GENERAL DESCRIPTION
Even a brief look at any control unit reveals that there are two basic components - one is the operation panel, full of rotary switches, toggle switches and push buttons. The other component is the display screen with a keyboard or a keypad. The programmer who does not normally work on the CNC machine will seldom, if ever, have a reason to use either the operation panel or the display screen. They are available at the machine to the CNC machine operator, and used for the machine setup as well as to control the activities of the machine.
Should the CNC programmer be interested in the ma-chine operation? Is it necessary for the programmer to know and understand all functions of the control system? There is only one answer to both questions - definitely yes.
The control unit - the CNC system - contains features that only work in conjunction with the program, it does not do anything useful on its own. Some features can be used only if the program itself supports them. All switches and but-tons and keys are used by the machine operator, to exercise control over the program execution and machining process.
♦ Operation Panel
Depending on the type of the CNC machine, the following table covers the most typical and common features found on the modern operation panel. There are some small differences for the operation of a machining center and a lathe, but both operation panels are similar. As with any general reference book, it is always a good idea to double check with the manufacturer specifications and recommendations. It is common that many machines used in the shop have some special features.
The screen display is the 'window' to the computer. Any active program can be viewed, including the status of the control, current tool position, various offsets, parameters, even a graphic representation of the Tool Path. On all CNC units, individual monochrome or color screens can be selected to have the desired display at any time, using the in-put keys (keyboard pads and soft keys). Setting for international languages is also possible. The keyboard pads and soft keys are used to input instructions to the control. Existing programs can be modified or deleted, new programs can be added. Using the key-board input, not only the machine axes motion can be controlled, but the spindle speed and feed rate as well. Changing the internal parameters and evaluating various diagnostics are more specific means of control, often restricted to service people. Keyboard and screen are used to set the program origin and to hook up to external devices, such as a connection with another computer. There are many other options. Every keyboard allows the use of letters, digits and symbols for data entry. Not every keyboard allows the use of all the alphabet letters or all available symbols. Some control panel keys have a description of an operation, rather than a letter, digit or symbol, for example, Read and Punch keys or the Offset key.
♦HANDLE
For the setup purposes, each CNC machine has a rotary handle that can move one selected axis by as little as the least increment of the control system. The official Fanuc name for the handle is Manual Pulse Generator. Associated with the handle is the Axis Select switch (often duplicated on the operation panel as well as on the handle) and the range of increment (that is the least increment XI, X10 and X100). The letter X in this case is the multiplier and stands for 'X times'. One handle division will move the selected axis by X times the minimum increment of the active units of measurement. the following table are the details of a typical handle.
SYSTEM FEATURES
The CNC unit is nothing more than a sophisticated special purpose computer. The 'special purpose' in this case is a computer capable of controlling the activities of a machine tool, such as a lathe or a machining center. It means the computer has to be designed by a company that has expertise in this type of special purpose computers. Unlike many business types of computers, each CNC unit is made for a particular customer. The customer is typically the machine manufacturer, not the end user. The manufacturer specifies certain requirements that the control system has to meet, requirements that reflect the uniqueness of the machines they build. The basic control does not change, but some customized features may be added (or taken away) or a specific machine. Once the control system is sold to the machine manufacturer, more features are added to the system. They mainly relate to the design and capabilities of the machine. A good example is a CNC unit for two machines that are the same in all respects except one. One machine has a manual tool changer, the other has an automatic tool changer. In order to support the automatic tool changer, the CNC unit must have special features built-in, that are not required for a machine without the tool changer. The more complex the CNC system is, the more expensive it is. Users that do not require all the sophisticated features, do not pay a premium for features they do not need.
♦ Memory Capacity
CNC programs can be stored in the control memory. The program size is only limited by the capacity of the control. This capacity is measured in a variety of ways, originally as the equivalent length of tape in meters or feet, lately as the number of bytes or the number of screen pages. A common minimum memory capacity of a CNC lathe control is 20 m of tape (66 ft). This is an old fashioned method that some-how persisted in staying with us. On CNC milling systems, the memory requirements based on the same criteria are generally larger and the typical minimum memory capacity is 80 m or about 263 ft. Optionally, larger memory capacity can be added to the control system. The minimum memory capacity of the control varies from one machine to another -always check control specifications carefully.
Modern methods of measuring memory capacity prefer to use bytes as the unit, rather that a length of an obsolete tape. A byte is the smallest unit of storage capacity and is very roughly equivalent to one character in the program. The memory capacity of the control system should be large enough to store the longest CNC program expected on a regular basis. That requires some planning before the CNC machine is purchased. For example, in three dimensional mold work or high speed machining, the cost of additional memory capacity may he very high. Although any cost is a relative term, there are reliable and inexpensive alternatives, well worth looking into.
One alternative is running the CNC program from a personal computer. An inexpensive communication software and cabling is required to connect the computer with the CNC system. The simplest version is to transfer the CNC program from one computer to the other. More sophisticated possibility includes software and cables that can actually run the machine from the personal computer, without loading it to the memory of the CNC first. This method is often called `drip feeding' or bitwise input'. When operated from the personal computer, the CNC program can be as long as the capacity of the storage device, typically the hard drive.
Most CNC programs will fit into the internal memory of the control system. Many controls use the number of avail-able characters or the equivalent length of tape. Here are some formulas that can be used to get at least the approximate memory capacity calculations:
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