Computer Age Machining

The development of the computer has made changes in our everyday life from retail sales, banking, and medicine to communications, transportation, science, and manufacturing. No other invention in history had such an impact on humanity in such a short period of time as the computer. The computer has made possible the exploration of space, world-wide television, improved health care, quality-controlled manufacturing, the use of robots, flexible manufacturing systems, and many others developments. This revolutionary invention has dramatically changed the way people live and work throughout the world.

The concept of computers and numerical control is historically a very recent development. The last fifty or sixty years are only a microscopic part of the whole technological evolution. Computers of today are nothing more than very powerful calculating devices.

Calculating, counting and the manipulation of numbers have been a natural human activity since the beginning of time. One of the most advanced devices of its time was an abacus. This device is still being used today, and has its roots in the Orient, more than 6000 years ago, Fig. 35-1.

In 1642 the first device that can be classified as a calculator was invented. It could only perform only addition and subtraction, in 1/5000 of a second.

The 20th century is considered to be the birth of the information age. In 1943, the United States government made a significant investment into a project requiring calculations of firing tables for various artillery weapons. The result of this project was the ENIAC Computer (Electronic Numerical Integrator And Computer), the first and largest computer in the world developed, at the University of Pennsylvania (Philadelphia).

ENIAC is actually an acronym for the world’s first computer that could actually be programmed. By today’s standards even the slowest personal computer currently in use is more powerful than the original ENIAC.

NC Technology

Numerical Control technology emerged in the mid-20th century, around the year 1952. Numerical control in its original concept was too expensive to implement, and was not applied in production manufacturing until the early 1960s. The real increase in machine shop installations came in the early 1970s, with the development of CNC,

Computer Numerical Control. Additional improvements came a decade later when affordable desktop microcomputers came on the market, Fig. 35-2.

Definition of Numerical Control

Many descriptions have been used over the years to define what Numerical Control is. The majority of all the known definitions can be summed up into a relatively simple statement: Numerical control is the operation of machine tools by specifically coded instructions.

The special coded instructions consist of combinations of the letters of the alphabet, numbers, and selected symbols. Typical symbols include a decimal point, percent sign, minus sign, or parenthesis. All the instructions are written in a logically planned sequential order, and must use a predetermined format. The combination of all the instructions necessary to machine a workpiece or perform certain operations on a workpiece is called a CNC Program or a Part Program. These programs can be stored for future use and can be used repeatedly to achieve identical machining results anytime.

CNC Technology

Computerized Numerical Control technology performs many machine tool oriented tasks, namely manipulation of data for the purpose of machining a workpiece. The design of the control system contains the logical functions that process the data, since it uses an internal microprocessor (a computer).

This computer contains memory registers storing a variety of routines capable of manipulating the logical functions that relate to machining operations. This all means that the part programmer (CNC Programmer) or the machine operator (CNC Operator or CNC Machinist) can change the program on the control, at the machine, Fig.35-3. The results of any change are immediate, and can be used right away. This flexibility is the greatest advantage of the systems and probably the single one that contributed to such a wide use of the technology in modern manufacturing. The CNC programs are stored on computer chips as software instructions, rather than used by hardware connections, such as wires, that control the logical functions.

The computer has brought many other advantages to the system, for example, various machining cycles, offsets and compensations, subprograms, and macros. Since the early years of CNC, programming has evolved through several stages into its current form. The evolution is not complete; after-all, technology advances at a rapid rate, and CNC will not be excluded.

What are Controllers?

In manufacturing the controller is a feature that operates and regulates various components of a manufacturing system. Specially designed industrial controllers, using desktop operating systems, such as Windows, control most modern production lines. Software, keypads or keyboards, and display screens show various selection menus to assist the operator. The whole manufacturing process can be controlled from a single computer.

Many parts of production processes can and must be controlled during manufacturing, for example:

• Spindles for machining operations

• Workpiece positioning

• Coolant and lubrication systems

• Gaging and inspection systems

• Chip removal devices

• Transfer of material and tools

• Monitoring of production

• Application of robots

Many other control types can be included here, but the one control that is the focus of this course is the control of machining operations by numbers known as Numerical Control.

Conventional and CNCMachining

What makes CNCmachining superior to conventional machining methods and what are the main benefits? Both the CNC and the conventional machining processes use a common general approach to machining a workpiece:

• Obtain and study the technical drawing (print)

• Select the most suitable machining method

• Decide on the setup method (work holding)

• Select the cutting tools

• Establish speeds and feeds

• Machine the workpiece

In conventional machining, Fig. 35-4A, the machine table slides are moved into position by manually turning the handwheels. In CNCmachining, Fig. 35-4B, the machine slide movements are controlled by the information contained in the CNC program. In both kinds of machining, a certain amount of knowledge on the part of the user is required. Working with metal requires a skill, but it also offers a great deal of personal flexibility. Like any skill, it has to be mastered to the last detail to be successful. It takes more than technical knowledge to be a CNC machinist or a CNC programmer; experience and a lot of practice are necessary.

In conventional machining, the machine operator sets up the machine and moves the handles that guide the cutting tool; the accuracy of the part depends upon the skill of the operator.

Numerical Control machining does away with the majority of variations and inconsistencies of manual machining. Information about the part is included in the program and controls the movement of the slides to produce the part automatically. Once the program has been proven, it can be used many times over producing consistent results. As machines and controls improve, this technology will continue to advance in the future.

Numerical Control Advantages

There are many advantages to having a numerically controlled machine tool. Some of the major areas where the CNC user can and should expect improvement are shown in Fig. 35-5.

Setup Time Reduction

In many cases, the setup time for a CNC machine can be greatly reduced. It is important to realize that setup is a manual operation, greatly dependent on the performance of the CNC operator, the type of fixturing, and general practices of the machine shop. Setup time is unproductive; although necessary, it is a part of the overhead costs of doing business. Keeping the setup time to a minimum should be one of the primary considerations of any machine shop manager, programmer, or operator.

Modular fixturing, standard tooling, fixed locators, automatic tool changing, pallets, automated vehicles, gantry systems, and other features make the setup time more convenient than a comparable setup of a conventional machine.

Lead Time Reduction

Although the lead-time for the first run on a CNC machine is usually longer than on a manual machine, this difference is virtually eliminated for any subsequent runs. Even if an engineering change of the part design requires the program to be changed, it can be done very quickly, reducing the lead-time. By preparing a part program and using simplified fixturing long lead-time used to design and manufacture special fixtures for conventional machines can be reduced.

Accuracy and Repeatability

Once the program is written and proven, it is ready to be used, or stored for use in the future. Whether the part program is stored on a disk or in the computer memory, it always remains the same; however, it can be changed whenever necessary. That means the program can be reused as many times as needed without losing a single bit of the data it contains. Generally, very little interference from the CNC programmer or operator will be required. The high accuracy of CNC machines and their repeatability makes it possible to produce high-quality parts consistently.

Contouring of Complex Shapes

CNC machine tools are capable of contouring a variety of complex shapes and forms. Good examples are CNC applications in the automotive and aircraft industries. The use of some form of computerized programming is virtually mandatory for any three-dimensional tool path generation.

Complex shapes such as multi-sided objects or molds can be manufactured without the additional expense of making a preliminary model. Mirrored parts can be produced at the switch of a button. Storage of programs is simpler than storage of patterns, templates, models, and other auxiliary tools.

Simplified Tooling and Work Holding Devices

Using standard tooling specially designed for numerical control applications can eliminate nonstandard tooling used for conventional machines. Multi-step tools such as pilot drills, step drills, or counter borers can be replaced with individual standard tools. These tools are cheaper and easier to replace than special and non standard tools. Standard, off-the-shelf tooling can usually be obtained faster than non standard tooling. Standard tools can be combined with special tools, increasing flexibility in the manufacturing process.

Fixturing and workholding for CNC machines are used to hold the workpiece in place rigidly, safely, and in the same position for all parts to be machined. Fixtures designed for CNC work do not normally require jigs, pilot holes, or other hole locating aids.

Consistent Cutting Time and Productivity Increase

The cutting time on the CNC machine known as the cycle time is always consistent. Unlike conventional machining, where the machine operator’s skill, experience, and personal fatigue are subject to change, the CNCmachining is under the control of a computer. The small amount of manual work is restricted to the initial setup, loading and unloading of the workpiece. For large batch runs (large number of the same parts), the cost of the unproductive time is spread over many parts, making time loss less significant. The main benefit of a consistent cutting time is for repetitive jobs, where the production scheduling and work allocation to individual machine tools can be done accurately.

Types of CNC Machine Tools

There are many different types of CNC machine tools used in industry, such as:

• Mills and machining centers

• Lathes and turning centers

• Drilling machines

• EDM machines

• Flame and Laser-cutting machines

• Waterjet profilers CNCmachining centers and turning centers (lathes) are the most commonly used machine tools.

Machine Axes

There are three standard axes on a milling machine, X axis, Y axis, and the Z axis, Fig 35-6. Unlike the machine tool designed for turning; the workpiece on a milling system is always stationary, mounted on a moving table. The cutting tool rotates, and it can move up and down (or in and out), but it does not follow the tool path.

CNC mills or CNC milling machines are usually small, simple machines without a tool changer or other automatic features. They are usually designed for contouring and are used in tool shops and maintenance departments for small part production.

CNC machining centers are far more popular and efficient mainly because of their flexibility. The main benefit the user gets from a CNCmachining center is the ability to group several 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. In addition, the flexibility is enhanced by automatic tool changing, using pallets to minimize idle time, indexing the workpiece, or using the rotary movement of additional axes.

Mills and Machining Centers

There are two major types of CNCmachining centers:

• Vertical Machining Centers (VMC)

• Horizontal Machining Centers (HMC)

The major difference between the two types is the type of work that can be done on them efficiently. For a vertical CNCmachining center, the most suitable type of work is flat parts, either mounted to the fixture on the table, or held in a vise or a chuck. Some multi-face machining of small parts can also be done on CNC vertical machining centers equipped with a rotary table.

For work that requires machining on two or more faces in a single setup, it may be more suitable to use a CNC horizontal machining center. Examples include an engine block, cylinder head, a pump housing, and other cubic-like shapes.

The programming process is the same for both types, except an additional axis (usually the B axis) is added to the horizontal design, to enable the part to rotate within the work area. This axis is either a simple positioning axis (indexing axis) for the table, or a fully rotary axis for simultaneous contouring.

CNC Lathes and Turning Centers

A CNC lathe is usually a machine tool with two directions of motion, called axes, the vertical X axis and the horizontal Z axis, Fig 35-7. The main difference between a CNC lathe and a machin- ing center is that the workpiece is rotating about the machine centerline, and the cutting tool is normally stationary, mounted in a sliding turret. The cutting tool follows the contour of the programmed tool path. For CNC lathes with milling attachments (live tooling), the milling tool has its own motor and rotates while the machine spindle is stationary.

The modern lathe design can be either horizontal or vertical. The horizontal type is the most common type. A typical horizontal CNC lathe can be designed with a flat bed or a slant bed, as a bar type, chucker type, or a universal type, many accessories can be added to a CNC lathe to make it an extremely flexible machine tool.

Employment Opportunities

Various industries, from small job shops to the large manufacturing companies, use skilled machinists and machine operators. Most jobs are in small machine shops or manufacturing companies, for example, automotive, metalworking, aerospace and industrial machinery. Studies indicate excellent employment opportunities for machinists, particularly those with complete skills, including manual and computer assisted CNC programming.

Even during times of slow economical growth, many employment opportunities will be available for machinists as a result of a retiring workforce. Because of automation, positions for CNC programmers will not grow as fast as those for CNC machinists. One major reason is the introduction of software that includes CNC instructions at the design stage.

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, the programmer and the machine operator. Their duties depend on the company, its size, and the product being manufactured. However, the function of each is quite distinct, although many companies combine the two functions into one, often called a CNC Programmer/Operator.

CNC Programmer

CNC programmer is often an experienced machinist, or at least someone knowledgeable in many aspects of tooling and machining. A Programmer is expected to plan and program a part, starting from the engineering drawing, and ending with the program ready to be loaded, Fig.35-8. The CNC programmer is also responsible for the selection of cutting tools, calculation of speeds and feeds, depth of cuts. The program consists of a carefully selected sequence of operations. Programmers work closely with the engineers who designed the part.

Programming Qualities

Many CNC programmers are experienced machinists who have the following qualities:

• Can read and interpret technical drawings.

• Able to visualize all tool motions and recognize any restricting factors.

• Able to collect, analyze, process and logically integrate all the collected data into a single, cohesive part program.

• Must be able to decide upon the best manufacturing methodology in all respects.

• Has an understanding of mathematical principles, mainly application of equations, solution of arcs and angles, and the use of trigonometry.

• Be able to listen to the engineers, CNC operators, and the managers.

CNC Machine Operator

The CNCmachine tool operator is a complementary position to the CNC programmer. The programmer and the operator may exist in a single person, as is the case in many small and mediumsize machine shops. Although the majority of duties performed by a conventional machine operator have been transferred to the CNC programmer, the CNC operator has many unique responsibilities such as:

• The setting-up of the machine, tooling, and the changing of the parts (workpiece).

• The control of the part quality.

• Reporting part quality and program problems to the part programmer, Fig.35-9.

Operator vs. Machinist

In many cases, the difference between a CNC operator and a machinist depends on the definition of a particular company. An operator may be an unskilled or semi-skilled person, performing less responsible duties than a machinist. Often, an operator only runs a production machine and is not involved in setup and programming. A typical machinist is responsible for all aspects of running a machine tool, including setup and general troubleshooting.

Education and Training

Many vocational schools offer elementary CAD/CAM and even CNC education, Fig 35-10.

• Community and technical colleges offer the most comprehensive programs.

• Courses are also offered by machine tool vendors, software vendors, and tooling companies.

• CAD/CAM and CNC related programs, Fig.35-10 offered in various institutions can fall into two groups,

· Programming courses that offer manual CNC programming and CAM programming,

· Operating courses for CNC machine setup, tooling, and machine operation.

• Numerical control is the operation of machine tools by specifically coded instructions, such as letters of the alphabet, digits, and selected symbols.

• Computerized Numerical Control technology uses program data to machine a workpiece.

• CNC and the conventional machining use the same basic procedure for machining a workpiece.

• CNCmachining uses the computer program to machine a part; in conventional machining, the machine slides are controlled by the machinist.

• Once the CNC program is written and proven, it is ready to be used, at any time in the future.

• Machining using Numerical Control technology does away with the majority of variations and inconsistencies of conventional machining.

Computers

1. What was considered the most advanced calculating device of its time?
2. What was ENIAC, and what does this acronym stand for?

NC Technology

3. Define the term Numerical Control.
4. Name three different coded instructions used in programming.

CNC Technology

5. What is the main advantage of CNC over NC machine tools?

Conventional and CNC Machines

6. State the main difference between conventional and CNC machines.
7. Name three advantages of CNC machine tools.
8. What two major industries use CNC machine tools?

CNC Machining Centers (mills)

9. List the three standard axes for a CNCmachining center (mill).
10. Name the two main type of CNCmachining centers.

CNC Turning Centers (lathes)

11. List the two standard axes of a CNC turning Center (lathe).
12. What are the two main types of CNC Turning centers?

Employment Opportunities

13. Define a CNC programmer, and state what job functions are performed.
14. Define a CNC machine operator, and state what job functions are performed.
15. What are three prerequisites for becoming a CNC programmer or operator?

Copyright 2004, Industrial Press, Inc., New York, NY