自动化生产线--外文翻译(1)教学内容

English Translation Material

1. Transfer Machine

The highest degree of automation with special-purpose, multifunction machines is achieved by using transfer machines. Transfer machine are essentially a combination of individual workstations arranged in the required sequence, connected by work transfer devices, and integrated with interlocked controls. Workplaces are automatically transferred between the stations, which are equipped with horizontal, vertical, or angular units to perform machining , gaging ,workplace repositioning, assembling, washing, or other operation. The two major classes of transfer machines are rotary and in-line types.

An important advantage of transfer machines is that they permit the maximum number of operations to be performed simultaneously. There is relatively no limitation on the number of workplace surface or planes that can be machined, since devices can be interposed in transfer machines at practically any point for inverting, rotating, or orienting the workplace, so as to complete the machining operations. Work repositioning also minimizes the need for angular machining heads and allows operations to be performed in optimum time. Complete processing from rough casting or forging to finished parts is often possible.

One or more finished parts are produced on a transfer machine with each index of the transfer system that moves the parts from stations to stations. Production efficiencies of such machines generally range from 50% for a machine variety of different parts to 85% for a machine producing one part, in high production, depending upon the workplace and how the machine is operated(material handling method, maintenance procedures, etc. )

All types of machining operations, such as drilling, tapping, reaming, boring, and milling, are economically combined on transfer machines. Lathe-type operations such as turning and facing are also being performed on in-line transfer machine, with the workplace being rotated in selected

machining stations. Turning operations are performed in lathe-type segments in which toolholders are fed on slides mounted on tunnel-type bridge units. Workplace are located on centers and rotated by chucks at each turning station. Turning stations with CNC are available for use on in-line transfer machine. The CNC units allow the machine cycles to be easily altered to accommodate changes in workplace design and can also be used for automatic tool adjustments.

Maximum production economy on transfer lines is often achieved by assembling

parts to the workplaces during their movement through the machine. such items as bushings, seals, welch plugs, and heat tubes can be assembled and then machine or tested during the transfer machining sequence. Automatic nut torquing following the application of part subassemblies can also be carried out.

Gundrilling or reaming on transfer machines is an ideal application provided that proper machining units are employed and good bushing practices are followed. Contour boring and turning of spherical seats and other surface can be done with tracer-controlled single-point inserts, thus eliminating the need for costly special form tools. In-process gaging of reamed or bored holes and automatic tool setting are done on transfer machines to maintain close tolerances.

Less conventional operations sometimes performed on transfer machines include grinding, induction heating of ring gears for shrink-fit pressing on flywheels, induction hardening of valve seats, deep rolling to apply compressive preloads, and burnishing.

Transfer machines have long been used in the automotive industry for production rates with a minimum of manual part handling. In addition to decreasing labor requirements, such machines ensure consistently uniform, high-quality parts at lower cost. They are no longer confined just to rough machining and now often eliminate the need for subsequent operations such as grinding and honing.

More recently, there has been an increasing demand for transfer machines to handle lower volumes of similar or even different parts in smaller sizes, with means for quick changeover between production runs. Built-in flexibility, the ability to rearrange and interchange machine units, and the provision of idle stations increases the cost of any transfer machine, but such feature are economically feasible when product redesigns are common. Many such machines are now being used in nonautomotive applications for lower production requirements.

Special feature now available to reduce the time required for part changeover include standardized dimensions, modular construction, interchangeable fixtures mounted on master pallets that remain on the machine, interchangeable fixture components, the ability to lock out certain stations for different parts by means of selector switches, and programmable controllers. Product design is also important, and common transfer and clamping surfaces should be provided on different parts whenever possible.

2. Programmable Logic Controllers

A programmable logic controller (PLC) is a solid-state device used to control machine motion or process operation by means of a stored program. The PLC sends output control signals output and receive input signals through input/output (I/O) devices. A PLC controls output in response to stimuli at the inputs according to the logic prescribed by the stored program. The inputs are made up of limit switches, pushbuttons, thumbwheels, switches, pulses, analog signal, ASCII serial data, and binary or BCD data from absolute position encoders. The output are voltage or current

level to drive end devices such as solenoids, motor starters, relays, lights, and so on. Other output device include analog devices, digital BCD displays, ASCII compatible devices, servo variable-speed drives, and even computers.

Programmable controllers were developed (circa in 1968) when General Motors Corps, and other automobile manufacturers were experimenting to see if there might be an alternative to scrapping all their hardwired control panel of machine tools and other production equipment during a model changeover. This annual tradition was necessary because rewriting of the panels was more expensive than buying new ones. The automotive companies approached a number of control equipment manufacturers and asked them to develop a control system that would have a longer productive life without major rewriting, but would still be understandable to and repairable by the plant personnel. The new product was named a “programmable controller”.

The processor part of the PLC contains a central processing unit and memory. The central processing unit (CPU) is the “traffic direction” of the processor, the memory stores information. Coming into the processor are the electrical signals from the input devices, as conditioned by the input module to voltage levels acceptable to processor logic. The processor scans the state of I/O and updates outputs stored in the memory of the PLC. For example, the processor may be programmed so that if an input connected to a limit switch is true (limit switch closed), then a corresponding output wired to an output module is to be energized. This processor remembers this command through its memory and compares on each scan to see if that limit switch is, in fact, closed. If it is closed ,the processor energizes the solenoid by turning on the output module.

The output device, such as a solenoid or motor starter, is wired to an output module’s terminal, and it receives its shift signal from the processor, in effect, the processor is performing a long and complicated series of logic decisions. The PLC performs such decisions sequentially and in according with the stored program. Similarly, analog I/O allows the processor to make decisions based on the magnitude of a signal, rather than just if it is on or off. For example, the processor may be programmed to increase or decrease the steam flow to a boiler (analog output) based on a comparison of the actual temperature in the boiler (analog input ) This is often performed by utilizing the built-in PID (proportional, integral, derivative) capabilities of the processor.

Because a PLC is “software based”, its control logic functions can be changed by reprogramming its memory. Keyboard programming devices facilitate entry of the revised program, which can be design to cause an existing machine or process to operate in a different sequence or to different level of, or combinations of stimuli. Hardware modifications are needed only if additional, changed, or relocated input/output device are involved.

3. Automated Assembly

Assembly in the manifacturing process consists of putting together all the component parts and sub-assemblies of a given product, fastening, performing inspections and function tests, labeling, separating good assembly from bad, and packaging and or preparing them for final use. Assembly is unique compared to the methods of manufacturing such as machining, grinding, and welding in that most of these processes invovle only a few disciplines and possibly only one. Most of these nonassembly operations cannot be performed weithout the aid of equipment; thus the development of automatic methods has been necessary rather than optional. Assembly, on the other hand, may involve in one machine many of the fastening methods,such as riveting, welding, screwdriving,and adhesive application,as well as automatic parts seletion, proding, gaging, functional testing, labeling,and packaging. The state of the art in assembly operations has not reached the level of standardization; much manual work is stillbeing performed in this area.

Assembly has traditionally been one of the highest areas of direct labor costs. In some cases, assembly accounts for 50% or more of manufacturing csosts and typically 20% ～50%. However, closer cooperation between design and manufacturing engineers has resulted in reducing and in a few cases eliminating altogether the need for assembly. When asssembly is required, improved design or products has simplified automated (semiautomatic or automatic) assembly.

Considerations for Automated Assembly

Before automated assembly is adopted, several factors should be considerd. These include practicality of the process for automation, simulation for economic considerations and justification, management involvement, and labor relations. Determining the practicality of automated assembly required careful evaluation of the following:

a)The number of parts in assembly.

b)Design of the parts with respect to producibility, assembility, automatic handling, and testability (materials, forms, dimensional tolerances, and weights).

c)Quality of parts to be assembled. Out-of-tolerance or defective parts can cause production losses and increase costs because of stoppages.

d)Availablity of qualiyied, technically competent personal to be responsible for equipment operation.

e) Total production and production-rate requipments.

Product variations and frequency of design changes.

f)Joining methods required.

g)Assembly times and costs.

h)Assembly lines or system configuration, using simulation, including material handling.