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TYPES OF CNC MACHINE TOOLS

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15 thg 4 2020

Different types of CNC machines cover rather large vari- ety. Number of installations is rapidly increasing, and the technology development advances at a rapid pace. It is impossible to identify all possible applications, they would make a long list. Here is a brief list of some of the groups CNC machines can be part of:

  • Mills and Machining centers
  • Lathes and Turning centers
  • Drilling machines
  • Boring mills and Profilers
  • EDM wire machines
  • Punch presses and Shears
  • Flame cutting machines
  • Routers
  • Water jet and Laser profilers
  • Cylindrical grinders
  • Welding machines
  • Benders, Winding and Spinning machines, etc.

  • Without a doubt, CNC machining centers and lathes dominate the number of installations in industry. These two groups share the market just about equally. Some industries may have a higher need for a particular type of machines, depending on their needs. One must remember there are many models of lathes available and equally many differ- ent models of machining centers. However, the program- ming process for a vertical machine is similar to the one for a horizontal machine or even a simple CNC mill, for example. Even between different machine groups, there is a great amount of general applications, while the program- ming process is generally unchanged. For example, a con- tour milled with an end mill has a lot in common with a contour cut with a wire on an EDM machine.

    Mills and Machining Centers

    Minimum number of axes on a milling machine is three - the X, Y and Z axes. Part set on a milling machine is always stationary, mounted on a moving machine table. The cut- ting tool rotates, it can move up and down (or in and out), but it does not physically follow the tool path.

    CNC mills - sometimes called CNC milling machines - are usually small, simple machines, without a tool changer or other automatic features. Their power rating is often low. In industry, they are used for toolroom work, maintenance purposes, or small part production. They are usually de- signed for simple contouring, unlike CNC drills.
    CNC machining centers are far more popular and effi- cient than drills and mills, mainly for their flexibility. The main benefit users get out of a CNC machining center is the ability to group several diverse operations into a single setup. For example, drilling, boring, counter boring, tapping, spot facing and contour milling can be incorporated into a single CNC program operation. In addition, the flexibility is enhanced by automatic tool changing, using pallets
    to minimize idle time, indexing to a different face of the part, using a rotary movement of additional axes, and number of other time saving features. CNC machining centers can be equipped with special software that controls cutting speeds and feeds, life of the cutting tool, automatic in-process gauging, broken tool detection, offset adjustment and other production enhancing and time saving devices.
    There are two basic designs of a typical CNC machining center. They are vertical and horizontal machining centers. The major difference between the two types is the nature of work that can be done on them efficiently. For a vertical CNC machining center, the most suitable type of work are flat parts, either mounted to the table fixture, or held in a vise or a chuck. The work that requires machining on two or more faces (sides) in a single setup is more desirable to be done on a CNC horizontal machining center. A good ex- ample is a pump housing and other cubic-like shapes, often irregular. Some multi-face machining of small parts can also be done on a CNC vertical machining center equipped with a rotary table.


    Programming process is the same, but an additional axis (usually B axis) is added to the horizontal version. This axis is either a simple table positioning axis (indexing axis), or a fully rotary axis for simultaneous contouring.

    This handbook concentrates on CNC vertical machining centers applications, with a special section dealing with the uniqueness of horizontal setup and machining. Suggested programming methods are also applicable to small CNC mills or drilling and/or tapping machines, but the part pro- grammer has to consider their often severe restrictions.

    Lathes and Turning Centers

    A CNC lathe in its basic form is a machine tool with two axes, vertical X axis and horizontal Z axis. The main fea- ture of a lathe that distinguishes it from a mill is that the part is rotating about the machine center line. In addition, the cutting tool is normally stationary, mounted in a sliding tur- ret. Cutting tool follows the contour of the programmed tool path. Many modern CNC lathes are much more than just turning centers - with a simple milling attachment, the so called live tooling, the milling cutter has its own motor and rotates while the spindle is stationary. More complex designs incorporate off-center milling, double spindles, double turrets, part transfer, and many other efficiency im- proving features. These machines are generally called the mill-turn centers or sometimes the turn-mill centers.


    Modern lathe design can be horizontal or vertical. Hori- zontal type is far more common than vertical type, but both designs have their purpose in manufacturing. Several dif- ferent designs exist for either group. For example, a typical CNC lathe of the horizontal group can be designed with a flat bed or a slant bed, as a bar type, chucker type or a uni- versal type. Added to these combinations are many acces- sories that make a CNC lathe an extremely flexible ma- chine tool. Typically, accessories such as tailstock, steady rests or follow-up rests, part catchers, pullout-fingers and a third axis milling attachment are popular components of CNC lathes. A CNC lathe can be very versatile - so versa- tile in fact, that it is often called a CNC Turning Center. All text and program examples in this handbook use the more traditional term CNC lathe, yet still recognizing all its mod- ern functions.


    PERSONNEL FOR CNC


    Computers and machine tools have no intelligence. They cannot think, they cannot evaluate a situation in a rational way. Only people with certain skills and knowledge can do that. In the field of numerical control, the skills are usually in the hands of two key people - one doing the program- ming, the other doing the actual setup and machining. Their respective numbers and duties typically depend on com- pany preferences, its size, as well as the product manufac- tured there. However, each position is quite distinct, al- though many companies combine the two functions into a one, often called a CNC Programmer/Operator.

    CNC Programmer

    CNC programmer is usually a person who has the most responsibility in the CNC machine shop. This person is of- ten responsible for the success of numerical control tech- nology in whole the plant. Equally, this person is also held responsible for problems related to CNC and related opera- tions. Although duties may vary, the programmer is also re- sponsible for a variety of tasks relating to the effective us- age of one or more CNC machines. In fact, this person is often accountable for the production and quality parts from all CNC operations.

    Many CNC programmers are experienced machinists who have had a practical, hands-on experience as machine tool operators. They know how to read technical drawings and they can comprehend engineering intent behind the de- sign. This practical experience is the main foundation for the ability to ‘machine’ a part in an off-machine environ- ment. A good CNC programmer must be able to visualize all tool motions and recognize all restricting factors that may be involved. The programmer must be able to collect, analyze, process and logically integrate all collected data into a single, cohesive and safe part program. In simple terms, the CNC programmer must be able to decide upon the best manufacturing methodology in all respects.

    In addition to machining skills, the CNC programmer has to have a good understanding of mathematical principles, mainly application of equations, solution of arcs and an- gles. Equally important is the knowledge of trigonometry. Even with computerized programming, the knowledge of manual programming methods is absolutely essential to deep and thorough understanding of computer output and to assure control over such output.

    The last important quality of a truly professional CNC programmer is his or her ability to listen to other people - the engineers, CNC operators, managers. Good listening skills are the first prerequisites to become flexible. Any professional CNC programmer must be flexible in order to offer high quality in programming.
    CNC Machine Operator


    The CNC machine tool operator is a complementary po- sition to that of CNC programmer. The programmer and the operator may exist in a single person, as is common in many smaller shops. Although the majority of duties per- formed by a conventional machine operator has been trans- ferred to CNC programmer, CNC operator also has many unique responsibilities. In typical cases, the operator is re- sponsible for tool and machine setup, for changing of com- pleted parts, often even for some in-process inspection. Many companies expect quality control at the machine - and the operator of any machine tool, manual or computer- ized, is also responsible for the quality of work done on that machine. One of the most important responsibilities of CNC machine operator is to report findings about each pro- gram to the programmer. Even with the best knowledge, skills, attitudes and good intentions, the ‘final’ program can always be improved. CNC operator, being the one who is the closest to actual machining, knows precisely what ex- tent such improvements can be.

    SAFETY RELATED TO CNC WORK


    On the walls of many machine shops may hang a safety poster with a simple, yet very powerful message:

    The first rule of safety is to follow all safety rules

    The heading of this section does not indicate whether safety is oriented at the programming or the machining level. There is no reason for it - safety is totally independ- ent. It stands on its own and it governs behavior and activi- ties of everybody in machine shop and outside of it. At first sight, it may appear that safety is something related to ma- chining and machine operations, perhaps to the machine setup as well. That is definitely true but hardly presents a complete picture.

    Safety is the most important element in programming, setup, machining, tooling, fixturing, inspection, shipping, and you-name-it operation within a typical machine shop daily work. Safety should never be compromised and can- not be overemphasized. Companies talk about safety, con- duct safety meetings, display posters, make speeches, call experts. This mass of information and instructions is pre- sented to all of us for some very good reasons. Quite a few are based on past tragic occurrences - many laws, rules and regulations have been written as a result of inquests and in- quiries into serious accidents.

    At first sight, it may seem that in CNC work, safety is a secondary issue, not as important as in manual machining. There is a lot of automation in CNC, a part program that is used over and over again, tooling that has been used in the past, a simple setup, etc. All this can lead to complacency and false assumption that safety is taken care of. This is a wrong view that can have serious consequences.

    Safety is quite a large subject but a few points that relate to CNC environment are very important. Every machinist should know the hazards of mechanical and electrical de- vices. The first step towards a safe work place is with a clean work area, where no chips, oil spills and other debris are allowed to accumulate on the floor. Taking care of per- sonal safety is equally important. Loose clothing, jewelry, ties, scarfs, unprotected long hair, improper use of gloves and similar infractions, is dangerous in any machining en- vironment. Protection of one's eyes, ears, hands and feet is strongly recommended.

    While a machine is operating, protective devices should be in place and no moving parts should be exposed. Special care should be taken around rotating spindles and auto- matic tool changers. Other devices that could pose a hazard are pallet changers, chip conveyors, high voltage areas, hoists, etc. Disconnecting any interlocks or other safety features is dangerous - and also illegal, without appropriate skills and authorization.

    Modern technology has brought machines that may have nine or more axes, tight work areas, special tool indexing, part transfers, etc. While these features dramatically in- crease company productivity, they also require additional safety training - and practicing all safety rules.

    In CNC programming (manual or computer based), ob- servation of safety rules is equally important. Atool motion can be programmed in many ways. Speeds and feeds have to be realistic, not just mathematically ‘correct’. Depth of cut, width of cut, various tool characteristics, they all have a profound effect on overall safety in the shop.

    All these ideas are just a very short summary and a re- minder that safety should be taken seriously at all times.

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