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英文原文
What computer-control programmers and operators do
Computer-control programmers and operators use computerized numerical control (CNC) machines to cut and shape precision products, such as automobile parts, machine parts, and compressors. CNC machines include machining tools such as lathes, multiaxis spindles, milling machines, and electrical discharge machines (EDM), but the functions formerly performed by human operators are performed by a computer-control module. CNC machines cut away material from a solid block of metal, plastic, or glass-known as a workpiece-to form a finished part. Computer-control programmers and operators normally produce large quantities of one part, although they may produce small batches or one-of-a-kind items. They use their knowledge of the working properties of metals and their skill with CNC programming to design and carry out the operations needed to make machined products that meet precise specifications.
Before CNC programmers-also referred to as numerical tool and process control programmers-machine a part, they must carefully plan and prepare the operation. First, these workers review three-dimensional computer aided/automated design (CAD) blueprints of the part. Next, they calculate where to cut or bore into the workpiece, how fast to feed the metal into the machine, and how much metal to remove .They then select tools and materials for the job and plan the sequence of cutting and finishing operations.
Next, CNC programmers turn the planned machining operations into a set of instructions, These instructions are translated into a computer aided/automated manufacturing (CAM) program containing a set of commands for the machine to follow. These commands normally are a series of numbers (hence, numerical control) that describes where cuts should occur, what type of cut should be used, and the speed of the cut. CNC programmers and operators check new programs to ensure that the machinery will function properly and that the output will meet specifications. Because a problem with the program could damage costly machinery and cutting tools, computer simulations may be used to check the program instead of a trial run. If errors found, the program must be changed and rested until the problem is resolved. In addition, growing connectivity between CAD/CAM software and CNC machine tools is raising productivity by automatically translating designs into instructions for the computer controller on the machine tool. These new CAM technologies enable programs to be easily modified for use on other jobs with similar specifications.
After the programming work is completed, CNC operators-also referred to as computer-controlled machine tool operators-perform the necessary machining operations. The CNC operators transfer the commands from the server to the CNC control module using a computer network link or floppy disk. Many advanced control modules are conversational, meaning that they ask the operator a series of questions about the nature of the task.. CNC operators position the metal stock on the CNC machine tool-lathe, milling machine, or other-set the controls, and let the computer make the cuts. Heavier objects may be loaded with the assistance of other workers, autoloaders, a crane, or a forklift. During the machining process, computer-control operators constantly monitor the readouts from the CNC control module, checking to see if any problems exist. Machine tools have unique characteristics, which can be problematic. During a machining operation, the operator modifies the cutting program to account for any problems encountered. Unique, modified CNC programs are saved for every different machine that performs a task.
CNC operators detect some problems by listening for specific sounds-for example, dull cutting tool or excessive vibration. Dull cutting tools are removed and replaced and replaced. Machine tools rotate at high speeds, which can create problems with harmonic vibrations in the workpiece. vibrations cause the machine tools to make minor cutting errors, hurting the quality of the product. Operators listen for vibrations and then adjust the cutting speed to compensate. in older, slower machine tools, the cutting speed would be reduced to eliminate the vibrations, but the amount of time needed to finish the product would increase at a result. in newer, high speed CNC machines, increasing the cutting speed normally eliminates the vibrations and reduces production time. CNC operators also ensure that the workpiece is being properly lubricated and cooled, because the machining of metal products generates a significant amount of heat.
Computer-control programmers and operators train in various ways-in apprenticeship programs, informally on the jop, and in secondary, vocational, or postsecondary schools. Due to a shortage of qualified applicants, many employers teach introductory courses, which provide a basic computers and electronics also is helpful .Experience with machine tools is extremely important. In fact, many entrants to these occupations have previously worked as machinists or machine setters, operators, and tenders. Persons interested in becoming computer-control programmers or operators should be mechanically inclined and able to work independently and do highly accurate work.
High school or vocational school courses in mathematics (trigonometry and algebra), blueprint reading, computer programming, metalworking, and drafting are recommended. Apprenticeship programs consist of shop training and related classroom instruction. In shop training, apprentices learn filing, handtapping, and dowel fitting, as well as the operation of various machine tools. Classroom instruction includes math, physics, programming, blueprint reading, CAD software, safely, and shop practices. Skilled computer-control programmers and operators need an understanding of the machining process, including the complex physics that occur at the cutting point. Thus, most training programs teach CNC operators and programmers to perform operations on manual machines prior to operating CNC machine. A growing number of computer-control programmers and operators receive most of their formal training from community or technical colleges. Less skilled CNC operators may need only a couple of weeks of on-the-job training.
Computer-control programmers and operators can advance in several ways. Experienced CNC operators may become CNC programmers, and some are promoted to supervisory or administrative position in their firms. A few open their own shops.
Computer-control programmers and operators should have excellent job opportunities. Due to the limited number of people entering training programs, employers are expected to have continue to have difficulty finding workers with the necessary skills and knowledge. Job growth in both occupations will be driven by the increasing use of CNC machine tools. Advances in CNC machine tools and manufacturing technology will further automate production, boosting CNC operators productivity and limiting employment growth. The demand for computer-control programmers will be negatively affected by the increasing use of software that automatically translates part and product designs into CNC machine tool instructions.
What is CNC
CNC stands for Computerized Numerical Control and has been around since the early 1970s. Prior to this, it was called NC, for numerical control. While people in most walks of life have never heard of this term, CNC has touched almost every form of manufacturing process in one way or another. If you’ll be working manufacturing, it’s likely that you’ll be dealing with CNC on a regular basis.
Before CNC
While there are exceptions to this statement, CNC machines replace (or work in conjunction with) some existing manufacturing process. Take one of the simplest manufacturing processes, drilling holes, for example.
A drill press can of course be used to machine holes. A person can place a drill chuck that is secured in the spindle of the drill press. They can then (manually) select the desired speed for rotation (commonly by switching belt pulleys), and activate the spindle. Then they manually pull on the quill lever to drive the drill into the workpiece being machined.
As you can easily see, there is a lot of manual intervention required to use a drill press to drill holes. A person is required to do something almost every step along the way! While this manual intervention may be acceptable for manufacturing companies if but a small number of holes or workpieces must be machined, as quantities grow, so does the likelihood for fatigue due to the tediousness of the operation. And do note that we’ve used one of the simplest machining operations (drilling) for our example. There are more complicated machining operations that would require a much higher skill level (and increase the potential for mistakes resulting in scrap workpieces) of the person running the conventional machine tool. (We commonly refer to the style of machine that CNC is replacing as the conventional machine.)
By comparison, the CNC equivalent for a drill press (possibly a CNC machining center or CNC drilling & tapping center) can be programmed to perform this operation in a much more automatic fashion. Everything that the drill press operator was doing manually will now be done by the CNC machine, including: placing the drill in the spindle, activating the spindle, positioning the workpiece under the drill, machining the hole, and turning off the spindle.
How CNC works
As you might already have guessed, everything that an operator would be required to do with conventional machine tools is programmable with CNC machines. Once the machine is setup and running, a CNC machine is quite simple to keep running. In fact CNC operators tend to get quite bored during lengthy production runs because there is so little to do. With some CNC machines, even the workpiece loading process has been automated. Let’s look at some of the specific programmable functions.
Motion control
All CNC machine types share this commonality: They all have two or more programmable directions of motion called axis. An axis of motion can be linear (along a straight line) or rotary (along a circular path). One of the first specifications that imply a CNC machine’s complexity is how many axes it has. Generally speaking, the more axes, the more complex the machine.
The axes of any CNC machine are required for the purpose of causing the motiona needed for the manufacturing process. In the drilling example, these (3) axes would position the tool over the hole to be machined (in two axes) and machine the hole (with the third axis). Axes are named with letters. Common linear axis names are X, Y, and Z. Common rotary axis names are A, B, and C. These are related to the coordinate system.
Programmable accessories
A CNC machine wouldn’t be very helpful if all it could only move the workpiece in two or more axes. Almost all CNC machines are programmable in several other ways. The specific CNC machine type has a lot to do with its appropriate programmable accessories. Again, any required function will be programmable on full-blown CNC machine tools. Here are some examples for one machine type (machining centers).
Automatic tool changer
Most machining centers can hold many tools in a tool magazine. When required, the required tool can be automatically placed in the spindle for machining.
Spindle speed and activation
The spindle speed (in revolutions per minute) can be easily specified can be turned on in a forward or reverse direction. It can also, of course, be turned off.
Coolant
Many machining operations require coolant for lubrication and cooling purposes. Coolant can be turned in and off from within the machine cycle.
The CNC program
Think of giving any series of step-by-step instructions. A CNC program is nothing more than another kind of instruction set. It’s written in sentence-like format and the control will execute it in sequential order, step by step.
A special series of CNC words are used to communicate what the machine is intended to do. CNC words begin with letter addresses (like F for federate, S for spindle speed, and X, Y & Z for axis motion). When placed together in a logical method, a group of CNC words make up a command that resemble a sentence.
The CNC control
The CNC control will interpret a CNC program and activate the series of commands in sequential order. As it reads the program, the CNC control will activate the appropriate machine functions, cause axis motion, and in general, follow the instructions given in the program.
Along with interpreting the CNC program, the CNC control has several other purposes. All current model CNC controls allow programs to be modified (edited) if mistakes are found. The CNC control allows special verification functions (like dry run) to confirm the correctness of the CNC program, The CNC control allows certain important operator inputs to be specified separate from the program, like tool length values. In general, the CNC control allows all functions of the machine to be manipulated.
What is a CAM system?
For simple applications (like drilling holes), the CNC program can be developed manually. That is, a programmer will sit down to write the program armed only with pencil, paper, and calculator. Again, for simple applications, this may be the very best way to develop CNC programs.
As applications get more and more complicated, and especially when new programs are required on a regular basis, writing programs manually becomes much more difficult. To simplify the programming process, a computer aided manufacturing (CAM) system can be used. A CAM system is a software program that runs on a computer (commonly a PC) that helps the CNC programmer with the programming process. Generally speaking, a CAM system will take the tediousness and drudgery out of programming.
In many companies the CAM system will work with the computer aided design (CAD) drawing developed by company’s design engineering department. This eliminates the need for redefining the workpiece configuration to the CAM system. The CNC programmer will simply specify (much like the manual programmer would have written) automatically.
What is a CNC system?
Once the program is developed (either manually or with a CAM system), it must be loaded into the CNC control. Though the setup person could type the program right into the control, this would be like using the CNC machine as a very expensive typewriter. If the CNC program is developed with the help of a CAM system, then it is already in the form of a text file. If the program is written manually, it can be typed into any computer using a common word processor (though most companies use a special CNC text editor for this purpose). Either way, the program is in the form of a text file that can be transferred right into the CNC machine. A distributive numerical control (DNC) system is used for this purpose.
A DNC system is nothing more than a computer that is networked with one or more CNC machines. Until only recently, rather crude serial communications protocol had to be used for transferring programs. Newer controls have more current communications capabilities and can be networked in more conventional ways (Ethernet, etc.). Regardless of methods, the CNC program must of course be loaded into the CNC machine before it can be run.
Fundamentals of CNC
While the specific intention and application for CNC machines vary from one machine type to another, all forms of CNC have common benefits. Here are but a few of the more important benefits offered by CNC equipment.
The first benefit offered by all forms of CNC machine tools is improved automation. The operators intervention related to producing workpieces can be reduced during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will be running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with conventional machine tools.
The second major benefit of CNC technology is consistent and accurate workpiece. Today’s CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verfied, two, ten, or one thousand identical workpiece can be easily produced identical with precision and consistency.
A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program.. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run, This leads to yet another bendfit, fast change overs. Since these machines are very easy to set upband run, and since programa can be easily loaded, they allow very short setup time. This is imperative with today’s just-in-time (JIT) product requirements.
Motion control-the heart of CNC
The most basic function of any CNC machine is automatic, precise, and consistent motion control. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion control in a revolutionary manner. All forms of CNC equipment have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a circular path).
Instead of causing motion by turning cranks and handwheels as is required on conventional machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type , the axes to move, the amount of motion and the motion rate (feedrate) are programmable with almost all CNC machine tools.
A CNC command executed within the control tells the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ball screw. And the ball screw drives the linear axis (slide). A feedback device (linear scale) on the slide allows the control to confirm that the commanded number of rotations has taken place.
Though a rather crude analogy, the same basic linear motion can be found on a common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn, drives the movable jaw on the vise. By comparison, a linear axis on a CNC machine tool is extremely precise. The number of revolution of the axis drive motor precisely controls the amount of linear motion along the axis.
How axis motion is commanded-understanding coordinate systems
It would be infeasible for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion amount. (This would be like having to figure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!). Instead all CNC controls allow axis motion to be commanded in a much simpler and more logical way by utilizing soma form of coordinate system. The two most popular coordinate systems used with CNC machines are the rectangular coordinate system and the polar coordinate system. By far the more popular of these two is the rectangular coordinate system.
The program zero point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a common location. If program zero is chosen wisely, usually coordinates needed for the program can be taken di
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