Optimization of cutter-location for 5-axis NC machining freeform surface with a flat-end cutter

axis NC machining freeform surface with a flat end cutter can theoretically improve material removal rate and surface finish, but it is very difficult to generate a gouge free cutter location. A new method of generating optimal cutter path is proposed to define the effective radius of a flat end cutter and determine the optimal step forward distance and step over distance. Thereby improving the NC machining efficiency and quality of freeform surfaces.

Experimental study on the influences of cutter geometry and material on scraper wear during shield TBM tunnelling in abrasive sandy ground

When shield TBM tunnelling in abrasive sandy ground,the rational design of cutter parameters is critical to reduce tool wear and improve tunnelling efficiency.However,the influence mechanism of cutter parameters on scraper wear remains unclear due to the lack of a reliable test method.Geometry and material optimisation are often based on subjective experience,which is unfavourable for improving scraper geological adaptability.In the present study,the newly developed WHU-SAT soil abrasion test was used to evaluate the variation in scraper wear with cutter geometry,material and hardness.The influence mechanism of cutter parameters on scraper wear has been revealed according to the scratch characteristics of the scraper surface.Cutter geometry and material parameters have been optimised to reduce scraper wear.The results indicate that the variation in scraper wear with cutter geometry is related to the cutting resistance,frictional resistance and stress distribution.An appropriate increase in the front angle(or back angle)reduces the cutting resistance(or frictional resistance),while an excessive increase in the front angle(or back angle)reduces the edge angle and causes stress concentration.The optimal front angle,back angle and edge angle for quartz sand samples areα=25°,β=10°andγ=55°,respectively.The wear resistance of the modelled scrapers made of different metal materials is related to the chemical elements and microstructure.The wear resistances of the modelled scrapers made of 45#,06Cr19Ni10,42CrMo4 and 40CrNiMoA are 0.569,0.661,0.691 and 0.728 times those made of WC-Co,respectively.When the alloy hardness is less than 47 HRC(or greater than 58 HRC),scraper wear decreases slowly with increasing alloy hardness as the scratch depth of the particle asperity on the metal surface stabilizes at a high(or low)level.However,when the alloy hardness is between 47 HRC and 58 HRC,scraper wear decreases rapidly with increasing alloy hardness as the scratch depth transitions from high to low levels.The sensitive hardness interval and recommended hardness interval for quartz sand are[47,58]and[58,62],respectively.The present study provides a reference for optimising scraper parameters and improving cutterhead adaptability in abrasive sandy ground tunnelling.

Research on the design method for uniform wear of shield cutters in sand-pebble strata

During shield tunneling in highly abrasive formations such as sand–pebble strata,nonuniform wear of shield cutters is inevitable due to the different cutting distances.Frequent downtimes and cutter replacements have become major obstacles to long-distance shield driving in sand–pebble strata.Based on the cutter wear characteristics in sand–pebble strata in Beijing,a design methodology for the cutterhead and cutters was established in this study to achieve uniform wear of all cutters by the principle of frictional wear.The applicability of the design method was verified through three-dimensional simulations using the engineering discrete element method.The results show that uniform wear of all cutters on the cutterhead could be achieved by installing different numbers of cutters on each trajectory radius and designing a curved spoke with a certain arch height according to the shield diameter.Under the uniform wear scheme,the cutter wear coefficient is greatly reduced,and the largest shield driving distance is increased by approximately 47%over the engineering scheme.The research results indicate that the problem of nonuniform cutter wear in shield excavation could be overcome,thereby providing guiding significance for theoretical innovation and construction of long-distance shield excavation in highly abrasive strata.

New Mathematical Method for the Determination of Cutter Runout Parameters in Flat-end Milling

The cutting force prediction is essential to optimize the process parameters of machining such as feed rate optimization, etc. Due to the significant influences of the runout effect on cutting force variation in milling process, it is necessary to incorporate the cutter runout parameters into the prediction model of cutting forces. However, the determination of cutter runout parameters is still a challenge task until now. In this paper, cutting process geometry models, such as uncut chip thickness and pitch angle, are established based on the true trajectory of the cutting edge considering the cutter runout effect. A new algorithm is then presented to compute the cutter runout parameters for flat-end mill utilizing the sampled data of cutting forces and derived process geometry parameters. Further, three-axis and five-axis milling experiments were conducted on a machining centre, and resulting cutting forces were sampled by a three-component dynamometer. After computing the corresponding cutter runout parameters, cutter forces are simulated embracing the cutter runout parameters obtained from the proposed algorithm. The predicted cutting forces show good agreements with the sampled data both in magnitude and shape, which validates the feasibility and effectivity of the proposed new algorithm of determining cutter runout parameters and the new way to accurately predict cutting forces. The proposed method for computing the cutter runout parameters provides the significant references for the cutting force prediction in the cutting process.

Experimental Study on Titanium Alloy Cutting Property and Wear Mechanism with Circular-arc Milling Cutters

Titanium alloy has been applied in the field of aerospace manufacturing for its high specific strength and hardness.Nonetheless,these properties also cause general problems in the machining,such as processing inefficiency,serious wear,poor workpiece face quality,etc.Aiming at the above problems,this paper carried out a comparative experimental study on titanium alloy milling based on the CAMCand BEMC.The variation law of cutting force and wear morphology of the two tools were obtained,and the wear mechanism and the effect of wear on machining quality were analyzed.The conclusion is that in contrast with BEMC,under the action of cutting thickness thinning mechanism,the force of CAMC was less,and its fluctuation was more stable.The flank wear was uniform and near the cutting edge,and the wear rate was slower.In the early period,the wear mechanism of CAMC was mainly adhesion.Gradually,oxidative wear also occurred with milling.Furthermore,the surface residual height of CAMC was lower.There is no obvious peak and trough accompanied by fewer surface defects.

Theoretical and numerical studies of rock breaking mechanism by double disc cutters

A plane mechanical model of rock breaking process by double disc cutter at the center of the cutterhead is established based on contact mechanics to analyze the stress evolution in the rock broken by cutters with different spacings. A continuous-discontinuous coupling numerical method based on zero-thickness cohesive elements is developed to simulate rock breaking using double cutters. The process, mechanism,and characteristics of rock breaking are comprehensively analyzed from five aspects: peak force, breaking form, breaking efficiency, crack mode, and breaking degree. The results show that under the penetrating action of cutters, dense cores are formed due to shear failure under respective cutters. The tensile cracks propagate in the rock, and then rock chips form with increasing penetration depth. When the cutter spacing is increased from 10 to 80 mm, the peak force gradually increases, the rock breaking range increases first and then decreases, the specific energy decreases first and then rises, and the breaking coefficient of intermediate rock decreases from 0.955 to 0.788. The area of rock breaking is positively correlated with the length of the tensile crack. Furthermore, the length of the tensile crack accounts for 14.4%–33.6% of the total crack length.

Performances of a Stinger PDC cutter breaking granite: Cutting force and mechanical specific energy in single cutter tests

The Stinger PDC cutter has high rock-breaking efficiency and excellent impact and wear resistance, which can significantly increase the rate of penetration (ROP) and extend PDC bit life for drilling hard and abrasive formation. The knowledge of force response and mechanical specific energy (MSE) for the Stinger PDC cutter is of great importance for improving the cutter's performance and optimizing the hybrid PDC bit design. In this paper, 87 single cutter tests were conducted on the granite. A new method for precisely obtaining the rock broken volume was proposed. The influences of cutting depth, cutting angle, and cutting speed on cutting force and MSE were analyzed. Besides, a phenomenological cutting force model of the Stinger PDC cutter was established by regression of experimental data. Moreover, the surface topography and fracture morphology of the cutting groove and large size cuttings were measured by a 3D profilometer and a scanning electron microscope (SEM). Finally, the rock-breaking mechanism of the Stinger PDC cutter was illustrated. The results indicated that the cutting depth has the greatest influence on the cutting force and MSE, while the cutting speed has no obvious effects, especially at low cutting speeds. As the increase of cutting depth, the cutting force increases linearly, and MSE reduces with a quadratic polynomial relationship. When the cutting angle raises from 10° to 30°, the cutting force increases linearly, and the MSE firstly decreases and then increases. The optimal cutting angle for breaking rock is approximately 20°. The Stinger PDC cutter breaks granite mainly by high concentrated point loading and tensile failure, which can observably improve the rock breaking efficiency. The key findings of this work will help to reveal the rock-breaking mechanisms and optimize the cutter arrangement for the Stinger PDC cutter.

Comparative Study of the Rock-breaking Mechanism of a Disc Cutter and Wedge Tooth Cutter by Discrete Element Modelling

The operation of a shield tunnel boring machine(TBM)in a high-strength hard rock stratum results in significant cutter damage,adversely affecting the thrust and torque of the cutter head.Therefore,it is very important to carry out the research on the stress characteristics and optimize the cutter parameters of cutters break high-strength hard rock.In this paper,the rock-breaking performance of cutters in an andesite stratum in the tunnel of Qingdao Metro Line No.8 was investigated using the discrete element method and theoretical analysis.The rock-breaking processes of a disc cutter and wedge tooth cutter were simulated by software particle flow code PFC^(3D),and the rock-breaking degree,stress of the cutter,and rock-breaking specific energy were analyzed.The rock damage caused by the cutter in a specific section was divided into three stages:the advanced influence,crushing,and stabilizing stages.The rock-breaking degree and the tangential and normal forces of the wedge tooth cutter are larger than that of the disc cutter under the same conditions.The disc cutter(wedge tooth cutter)has the highest rock-breaking efficiency at a cutter spacing of 100 mm(110 mm)and a penetration depth of 8 mm(10 mm),and the rock-breaking specific energy is 11.48 MJ/m^(3)(12.05 MJ/m^(3)).Therefore,two types of cutters with different penetration depths or cutter spacing should be considered.The number of teeth of wedge tooth cutters can be increased in hard strata to improve the rock-breaking efficiency of the shield.The research results provide a reference for shield cutterhead selection and cutter layout in similar projects.

Experimental investigation on the cuttings formation process and its relationship with cutting force in single PDC cutter tests

The single polycrystalline diamond compact(PDC)cutter test is widely used to investigate the mecha-nism of rock-breaking.The generated cuttings and cutting force are important indexes reflecting the rock failure process.However,they were treated as two separate parameters in previous publications.In this study,through a series of rock block cutting tests,the relationship between them was investigated to obtain an in-depth understanding of the formation of cuttings.In addition,to validate the standpoints obtained in the aforementioned experiments,rock sheet cutting tests were conducted and the rock failure process was monitored by a high-speed camera frame by frame.The cutting force was recorded with the same sampling rate as the camera.By this design,every sampled point of cutting force can match a picture captured by the camera,which reflects the interaction between the rock and the cutter.The results indicate that the increase in cutting depth results in a transition of rock failure modes.At shallow cutting depth,ductile failure dominates and all the cuttings are produced by the compression of the cutter.The corresponding cutting force fluctuates slightly.However,beyond the critical depth,brittle failure occurs and chunk-like cuttings appear,which leads to a sharp decrease in cutting force.After that,the generation of new surface results in a significant decrease in actual cutting depth,a parameter proposed to reflect the interaction between the rock and the cutter.Consequently,ductile failure dominates again and a slight fluctuation of cutting force can be detected.As the cutter moves to the rock,the actual cutting depth gradually increases,which results in the subsequent generation of chunk-like cuttings.It is accompanied by an obvious cutting force drop.That is,ductile failure and brittle failure,one following another,present at large cutting depth.The transition of rock failure mode can be correlated with the variation of cutting force.Based on the results of this paper,the real-time monitoring of torque may be helpful to determine the efficiency of PDc bits in the downhole.

Numerical simulation of rock-breaking and influence laws of dynamic load parameters during axial-torsional coupled impact drilling with a single PDC cutter

Axial and torsional impact drilling technology is used to improve the drilling efficiency of hard rock formation in the deep underground.Still,the corresponding theory is not mature,and there are few correlative research reports on the rock-breaking mechanism of axial and torsional coupled impact drilling tools.Considering the influence of the impact hammer geometry and movement on the dynamic load parameters(i.e.,wavelength,amplitude,frequency,and phase difference),a numerical model that includes a hard formation and single polycrystalline diamond compact cutter was established.The Riedel-Hiermaier-Thoma model,which considers the dynamic damage and strength behavior of rocks,was adopted to analyze the rock damage under axial and torsional impact loads.The numerical simu-lation results were verified by the experimental results.It was found that compared with conventional drilling,the penetration depths of axial,torsional,and axial-torsional coupled impact drilling increased by 31.3%,5.6%,and 34.7%,respectively.Increasing the wavelength and amplitude of the axial impact stress wave improved the penetration depth.When the bit rotation speed remained unchanged,increasing the frequency in the axial and circumferential directions had little effect on the penetration depth.However,as the frequency increased,the cutting surface became increasingly smooth,which reduced the occurrence of bit vibration.When the phase difference between the axial and circumfer-ential stress waves was 25%,the penetration depth significantly increased.In addition,the bit vibration problem can be effectively reduced.Finally,the adjustment of engineering and tool structure parameters is proposed to optimize the efficiency of the axial-torsional coupled impact drilling tool.

Experimental studies on rock failure mechanisms under impact load by single polycrystalline diamond compact cutter

Percussive drilling shows excellent potential for promoting the rate of penetration(ROP)in drilling hard formations.Polycrystalline diamond compact(PDC)bits account for most of the footage drilled in the oil and gas fields.To reveal the rock failure mechanisms under the impact load by PDC bits,a series of drop tests with a single PDC cutter were conducted to four kinds of rocks at different back rake angles,drop heights,drop mass,and drop times.Then the morphology characteristics of the craters were obtained and quantified by using a three-dimensional profilometer.The fracture micrographs can be observed by using scanning electron microscope(SEM).The distribution and propagation process of subsurface cracks were captured in rock-like silica glass by a high-speed photography system.The results can indicate that percussive drilling has a higher efficiency and ROP when the rock fractures in brittle mode.The failure mode of rock is related with the type of rock,the impact speed,and the back rake angle of the cutter.Both the penetration depth and fragmentation volume get the maximum values at a back rake angle of about 45°.Increasing the weight and speed of falling hammer is beneficial to improving the rock breaking effects and efficiency.The subsurface cracks under the impact load by a single PDC cutter is shaped like a clamshell,and its size is much larger than the crater volume.These findings can help to shed light on the rock failure mechanisms under the impact of load by a single PDC cutter and provide a theoretical explanation for better field application of percussive drilling.

分体刀具式名优茶采摘末端执行器设计与试验优化

针对名优茶机械化采摘过程中侧芽无法采摘的问题,根据顶芽、侧芽及茶梗的相关参数并结合茶园环境设计了一种末端执行器,利用分体式刀具的刀齿弯曲变形适应茶梗的干扰从而采摘侧芽。通过有限元仿真刀具切割侧芽得到采摘成功率的影响因素为刀齿宽度、刀齿长度及刀具厚度;采用三因素三水平的中心组合设计与响应面分析法研究各因素对采摘成功率的交互影响;以采摘成功率为响应值建立二次回归模型,确定各因素对采摘成功率的影响显著性主次排序为:刀齿长度、刀齿宽度、刀具厚度。以采摘成功率为目标对各试验因素进行优化,得到优化后的刀齿宽度、刀具厚度、刀齿长度分别为2.6、0.9、20.0 mm。采用优化后的参数进行茶园采摘试验,结果表明,末端执行器能够有效完成茶叶采摘工作,顶芽、侧芽采摘成功率分别为93%、63%,试验值与预测值的相对误差小于5%,优化模型结果可靠。

具有内阻的旋转锥形复合材料刀杆铣削过程稳定性的仿真分析

研究考虑材料内阻的旋转圆锥形复合材料刀杆铣削系统的再生颤振稳定性。基于复合材料粘弹性本构关系、Hamilton原理和Rayleigh梁理论,结合再生时滞铣削力模型,分别导出旋转坐标以及惯性坐标表示的颤振偏微分方程。采用Galerkin法对颤振偏微分方程进行离散化。基于自由振动分析和能量法计算复合材料刀杆的模态损耗因子;基于特征值分析技术计算旋转复合材料刀杆的临界速度和失稳阈。采用时域半离散法和频域零阶近似解法,预测切削系统的稳定性。分析锥度比、长径比、铺层角、铺层方式,以及内、外阻对切削稳定性的影响。计算结果表明,轴对称旋转刀杆铣削过程在两种坐标系下的稳定性预测结果是一致的。研究发现,在高转速下,由于旋转复合材料刀杆内阻的作用,切削系统出现新的不稳定区域。这些新的不稳定区域的起始转速等于旋转复合材料刀杆的失稳阈。

基于层次分析法的TBM滚刀评价方式

[目的]为了减少TBM(硬岩隧道掘进机)在高磨蚀性岩层中掘进时的换刀次数,以及选出最适用于高磨蚀性岩层的滚刀,提出一种基于层次分析法的滚刀评价方法。[方法]建立了滚刀磨损失效危害评价模型;选取7个磨损失效类型因素,基于层次分析法建立了滚刀磨损失效危害评价模型和比较判断矩阵,计算出各失效类型危害的占比;以广佛东环城际铁路大源站至太和站区间线路为例,对4个常用品牌的滚刀进行定量分析;建立风险评价分值表,分析4个品牌的滚刀在使用过程中的失效影响评价值。[结果及结论]高磨蚀岩层中的滚刀非正常磨损占比为23.29%,正常磨损占比为76.71%。c品牌和d品牌滚刀的失效影响评价值分别为7.141和4.816,远小于a品牌滚刀的失效影响评价值7.587和b品牌滚刀的失效影响评价值9.058。d品牌滚刀的相对磨损速率大于c品牌滚刀,因此c品牌滚刀最适用于高磨蚀性岩层。a品牌滚刀适用于硬度极大的岩层,b品牌滚刀不推荐使用,d品牌滚刀适用于硬度较小的岩层。

PDC齿切削砾岩的细观损伤模拟

砾岩一般具有孔隙结构复杂、非均匀性严重等特点,PDC钻头钻进砾岩地层时破岩效率普遍较低,室内试验很难有效地研究PDC齿的破岩过程。为此,采用离散元软件PFC2D建立了PDC齿切削砾岩的数值模型,研究不同切削深度、前倾角及岩石非均质度情况下,砾岩的破碎形式和过程,引入破岩比功评价了砾岩切削过程中的破岩效率。研究结果表明:一定范围内增加切削深度,砾岩内部剪切裂纹更快萌生且剪切裂纹占比显著增加,主裂纹沿自由面方向和切削方向绕砾扩展;提高砾岩非均质度,岩石逐渐由塑性破坏转变为脆性破坏,非均质度过高会增加砾岩的整体强度,且使作用在PDC齿上的冲击破坏频率增加,不利于破岩;当PDC齿以1 mm切削深度、5°前倾角切削砾岩时,可获得最高的破岩效率。所得结论可为PDC齿设计及切削参数选择提供理论支持。

铡草机切碎辊作物喂入矢角参数模型建立与试验

饲草收获机作物喂入矢角(辊(叶片)轴心至喂入点矢径的水平夹角)对切碎辊转矩负荷及切碎性能有重要影响,通过检测传动带压力与作物喂入矢角的关系,对收获机切碎特性与作物切碎过程进行了运动学、动力学分析,揭示了饲草作物切碎质量、能耗与作物喂入矢角的关系,建立收获机切碎辊作物喂入矢角数学模型并进行了优化。试验结果表明,当优化参数组作物单位长度喂入量为5.5 kg/s、切碎辊转速为1600 r/min、传动带初始压力为1.2 MPa时,该作业参数组下切碎作物喂入矢角数学模型精度最高,准确率达79.52%,模型偏差为0.491 MPa(0.957 rad),能够有效表征铡草机切碎作业负荷,同时在保证作物切碎质量下提高了铡草机作业能力,优化的切碎喂入矢角模型下切碎扰动响应时间缩短约26.41%,单位作业能耗减小约11.36%。该切碎作物喂入矢角模型丰富了铡草机切碎作业的建模方法,为饲草作物类铡草机负荷控制系统设计提供了参考。

铣削高温合金GH3039刀具寿命预测及磨损机理

为揭示高温合金GH3039铣削参数对刀具寿命的影响规律,采用正交试验法对其进行铣削试验,运用多元回归法建立刀具寿命预测模型,分析铣削参数对刀具寿命的影响规律,通过观测铣刀后刀面的磨损情况,探究不同主轴转速下铣刀的磨损机理。结果表明:主轴转速对刀具寿命的影响程度最大,最小为轴向切深;当主轴转速为750 r/min时,铣刀磨损区域主要以黏结磨损和磨粒磨损为主;当铣削速度为950 r/min时,氧化磨损与扩散磨损开始增强;当铣削速度为1 180 r/min时,磨粒磨损与黏结磨损开始减弱甚至消失,氧化磨损与扩散磨损占据主导地位。

烟台港龙口港区27^(#)~29^(#)通用泊位疏浚工程绞吸船施工工艺优化

烟台港龙口港区27^(#)~29^(#)通用泊位疏浚工程土质主要为起球的黏性土,混有少量粗砂,绞吸船在施工时受黏土起球及黏性等影响,容易出现堵管、绞刀糊堵、输送困难等情况,施工作业关键参数波动幅度较大,作业人员操作控制难度高,平均生产效率低。重点围绕绞吸船的挖掘和输送分析,确定关键施工参数,优化施工工艺,指导作业人员施工。优化后的施工工艺使烟台港龙口港区27^(#)~29^(#)通用泊位疏浚工程绞吸船施工产能提高20%以上,节约了施工成本。

最大熵试验法对切割器功能可靠性的评估

针对切割器设计裕度大且刺激能量服从正态分布的特点,提出采用最大熵试验法对切割器的功能可靠性进行评估。以某型切割器为实例,通过临界药量摸底试验、升降法试验及成败型试验对其功能可靠性进行了评估。结果表明:运用最大熵试验法,采用46个试验样本即可完成切割器在置信度γ=0.95下,可靠性指标不小于0.999 995的可靠性评估试验,为航天火工装置(尤其是切割器)的小样本量高可靠性评估提供了技术途径。

预切缝位置及深度对SBM滚刀破岩的影响

利用水射流对岩体进行预切缝是提高竖井全断面掘进机破岩效率、实现深部资源机械化开发的关键技术。针对预切缝位置及深度合理参数选取的难题,首先指出了锥面刀盘带动下滚刀破岩的工况特征,进而基于Cohesive单元方法,建立了预切缝条件下的滚刀破岩离散-连续耦合数值模型。以完整岩体为对照组,对比分析了不同预切缝距离和深度对滚刀破岩的受力特征和岩石破碎特征的影响规律。阐述了预切缝条件下,滚刀破岩时密实核形成、块状岩渣形成、Hertz裂纹扩展3个阶段性特征,指出了岩体的非对称破碎特性和不同区域岩渣的形成机制。最终从滚刀受力、破岩面积、破岩比能耗、岩渣破碎程度4个方面综合分析,确定了预切缝的最佳距离和深度取值范围。研究结果表明:预切缝可以显著减小竖井全断面掘进机滚刀破岩过程中的贯入力和侧向力。与完整岩体相比,贯入力峰值降低了44.0%~10.3%。侧向力峰值降低了35.2%~6.5%。预切缝间距及深度较小时,会限制滚刀破岩范围,同时使得破碎岩体块度更小,导致破岩比能耗增高。随着预切缝深度的增大,破岩体积先增大后趋于稳定,比能耗先减小后保持不变。最终,确定了切缝距离为70~90 mm,深度为60~80 mm时破岩效果最佳,破岩体积较无切缝时提高约1倍,破岩比能耗降低69.2%。研究结果为刀盘辅助破岩装备的设计及参数确定提供了参考。