NURBS interpolator for constant material removal rate in open NC machine tools

Conventionally used linear or circular interpolators are undesirable for the precision machining of 3D free-form surfaces for the following reasons: the transmission errors due to the huge number of point data, discontinuity of curve segmentation, and unsmooth motion speed. In this regard, modern CNC machine tools are designed with a function for machining arbitrary parametric curves. However, these systems do not consider controlling feedrate adaptively, which dominates the quality of the machining process. This paper proposes a NURBS interpolator based on the adaptive feedrate control for the constant material removal rate. This is accomplished by varying feedrate using the curvature of a surface. The curvature-compensated feedrate system has important potential applications in ensuring part accuracy and protecting the cutting tool. The simulated and experimental results show it is applicable to real machining.     

Robust real-time NURBS path interpolators

In a review of the real-time non-uniform rational B-splines (NURBS) path interpolation method in CNC controllers, it was found that none of the NURBS interpolators described in the literature has the necessary robustness against an extreme knot distribution. The problems begin with the calculation of the total length of the NURBS path: most interpolators handle knots as a global curve parameter and may deliver incorrect results if the knots are of an extreme distribution. Further, Taylor's-expansion-based NURBS interpolators may overlook the path portion of extremely small knot spans. To solve these problems, methods for improving the robustness of Taylor's-expansion-based NURBS interpolators are proposed in this study, and an improved robust fast NURBS path interpolator is described. These new methods process NURBS data on the basis of knot spans: the adaptive quadrature method is applied to each knot span, and the calculated lengths of all knot spans are summed to build the total path length. Further, the inverse length functions are also generated based on the knot spans, and a control mechanism is introduced to prove the validity of the resulting inverse length functions. Experimental results have proved the effectiveness of the proposed robust fast NURBS path interpolator.     

CNC machine tool surface interpolator for ball-end milling of free-form surfaces

This paper presents a new type of CNC machine tool interpolator that is capable of generating the cutter path for ball-end milling of a free-form surface. The surface interpolator comprises on-line algorithms for cutter-contact (CC) path scheduling, CC path interpolation, and tool offsetting. The interpolator algorithms for iso-parametric, iso-scallop and iso-planar machining methods are developed, respectively. The proposed surface interpolator method gains the advantages for minimizing the data loaded to the CNC machine tool and maintaining the desired feedrate and position accuracy along the CC path.     

自由曲面数控加工智能选刀方法的研究与开发

由于自由曲面的几何复杂性,其数控加工的刀具干涉判别和刀具选取往往依靠数控机床编程工程师的经脸,致使加工的安全性、精度、效率及可靠性得不到有效保证.采用改进的遗传算法获取自由曲面的点云数据,在自主开发的系统中重构自由曲面.研究自由曲面加工刀位面生成的几何机理,根据刀具类型和几何尺寸,自动生成自由曲面数控加工的刀位面,可视化判别是否发生刀具干涉.将改进的快速排序算法应用到智能选刀模块,通过对常用类型和尺寸的刀具进行干涉排查,在避免干涉的前提下,智能选取最优化的加工刀具.对三种加工方案进行了测试,验证了智能选刀方法的正确性.     

A PC–NC milling machine with new simultaneous 3-axis control algorithm

Increasing demands on precision machining have necessitated that the tool move not only with a position error as small as possible, but also with smoothly varying feed rates. In this paper, a 3-axis PC–NC milling system, which is capable of synchronized simultaneous 3-dimensional (3D) machining, is developed. To achieve the synchronous 3D linear and circular motions, new interpolation algorithms based on the intersection criteria are presented. A real-time reference-pulse 3D linear and circular interpolator is developed using a PC to implement in the framework of the PC–NC milling machine reconfigured in this research. The performance test via computer simulation and actual machining have shown that the developed PC–NC milling system is useful for the machining of arbitrary lines and circles in 3D space.     

叶轮高速开粗加工路径优化设置研究

分析叶轮五轴数控加工刀具路径优化及NURBS曲面优化算法,应用MasterCAM X6软件中的叶轮专家系统,完成叶轮的高速五轴开粗加工优化参数设置,模拟仿真得到无碰撞、无过切的五轴叶轮开粗刀路轨迹。经实际切削验证,得到开粗的叶轮。     

Real-time surface interpolator for 3-D parametric surface machining on 3-axis machine tools

The conventional approaches for surface machining use a sequence of straight lines to approximate the part surface. These straight lines are subsequently translated into linear g-codes by a post processor, and then sent to the CNC system. This process limits the surface machining capability of CNC systems. For example, the CNC machine tool can be controlled only to follow these approximated straight lines since the machine loses the surface information. In contrast, this research utilised a real-time approach for 3-D parametric surface machining on 3-axis CNC machine tools. This real-time approach, called CNC surface interpolator, can read surface g-codes and perform surface machining interpolation. The input to the CNC surface interpolator is the surface g-code, which contains geometric information, such as the coefficients of the parametric surface, as well as cutting conditions. This results in better machining feedrate controls, interpolates more precise machining commands, and requires less machining time compared to that produced by off-line approaches.     

Feedrate scheduling for free-form surface using an NC verification model

This paper describes feedrate scheduling for a free-form surface using an NC verification model that can describe the actual cutting motions. This is important from the viewpoint of verification of the free-form surface and feedrate scheduling in the same simulator. In this paper, feedrate scheduling is conducted using the chip volume per tooth and per NC block using an NC verification model in order to obtain a constant chip load. A strategy for generating new NC commands at a set number of rotations between two existing NC blocks with a constant chip volume per tooth is proposed. The travel length of acceleration/deceleration is compensated in generating a new NC command in an NC block. Chip loads for feedrate scheduling are rapidly obtained in all tool paths considering surface curvature, feedrate, and cusp height formed by the previous tool path. Experiments validated that the presented feedrate scheduling obtained a constant chip load in all tool paths by inserting new NC commands in the existing NC file.     

Feedrate scheduling strategies for free-form surfaces

Free-form machining is one of the commonly used manufacturing processes for several industries such as automobile, aerospace, die and mold industries. In 3D complicated free-form surfaces, it is critical, but often difficult, to select applicable cutting conditions to achieve high productivity while maintaining high quality of parts. It is essential to optimize the feedrate in order to improve the machining efficiency of the ball-end milling. Conservative constant feedrate values have been mostly used up to now since there was a lock of physical models and optimization tools for the machining processes.The common approach used in feedrate scheduling is material removal rate (MRR) model. In the MRR based approach, feedrate is inversely proportional to either average or instantaneous volumetric removal rate. Commonly used CAM programs and NC code generators based on only the geometric and volumetric analysis, but they do not concern the physics of the free-form machining process yet. The new approach that is also introduced in this paper is based on the mechanics of the process. In other words, the force-based models in which feedrate is set to values which keep either average or instantaneous machining forces to prescribed values. In this study, both feedrate scheduling strategies are compared theoretically and experimentally for 3D ball-end milling of free-form surfaces. It is shown that MRR based feedrate strategy outputs higher feedrate values compared to force based feedrate strategy. High feedrate values of the MRR strategy increase the cutting forces extensively which can be damaging to the part quality and to the CNC Machine.When the new force based feedrate-scheduling strategy introduced in this paper is used, it is shown that the machining time can be decreased significantly along the tool path. The force-based feedrate scheduling strategy is tested under various cutting conditions and some of the results are presented in the paper.     

基于反求工程的自由曲面数控加工

介绍了以反求工程为技术支撑的自由曲面数控加工的基本原理和全过程，并对其中的数字化扫描、CAD建模及刀具轨迹生成等关键技术作了进一步的分析。最后描述了采用逆向工程技术进行国际象棋的棋子——马从模型到成品零件数控加工的实例。     

自由曲面数控加工走刀方向的优化研究

走刀方向对数控加工的效率有重要影响，尤其对于多曲面拼接自由曲面不规则轮廓区域的数控加工。针对现有自由曲面数控加工过程走刀方向优化研究存在的问题，提出了基于曲面数控加工区域划分的走刀方向优化策略，建立了简化的刀具轨迹计算模型和加工时间模型，并用实例加以验证，取得了满意的效果。     

模具数控磨削技术开发研究

研究了模具研腔的数控磨削加工技术,开发了安装于数控铣床主轴上的Ｔ／Ｄ牵引驱动增速装置和各种ＣＢＮ磨头,利用开发的增速器和ＣＢＮ磨头可进行平面、斜同、竖直面及空间自由曲面的磨削加工,为在ＮＣ机床和加工中心实现一次装夹完成模具的铣削和磨削加工提供了新的工艺装备和工艺条件。     

A real-time NURBS surface interpolator for precision three-axis CNC machining

Due to the fact that the cutting occurs around the cutter contact (CC) point, the efficiency and quality of CNC machining can be improved significantly if the CC velocity along the surface is kept costant. Conventional approaches to machining mainly maintain a constant cutter location (CL) velocity, so that the CC velocity along the surface is often not constant and usually results in non-uniform machining and unsatisfactory quality. To overcome this difficulty, this paper presents a novel NURBS surface interpolator that is capable of real-time generation of CL motion command for ball-end milling of NURBS surfaces and maintaining a constant CC velocity along the CC path and its intervals. For performance evaluation, a three-axis servomechanism driven by three servomotors is controlled to track segments represented by NURBS surfaces. Experimental results verify the effectiveness of the proposed method.     

The application of an inclined end mill machining strategy on 3-axis machining centres

An inclined end mill machining strategy offers an effective machining method for manufacturing low curvature free-form surfaces. The machined surface profile is a close match to the required surface geometry, and standard end mills are available at competitive prices. The amount of surface finishing work is therefore reduced, and the inclined cutting action prolongs the tool life. Although this machining strategy has these advantages, it is not commonly used in manufacturing industry, because it needs the facilities of costly 5-axis machining centres and appropriate post processors are required to generate the NC codes for these advanced machine tools. This paper proposes a simple method which extends the machining capability of the conventional 3-axis machining centre to give the same specifications as a 5-axis machine, and therefore allow the inclined end mill machining strategy to be used on 3-axis machines. The method involves tilting the workpiece instead of tilting the end mill during the machining process. The tool paths are determined by the workpiece rotation and cutter contact point offset equations. The amount of excess material on the machined surface is minimized by choosing optimum pitching and rolling angles at the work set-up stage. The experimental results indicate that the quality of the machined surfaces is improved in comparison with the surfaces machined by ball-nosed cutters, e.g. the resulting cusp volume is reduced by 64%. The research suggests that the need for advanced and expensive 5-axis machining could be satisfied in a more economical machining environment. As a result, the machining cost of the products may be significantly reduced.     

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.     

CNC机床指示输入及其系统

CNC机床指示输入是依据机械加工人员的操作习惯而设计的加工程序自动生成模式，CNC机床操作人员用加工任务指示器指示机床的运动部件和刀具，在完成加工过程空运转及试切的同时自动生成数控码，文章论述了实现这种方法的系统构成，运动参数的输入与处理。     

数控加工轨迹模拟器的研究

轨迹模拟是微机环境下进行数控程序检验的重要环节,它通过对数控程序进行翻译和动态绘制加工刀具运动轨迹,不仅实现了编程出错的检查,而且为系统仿真准备了信息完备、结构统一的轨迹数据。本文以ＳＩＮＵＭＥＲＩＫ８１０Ｔ系统的双刀架数控车为研究对象,介绍数控加工轨迹模拟器的功能和结构,并着重讨论系统研制方面的关键技术。     

高速切削CNC系统的研究

比较了高速切削加工与传统数控加工的区别 ,指出了高速切削对CNC系统的特殊要求 ,基于这些要求分析了目前高速切削CNC系统存在的四个问题 :①CNC体系结构封闭 ;②与CAM系统集成不够 ;③插补器和进给伺服控制器存在局限 ;④CNC缺乏对已知信息的综合考虑。针对这些问题的解决 ,提出基于PC的开放式CNC系统是解决问题的可行方案 ,应用面向对象的方法建立了这种方案的层次化参考模型 ,最后 ,给出了其实现的硬件拓扑结构     

Algorithms to support point-based cadcam

This paper presents a system that supports a point-based approach to CADCAM that was first suggested by McLaughlin. The core to the approach is the ability to interpolate a grid of points in a geometrically meaningful way that does not suffer from the disadvantages of the standard interpolation schemes based on NURBS. The essential features of the approach are based on recursive subdivision between existing points to generate intermediate points that are consistent with a smooth curvature profile. There are many advantages to this approach. Time-consuming surface fitting is not necessary; there are no problems with ensuring continuity between surfaces; data exchange can be carried out exactly. By chosing to work entirely with points, all of the standard interrogations available in CAE systems, including visualisations, FE analysis and NC machining, which are dependent on NURBS curves and surfaces have to be replicated. The feasibility of the point-based approach is illustrated by showing how geometric interrogations and machine tool paths can be accurately generated using just points.     

Enhanced virtual machining for sculptured surfaces by integrating machine tool error models into NC machining simulation

Sculptured surface machining is a time-consuming and costly process. It requires simultaneously controlled motion of the machine axes. However, positioning inaccuracies or errors exist in machine tools. The combination of error motions of the machine axes will result in a complicated pattern of part geometry errors. In order to quantitatively predict these part geometry errors, a new application framework ‘enhanced virtual machining’ is developed. It integrates machine tool error models into NC machining simulation. The ideal cutter path in the NC program for surface machining is discretized into sub-paths. For each interpolated cutter location, the machine geometric errors are predicted from the machine tool error model. Both the solid modeling approach and the surface modeling approach are used to translate machine geometric errors into part geometry errors for sculptured surface machining. The solid modeling approach obtains the final part geometry by subtracting the tool swept volume from the stock geometric model. The surface modeling approach approximates the actual cutter contact points by calculating the cutting tool motion and geometry. The simulation results show that the machine tool error model can be effectively integrated into sculptured surface machining to predict part geometry errors before the real cutting begins.