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Numerical Control technology

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15 thg 4 2020

Numerical Control technology as known today, emerged in the mid 20th century. It can be traced to the year of 1952, the U.S. Air Force, Massachusetts Institute of Technology in Cambridge, MA, USA, and the name of John Parsons (1913-2007), who is closely associated with the invention of numerical control. It was not applied in production man- ufacturing until the early 1960’s. The real boom came in the form of CNC, around the year of 1972, and a decade later with the introduction of affordable micro computers. History and development of this fascinating technology has been well documented in many publications.

In manufacturing field, and particularly in the area of metal working, Numerical Control technology has caused something of a revolution. Even in the days before comput- ers became standard fixtures in every company and many homes, machine tools equipped with Numerical Control system found their special place in many machine shops. The relatively recent evolution of micro electronics and the never ceasing computer development, including its impact on Numerical Control, has brought enormously significant changes to manufacturing sector in general and metal- working industry in particular.

DEFINITION OF NUMERICAL CONTROL

In various publications and articles, many descriptions have been used during the years, to define what Numerical Control actually is. It would be pointless to try to find yet another definition, just for the purpose of this handbook. Many of these definitions share the same idea, same basic concept, just use different wording.

The majority of all the known definitions can be summed up into a relatively simple statement:

Numerical Control can be defined as an operation of machine tools by means of specifically coded instructions to the machine control system

The 'specifically coded instructions' are combinations of the letters of alphabet, digits and selected symbols, for ex- ample, a decimal point, the percent sign, or the parenthesis symbols. All instructions are written in a logical order and in predetermined form. The collection of all instructions necessary to machine a single part or operation is called an NC Program, CNC Program, or a Part Program. Such a program can be stored for future use and used repeatedly to achieve identical machining results at any time.

NC and CNC Technology

In strict adherence to terminology, there is a difference in the meaning of abbreviations NC and CNC. The NC stands for the older and original Numerical Control technology, whereby the abbreviation CNC stands for the newer Com- puterized Numerical Control technology - a modern suc- cessor to its older relative. However, in everyday practice, CNC is the preferred abbreviation. To clarify the proper us- age of each term, look at the major differences between NC and CNC systems.

Both systems perform the same tasks, namely manipula- tion of data for the sole purpose of machining a part. In both cases, the control system internal design contains all logical instructions that process the input data. At this point the similarity ends.

The NC system (as opposed to the CNC system) uses a fixed logical functions, those that are built-in and perma- nently wired within the control unit. These functions can- not be changed by the part programmer or the machine op- erator. Because of the fixed wiring of control logic, NC control system is synonymous with the term ‘hardwired’. The system can interpret a part program, but it does not al- low any changes to the program at the control (using the control features). All required program changes must be made away from the control, typically in an office environ- ment. Also, NC system typically requires the compulsory use of punched tapes for input of the program information.

The modern CNC system (but not the old NC system), uses an internal micro processor (i.e., a computer). This computer contains memory registers storing a variety of routines that are capable of manipulating logical functions. That means the part programmer or machine operator can change any program at the control unit (at the machine), with instantaneous results. This flexibility is the greatest advantage of CNC systems and probably the key element that contributed to such a wide use of the technology in modern manufacturing. Typically, CNC programs and the logical functions are stored on special computer chips, as software instructions, rather than used by the hardware connections, such as wires, that control the logical func- tions. In contrast to the NC system, the CNC system is syn- onymous with the term ‘softwired’.

When describing a particular subject that relates to nu- merical control technology, it is customary to use either the term NC or CNC. Keep in mind that NC can also mean CNC in everyday talk, but CNC can never refer to the older technology, described in this handbook under the abbrevia- tion of NC. The letter ‘C’stands for Computerized, and it is not applicable to the hardwired system. All control systems manufactured today are of the CNC design. Abbreviations such as C&C or C’n’C are not correct and reflect poorly on anybody that uses them.

CONVENTIONAL AND CNC MACHINING

typical problems encountered in conventional machining. Individual machinists may have their own 'time proven’ methods, different from those of their fellow colleagues. Combination of these and other factors create a large field of inconsistency.

Machining under numerical control does away with the majority of inconsistencies. It does not require the same  physical involvement as manual machining. Numerically controlled machining does not need any levers or dials or

What makes CNC machining methods superior to con- ventional methods? Are they superior at all? Where are the main benefits? While comparing CNC and conventional machining processes, common general approach to ma- chining a typical part will emerge:

1. Obtain and study the engineering drawing

2. Select the most suitable machining method

3. Decide on the setup method (work holding)

4. Select cutting tools and holders

5. Establish spindle speeds and cutting feedrates

6. Machine the part

This general approach is the same for both types of ma- chining. One major difference is how various data are in- put. A feedrate of 10 inches per minute (10 in/min) is the same in manual or CNC applications, but the method of ap- plying it is not. The same can be said about a coolant - it can be activated by physically turning a knob, pushing a switch or programming a special code. All these actions will result in coolant rushing out of a nozzle. In both kinds of machin- ing, a certain amount of knowledge by the user is required. After all, metal working, and metal cutting specifically, is mainly a skill, but it is also, to a great degree, an art and a profession of large number of people. So is the application of Computerized Numerical Control. Like any skill, or art, or profession, mastering it to the last detail is necessary to be successful. It takes a lot more than just technical knowl- edge to be a CNC machinist, operator or CNC programmer. Work experience, intuition, and what is sometimes called a ‘gut-feel’, are much needed supplements to any skill.

In conventional machining, the operator sets up the ma- chine and moves each cutting tool, using one or both hands, to produce the required part. Design of a manual machine tool offers many features that help the process of handles, at least not in the same sense as conventional machining. Once the part program has been proven, it can be used any number of times over, always returning consistent results. That does not mean there are no limiting factors.

Cutting tools do wear out, material blank in one batch is not identical to the material blank in another batch, setups may vary, etc. These factors should be considered and compensated for, whenever necessary.

Emergence of numerical control technology does not mean an instant - or even a long term - demise of all manual machines. There are times when a traditional machining method is preferable to a computerized method. For exam- ple, a simple one time job may be done more efficiently on a manual machine than on a CNC machine. Certain types of machining jobs will benefit from manual, semiautomatic or automatic machining, rather than machining under nu- merical control. CNC machine tools are not meant to re- place every manual machine, only to supplement them.

In many instances, the decision whether certain machin- ing will be done on a CNC machine or not is based on the number of required parts and nothing else. Although the volume of parts machined as a batch is always an important criteria, it should never be the only factor. Consideration should also be given to the part complexity, its tolerances, the required quality of surface finish, etc. Often, a single complex part will benefit from CNC machining, while fifty relatively simple parts will not.

Keep in mind that numerical control has never machined a single part by itself. Numerical control is only a process or a method that enables a machine tool to be used in a pro- ductive, accurate and consistent way.

NUMERICAL CONTROL ADVANTAGES

machining a part - levers, handles, gears and dials, to name just a few. The same body motions are repeated by the operator for every part machined. However, the word ‘same’ in this context really means ‘similar’rather than ‘identical’. Humans are not capable to repeat every process exactly the same at all times - that is the job of machines. People can- not work at the same performance level all the time, with- out a rest. All of us have some good and some bad mo- ments. Such moments, when applied to machining a part, are difficult to predict. There will always be some differ- ences and inconsistencies within each batch of parts. Parts will not always be exactly the same. Maintaining dimen- sional tolerances and surface finish quality are the most

What are the main advantages of numerical control?

It is important to establish which areas of machining will benefit from it and which are better done the conventional way. It is absurd to think that a two horse power CNC mill will win over jobs that are currently done on a twenty times more powerful manual mill. Equally unreasonable are ex- pectations of super improvements in cutting speeds and feedrates over a conventional machine. If the machining and tooling conditions are the same, the total cutting time will always be very close in both cases.

A list of some major areas where CNC users can and should expect improvement includes:

  • Setup time reduction
  • Lead time reduction
  • Accuracy and repeatability
  • Contouring of complex shapes
  • Simplified tooling and work holding
  • Consistent cutting time
  • General productivity increase
  • Each area offers only a potential improvement. Individ- ual CNC users will experience different levels of actual improvement, depending on the product manufactured, CNC machine used, setup methods applied, complexity of fixturing, quality of cutting tools, management philosophy and engineering design, experience level of the workforce, individual attitudes, and many others.

    Setup Time Reduction

    In many cases, actual setup times for CNC machines can be reduced, sometimes quite dramatically. It is important to realize that setup is a manual operation, greatly dependent on the performance of CNC operators, the type of fixturing and general machine shop practices. Setup time is unpro- ductive, but necessary - it is part of the overall costs of do- ing business. To keep setup time to minimum should be the primary consideration of any machine shop supervisor, programmer and operator.

    Because of the design of CNC machines, real setup time should not be a major problem. Modular fixturing, stan- dardized tooling, fixed locators, automatic tool changing, pallets, and other advanced features, make the setup time more efficient than a comparable setup of conventional machines. With good knowledge of modern manufactur- ing, productivity can be increased quite significantly.

    The number of parts machined in a single setup is also important, in order to assess the actual cost of setup time. If a great number of parts is machined in one setup, the setup cost per part can be rather insignificant. A very similar re- duction can be achieved by grouping several different op- erations into a single setup. Even if the setup time is longer, it may be justified when compared to the time required to setup several conventional machines and operations.

    Lead Time Reduction

    Once a part program is written and proven correct, it is ready to be used again in the future, even at a short notice. Although the first run lead time is usually longer, it is virtu- ally nil for all subsequent runs. Even if an engineering change of the part design requires program modification, it can be done usually quickly, reducing the lead time.

    Long lead time, required to design and manufacture sev- eral special fixtures for conventional machines, can often be reduced by using simplified fixturing.

    Accuracy and Repeatability

    The high degree of accuracy and repeatability of modern CNC machines has been the single major benefit to many users. Whether part program is stored on a disk or in the computer memory, or even on a tape (the original method, now obsolete), it always remains the same. Any program can be changed at will, but once proven, no changes are usually required any more. A given program can be reused as many times as needed, without losing a single bit of data it contains. True, program has to allow for such changeable factors as tool wear and operating temperatures, it has to be stored safely, but generally very little interference from the CNC programmer or operator will be required. The accu- racy of modern CNC machines and their repeatability al- lows high quality parts to be produced consistently, time after time.

    Contouring of Complex Shapes

    CNC lathes and machining centers are capable of con- touring a large variety of different shapes. Many CNC us- ers acquired their machines only to be able to handle com- plex parts. A good examples are CNC applications in the aircraft and automotive industries. Any use of some kind of computerized programming is virtually mandatory for any three dimensional tool path generation.

    Complex shapes, such as molds, manifolds, dies, etc., can be manufactured without the additional expense of making a model for tracing. Mirrored parts can be achieved literally at the switch of a button. Storage of part programs is a lot simpler than storage of paper patterns, templates, wooden models, and other pattern making tools.

    Simplified Tooling and Work Holding

    Non-standard and ‘homemade’ tooling that clutters the benches and drawers around a conventional machine can be eliminated by using standard tooling, specially designed for numerical control applications. Multi-step tools such as pilot drills, step drills, combination tools, counter borers and others, are replaced with several individual standard tools. These tools are often cheaper and easier to replace than special and non-standard tools. Cost-cutting measures have forced many tool suppliers to keep a low or even a nonexistent inventory, while increasing delivery time to the customer. Standard, off-the-shelf tooling can usually be obtained faster then non-standard tooling.

    Fixturing and work holding for CNC machines have only one major purpose - to hold the part rigidly and in the same position for all parts within a batch. Fixtures designed for CNC work do not normally require special jigs, pilot holes and other hole locating aids.

    Cutting Time and Productivity Increase

    Cutting time on a CNC machine is commonly known as the cycle time - and is always consistent. Unlike a conven- tional machining, where the operator’s skill, experience and personal fatigue are subject to changes, CNC machin- ing is under the control of a computer. Only a small amount of manual work is restricted to the setup and part loading and unloading. For large batch runs, the high cost of unpro- ductive time is spread among many parts, making it less significant. The main benefit of a consistent cutting time is for repetitive jobs, where production scheduling and work allocation to individual machine tools can be done very ef- ficiently and accurately.

    One of the main reasons companies often purchase CNC machines is strictly economic - it is a serious investment with great potential. Also, having a competitive edge is al- ways on the mind of every plant manager. Numerical control technology offers excellent means to achieve sig- nificant improvements in manufacturing productivity and increasing the overall quality of manufactured parts. Like any means to an end, it has to be used wisely and knowl- edgeably. When more and more companies use CNC tech- nology, just having a CNC machine does not offer the extra edge anymore. Companies that grow and get forward are those where the use of technology is managed efficiently, with the goal to be competitive in the global economy.

    To reach the goal of major increase in productivity, it is essential that users understand the fundamental principles on which CNC technology is based. These principles take many forms, for example, understanding the electronic cir- cuitry, complex ladder diagrams, computer logic, metrol- ogy, machine design, machining principles and practices, and many others. Each discipline has to be studied and mastered by all persons in charge. In this handbook, the main emphasis is on topics relating directly to CNC pro- gramming and understanding the most common CNC ma- chines - Machining Centers and Lathes (sometimes called Turning Centers). Part quality consideration should be very important to every programmer and machine operator and this goal is also reflected in the handbook approach as well as in numerous examples.

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