BHIL Varanasi CNC Report

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summer training report for mech. engg. from BHEL Varanasi



CNC MACHINESDevelopment 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. In modern CNC systems, end-to-end component design is highly automated using computer-aided design (CAD) and computer-aided manufacturing (CAM) programs. The programs produce a computer file that is interpreted to extract the commands needed to operate a particular machine via a postprocessor, and then loaded into the CNC machines for production. Since any particular component might require the use of a number of different toolsdrills, saws, etc., modern machines often combine multiple tools into a single "cell". In other cases, a number of different machines are used with an external controller and human or robotic operators that move the component from machine to machine. In either case, the complex series of steps needed to produce any part is highly automated and produces a part that closely matches the original CAD design.

ADVANTAGE OF CNC MACHINES Higher flexibility increased productivity Consistent quality Reduced scrap rate Reliable operation Reduced non productive time Reduced manpower Shorter cycle time High accuracy Reduced lead time Just in time (JIT) manufacture Automatic material handling Lesser floor space Increased operation safety


CNC SYSTEMSINTRODUCTIONNumerical control (NC) refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to manually controlled via handwheels or levers, or mechanically automated via cams alone. The first NC machines were built in the 1940s and 1950s, based on existing tools that were modified with motors that moved the controls to follow points fed into the system on punched tape. These early servomechanisms were rapidly augmented with analog and digital computers, creating the modern computer numerical control (CNC) machine tools that have revolutionized the machining processes. 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 inaccuracies are present, it will result in poorer part accuracy. CONFIGURATION OF THE CNC SYSTEM . CNC system PL C NCCommand value Spindle Head Servo Drive Servo Motor Encoder

Velocity Feedback

Tacho Generator

Lead Screw

Work piece Table

Tape Reader Tape Punch Other Devices Inputs Outputs

Position Feedback

Machine Elements

Proximity switches Limit switches Relay coils Pressure switches Float switches


A CNC system basically consists of the following: Central processing unit (CPU) Servo-control unit Operator control panel Machine control panel Other peripheral device Programmable logic controller (PLC)

Central Processing Unit (CPU)The CPU is the heart and brain of a CNC system. It accepts the information stored in the memory as part program. This data is decoded and transformed into specific position control and velocity control signals. It also oversees the movement of the control axis or spindle whenever this does not match the programmed values, a corrective action is taken. All the compensations required for machine accuracy (like lead screw pitch error, tool wear out, backlash, etc.) are calculated by the CPU depending upon the corresponding inputs made available to the system. The same will be taken care of during the generation of control signals for the axis movement. Also, some safety checks are built into the system through this unit and the CPU unit will provide continuous necessary corrective actions. Whenever the situation goes beyond control of the CPU, it takes the final action of shutting down the system in turn the machine. Speed Control Unit This unit acts in unison with the CPU for the movement of the machine axes. The CPU sends the control signals generated for the movement of the axis to the servo control unit and the servo control unit convert these signals into the suitable digital or analog signal to be fed to the machine tool axis movement. This also checks whether machine tool axis movement is at the same speed as directed by the CPU. In case any safety conditions related to the axis are overruled during movement or otherwise they are reported to the CPU for corrective action.

Servo-Control UnitThe decoded position and velocity control signals, generated by the CPU for the axis movement forms the input to the servo-control unit. This unit in turn generates suitable signals as command values. The servo-drive unit converts the command values, which are interfaced with the axis and the spindle motors. The servo-control unit receives the position feedback signals for actual movement of the machine tool axes from the feedback devices (like linear scales, rotary encoders, resolves, etc.). The velocity feedback is generally obtained through tacho generators. The feedback signals are passed on to the CPU for further processing. Thus the servo-control unit performs the data communication between the machine tool and the CPU. As explained earlier, the actual movements of the slides on the machine tool is achieved through servo drives. The amount of movement and the rate of movement are controlled by the CNC system depending upon the type of feedback system used, i.e. closed-loop or open-loop system. Closed-loop System The closed-loop system is characterized by the presence of feedback. In this system, the CNC system send out commands for movement and the result is continuously monitored by the system through various feedback devices. There are generally two types of feedback to a CNC system -- position feedback and velocity feedback.




LSMLogic Sub module


Emergency Stop

X+ ZXZ+ Machine Control Panel Cycle Machine Control Panel Expansion


Power Supply

PLC 2, external NC PLC1 Logic Unit

Tape Puncher

Tape Reader

Fig.Typical numerical control configuration of Hinumerik 3100 CNC system


Position Feedback A closed-loop system, regardless of the type of feedback device, will constantly try to achieve and maintain a given position by self-correcting. As the slide of the machine tool moves, its movement is fed back to the CNC system for determining the position of the slide to decide how much is yet to be traveled and also to decide whether the movement is as per the commanded rate. If the actual rate is not as per the required rate, the system tries to correct it. In case this is not possible, the system declares fault and initiates action for disabling the drives and if necessary, switches off the machine.Table Command Counter Subtraction Circuit Position Control Comparison Circuit Stop at Zero


Servo Motor

Lead Screw

Tape reader


Open-loop positioning control

Error Signal

Table Transducer

Active Buffer Storage

Count Comparator Amplifier

Servo Motor

Lead Screw

Tape reader Position feedback signal

Close-loop positioning control

Fig. Open-and Closed-loop positioning system


Velocity feedback In case no time constraint is put on the system to reach the final programmed position, then the system may not produce the required path or the surface finish accuracy. Hence, velocity feedback must be present along with the position feedback whenever CNC system are used for contouring, in order to produce correct interpolation and also specified acceleration and deceleration velocities. The tacho generator used for velocity feedback is normally connected to the motor and it rotates whenever the motor rotates, thus giving an analog output proportional to the speed of motor. The analog voltage is taken as speed feedback by the servo-controller and swift action is taken by the controller to maintain the speed of the motor within the required limits. Open-loop system The open loop system lacks feedback. In this system, the CNC system send out signals for movement but does not check whether