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Introduction GENERAL In modern manufacturing scenarios, the complex nature of industrial processes, the high demand for quality products at ever reducing lead times and the strive for attaining higher profitability has encouraged researchers to develop Computer Numerically Controlled (CNC) manufacturing systems. The first Numerically Controlled (NC) machine was developed at Massachusetts Institute of Technology, USA in 1950s. The continuous development of computer technology provided the beneficiary and the first CNC machine was developed in late 1960s. From this humble beginning, today’s CNC multi-process workstation configurations have evolved to support production from high-volume car engine manufacture to low-volume volatile component production. [1]	DEVELOPMENT OF CNC One of the major drawbacks of traditional CNC manufacturing is the reliance over the experience and the knowledge of the operator to re-adjust the process parameters manually based on his observations. This bespoke approach reduces the repeatability and the effectiveness of a manufacturing company in realizing full production potential. In addition to process parameters, variables such as tool wear, thermal changes, dimensional inaccuracies and other disturbances affect the quality of the manufactured products. The invention of mini-computers and micro-computers later brought a massive improvement in the capabilities of CNC machines with the ability of bi-directional information exchange and multi-axis, multi-tool, and multi-process manufacturing. [1, 2]	ADVANTAGES OF CNC. High productivity rates Uniformity of products Reduced component rejection Reduced tooling costs Less operator involvement Complex shapes machined easily [3]

IMPORTANCE OF ACCURASY In the process of production of complex components, it is essential to maintain the uniformity of the product. Uniformity is an essential factor to be maintained throughout the process of production of the component so that component rejection will be minimized at quality control level. In this context; CNC plays a vital role in today’s manufacturing process.

Machine and machining accuracy. The accuracy of a product turn out from a CNC is largely depending on: the machine accuracy the machining accuracy Machine accuracy is depending on the performance of various components associated in a machine that facilitate the productions of a quality product. Machining accuracy is the accuracy of a product turned out from CNC machining process. Basically it depends upon the machine accuracy. During the machining process various types of deviations due to operation conditions such as forces, tool tip temperature, rate of tool wear etc may occur. These deviations may cause inaccuracies in the product from the original set conditions. Therefore, to get an expected quality product it is necessary to compensate those deviations. This is called the maintaining of machining accuracy.

Machine accuracy With today’s highly competitive global manufacturing market place, the pressure for right-first-time manufacturing of accurate quality product is vital. The quality and machining accuracy of a complex product is completely dependent on the performance of the machine. The performance of the machine or the machine accuracy depends on characteristics and performance of various components incorporated in the driving system of the machine and following factors are to be considered. REPEATABILITY With a CNC machine tool responding to a predetermined program, the capacity for readily varying the conditions when machining is underway is limited, and to make changes is inconvenient. As far as possible conditions have to be correctly determined at the time the program is produced and the machine is set up. The slide movements of the machine must be precise, and this precision must continue throughout a machining program, which may involve thousands of components. [3] Repeatability is the ability of the machine to produce continually accurate slide movements, the maximum difference that can occur between the shortest and longest positions achieved in a number of attempted moves to any programmed target position. A typical figure for repeatability would be ±0.008mm. [3] The repeatability depends on the following features being incorporated in the design of the machine: Adequate strength Rigidity Minimum of vibration Dimensional stability Accurate control of the slide movements

BASIC STRUCTURE In order to increase the repeatability, the vibrations of the machine should be limited to a minimum level. Therefore concrete or ceramics is used as a machine base nowadays. The advantages of concrete are its low cost and good damping characteristics. MACHINE SPINDLES Machine spindle is a very important design feature. The design of spindle assembly must be such that thrust loads acting along its axis are to be adequately supported. Otherwise, dimensional inaccuracies and poor surface finish and chatter may occur. A well supported spindle assembly is shown in figure 01.In turning and horizontal machines spindle overhang should be kept to a minimum. The spindle of vertical machining centers presents a risk of deflection a when moving up and down. This could be reduced by moving away from the moving-spindle concept and instead the whole head assembly moves up and down. The forces that cause deflection of the spindle also result in a tendency for the complete spindle-housing assembly to twist. A bifurcated or two-pillar structure may be located in between two substantial slide ways to reduce the tendency to twist as shown in figure 02.

Figure 01								Figure 02 SPINDLE DRIVES The accuracy and repeatability of the spindle drives is very much depending on the characteristics and performance of the driving system. Driving system has to response accurately according to the programmed instructions. Most of the CNC machines now use AC servo motors and Linear motors. AC servo motors are regulated by the frequency converters and its movements could be controlled by feedbacks. The advantages of Linear motors are of its high precision, fast response and high speed with rigidity of linear races up to 120m/min rapid transverse. LEADSCREWS CNC machines are fitted with recirculation ball screws (as shown in figure 03), which replace sliding motion with rolling. The balls make opposing point contact which virtually eliminates backlash. Therefore it raises the repeatability. The figure 04 and 05 show external and internal ball returns respectively. The advantages of recirculation ball screws are: Longer life Less wear Low frictional resistance Less drive frictional power High traversing speeds No stick slip effect More precise positioning over the total of the machine. Less backlash

Figure 03

Figure 04

Figure 05 MACHINE SLIDES The movement of a machine slide must be smooth and responsive. There must be the minimum frictional resistance to motion. Many CNC machines have flat bearing surfaces. These surfaces are usually hardened and ground and coated with polytetrafluorethylene. For higher repeatability, some machines have hydrostatic bearings, where the bearing surfaces are separated by oil or air supplied under pressure. SLIDE DRIVES Both electric and hydraulic power is used to achieve slide motion. There are a number of very effective, responsive and thoroughly proved hydraulic systems currently in use. Figure 06 shows a block diagram of a closed loop servo system. With this type of drive system resolutions of 50 millionths and speed higher than 400 inches per minute are possible.

Figure 06 POSITIONAL FEEDBACK In CNC machines closed loop positioning system is an important feature and the concept can be summarized as instruction-movement-information-confirmation. The crucial feedback information is provided by a transducer. Two of the more common types of transducers are described below. Rotary-type transducers Rotary-type transducer transmits angular displacement as a voltage. The transducer is mounted at the end of the motor shaft or screw to measure the angular displacement as shown in figure 07, 08 and 09. The accuracy of this method could be increased by attaching the transducer through a position gear. An error may occur due to the backlash of screw and motor etc. and could be compensated during calibration of the machine.

Figure 07

Figure 08

Figure 09 Linear transducers (Optical grating) Linear transducer is mounted in the machine table as in the figure 10, to measure the actual displacement of the slide in such a way that backlash of screws; motors etc. would not cause any error in the feedback data. An optical grating transducer transmits linear movement as a voltage signal in the form of a series of pulses. As appears in figure 11; a pair of gratings is used and each consisting of a number of evenly spaced parallel lines. One grating is fixed and other is caused to move along. When one is moved across the other a fringe pattern moves across the fixed grating. This pattern of firing movement is measured using a photo transistor. The number of pulses are countered and this information is fed back to the control unit as confirmation that the correct movement has been made.

Figure 10

Figure 11 Both the transducers have a weakness. One monitors revelations of the lead screw and the other movements made by a slide. Neither of these factors may be a precise indication of the position of the tool in relation to work which would be the function of a perfect transducer. Such a transducer poses many design problems and has yet to be developed.

Machining Accuracy Machining accuracy is obtained by the introduction of adaptive control to compensate the deviations which may occur during the machining process. Adaptive control is the facility that enables a machine control unit to recognize certain variations from the original conditions which may occur during machining process and to make compensating response. Machining accuracy is directly affected by the accuracy and precision of CNC machine tools. In this context, it is necessary to investigate machine tool capability. And the accuracy along the manufacturing errors that can be eliminated and lead towards a regular quality production [4, 5, 6, 7 & 8]. These errors [8] can be further classified into geometric errors, static/loadings, thermal effects, machine tool vibration, chatter, spindle vibration, spindle deflection and control error. These errors may occur randomly as well as stochastic manner and need to be compensated during machining process. In order to achieve the machining accuracy on-line sensors, which use acoustics, optical, electrical, thermal and magnetic sensing systems may be used. These sensors help in direct observation of the process variables or assist in arriving judgments based on indirect measurements such as condition monitoring of tools, predicting wear, surface roughness and chatter detection. Further developments on machining accuracy such as fuzzy-based techniques, neural networks, genetic algorithms, FEM based techniques [9, 10] are in pipeline.

The machining forces [11, 12] also play a vital role in increasing the operation productivity, minimizing the tool breakage, managing the tool deflections that control the machining errors and improving the part quality. Since adequate regulation of the machining force is a challenging task, a number of adaptive techniques have been investigated by the researchers [13] to eliminate off-line calibration and by evaluating the force process model parameters on-line.

Machining conditions to reduce machining costs and increase machine tool efficiency have been achieved [14] by the introduction of adaptive control with constraints (ACC), adaptive control with optimization (ACO) and geometric adaptive controller (GAC).

In the CNC machining process following three types of process control elements have been identified and indicated in the fish bone diagram shown in figure 12.

Static errors Dimensional errors Surface roughness errors.

TEMPERATURE VARIATIONS THAT AFFECT DIMENTIONAL STABILITY

Correct alignments of the machines are achieved at a stated temperature. Therefore “Warm up” times are often quoted or sometimes warning lights are built into the control system. Deviation from the stated temperature can cause twisting or distortion of the machine casting and can have a considerable effect on the accuracy of the work produced. Following heat sources to be eliminated: Heat due to friction in motors, bearings and slides Heat due to metal cutting action Heat due to accumulated chips or swarf Heat in the environment.

Heat due to friction is eliminated or its effect is reduced by varies methods: Placing the motor outside the main structure that in turn reduce the vibration as well. Using a ducted air flow Spindles may be air or oil cooled Using of frictionless slides.

Heat due to metal cutting action: Using tools with correct geometry Ensuring correct cutting speed and feed rate Applying a coolant as a flood or spray mist.

Heat due to accumulated chips or swarf: Making chips falling away from the machine Chips continuously removed by conveyer.

Heat in the environment: Maintaining room temperature at a constant 200C. Presence of a radiator.

CNC BASED PRECISION WATER JET By using precision water jet principle, a machining accuracy of ±1 micron could be obtained, especially in cutting machines. That is 20 times more accurate than competitive machines. It consumes considerably less water and abrasives than traditional systems and looks set to open up entirely new application areas for this form of materials processing technology. Using a jet of water to perform the cutting operation ensures that the material structure remains unchanged but the heat generated while the operation will be absorbed to the water. It can handle land widths as narrow as 20 microns (0.02mm). The machine uses a very high precision water jet, with a diameter of less than 300 microns, and cut materials at a rate of up to 4000mm per minute. REAL-TIME CUTTING PATH RECTIFICATION Real-time cutter path rectification offers an effective means to overcome the serious problem of the thermal deformation of work pieces. In this case, it is necessary to take many factors into consideration; the diversity of shapes, the change of cutting conditions, the unstable thermal situation, and so on. Therefore, the adaptive control is applied to compensate the thermal displacement of the contour during the cutting process. Relating to this subject, the effective cutter radius, which depends on cutter wear, is also evaluated in real-time operation; and the cutter diameter compensation is included in the cutter path rectification.

ADDAPTIVE CONTROL Adaptive control is basically a data feedback system of unacceptable things that can occur during machining such as lose of a cutting edge of a tool, deflection of a tool etc. On manually operated machines this would immediately be obvious to the operator, who would react accordingly. But in CNC machines, it is important to detect such faults automatically and correct them immediately. This area of computerized numerically controlled machining is the subject of much research and experiment are needed. 4.4.1 Monitoring of torque Torque monitoring of spindles and servo motors is one method of adaptive control that is used. Here, power consumption is monitored electronically and sees whether they are within the permitted maximum and minimum torque values. If maximum value is reached; the cutting may be dull or component material harder than anticipated. Accordingly, control unit will respond to the feedback signal by lowering the feedback rate and/or varying the spindle speed. If the programmed torque does not occur, means that cutting tool is broken or not made contact or else, the drill is broken. Then a sister too may be called upon. 4.4.2 Monitoring of work pieces Another approach is to monitoring the presence of work pieces by the use surface sensing probes that can be used in milling operations. 4.4.3 Detecting the presence of cutting tools This combines with pneumatic or light. It is design primarily for use on machining centers to monitor small diameter drills, taps and reamers. STATIC AND DYNAMIC LOADING In static loading; forces act on the structure or the part of the machine when it is not in motion, while in dynamic loading forces act when a motion is taking place which could cause a slight deflection of its parts. These slight deflections affect the dimensional accuracy of the work piece as shown in figure 13 and 14. [3]