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徐州工程學(xué)院畢業(yè)設(shè)計(jì)(論文)
附錄
附錄1
英文原文
CNC TECHNOLOGY
Numerical control (NC) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters, and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or job. When the job changes, the program of instructions is changed. The capability to change the program is what makes NC suitable for low-and medium-volume production. It is much easier to write new programs than to make major alterations of the processing equipment.
BASIC COMPONENTS OF NC
A numerical control system consists of the following three basic components:
·Program of instructions
·Machine control unit
·Processing equipment
The general relationship among the three components is: the program is fed into the control unit, which directs the processing equipment accordingly.
The program of instructions is the detailed step-by-step commands that direct the processing equipment. In its most common form, the commands refer to positions of a machine tool spindle with respect to the worktable on which the part is fixtured. More advanced instructions include selection of spindle speeds, cutting tool, and other function. The most common medium in use over the last several decades has been 1-in. -wide punched tape. Because of the widespread use of the punched tape, NC is sometimes called “tape control”. However, this is a misnomer in modern usage of numerical control. Coming into use more recently have been magnetic tape cassettes and floppy diskettes.
The machine control unit (MCU) consists of the electronics and control hardware that read and interpret the program of instruction and convert it into mechanical actions of the machine tool or other processing equipment.
The processing equipment is the third basic component of an NC system. It is the component that performs useful work. In the most common example of numerical control, one that performs machining operations, the processing equipment consists of the worktable and spindle as well as the motors and controls needed to drive them.
Types Of Control Systems
There are two basic types of control systems in numerical control: point-to-point and contouring. In the point-to-point system, also called positioning, each axis of the machine is driven separately by leadscrews and, depending on the type of operation, at different velocities. The machine moves initially at maximum velocity in order to reduce nonproductive time but decelerates as the tool reaches its numerically defined position. Thus in an potation such as drilling or punching, the positioning and cutting take place sequentially. After the hole is drilled or punched, the tool retracts, moves rapidly to another position, and repeats the operation. The path followed from one position to another is important in only one respect: The time required should be minimized for efficiency. Point-to-point systems are used mainly in drilling, punching, and straight milling operations.
In the contouring system, also known as the continuous path system, positioning and cutting operations are both along controlled paths but at different velocities. Because the tool cuts as it travels along a prescribed path, accurate control and synchronization of velocities and movements are important. The contouring system is used on lathes, milling machines, grinders, welding machinery, and machining centers.
Movement along the path, or interpolation, occurs incrementally, by one of several basic methods. In all interpolations, the path controlled is that of the center of rotation of the tool. Compensation for different tools, different diameter tools, or tool wear during machining, can be made in the NC program.
There are a number of interpolation schemes that have been developed to deal with the various problems that are encountered in generating a smooth continuous path with a contouring-type NC system. They include:
·Linear interpolation
·Circular interpolation
·Helical interpolation
·Parabolic interpolation
·Cubic interpolation
Each of these interpolation procedures permits the programmer (or operator) to generate machine instructions for linear or curvilinear paths, using a relatively few input parameters. The interpolation module in the MCU performs the calculations and directs the tool along the path.
Linear interpolation is the most basic and is used when a straight-line path is to be generated in continuous-path NC. Two-axis and three-axis linear interpolation routines are sometimes distinguished in practice, but conceptually they are the same. The program is required to specify the beginning point and end point of the straight line, and the feed rate that is to be followed along the straight line. The interpolator computes the feed rates for each of the two (or three) axes in order to achieve the specified feed rate.
Linear interpolation for creating a circular path would be quite inappropriate because the programmer would be required to specify the line segments and their respective end points that are to be used to approximate the circle. Circular interpolation schemes have been developed that permit the programming of a path consisting of a circular arc by specifying the following parameters of the arc: the coordinates of its end points, the coordinates of its center, its radius, and the direction of the cutter along the arc. The tool path that is created consists of a series of straight-line segments, but the segments are calculated by the interpolation module rather than the programmer. The cutter is directed to move along each line segment one by one in order to generate the smooth circular path. A limitation of circular interpolation is that the plane in which the circular arc exists must be a plane defined by two axes of the NC system.
Helical interpolation combines the circular interpolation scheme for two axes described above with linear movement of a third axis. This permits the definition of a helical path in three-dimensional space.
Parabolic and cubic interpolation routines are used to provide approximations of free-form curves using higher-order equations. They generally require considerable computational power and are not as common as linear and circular interpolation. Their applications are concentrated in the automobile industry for fabricating dies for car body panels styled with free-form designs that cannot accurately and conveniently be approximated by combining linear and circular interpolations.
Programming For NC
A program for numerical control consists of a sequence of directions that causes an NC machine to carry out a certain operation, machining being the most commonly used process. Programming for NC may be done by an internal programming department, on the shop floor, or purchased from an outside source. Also, programming may be done manually or with computer assistance.
The program contains instructions and commands. Geometric instructions pertain to relative movements between the tool and the work piece. Processing instructions pertain to spindle speeds, feeds, tools, and so on. Travel instructions pertain to the type of interpolation and slow or rapid movements of the tool or worktable. Switching commands pertain to on/off position for coolant supplies, spindle rotation, direction of spindle rotation, tool changes, work piece feeding, clamping, and so on.
(1) Manual Programming??
Manual part programming consists of first calculating dimensional relationships of the tool, work piece, and work table, based on the engineering drawings of the part, and manufacturing operations to be performed and their sequence. A program sheet is then prepared, which consists of the necessary information to carry out the operation, such as cutting tools, spindle speeds, feeds, depth of cut, cutting fluids, power, and tool or work piece ally a paper tape is first prepared for trying out and debugging the program. Depending on how often it is to be used, the tape may be made of more durable Mylar.
Manual programming can be done by someone knowledgeable about the particular process and able to understand, read, and change part programs. Because they are familiar with machine tools and process capabilities, skilled machinists can do manual programming with some training in programming. However, the work is tedious, time consuming, and uneconomical-and is used mostly in simple point-to-point applications.
(2) Computer-Aided Programming??
Computer-aided part programming involves special symbolic programming languages that determine the coordinate points of corners, edges, and surfaces of the part. Programming language is the means of communicating with the computer and involves the use of symbolic characters. The programmer describes the component to be processed in this language, and the computer converts it to commands for the NC machine. Several languages having various features and applications are commercially available. The first language that used English-like statements was developed in the late 1950s and is called APT (for Automatically Programmed Tools). This language, in its various expanded forms, is still the most widely used for both point-to-point and continuous-path programming.
Computer-aided part programming has the following significant advantages over manual methods:
· Use of relatively easy to use symbolic language
·Reduced programming time. Programming is capable of accommodating a large amount of data concerning machine characteristics and process variables, such as power, speeds, feed, tool shape, compensation for tool shape changes, tool wear, deflections, and coolant use.
· Reduced possibility of human error, which can occur in manual programming
· Capability of simple changeover of machining sequence or from machine to machine.
· Lower cost because less time is required for programming.
Selection of a particular NC programming language depends on the following factors:
a)???Level of expertise of the personnel in the manufacturing facility.
b)???Complexity of the part.
c)???Type of equipment and computers available.
d)???Time and costs involved in programming.
Because numerical control involves the insertion of data concerning work piece materials and processing parameters, programming must be done by operators or programmers who are knowledgeable about the relevant aspects of the manufacturing processes being used. Before production begins, programs should be verified, either by viewing a simulation of the process on a CRT screen or by making the part from an inexpensive material, such as aluminum, wood, or plastic, rather than the material specified for the finished part.
Cutting tool choice and cutting specifications determination in CNC processing
The cutting tool choice and the cutting specifications determination is in the numerical control processing craft important content, it not only influence numerical control engine bed processing efficiency, moreover affects the processing quality directly. CAD/The CAM technology development, enables in the numerical control processing to become directly using the CAD design data possibly, specially the microcomputer and the numerical control engine bed joint, causes the design, the craft plan and the programming entire process completes completely on the computer, does not need to output the special technological document generally.
Now, many CAD/The CAM software package all provides the automatic programming function, these software are generally prompt the craft plan in the programming contact surface the related question, for instance, cutting tool choice, processing way plan, cutting specifications hypothesis and so on, programmers so long as have established the related parameter, may automatically produce completes the processing the NC procedure and the transmission to the numerical control engine bed. Therefore, in the numerical control processing cutting tool choice and the cutting specifications determination is completes under the man-machine interactive condition, this forms the sharp contrast with the ordinary engine bed processing, at the same time also requests the programmers to have to grasp the cutting tool choice and the cutting specifications determination basic principle, when programming full consideration numerical control processing characteristic. This article the cutting tool choice and the cutting specifications which must face to the numerical control programming in determined the question has carried on the discussion, has produced certain principles and the suggestion, and to the question which should pay attention has carried on the discussion.
First, numerical control processing commonly used cutting tool type and characteristic
The numerical control processing cutting tool must adapt the numerical control engine bed high speed, is highly effective and the automatic high characteristic, should include the general cutting
tool, the general connection hilt and the few special-purpose hilts generally. The hilt must join the cutting tool and install on the engine bed power head, therefore already gradual
standardization and seriation. The numerical control cutting tool classification has the many kinds of methods. May divide into according to the cutting tool structure: (1) Integral type; (2) The mosaic, uses the welding or machine clamps the type connection, machine clamps the type to be possible to divide into does not index and may index two kinds; (3) Special pattern, like compound expression cutting tool, absorption of shock type cutting tool and so on. According to makes the material
which the cutting tool uses to be possible to divide into: (1) High-speed steel cutting tool; (2) Hard alloy tools; (3) Diamond cutting tool; (4) Other material cutting tools, like cubic boron nitride cutting tool, ceramic cutting tool and so on. May divide into from the cutting craft: (1) The turning cutting tool, divides the outer annulus, in the hole, the thread, cuts the cutting tool many kinds of and so on; (2) Drills truncates the cutting tool, including drill bit, reamer, screw tap and so on; (3) Boring cutting tool; (4) Milling cutting tool and so on. In order to adapt the numerical control engine bed durably to the cutting tool, is stable, easy change, may trade and so on the request, in recent years machine clamps the type to be possible to index the cutting tool to obtain the widespread application, reaches higher authorities in the quantity to the entire numerical control cutting tool 30% ~ 40%, the metal excision quantity accounts for the total 80% ~ 90%.
Machining Centers
Many of today’s more sophisticated lathes are called machining centers since they are capable of performing, in addition to the normal turning operations, certain milling and drilling operations. Basically, a machining center can be thought of as being a combination turret lathe and milling machine. Additional features are sometimes included by manufacturers to increase the versatility of their machines.
Numerical Control
One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control (NC). Prior to the advent of NC, all machine tools were manually operated and controlled .Among the many limitations associated with manual control machine tools, perhaps none is more prominent than the limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major step away from human control of machine tools.
Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician writes a program that issues operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a reader.
Numerical control was developed to overcome the limitation of human operators, and it has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology:
1. Electrical discharge machining.
2. Laser cutting.
3. Electron beam welding.
Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes.
Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U. S. Air force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively.
However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter is the straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated.
This problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the further development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development.
A major problem was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader. If it was necessary to produce 100 copies of a given part, it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use.
This led to the development of a special magnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper taps, which solved the problem of frequent tearing and breakage. However, it still left two other problems.
The most important of these was that it was difficult or impossible to change the instructions entered on the tape. To make even the most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a new tape .It was also still necessary to run the tape th