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From Peter Smid, CNC Programming Handbook: A Comprehensive Guide to Practical CNC Programming, Second Edition, Industrial Press, Inc. 200 Madison Avenue, New York, 2003. CHAPTER 6: PROGRAMMING PLANNINGThe development of any CNC program begins with a very carefully planned process. Such a process starts with the engineering drawing of the required part. Before the part is machined, several steps have to considered and carefully evaluated. The more effort is put into the planning stage of the program, the better results may be expected at the end.STEPS IN PROGRAM PLANNINGThe steps required in program planning are decided by the nature of the work. There is no formula for all the jobs, but some basic steps should be considered:* Initial information / Machine tools features* Part complexity* Manual programming / computerized programming* Typical programming procedure* Part drawing / Engineering data* Methods sheet / Materials specifications* Machining sequence* Tooling selection* Part setup* Technological decisions* Work sketch and calculations* Quality considerations in CNC programmingThe steps in the list are suggestions only a guideline. They are quite flexible and should always be adapted for each job and to the specific conditions of the work.INITIAL INFORMATIONMost drawings define only the shape and size of the completed part and normally do not specify data about the initial blank material. For programming, a good knowledge of the material is an essential start - mainly in terms of its size, type, shape, condition, hardness, etc. The drawing and material data are the primary information about the part. At this point, CNC program can be planned. The objective of such a plan is to use the initial information and establish the most efficient method of machining, with all related consideration mainly part accuracy, productivity, safety and convenience.The initial part information is not limited to the drawing and the material data, it also includes conditions not covered in the drawing, such as pro- and post- machining, grinding allowances, assembly features, requirements for hardening, next machine setup, and others. Collecting all this information provides enough material to start planning the CNC program.MACHINE TOOLS FEATURESNo amount of initial information is useful if the CNC machine is not suitable for the job. During program planning, programmer concentrates on a particular machine tool, using a particular CNC system. Each part has to be setup in a fixture, the CNC machine has to be large enough to handle the size of the part, the part should not be heavier than the maximum weight allowed. The control system must be capable to provide the needed tool path, and so on. In the most cases, the CNC equipment is already available in the shop. Very few companies go and buy a new CNC machine just to suit a particular job. Such cases are rather rare and happen on if they make economic sense.* Machine Type and SizeThe most important considerations in program planning are the type and the size of the CNC machine, particularly its work space or work area. Other features, equally important, are the machine power rating, spindle speed and feed rate range, number of tool stations, tool changing system, available accessories, etc. Typically, small CNC machines have higher spindle speeds and lower power rating, large machines have lower spindle speeds available, but their power rating is higher.* Control SystemThe control system is the heart of a CNC machine. Being familiar with all the standard and optional features available on the control is a must. This knowledge allows the use of a variety of advanced programming methods, such as the machining cycles, subprograms, macros and other timesaving features of a modern CNC system.A programmer does not have to physically run a CNC machine. Yet, the programs will become better and more creative with good understanding of the machine and its control system. Program development reflects programmers knowledge of the CNC machine operation.One of the main concerns in program planning should be the operators perception of the program. To a large degree, such a perception is quite subjective, in the sense that different operators will express their personal preferences. On the other hand, every operator appreciates an err-free, concise, well-documented and professionally prepared part program, consistently and one after another. A poorly designed program is disliked by any operator, regardless of personal preferences.PART COMPLEXITYAt the time the drawing, material and the available CNC equipment are evaluated, the complexity of the programming task becomes much clearer. How difficult is to program the part manually? What are the capabilities of the machines? What are the costs? Many questions have to be answered before starting the program.Simple programming jobs may be assigned to a less experienced programmer of the CNC operator. It makes sense from the management perspective and it is also a good way to gain experience.Difficult or complex jobs will benefit from a computerized programming system. Technologies such as Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) have been a strong part of the manufacturing process for many years. The cost of a CAD/CAM system is only a fraction of what it used to be only a few years ago. Even small shops now find that the benefits offered by modern technology are too significant to be ignored. Several programming systems are available various computers and can handle any job. For a typical machine shop, a Windows based programming software can be very beneficial. A typical example of this kind of application is the very popular and powerful MasterCAM, form CNC Software, Inc. Tolland. CT. There are several others.MANUAL PROGRAMMINGManual programming (without a computer) has the most common method of preparing a part program for many years. The latest CNC controls make manual programming much easier than ever before by using fixed or repetitive machining cycles, variable type programming, graphic tool motion simulation, standard mathematical input and other timesaving features. In manual programming, all calculations are done by hand with the aid of a pocket calculator no computer programming is used. Programmed data can be transferred to the CNC machine via a cable, using an inexpensive desktop or a laptop computer. This process is faster and more reliable than other methods. Short programs can also be entered manually, by keyboard entry, directly at the machine. A punched tape used to be the popular media of the past but has virtually disappeared from machine shops.* DisadvantagesThere are some disadvantages associated with manual programming. Perhaps the most common is the length of time required to actually develop a fully functioning CNC program. The manual calculation, verification and other related activities in manual programming are very time consuming. Other disadvantages, also very high on the list, are a large percentage of errors, a lack of tool path verification, the difficulty is making changes to a program, and many others.* AdvantagesOn the positive side, manual part programming does have quite a few unmatched qualities. Manual programming is so intense that it requires the total involvement of the CNC programmer and yet offers virtually unlimited freedom in the development of the program structure. Programming manually does have some disadvantages, but it teaches a tight discipline and organization in program development. It forces the programmer to understand programming techniques to the last detail. In fact, many useful skill learned in manual programming are directly applied to CAD/CAM programming. Programmer has to know what is happening at all times and why it is happening. Very important is the in-depth understanding of every detail during the program development.Contrary to many beliefs, a thorough knowledge of manual programming methods is absolutely essential for efficient management of CAD/CAM programming.CAD/CAM AND CNCThe need for important improved efficiency and accuracy in CNC programming has been the major reason for development of a variety of methods that use a computer to prepare part programs. Computer assisted CNC programming has been around for many years. First, in the form of language based programming, such as APT or Compact . Since the late 1970s, CAD/CAM has played a significant role by adding the visual aspect to the programming process. The acronym CAD/CAM means Computer Aided Design and Computer Aided Manufacturing. The first three letters (CAD) cover the area of engineering design and drafting. The second three letters (CAM), covers the area of computerize manufacturing, where CNC programming is only a small part. The whole subject of CAD/CAM covers much more than just design, drafting and programming. It is a part of modern technology also known as CIM Computer Integrated Manufacturing.In the area of numerical control, computers have played a major role for a long time. Machine controls have become more sophisticated, incorporating the latest techniques of data processing, storage, tool path graphics, machining cycles, etc. Programs can now be prepared with the use of inexpensive computers, using graphical interface. Cost is no longer an issue; even small machine shops can afford a programming system in house. These systems are also popular because of their flexibility. A typical computerized programming system does not have to be dedicated only to programming all related tasks, often done by the programmer, can be implemented on the same computer, for example, cutting tool inventory management, database of part programs, material information sheet, setup sheets and tooling sheets, etc. The same computer could also be used for uploading and downloading CNC programs.* IntegrationThe keyword in the acronym CIM is integration. It means putting all the elements of manufacturing together and work with them as a single unit and more efficiently. The main idea behind a successful integration is to avoid duplication. One of the most important rules of using a CAD/CAM computer software is: Never Do Anything Twice!When a drawing is made in a CAD software (such as AutoCAD), then done again in a CAM software (such as MasterCAM), there is a duplication. Duplication breeds errors. In order to avoid duplication, most of the CAD system incorporate a transfer method of the design to the selected CAM system to be use for CNC programming. Typical transfers are achieved through special DXF or IGES files. The DXF stands for Data Exchange Files or Drawing Exchange Files, and the IGES abbreviation is a short form of Initial Graphics Exchange Specification files. Once the geometry is transferred form the CAD system to the CAM system, only the tool path related process is needed. Using a post processor (special kind of formatter), the computer software will prepare a part program, ready to be loaded directly to the CNC machine.* Future of Manual ProgrammingIt may seem that the manual programming is on the decline. In terms of actual use, this is probably true. However, it is necessary to keep in perspective that any computerized technology is based on the already well established methods of manual programming. Manual programming for CNC machines serves as the source of the new technology it is the very elementary concept on which the computerized programming is based. This knowledge base opens the door for development of more powerful hardware and software applications.The manual programming may be use somewhat less frequently today and eventually will be used even less but knowing it well really understanding it is and always will be the key to control the power of CAM software. Even computers cannot do everything. There are some special programming projects that a CAM software, regardless of the price, may handle to an absolute satisfaction. If the control system can handle it, manual programming is the way to the ultimate control over such a project, when any other methods may not be suitable.Even with a well customized and organized computerized programming system, how can the generated program output be exactly as intended? How can the CNC operator change any part of the program on the machine, without knowing its rules and structure? Successful use of computerized programming requires understanding of manual programming methods.TYPICAL PROGRAMMING PROCEDUREPlanning of a CNC program is no different than any other planning; it must be approached in a logical and methodical way. The first decisions relate to what tasks have to be done and what goals have to be reached. The other decisions relate to how to achieve the set goals in an efficient and safe manner. Such a progressive method not only isolates individual problems as they develop, it also forces their solution before the next step can be taken.The following items form a fairly common and logical sequence of tasks done in CNC programming. The items are only in a suggested order, offered for further evaluation. This order may be changed to reflect special conditions or working habits. Some items may be missing or redundant:1. Study of initial information (drawing and methods)2. Material stock (blank) evaluation3. Machine tool specifications4. Control system features5. Sequence of machining operations6. Tooling selection and arrangement of cutting tools7. Set of the part8. Technological data (speed, feed rates, etc.)9. Determination of the tool path10. Working sketches and mathematical calculations11. Program writing and preparation for transfer to CNC12. Program testing and debugging13. Program documentationThere is only one goal in CNC program planning and that is the completion of all instructions in the form of a program that will result in an error- free, safe and efficient CNC machining. The suggested procedures may require some changes for example, should the tooling be selected before or after the part setup is determined? Can the manual part programming methods be used efficiently? Can the manual part programming methods be used efficiently? Are the working sketches necessary? Do not be afraid to modify any so called ideal procedure either temporarily, for a given job, or permanently, to reflect a particular CNC programming style. Remember, there are no ideal procedures.第六章：制定编程计划编写任何CNC程序都必须经过周密计划后开始。这个过程的起点就是所需工件的工程图。在实施工件加工之前一些步骤必须仔细考虑和评估。在程序的计划阶段付出的努力越多，最后预期的结果可能就越好。制定编程计划的步骤具体采用哪些步骤要根据实际的工作条件而定，没有国定的程式。但还是有一些基本的步骤可供参考：l 初始信息、机床的加工特性l 工件的复杂程度l 手工编程、电脑编程l 典型的编程工艺l 工件图纸、工程数据l 处理表、材料规格l 加工工序l 刀具选择l 工件的夹装l 技术要求l 工程草图和计算l 编程要达到的质量要求这些建议只是提供了一点基本的方向。具体的应用变化很多，必须根据对应的加工和工作特定的条件灵活的调整。初始信息大多数工程图只是定义了零件的形状和尺寸，一般不标明最初毛坯料的数据。对于编程来说，基本的起点是对原材料的掌握，诸如尺寸、形状、硬度、加工性能等等。对于工件来说最主要的信息就是工程图和原材料数据，在这基础上程序才得于建立。这个计划的目的就是利用最初的信息去建立最高效的加工方法，要考虑到所有的相关事项主要是工件的精度、生产率、安全性和方便性。最初的工件信息并不限于零件图的材料信息，它还包括一些没有涵盖在零件图中的内容，比如前加工和后加工、磨削余量、装配特点、硬化要求、下一步加工准备以及其他信息。收集所有的信息给开始给CNC编程制定计划供了充足的材料。机床加工特性如果CNC不能适合某个作业的话再多的初始信息也没有用。在计划编程阶段程序员精力都集中在特定的CNC系统相应的机床刀具上。所有的工件都必须在固定在夹具上，CNC机床的加工极限要大于工件尺寸，工件的重量不能超出容许的范围。控制系统必须可以提供所需的走刀路线，诸如此类。在通常的情况下，CNC设备已经在工厂设置好了，很少有公司为了适应某个特殊的作业而购置新的CNC机床，这种情况非常罕见，除非这样做符合经济意义。l 机床的型号和尺寸给编程制定计划时最重要的考虑是机床的型号和尺寸，尤其是它的加工空间或是加工区域。其它特性也非常重要诸如机床的功率等级、主轴速度和进给范围、刀库的数量、换刀系统、配件等等。通常小型CNC主轴转速高功率小，而大型机床功率等级高主轴转速则偏低。l 控制系统控制系统是CNC机床的心脏。必须熟悉所有控制器的标准功能和可选功能。有了这些知识，就可以使用各种高级编程方法，比如加工循环、子程序、宏和现代CNC系统其它节约时间的功能。尽管编程人员不必实际操作CNC机床，然而熟悉机床及其控制系统有助于使编程更合理更具有创造性。程序反映出了编程员对CNC机床运行知识的理解程度。在编制计划时有一点也很重要，那就是机床操作人员对程序的看法。在很大程度上，他们的观点相当的主观，不同的操作员个人的偏好不同。但另一方面，每个作业员都喜欢没有错误、简练、文档完备以及专业的零件加工程序。排除个人偏好，没有人喜欢一个设计有缺陷的程序。零件复杂程度对工件图、材料和可用的CNC设备进行评估之后，编程工作的复杂程度就变得十分清楚。在开始编程之前还要考虑清楚零件手工编程的难度有多大？机床是否适合？成本是多少？简单的编程任务可以分配给初具经验的程序员或者是操作员。这是符合管理理念的，也可以帮助他们获得经验。困难或复杂的作业得益于计算机编程系统。计算机辅助设计（CAD）、计算机辅助加工（CAM）在很多年前就成为了生产工序的重要部分。CAD/CAM系统的成本也降到几年前的一小部分。即便是小车间也发现现代技术带来的巨大优势不容忽视。有几种编程系统在不同计算机上都可以使用，可以处理任何作业。对于常见的加工车间，一个基于Windows的编程系统非常有益。典型的应用软件诸如使用广泛功能强大的MasterCAM，还有很多其它软件。手工编程很多年前零件加工程序最常用的编程方法就是手工编程。新型的CNC控制器可以提供固定循环、多种类型的编程、刀具运动的图型模拟、标准的数学输入以及其它节省时间的特性，这使得手工编程比以前简单了许多。手工编程时所有的计算都是通过便携式计算器不用计算机而手工完成的。通过电缆，程序数据可以从便宜的台式机或笔记本电脑传送到CNC机床上。比较而言这种方法速度快而且可靠。更简短的程序可以通过机床的键盘直接输入。穿孔纸带是过去常用介质，目前基本上从机加工车间消失了。l 缺点手工编程有一些缺点。或许最突出的就是编制一个完整功能CNC程序消耗的时间量过于庞大。手工计算、核对等等工作都很费时间。其它的缺点也是很明显，诸如错误率高、不能进行刀路检查、程序的修改也是相当不便。l 优点从有利的一面看，手工编程也有不少不可比拟的优点。手工编程如此紧凑，需要程序员全身心的投入，而且确实提供了建立程序结构无限的自由空间。程序的手工编制肯定会有弊端，但对于养成良好的编程习惯和