重型板式给料机论文(13)

The shear stress contours are presented in Fig. 8. The shear stress changes from tensile in the secondary deformation zone, ranging from 323 to 437 MPa, to compressive in the primary deformation zone, from 357 MPa up to a maximum of 470 MPa. Note that the residual shear stresses in the sub-surface of the workpiece are also compressive, ranging from 17 to 243 MPa.

Fig.8. Contours of the shear stress at steady state in simulated orthogonal machining.

The variation of the simulated cutting and thrust forces with tool travel is shown in Fig. 9. The simulated forces are obtained by summing the horizontal, x, and the vertical, y, external forces of all nodes in contact with the tool rake face over several incremental movements of the cutting tool. Both the cutting and the thrust forces have reached the steady state after a tool path of about 2 mm, their values being 1250 and 420 N, respectively. The experimental cutting force at steady state under the same cutting conditions, taken from [23], is also shown in Fig. 9 and has a value of 1403 N. Comparison between the predicted and the experimental cutting force shows a divergence of

38

洛阳理工学院毕业设计论文 about 11%,which may be considered as an acceptable level of agreement. Comparison between predicted and experimental thrust forces was not made, since experimental thrust force data were not available. Note, however, that the ratio of predicted cutting to thrust force was about 3, which is realistic for a cutting process.

Fig.9. Variation of cutting and thrust forces with tool travel for orthogonal machining: (a) predicted cutting force; (b) predicted thrust force; (c) experimental cutting force, at steady state. The presented FE cutting model is a first attempt to develop a design tool capable of quantitatively predicting the performance of machining operations. It is believed that this model, with suitable adjustments if necessary, will be applicable to other cutting procedures of ductile materials with continuous chip formation, i.e. high-speed machining and micro-machining. In addition, the implementation of the geometrical chip-separation criterion in the MARC code, which is the basis of the constructed model, has been substantiated so as to allow, properly modified, for its extension to 3D cutting procedures.

4. Conclusions

In this paper, a coupled thermo-mechanical model of plane-strain orthogonal metal cutting with continuous chip formation is presented. In order to allow for the separation of the chip from the workpiece, a chip-separation criterion based on a critical distance consideration has been implemented into the MARC finite element code. The model is able to predict the stress, strain, strain-rate and temperature distribution in the chip, the workpiece and the tool, as well as the developed cutting forces. A good agreement was found between predicted and experimental cutting forces, indicating the validation of the proposed model.

The numerical results of the constructed finite element machining model indicate that the FE method may be more reliable for machining operations than the related analytical methods, since the effect of parameters such as large strain, strain-rate and temperature, on the workpiece material properties can be taken into account. However, the properties of the materials under such conditions are difficult to be obtained, which limits the accuracy of the results of any FE machining model. Nevertheless, computer modelling of machining operations, which belongs to the new concept of computational machining or virtual machining simulation [24], especially using a commercial FE code widely available to engineers and industry, constitutes a very useful tool for the prediction of the machining behaviour of the workpiece as well as the optimum cutting tool design, thus reducing the need for resorting to extensive cutting experiments.

39

洛阳理工学院毕业设计论文

正交金属切削切屑形成的有限元模拟

摘要：

一对平面应力正交金属切削连续切屑形成的热力耦合模型的呈现使用了一种商业隐式有限元程序MARC。工件材料流动应力则被作为应变、应变率和温度的根据,以考虑较大应变、应变率和温度的影响与相关切削和材料性能的关系。切削过程通过刀具的缓慢进给,从初期到稳态切削力进行模拟,而几何切屑分离间隙标准,基于临界距离为刀尖的状态,通过重新划分流程执行到MARC编码。切屑的形状与应力、应变率在切片和工件上的分布以及工件、切屑和工具上的温度场已经定了。计算出的切削力与已公布的实验数据进行比较,结果发现吻合较好,验证,因此,提出了一种有限元模型。 关键词：有限元、模拟、切削、切屑形成 1. 介绍

切削是一种经常用于生产部分所需的尺寸和形状配置的生产工艺，用楔形的工具除去多余的材料来达到目的。由于它的工业上的重要性和广泛使用，平面应力正交金属切削的技术早在20世纪40年代就被研究，因为它与切削过程近似值很接近。在发展能够预测工件材料切削行为的模型上，付出了巨大的努力。 正交金属切削的简化分析首先被引进剪切角的概念的商人李·谢弗考虑，他们提出一种解析模型的应用slip-line理论。在这些模型基础上,一直尝试发展出更准确和精致的模型来同时反应摩擦、加工硬化、应变率和温度的特征([3 - 5]).尽管所有这些分析模型提供有用的切削加工过程,但是由于其单一化，导致分析与实验结果有显著的差异。有限元方法已经被广泛应用于切削过程的模拟[6 - 8],并且Usui和Shirakashi、岩田吴景之,已经分析了稳态正交切削。关于正交金属切削中切屑的形成和分离的有限元模拟,最初是由Strenkowski和卡罗尔尝试的。他们从最初的稳定状态进行了模拟切削,而为了模拟切屑的形成,开发了一种基于有效塑性应变的临界值，元素在刀尖前面分离的技术,从而引入一个元素或切屑分离判据的概念。

40