18-07-2014, 12:57 PM
CNC Machines
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INTRODUCTION
Numerical control for machines .tools were introduced in 1950’s by Prof.John T Parsons. The first NC machine was built at the Massachusetts institute of Technology in 1953 by joint efforts of US Air Force, the MIT and parson’s cooperation. NC is control by numbers .NC is control recorded information called part program, which is set of coded instructions given as numbers for automatic control of am machine in a pre-determined sequence.
Numerical control can be defined as a technique of controlling a machine tool by the direct insertion of numerical data at some point of the system .The functions that are controlled on the machine tool are displacement of the slide members, spindle speeds ,tool selection etc.At first ,the numerical control was used to produce geocentrically complex parts ,but later used for added efficiency in medium batch production of turned and milled parts presently, Numerical control is employed in all sectors of production .
Rapid development in the field of electronics such as integrated circuit, large scale integrated circuits and development of minicomputer lead to the development of minicomputers based CNC systems. Further development and the electronic “chip” revolution have ushered in the current generation “compact and powerful” Microprocessor based CNC systems.
Development of computer numerically controlled (CNC) machines is an outstanding contribution to the manufacturing industries. It has made possible the automation of the machining process with flexibility to handle small to medium batch of quantities in part production.
Initially, the CNC technology was applied on basic metal cutting machine like lathes, milling machines, etc. Later, to increase the flexibility of the machines in handling a variety of components and to finish them in a single setup on the same machine, CNC machines capable of performing multiple operations were developed. To start with, this concept was applied to develop a CNC machining centre for machining prismatic components combining operations like milling, drilling, boring and taping. Further, the concept of multi-operations was also extended for machining cylindrical components, which led to the development of turning centers.
Computer Numerical Control (CNC) is a specialized
CNC SYSTEMS
Numerical control (NC) is a method employed for controlling the motions of a machine tool slide and its auxiliary functions with input in the form of numerical data. A computer numerical control (CNC) is a microprocessor-based system to store and process the data for the control of slide motions and auxiliary functions of the machine tools. The CNC system is the heart and brain of a CNC machine which enables the operation of various machine members such as slides, spindles, etc. as per the sequence programmed into it, depending on the machining operations.
The main advantage of a CNC system lies in the fact that the skills of the operator hitherto required in the operation of a conventional machine is removed and the part production is made automatic.
The CNC systems are constructed with a NC unit integrated with a programmable logic controller (PLC) and some times with an additional external PLC (non-integrated). The NC controls the spindle movement and the speeds and feeds in machining. It calculates the traversing path of the axes as defined by the inputs. The PLC controls the peripheral actuating elements of the machine such as solenoids, relay coils, etc. Working together, the NC and PLC enable the machine tool to operate automatically. Positioning and part accuracy depend on the CNC system's computer control algorithms, the system resolution and the basic mechanical machine accuracy. Control algorithm may cause errors while computing, which will reflect during contouring, but they are very negligible. Though this does not cause point to point positioning error, but when mechanical machine inaccuracy is present, it will result in poorer part accuracy.
Control Systems
Open loop systems have no access to the real time data about the performance of the system and therefore no immediate corrective action can be taken in case of system disturbance. This system is normally applied only to the case where the output is almost constant and predictable. Therefore, an open loop system is unlikely to be used to control machine tools since the cutting force and loading of a machine tool is never a constant. The only exception is the wirecut machine for which some machine tool builders still prefer to use an open loop system because there is virtually no cutting force in wirecut machining.
Close Loop Systems
In a close loop system, feedback devices closely monitor the output and any disturbance will be corrected in the first instance. Therefore high system accuracy is achievable. This system is more powerful than the open loop system and can be applied to the case where the output is subjected to frequent change. Nowadays, almost all CNC machines use this control system.
Serial communication
The data transfer between a computer and a CNC machine tool is often accomplished through a serial communication port. International standards for serial communications are established so that information can be exchanged in an orderly way. The most common interface between computers and CNC machine tools is referred to the EIA Standard RS-232. Most of the personal computers and CNC machine tools have built in RS232 port and a standard RS-232 cable is used to connect a CNC machine to a computer which enables the data transfer in reliable way. Part programs can be downloaded into the memory of a machine tool or uploaded to the computer for temporary storage by running a communication program on the computer and setting up the machine control to interact with the communication software.
Machine Tool
This can be any type of machine tool or equipment. In order to obtain high accuracy and repeatability, the design and make of the machine slide and the driving lead screw of a CNC machine is of vital importance. The slides are usually machined to high accuracy and coated with anti-friction material such as PTFE and Turcite in order to reduce the stick and slip phenomenon. Large diameter recirculating ball screws are employed to eliminate the backlash and lost motion. Other design features such as rigid and heavy machine structure; short machine table overhang, quick change tooling system, etc also contribute to the high accuracy and high repeatability of CNC machines.
Driving System
The driving system is an important component of a CNC machine as the accuracy and repeatability depend very much on the characteristics and performance of the driving system. The requirement is that the driving system has to response accurately according to the programmed instructions. This system usually uses electric motors although hydraulic motors are sometimes used for large machine tools. The motor is coupled either directly or through a gear box to the machine lead screw to moves the machine slide or the spindle. Three types of electrical motors are commonly used.
DC Servo Motor
This is the most common type of feed motors used in CNC machines. The principle of operation is based on the rotation of an armature winding in a permanently energized magnetic field. The armature winding is connected to a commutator, which is a cylinder of insulated copper segments mounted on the shaft. DC current is passed to the commutator through carbon brushes, which are connected to the
machine terminals. The change of the motor speed is by varying the armature voltage and the control of motor torque is achieved by controlling the motor's armature current. In order to achieve the necessary dynamic behaviour it is operated in a closed loop system equipped with sensors to obtain the velocity and position feedback signals
Display Unit The Display Unit serves as an interactive device between the machine and the operator. When the machine is running, the Display Unit displays the present status such as the position of the machine slide, the spindle RPM, the feed rate, the part programmes, etc. In an advanced CNC machine, the Display Unit can show the graphics simulation of the tool path so that part programmes can be verified before the actually
machining. Much other important information about the CNC system can also displayed for maintenance and installation work such as machine parameters, logic diagram of the programmer controller, error massages and diagnostic data.
Servo Drive
A servo drive consists of a servo amplifier and a servo motor. The main task of a servo amplifier (also called amplifier, servo controller, or just controller) is the control of the motor current. In addition, ESR servo amplifiers offer a broad spectrum of functionality
While most of the electrical drives are operated at constant speed, a servo drive has a rather "hectic" life. Often it has to accelerate to the rated speed within a few milliseconds only to decelerate a short time later just as quick. And of course the target position is to be reached exactly with an error of a few hundredths of a milli -meter.
Compared to other controlled drives servo drives have the advantage of high dynamics and accuracy, full stall torque, and compact motors with high power density.
Servo drives are used where high dynamics (i. e. fast acceleration and deceleration) and good accuracy at reaching target positions are important. The good control behaviour allows the optimal adaptation to the application (e. g. positioning without overshoot). But also the smooth run (due to sinusoidal commutation) and the possibility of exact synchronisation of two or more drives open a wide field. Because of their wide speed range servo drives can be used in a huge number of applications.
MACHINE DATA
Generally, a CNC system is designed as a general-purpose control unit, which has to be matched with the particular machine to which the system is interfaced. The CNC is interfaced to the machine by means of data, which is machine specific. The NC and PLC machine data can be entered and changed by means of external equipment or manually by the keyboard. These data are fixed and entered during commissioning of the machine and generally left unaltered during machine operations.
Machine data entered is usually relevant to the axis travel limits, feed rates, rapid traverse speeds and spindle speeds, position control multiplication factor, Kv factor, acceleration, drift compensation, adjustment of reference point, backlash compensation, pitch error compensation, etc. Also the optional features of the control system are made available to the machine tool builder by enabling some of the bits of machine data
PLC PROGRAMMING
A PLC matches the NC to the machine. PLCs were basically introduced as replacement for hard wired relay control panels. They were developed to be reprogrammed without hardware changes when requirements were altered and thus are reusable. PLCs are now available with increased functions, more memory and large input/output capabilities. Fig.7 gives the generalized PLC block diagram.
In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU receives input data, performs logical decisions based upon stored programs and drives the outputs. Connections to a computer for hierarchical control are done via the CPU.
The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons, limit switches, relay contacts, analog sensor, selector switches, proximity switches, float switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils, indicator lights, LED displays, etc.
The field devices are typically selected, supplied and installed by the machine tool builder or the end user. The voltage level of the field devices thus normally determines the type of I/O. So, power to actuate these devices must also be supplied external to the PLC. The PLC power supply is designated and rated only to operate the
internal portions of the I/O structures, and not the field devices. A wide variety of voltages, current capacities and types of I/O modules are available.
The principle of operation of a PLC is determined