CNC is an abbreviation of Computer Numerically Controlled. It is a new feature widely available for machine tools like lathe, milling, drilling etc. to enhance their working ability and efficiency with the aid of a computer. Till date, the working of some these machines is manually operated which give no chance for operators to rest or concentrate on other tasks. CNC machines are considered as it offered high average precision and accuracy and also time efficiency.
Today we’ll be looking at the definition, history, working principle, manufacturing process, types, as well as futures of CNC machines.
CNC or "computer numerical controlled" machines are sophisticated metalworking tools that can create complicated parts required by modern technology. Growing rapidly with the advances in computers, CNCs can be found performing work as lathes, milling machines, laser cutters, abrasive jet cutters, punch presses, press brakes, and other industrial tools. The CNC term refers to a large group of these machines that utilize computer logic to control movements and perform metalworking.
Although wood-working lathes have been in use since Biblical times, the first practical metalworking lathe was invented in 1800 by Henry Maudslay. It was simply a machine tool that held the piece of material being worked, or workpiece, in a clamp, or spindle, and rotated it so a cutting tool could machine the surface to the desired contour. The cutting tool was manipulated by the operator through the use of cranks and handwheels. Dimensional accuracy was controlled by the operator who observed the graduated dials on the handwheels and moved the cutting tool the appropriate amount. Each part that was produced required the operator to repeat the movements in the same sequence and to the same dimensions.
The first milling machine was operated in much the same manner, except the cutting tool was placed in the rotating spindle. The workpiece was mounted to the machine bed or worktable and was moved about under the cutting tool, again through the use of handwheels, to machine the workpiece contour. This early milling machine was invented by Eli Whitney in 1818.
The motions that are used in machine tools are called "axis," and are referred to as "X" (usually left to right), "Y" (usually front to back), and "Z" (up and down). The work-table may also be rotated in the horizontal or vertical plane, creating a fourth axis of motion. Some machines have a fifth axis, which allows the spindle to pivot at an angle.
One of the problems with these early machines was that they required the operator to manipulate the handwheels to make each part. Besides being monotonous and physically exhausting work, the ability of the operator to make identical parts was limited. Slight differences in operation resulted in variation of the axis dimensions, which, in turn, created poorly fitting or unusable parts. Scrap levels for the operations were high, wasting raw materials and labor time. As production quantities increased, the number of usable parts produced per operator per day were no longer economical. What was needed was a means to operate the motions of the machine automatically. Early attempts to "automate" these operations used a series of cams that moved the tools or worktable through linkages. As the cam rotated, a link followed the surface of the cam face, moving the cutting tool or the workpiece through a series of motions. The cam face was shaped to control the amount of linkage movement, and the rate at which the cam turned controlled the feedrate of the tool. These early machines were difficult to set correctly, but once set, they offered excellent repeatability for their day. Some have survived to this day and are called "Swiss" machines, a name synonymous with precision machining.
The modern CNC machine works by reading the thousands of bits of information stored in the program computer memory. To place this information in the memory, the programmer creates a series of instructions that the machine can understand. The program may consist of "code" commands, such as "M03" which instructs the controller to move the spindle to a new position, or "G99," which instructs the controller to read an auxiliary input from some process inside the machine. Code commands are the most common way to program a CNC machine tool. However, the advancement in computers has allowed the machine tool manufacturer to offer "conversational programming," where the instructions are more like plain words. In conversational programming, the "M03" command is entered simply as "MOVE," and the "G99" command is simply "READ." This type of programming allows faster training and less memorizing of the code meanings by the programmers. It is important to note, however, that most conversational machines still read code programs, since the industry relies on that form of programming quite heavily.
The controller also offers help to the programmer to speed up the machine use. In some machines, for example, the programmer can simply type in the location, diameter, and depth of a feature and the computer will select the best machining method for producing the feature in the workpiece. The latest equipment can take a computer-generated engineering model; calculate the correct tool speeds, feeds, and paths; and produce the part without a drawing or program ever being created.
The mechanical components of the machine must be rigid and strong to support the quickly moving parts. The spindle is usually the strongest part and is supported by large bearings. Whether the spindle holds the work or the tool, an automatic clamping feature allows the spindle to rapidly clamp and unclamp during the program run. A transfer arm, sometimes called the tool bar, removes a tool from the machine, places it into the magazine, selects a different tool from the magazine, and returns it to the machine through instructions in the program. Typical cycle time required for this procedure is two to eight seconds. Some machines may contain up to 400 tools in large "hives," each automatically loaded in sequence as the program runs.
The bed or worktable of the machine is supported on hardened steel "ways" which are usually protected by flexible guards.
Cast iron or Meehanite used to be the material of choice for metal working machines. Today, most machines make liberal use of weldments of hot-rolled steel and wrought products such as stainless steel to reduce cost and allow fabrication of more intricate frame designs.
Some machines are designed as cells, which means they have a specific group of parts they are designed to manufacture. Cell machines have large tool magazines to carry enough tools to do all of the various operations on each of the different parts, large worktables or the ability to change worktables, and special provisions in the controller for data inputs from other CNC machines. This allows the CNC machine to be assembled with other similarly equipped machines into a Flexible Machining Cell, which can produce more than one part simultaneously. A group of cells, some containing 20 or 30 machines, is called a Flexible Machining System. These systems can produce literally hundreds of different parts at the same time with little human intervention. Some are designed to run day and night without supervision in what is referred to as "lights out" manufacturing.
Until recently, most machining centers were built to customer specifications by the machine tool builder. Now, standardized tooling design has allowed machines to be built for stock or later sale, since the new designs can perform all the needed operations of most users. The cost of a new CNC machine runs from about $50,000 for a vertical center to $5 million for a Flexible Machining System for engine blocks. The actual manufacturing process proceeds as follows.
CNC machines are can be classified into the following types:
The future of CNC machines is exploding. One idea under development is a spider-like machine whose spindle is suspended by six telescoping ballscrew struts. The struts are like the ways in a conventional machine, but they are round with the ballscrew assembly in the center. The motions of the spindle are controlled by a sophisticated computer performing millions of calculations to assure proper part contour. Costing several million dollars to develop and using high level, proprietary mathematics, this machine promises to perform previously unheard of operations in metal machining. Advancement in computers and artificial intelligence will make CNC machines of the future faster and easier to operate. This will not come cheaply, and the price of sophisticated CNC machines will be beyond the reach of many companies. It will, how-ever, reduce the prices of the basic CNC machines performing the original three-axis movements.
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