserts by rapid pulse electric current sintering (PECS), TiC and Mo₂C additions and laser surface modification (LSM) was done. Use of a nickel binder and additions TiC and Mo₂C to liquid phase sintered (LPS) NbC based samples led to comparable hardness (>13 GPa) and K_IC (~10 MPa.m^1/2) to LPS WC-Co/Ni samples. The laser surface modification (LSM) technique produced a ~2.5 μm thick self-carbide coating, increasing the surface hardness of all the samples. Laser surface modification was done to improve abrasion and attrition wear resistance. Face-milling of grade 17 grey cast iron (BS 1452/GG35) was conducted at 100–500 m/min cutting speeds (v_c) and 0.25–1.5 mm depths of cut (a_p). The insert wear was measured after every pass, and analyzed by annular dark field scanning transmission electron microscopy (ADF-STEM). During roughing, WC-Co based inserts had the lowest flank wear rate (FWR) values, with the WC-10Co (LPS) insert having a FWR of 10.15 μm/min after 20 min cutting time. However, during semi-finishing and finishing, NbC-4TiC-12Ni (PECS) and NbC-4Mo₂C-4TiC-12Ni (PECS) inserts had the lowest FWR values, showing up to six times longer tool life than the WC-Co (LPS) inserts based inserts and 12 times longer life than the WC-Ni based inserts. Generally, LSM improved the NbC inserts' tool life, reducing the FWR values in all NbC based inserts in all cutting tests.     

Grinding of PCBN cutting inserts

PCBN cutting inserts have been more often used in order to attend to the demands of an economically viable process and to lead to a proper workpiece surface quality. A proper application of this cutting material requires its adequate processing. Plunge-face grinding is used for finishing the inserts after sintering. To choose a suitable grinding tool and process parameters, the properties of the ground cutting inserts must be taken into account. Therefore, the influence of PCBN grain size and composition on the insert cutting edge and surface quality has to be investigated. This work aims to give an overview of material removal mechanisms, process forces and abrasive grain wear during grinding different PCBN inserts. It was found that the insert quality depends mainly on the material removal mechanism, which in the studied case is defined by the PCBN grain size.     

Time domain model of plunge milling operation

Plunge milling operations are used to remove excess material rapidly in roughing operations. The cutter is fed in the direction of spindle axis which has the highest structural rigidity. This paper presents time domain modeling of mechanics and dynamics of plunge milling process. The cutter is assumed to be flexible in lateral, axial, and torsional directions. The rigid body feed motion of the cutter and structural vibrations of the tool are combined to evaluate time varying dynamic chip load distribution along the cutting edge. The cutting forces in lateral and axial directions and torque are predicted by considering the feed, radial engagement, tool geometry, spindle speed, and the regeneration of the chip load due to vibrations. The mathematical model is experimentally validated by comparing simulated forces and vibrations against measurements collected from plunge milling tests. The study shows that the lateral forces and vibrations exist only if the inserts are not symmetric, and the primary source of chatter is the torsional–axial vibrations of the plunge mill. The chatter vibrations can be reduced by increasing the torsional stiffness with strengthened flute cavities.     

Feedrate scheduling for indexable end milling process based on an improved cutting force model

In CNC machining, an optimal process plan is needed for higher productivity and machining performance. This paper proposes a mechanistic cutting force model to perform feedrate scheduling that is useful in process planning for indexable end milling. Indexable end mills, which consist of inserts and a cutter body, have been widely used in the roughing of parts in the mold industry. The geometry and distribution of inserts compose a discontinuous cutting edge on the cutter body, and tool geometry of indexable end mill varies with axial position due to the geometry and distribution of inserts. Thus, an algorithm that calculates tool geometry data at an arbitrary axial position was developed. The developed cutting force model uses cutting-condition-independent cutting force coefficients and considers run out, cutter deflection, geometry variation and size effect for accurate cutting force prediction. Through feedrate scheduling, NC code is optimized to regulate cutting forces at given reference force. Experiments with general NC codes show the effectiveness of feedrate scheduling in process planning.     

螺旋锥齿轮端铣加工刀位计算方法

针对螺旋锥齿轮在数控加工中心上加工效率低的问题,提出利用盘状铣刀对螺旋锥齿轮进行端铣的加工方法.由螺旋锥齿轮齿面及刀具的几何特征,调整铣刀与被加工齿面的相对位置,使齿面与刀具包络面的法截面相对法曲率最小,增大切削带宽,同时控制刀轴矢量调整切削深度避免加工干涉.最后将规划的加工刀位转化到数控机床轴上,利用VERICUT进行加工仿真验证该方法的可行性与正确性.     

Milling force prediction using a dynamic shear length model

Modelling of cutting forces in milling is often needed in machining automation. In this paper, a new method for the determination of the cutting forces in face milling is presented, which applies a predictive machining theory originally developed for orthogonal cutting to milling operations, with a dynamic shear length model developed and incorporated. The proposed dynamic shear length model is developed based on the analysis for the true tooth trajectories of a milling cutter, taking into account of the characteristic wavy surface effects in milling. The prediction for the cutting forces is carried out at each step of the angular increment of cutter rotation from input data of fundamental workpiece material properties, tool geometry and cutting conditions. Cutting forces at a cutter tooth can be predicted once the shear angle, shear length, shear plane area, and the shear flow stress along the shear length have been determined. The milling force prediction using the dynamic shear length model is verified through milling experimental tests. The sensitivity of the difference between the static and dynamic shear length models with respect to the feed per tooth and the cutter diameter is discussed.     

抛光涂层硬质合金刀片加工钛合金的耐用度分析

目的提高涂层硬质合金刀具加工钛合金的切削性能及加工效率。方法采用化学机械抛光(Chemical Mechanical Polishing,CMP)对经过磨削加工的YG8硬质合金车削刀片前刀面进行抛光预处理,并使用CVD与PVD涂层工艺制备涂层。运用单因素试验法,对抛光涂层硬质合金刀片进行切削TC4钛合金的刀片耐用度试验,分析钛合金加工过程中刀具种类及切削参数变化对刀片耐用度的影响规律。采用扫描电子显微镜(SEM)和能谱仪(EDS)分析刀片的磨损机理。结果经过化学机械抛光处理后,硬质合金刀片的平均粗糙度由87 nm降低为19 nm,降低幅度达78.2%。相同切削参数时,抛光CVD硬质合金刀片的耐用度最大程度上比磨削CVD硬质合金刀片提高了75%,抛光PVD硬质合金刀片的耐用度最大程度上比磨削PVD硬质合金刀片提高了8.3%。可见采用化学机械抛光对硬质合金刀片进行加工是提高刀片表面平整度及耐用度的重要途径。结论抛光CVD硬质合金刀片的耐用度优于磨削CVD硬质合金刀片,抛光PVD硬质合金刀片的耐用度优于磨削PVD硬质合金刀片。