| Facilities | Synopsis of Lectures in CNC Course at DMW, Patiala (From 8-Dec-2003 to 12-Dec2003) Course Organised and Conducted by Shri Manish K Gupta, Professor(MIS), IRIMEE Thanks are due to the entire team of DMW, Patiala, especially Shri Rishi Lal, DyCME(P)/DMW/PTA
The paper gives an overview of the various controls that are available in a CNC machine and their interfacing with the Mechanical portion of the machine. The paper discusses in details the function of various control units such as Central Processing Unit, Servo Control, Operator Control Panel, Machine Control Panel, Other peripheral devices, and Programmable Logic Controller (PLC). The paper also deals with on-line, off-line and remote diagnostics of CNC machines.
CNC machines, which employ sophisticated electronics, hydraulics/pneumatics and computerized systems, require a different maintenance management in terms of knowledge, skill, equipment and systems. Since the maintenance philosophy of CNC machines is essentially different from that of conventional machines, a thorough knowledge of various systems and sub-assemblies is required. To maintain these machines reliably, it is imperative for the Railways to take suitable steps for creating and organizing an effective CNC maintenance group with proper equipment and trained manpower. The areas of CNC maintenance that are discussed in this paper are: · Mechanical and Hydraulics/Pneumatics · Electrical Control and Power Supply Circuits. · Electronics including Computer Numerical Controls, Servo drives, Measurement Systems and Instrumentation. As the maintenance requirement in each of the above disciplines vary, the paper suggests as to how a workshop or Production unit can go about formulating a strategic maintenance policy to fulfill the essential requirements of availability of trained staff, availability of appropriate maintenance documentation, availability of essential spares and consumables, and availability of test/trouble shooting, calibration and repair equipment. 3. CNC MACHINES - ADVANTAGES AND DISADVANTAGES by Shri.Rishi Lal, Dy.CME(P)/DMW The paper compares and enumerates the advantages of CNC machines vis-à-vis conventional machines. It also points out the limitations of CNC machines and some of their disadvantages.
The successful repair of electronics units, sub-assemblies and printed circuit board lies in locating the defective device(s) through testing and troubleshooting. Most of the machine builders do not supply circuit diagram for all electronic units, sub-assemblies and printed circuit boards mounted on the machine. In such a situation absence of circuit diagram leads to the need of retrieval of circuit diagram from printed circuit board. The activity of retrieval or reverse engineering from assembled printed circuit board requires the services of a skilled and experienced engineer having adequate knowledge of circuit types and design employed on CNC control, servo drives and other electronic units. The reverse engineering of printed circuit boards and modules forms a major activity in any electronics repair laboratory. The paper details the maintenance procedures for analog, digital and hybrid printed circuit board. Various special function test and trouble shooting equipment for digital circuit, test and measurement equipment required for measuring quantity of any function and characterization and Inspection equipment for maintenance of electronic equipment such as Oscilloscope, Universal Timer/Counter, Comparators, Micro System Trouble Shooter, Logic Analyser, Signature Analyser, Impedance Analyser, Storage Type Curve Tracer, Digital IC Tester, Data Acquisition System, Automatic temperature controlled soldering/de-soldering station etc. are also explained.
Feedback devices play an important role in the machine tool control. The main function of these devices is to provide output in the form of train of electrical pulses or an analog electrical signal by processing of which the exact position / speed of the machine tool can be obtained. The paper discusses the various types of position and velocity feedback devices both analog as well as digital such as inductosyns, encoders, optical scale etc. that are used in CNC machines. Faults related to these feedback devices and remedial actions are also dwelt upon.
The primary function of the drive is to cause motion of the controlled machine tool member (spindle, slide, etc.) to conform as closely as possible to the motion commands issued by the CNC system. In a metal cutting machine the metal is removed as a result of the movement of job and the cutting tool. In order to maintain a constant material removal rate, the spindle and the tool movements have to be co-coordinated such that the spindle has a constant power and the slide has a constant torque. Modern CNC machine are built with higher control accuracies. In order to ensure a high degree of consistency of production, variable speed drives are necessary. Most of the drives used in machine tools should have provisions such that both the spindle and feed motors should have the facility to vary the speeds infinitely (zero to maximum) in both the directions. The lecture discusses various spindle drives and feed drives in detail. Principle and construction of servo drives along with their evolution have been included to give an in-depth knowledge of these drives. For a quick and efficient trouble shooting in a servo system, a clear understanding of the principles and concepts is essential for problem solving in servo drive systems. Troubleshooting procedures and general guidelines for rectification of servo drives malfunction based on the DMW’s experience is a valuable resource that is available in this paper.
NC is control by recorded information called part program, which is a set of coded instructions for automatic control of a machine in a pre-determined sequences. The paper details structure of a Part Program, Preparatory Functions (G Codes) and Miscellaneous or Auxiliary Function (M codes). Various Programming Modes used in CNC machines such as absolute Mode, Incremental Mode are explained in detail. Understanding Program Zero, Measuring Program Zero and Motion Types employed in CNC machines are also discussed in detail.
The design and construction of CNC machines differ greatly compared to a conventional machine tool. This basically arises due to the requirements of higher performance levels. These CNC machines often employ the various Mechatronics elements that have been developed over the years. The shape and size of the work produced depends to a great extent on geometric and kinematics accuracies of the guide way. The geometric relationship of the guide (the moving part) and the guide way (stationary part) to the machine base determines the geometric accuracy of the machine. Kinematics accuracy depends on the straightness, flatness and parallelism errors in the guide ways. These types of errors result in a variety of tracking errors like pitch, yaw and roll that are difficult to measure and correct. Further, over a period of usage, any kind of wear in the guide ways reduces the accuracy of the guide motion resulting in errors in the movement as well as position. The paper discusses in detail features of various guide ways such as friction guide ways, antifriction linear motion guide ways, hydrostatic guide ways, aerostatic guide ways, their applications and limitations.
Safety is an important aspect, it needs to be considered at the machine design stage itself. Various types of safety features are designed to protect the machine, the operator, and the environment. The safety device must be simple and serve the purpose to the fullest extent, without itself being the source of an accident. The paper enumerates various safety features available on CNC machines. 10.DCW EXPERIENCE REGARDING MAINTENANCE OF CNC MACHINES by Shri.Paramjit Singh, AWM/DMW This paper complements the previous paper on Maintenance of CNC machines. DMW was the first Production Unit of the Railways to go in for such a large number of CNC machines and their performance in DMW have been quite satisfactory with machine uptime exceeding 92%. DMW also developed in-house facility for component level repair of PCBs and has also extended this help to various other Railways/PUs. Based on their experience DMW has compiled this exhaustive list of typical problems faced on the CNC machines, controls / drives in mechanical and electrical and electronics portion.
This paper documents the strength of DMW in the areas of CNC maintenance. It showcases the resources that are available at DMW for better upkeep of CNC machines. The area of strength discussed are in the field of trained manpower, centralised control, spares management, and Electronics Lab.
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