CNC MACHINE Introduction-1

AN OVERVIEW OF CNC MACHINES

( 1 ) Historical Perspective 

The word NC which stands for numerical control refer to control of a machine or a process using symbolic codes consisting of characters and numerals. The word CNC came into existence in seventies when microprocessors and microcomputers replaced integrated circuit IC based controls used for NC machines. The development of numerical control owes much to the United States air force. The concept of NC was proposed in the late 1940s by John Parsons who recommended a method of automatic machine control that would guide a milling cutter to produce a curvilinear motion in order to generate smooth profiles on the work-pieces. In 1949, the U.S Air Force awarded Parsons a contract to develop new type of machine tool that would be able to speed up production methods. 
Parsons sub-contracted the Massachusetts Institute of Technology (MIT) to develop a practical implementation of his concept. Scientists and engineers at M.I.T built a control system for a two axis milling machine that used a perforated paper tape as the input media. This prototype was produced by retrofitting a conventional tracer mill with numerical control servomechanisms for the three axes of the machine. By 1955, these machines were available to industries with some small modifications.
The machine tool builders gradually began developing their own projects to introduce commercial NC units. Also, certain industry users, especially airframe builders, worked to devise numerical control machines to satisfy their own particular production needs. The Air force continued its encouragement of NC development by sponsoring additional research at MIT to design a part programming language that could be used in controlling N.C. machines.
In a short period of time, all the major machine tool manufacturers were producing some machines with NC, but it was not until late 1970s that computer-based NC became widely used. NC matured as an automation technology when electronics industry developed new products. At first, miniature electronic tubes were developed, but the controls were big, bulky, and not very reliable. Then solid-state circuitry and eventually modular or integrated circuits were developed. The control unit became smaller, more reliable, and less expensive.

(2) Computer Numerical Control Computer numerical control (CNC) is the numerical control system in which a dedicated computer is built into the control to perform basic and advanced NC functions. CNC controls are also referred to as soft-wired NC systems because most of their control functions are implemented by the control software programs. CNC is a computer assisted process to control general purpose machines from instructions generated by a processor and stored in a memory system. It is a specific form of control system where position is the principal controlled variable. All numerical control machines manufactured since the seventies are of CNC type. The computer allows for the following: storage of additional programs, program editing, running of program from memory, machine and control diagnostics, special routines, inch/metric, incremental/absolute switchability. CNC machines can be used as stand alone units or in a network of machines such as flexible machine centres. The controller uses a permanent resident program called an executive program to process the codes into the electrical pulses that control the machine. In any CNC machine, executive program resides in ROM and all the NC codes in RAM. The information in ROM is written into the electronic chips and cannot be erased and they become active whenever the machine is on. The contents in RAM are lost when the controller is turned off. Some use special type of RAM called CMOS memory, which retains its contents even when the power is turned off.

Figure 21.1: CNC milling machine

( 1.3 ) Direct Numerical Control

In a Direct Numerical Control system (DNC), a mainframe computer is used to coordinate the simultaneous operations of a number NC machines as shown in the figures 21.2 & 21.3. The main tasks performed by the computer are to program and edit part programs as well as download part programs to NC machines. Machine tool controllers have limited memory and a part program may contain few thousands of blocks.So the program is stored in a separate computer and sent directly to the machine, one block at a time.
First DNC system developed was Molins System 24 in 1967 by Cincinnati Milacron and General Electric. They are now referred to as flexible manufacturing systems (FMS). The computers that were used at those times were quite expensive.

Figure 21.2: DNC system

Figure 21.3: DNC system

21.4 Advantages & Disadvantages of CNC machine tools

Figure 21.4 (a) Manually operated milling Figure 21.4 (b) Computer controlled machine milling machine

Some of the dominant advantages of the CNC machines are: CNC machines can be used continuously and only need to be switched off for occasional maintenance.  These machines require less skilled people to operate unlike manual lathes / milling machines etc. CNC machines can be updated by improving the software used to drive the machines. Training for the use of CNC machines can be done through the use of 'virtual software'.  The manufacturing process can be simulated virtually and no need to make a prototype or a model. This saves time and money. Once programmed, these machines can be left and do not require any human intervention, except for work loading and unloading. These machines can manufacture several components to the required accuracy without any fatigue as in the case of manually operated machines.  Savings in time that could be achieved with the CNC machines are quite significant. Some of the disadvantages of the CNC machines are: CNC machines are generally more expensive than manually operated machines. The CNC machine operator only needs basic training and skills, enough to supervise several machines.  Increase in electrical maintenance, high initial investment and high per hour operating costs than the traditional systems. Fewer workers are required to operate CNC machines compared to manually operated machines. Investment in CNC machines can lead to unemployment.

( 5 ) Applications of NC/CNC machine tools CNC was initially applied to metal working machinery: Mills, Drills, boring machines, punch presses etc and now expanded to robotics, grinders, welding machinery, EDM's, flame cutters and also for inspection equipment etc. The machines controlled by CNC can be classified into the following categories: CNC mills and machining centres.  CNC lathes and turning centers  CNC EDM CNC grinding machines  CNC cutting machines (laser, plasma, electron, or flame)  CNC fabrication machines (sheet metal punch press, bending machine, or press brake)  CNC welding machines  CNC coordinate measuring machines

CNC Coordinate Measuring Machines: A coordinate measuring machine is a dimensional measuring device, designed to move the measuring probe to determine the coordinates along the surface of the work piece. Apart from dimensional measurement, these machines are also used for profile measurement, angularity, digitizing or imaging. A CMM consists of four main components: the machine, measuring probe, control system and the measuring software. The control system in a CMM performs the function of a live interaction between various machine drives, displacement transducers, probing systems and the peripheral devices. Control systems can be classified according to the following groups of CMMs. 1. Manually driven CMMs 2. Motorized CMMs with automatic probing systems 3. Direct computer controlled (DCC) CMMs 4. CMMs linked with CAD, CAM and FMS etc. The first two methods are very common and self explanatory. In the case of DCC CMMs, the computer control is responsible for the movement of the slides, readout from displacement transducers and data communication. CMM are of different configurations-fixed bridge, moving bridge, cantilever arm figure 21.5(a), horizontal arm and gantry type CMM as shown in figure 21.5(b).

Figure 21.5(a) Cantilever type CMM Figure 21.5(b) Gantry type CMM

(6 ) CNC welding machines:

Figure 21.6 4 axis CNC Tig welding machine

The salient features of CNC welding machines are: Superior quality and weld precision. These machines are also equipped with rotary tables. Weld moves, welding feed rate, wire feed, torch heights & welding current can be programmed. CNC welding machines are used for laser welding, welding of plastics, submerged arc welding, wire welding machines, butt welding, flash butt welding etc. These machines are generally used in automobile work shops Cost of these machines will be twice than the conventional welding machines.

CNC EDM & WEDM machines: EDM is a nontraditional machining method primarily used to machine hard metals that could not be machined by traditional machining methods. Material removal will be taking place by a series of electric arcs discharging across the gap between the electrode and the work piece. There are two main types- ram EDM & wire cut EDM. In wire-cut EDM, a thin wire is fed through the work piece and is constantly fed from a spool and is held between upper and lower guides. These guides move in the x-y plane and are precisely controlled by the CNC. Wire feed rate is also controlled by the CNC.

Figure 21.6 (a) Ram EDM

Figure 21.6 (b) Wire cut EDM

CLASSIFICATION OF CNC MACHINE TOOLS

( 1) Based on the motion type ' Point-to-point & Contouring systems

There are two main types of machine tools and the control systems required for use with them differ because of the basic differences in the functions of the machines to be controlled. They are known as point-to-point and contouring controls.

( 1.1) Point-to-point systems

Some machine tools for example drilling, boring and tapping machines etc, require the cutter and the work piece to be placed at a certain fixed relative positions at which they must remain while the cutter does its work. These machines are known as point-to-point machines as shown in figure 22.1 (a) and the control equipment for use with them are known as point-to-point control equipment. Feed rates need not to be programmed. In theses machine tools, each axis is driven separately. In a point-to-point control system, the dimensional information that must be given to the machine tool will be a series of required position of the two slides. Servo systems can be used to move the slides and no attempt is made to move the slide until the cutter has been retracted back.

( 1.2) Contouring systems (Continuous path systems)

Other type of machine tools involves motion of work piece with respect to the cutter while cutting operation is taking place. These machine tools include milling, routing machines etc. and are known as contouring machines as shown in figure 22.1 (b) and the controls required for their control are known as contouring control.
Contouring machines can also be used as point-to-point machines, but it will be uneconomical to use them unless the work piece also requires having a contouring operation to be performed on it. These machines require simultaneous control of axes. In contouring machines, relative positions of the work piece and the tool should be continuously controlled. The control system must be able to accept information regarding velocities and positions of the machines slides. Feed rates should be programmed.

Figure 22.1 (a) Point-to-point system

Figure 22.1 (b) Contouring system

Figure 22.1 (c) Contouring systems

22.2 Based on the control loops ' Open loop & Closed loop systems

22.2.1 Open loop systems:
Programmed instructions are fed into the c
ontroller through an input device. These instructions are then converted to electrical pulses (signals) by the controller and sent to the servo amplifier to energize the servo motors. The primary drawback of the open-loop system is that there is no feedback system to check whether the program position and velocity has been achieved. If the system performance is affected by load, temperature, humidity, or lubrication then the actual output could deviate from the desired output. For these reasons the open -loop system is generally used in point-to-point systems where the accuracy requirements are not critical. Very few continuous-path systems utilize open-loop control.

Figure 22.2 (a) Open loop control system Figure 22.2 (b) Closed loop control system Courtesy: http://jjjtrain.kanabco.com/vms/Media/glossary_o/cnc_opencloseloop.gif Courtesy: http://jjjtrain.kanabco.com/vms/Media/glossary_o/cnc_opencloseloop.gif

Figure 22.2 (c) Open loop system

22.2.1 Closed loop systems:

The closed-loop system has a feedback subsystem to monitor the actual output and correct any discrepancy from the programmed input. These systems use position and velocity feed back. The feedback system could be either analog or digital. The analog systems measure the variation of physical variables such as position and velocity in terms of voltage levels. Digital systems monitor output variations by means of electrical pulses. To control the dynamic behavior and the final position of the machine slides, a variety of position transducers are employed. Majority of CNC systems operate on servo mechanism, a closed loop principle. If a discrepancy is revealed between where the machine element should be and where it actually is, the sensing device signals the driving unit to make an adjustment, bringing the movable component to the required location.
Closed-loop systems are very powerful and accurate because they are capable of monitoring operating conditions through feedback subsystems and automatically compensating for any variations in real-time.

Figure 22.2 (d) Closed loop system

(3 ) Based on the number of axes ' 2, 3, 4 & 5 axes CNC machines. ( 3.1) 2& 3 axes CNC machines:

CNC lathes will be coming under 2 axes machines. There will be two axes along which motion takes place. The saddle will be moving longitudinally on the bed (Z-axis) and the cross slide moves transversely on the saddle (along X-axis). In 3-axes machines, there will be one more axis, perpendicular to the above two axes. By the simultaneous control of all the 3 axes, complex surfaces can be machined.

( 3.2 ) 4 & 5 axes CNC machines:

4 and 5 axes CNC machines provide multi-axis machining capabilities beyond the standard 3-axis CNC tool path movements. A 5-axis milling centre includes the three X, Y, Z axes, the A axis which is rotary tilting of the spindle and the B-axis, which can be a rotary index table.

Figure 22.3 Five axes CNC machine

Importance of higher axes machining : Reduced cycle time by machining complex components using a single setup. In addition to time savings, improved accuracy can also be achieved as positioning errors between setups are eliminated. Improved surface finish and tool life by tilting the tool to maintain optimum tool to part contact all the times. Improved access to under cuts and deep pockets. By tilting the tool, the tool can be made normal to the work surface and the errors may be reduced as the major component of cutting force will be along the tool axis. Higher axes machining has been widely used for machining sculptures surfaces in aerospace and automobile industry.

(3.3) Turning centre: Traditional centre lathes have horizontal beds. The saddle moves longitudinally and the cross slide moves transversely. Although the tools can be clearly seen, the operator must lean over the tool post to position them accurately. Concentration of chips may be creating a heat source and there may be temperature gradients in the machine tool. Keeping the above points in view, developments in the structure of the turning centres lead to the positioning the saddle and the cross slide behind the spindle on a slant bed as shown in the figure 22.4. Chips fall freely because of slant bed configuration which is more ergonomically acceptable from operator's point of view. Figure 22.4 Slant bed turning centre

22.4 Based on the power supply ' Electric, Hydraulic & Pneumatic systems

Mechanical power unit refers to a device which transforms some form of energy to mechanical power which may be used for driving slides, saddles or gantries forming a part of machine tool. The input power may be of electrical, hydraulic or pneumatic.

22.4.1 Electric systems:

Electric motors may be used for controlling both positioning and contouring machines. They may be either a.c. or d.c. motor and the torque and direction of rotation need to be controlled. The speed of a d.c. motor can be controlled by varying either the field or the armature supply. The clutch-controlled motor can either be an a.c. or d.c. motor. They are generally used for small machine tools because of heat losses in the clutches. Split field motors are the simplest form of motors and can be controlled in a manner according to the machine tool. These are small and generally run at high maximum speeds and so require reduction gears of high ratio. Separately excited motors are used with control systems for driving the slides of large machine tools.
22.4.2 Hydraulic systems:

These hydraulic systems may be used with positioning and contouring machine tools of all sizes. These systems may be either in the form of rams or motors. Hydraulic motors are smaller than electric motors of equivalent power. There are several types of hydraulic motors. The advantage of using hydraulic motors is that they can be very small and have considerable torque. This means that they may be incorporated in servosystems which require having a rapid response.

( 1 ) Different components related to CNC machine tools

Any CNC machine tool essentially consists of the following parts:

( 1.1 ) Part program:

A part program is a series of coded instructions required to produce a part. It controls the movement of the machine tool and on/off control of auxiliary functions such as spindle rotation and coolant. The coded instructions are composed of letters, numbers and symbols.

( 1.2 ) Program input device:

The program input device is the means for part program to be entered into the CNC control. Three commonly used program input devices are punch tape reader, magnetic tape reader, and computer via RS-232-C communication.

( 1.3 ) Machine Control Unit:

The machine control unit (MCU) is the heart of a CNC system. It is used to perform the following functions:  To read the coded instructions. To decode the coded instructions.  To implement interpolations (linear, circular, and helical) to generate axis motion commands. To feed the axis motion commands to the amplifier circuits for driving the axis mechanisms. To receive the feedback signals of position and speed for each drive axis. To implement auxiliary control functions such as coolant or spindle on/off and tool change.

( 1.4 ) Drive System:

A drive system consists of amplifier circuits, drive motors, and ball lead-screws. The MCU feeds the control signals (position and speed) of each axis to the amplifier circuits. The control signals are augmented to actuate drive motors which in turn rotate the ball lead-screws to position the machine table.

( 1.5 ) Machine Tool:

CNC controls are used to control various types of machine tools. Regardless of which type of machine tool is controlled, it always has a slide table and a spindle to control of position and speed. The machine table is controlled in the X and Y axes, while the spindle runs along the Z axis.

( 1.6 ) Feed Back System:

The feedback system is also referred to as the measuring system. It uses position and speed transducers to continuously monitor the position at which the cutting tool is located at any particular instant. The MCU uses the difference between reference signals and feedback signals to generate the control signals for correcting position and speed errors.

( 2 ) Machine axes designation

Machine axes are designated according to the "right-hand rule", When the thumb of right hand points in the direction of the positive X axis, the index finger points toward the positive Y axis, and the middle finger toward the positive Z axis. Figure 10 shows the right-hand rule applied to vertical machines, while Figure 23.1 applies to horizontal machines.

Figure 23.1: Right hand rule for vertical and horizontal machine

CNC SYSTEMS - ELECTRICAL COMPONENTS (1) Power units In machine tools, power is generally required for  For driving the main spindle For driving the saddles and carriages. For providing power for some ancillary units. The motors used for CNC system are of two kinds Electrical - AC , DC or Stepper motors Fluid - Hydraulic or Pneumatic Electric motors are by far the most common component to supply mechanical input to a linear motion system. Stepper motors and servo motors are the popular choices in linear motion machinery due to their accuracy and controllability. They exhibit favourable torque-speed characteristics and are relatively inexpensive.

(1.1) Stepper motors

Stepper motors convert digital pulse and direction signals into rotary motion and are easily controlled. Although stepper motors can be used in combination with analog or digital feedback signals, they are usually used without feedback (open loop). Stepper motors require motor driving voltage and control electronics. The rotor of a typical hybrid stepper motor has two soft iron cups that surround a permanent magnet which is axially magnetized. The rotor cups have 50 teeth on their surfaces and guide the flux through the rotor- stator air gap. In most cases, the teeth of one set are offset from the teeth of the other by one-half tooth pitch for a two phase stepper motor.

Figure 24.1 Unipolar and Bipolar Stepper Motor

The stator generally has the same number of teeth as the rotor, but can have two fewer depending upon the motor's design. When the teeth on the stator pole are energized with North polarity, the corresponding teeth on the rotor with South polarity align with them. Similarly, teeth on the stator pole energized with South polarity attract corresponding teeth on the rotor that are energized with North polarity. By changing the polarity of neighbouring stator teeth one after the other in a rotating sequence, the rotor begins to turn correspondingly as its teeth try to align themselves with the stator teeth. The strength of the magnetic fields can be precisely controlled by the amount of current through the windings, thus the position of the rotor can be precisely controlled by these attractive and repulsive forces. There are many advantages to using stepper motors. Since maximum dynamic torque occurs at low pulse rates (low speeds), stepper motors can easily accelerate a load. Stepper motors have large holding torque and stiffness, so there is usually no need for clutches and brakes (unless a large external load is acting, such as gravity). Stepper motors are inherently digital. The number of pulses determines position while the pulse frequency determines velocity. Additional advantages are that they are inexpensive, easily and accurately controlled, and there are no brushes to maintain. Also, they offer excellent heat dissipation, and they are very stiff motors with high holding torques for their size. The digital nature of stepper motors also eliminates tuning parameters. There are disadvantages associated with stepper motors. One of the largest disadvantages is that the torque decreases as velocity is increased. Because most stepper motors operate open loop with no position sensing devices, the motor can stall or lose position if the load torque exceeds the motor's available torque. Open loop stepper motor systems should not be used for high-performance or high-load applications, unless they are significantly derated. Another drawback is that damping may be required when load inertia is very high to prevent motor shaft oscillation at resonance points. Finally, stepper motors may perform poorly in high-speed applications. The maximum steps/sec rate of the motor and drive system should be considered, carefully.

( 1.2) Servo Motors

Servo motors are more robust than stepper motors, but pose a more difficult control problem. They are primarily used in applications where speed, power, noise level as well as velocity and positional accuracy are important. Servo motors are not functional without sensor feedback. They are designed and intended to be applied in combination with resolvers, tachometers, or encoders (closed loop). There are several types of servo motors, and three of the more common types are described as follows. The DC brush type servo motors are most commonly found in low-end to mid-range CNC machinery. The "brush" refers to brushes that pass electric current to the rotor of the rotating core of the motor. The construction consists of a magnet stator outside and a coil rotor inside. A brush DC motor has more than one coil. Each coil is angularly displaced from one another so when the torque from one coil has dropped off, current is automatically switched to another coil which is properly located to produce maximum torque. The switching is accomplished mechanically by the brushes and a commutator as shown below. There are distinct advantages to using DC brush servo motors. They are very inexpensive to apply. The motor commutates itself with the brushes and it appears as a simple, two-terminal device that is easily controlled. Among the disadvantages it is the fact that they are thermally inefficient, because the heat must dissipate through the external magnets. This condition reduces the torque to volume ratio, and the motor performance may suffer inefficiencies. Also, the brushed motor will require maintenance, as the brushes will wear and need replacement. Brushed servo motors are usually operated under 5000 rpm. The DC brushless type offers a higher level of performance. They are often referred to as "inside out" DC motors because of their design. The windings of a brushless motor are located in the outer portion of the motor (stator), and the rotor is constructed from permanent magnets as shown below. DC brushless motors are typically applied to high-end CNC machinery, but the future may see midrange machinery use brushless technology due to the narrowing cost gap. AC servo motors are another variety that offers high-end performance. Their physical construction is similar to that of the brushless DC motor; however, there are no magnets in the AC motor. Instead, both the rotor and stator are constructed from coils. Again, there are no brushes or contacts anywhere in the motor which means they are maintenance-free. They are capable of delivering very high torque at very high speeds; they are very light and there is no possibility of demagnetization. .However, due to the electronic commutation, they are extremely complex and expensive to control. Perhaps the largest advantage of using servo motors is that they are used in closed loop form, which allows for very accurate position information and also allows for high output torque to be realized at high speeds. The motor will draw the required current to maintain the desired path, velocity, or torque, and is controlled according to the requirements of the application rather than by the limitations of the motor. Servo motors put out enormous peak torque at or near stall conditions. They provide smooth, quiet operation, and depending upon the resolution of the feedback mechanism, can have very small resolutions. Among the disadvantages of servo motors are the increased cost, the added feedback component, and the increased control complexity. The closed loop feature can be a disadvantage for the case when there is a physical obstacle blocking the path of motion. Rather than stalling, the servo motor will continue to draw current to overcome the obstacle. As a result, the system hardware, control electronics, signal amplifier and motor may become damaged unless safety precautions are taken.

( 2 ) Encoders

An encoder is a device used to change a signal or data into a code. These encoders are used in metrology instruments and high precision machining tools ranging from digital calipers to CNC machine tools.

( 2.1) Incremental encoders

With incremental linear encoders, the current position is determined by stating a datum and counting measuring steps. The output signals of incremental rotary encoders are evaluated by an electronic counter in which the measured value is determined by counting "increments". These encoders form the majority of all rotary encoders. Incremental rotary encoders with integral couplings used for length measurement are also in the market. The resolution of these encoders can be increased by means of electronic interpolation. There are, of course, the precision rotary encoders specifically designed for angle measurement. If finer resolution is required, standard rotary encoders often utilize electronic signal interpolation. Rotary encoders for applications in dividing heads and rotary tables, with very small measuring steps (down to 0.36 arc second) have in principle the same basic design features as standard rotary encoders, but incorporate some overall varying construction.

Figure 24.2 Rotary encoders

( 2.2 ) Absolute encoders

Absolute linear encoders require no previous transfer to provide the current position value. Absolute rotary encoders provide an angular position value which is derived from the pattern of the coded disc. The code signal is processed within a computer or in a numerical control. After system switch-on, such as following a power interruption, the position value is immediately available. Since these encoder types require more sophisticated optics and electronics than incremental versions, a higher price is normally to be expected. Apart from these two codes, a range of other codes have been employed, though they are losing their significance since modern computer programs usually are based on the binary system for reasons of high speed. There are many versions of absolute encoders available today, such as single-turn or multi-stage versions to name only two, and each must be evaluated based on its intended application.

( 2.3 ) Rotary and Linear encoders

A linear encoder is a sensor, transducer paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position by a digital readout (DRO). Linear encoder technologies include capacitive, inductive, eddy current, magnetic and optical. A rotary encoder, also called a shaft encoder, is an electro-mechanical device used to convert the angular position of a shaft to a digital code, making it a sort of a transducer. Rotary encoders serve as measuring sensors for rotary motion, and for linear motion when used in conjunction with mechanical measuring standards such as lead screws. There are two main types: absolute and relative rotary encoders. Incremental rotary encoder uses a disc attached to a shaft. The disc has several radial lines. An optical switch, such as a photodiode, generates an electric pulse whenever one of the lines passes through its field of view. An electronic control circuit counts the pulses to determine the angle through which the shaft has turned. As the present trend of machine tools evolves toward increasingly higher accuracy and resolution, increased reliability and speeds, and more efficient working ranges, so too must feedback systems. Currently, linear feedback systems are available that will achieve resolutions in the submicron range.

Figure 24.3: Exposed and sealed linear encoders

Submicron resolutions, for example, are required in the semiconductor industry and in ultra-precision machining. Achieving these resolutions is possible with the use of linear scales which transmit displacement information directly to a digital readout. As in rotary, linear scales operate on the same photoelectric scanning principle, but the linear scales are comprised in an overall straight construction, and their output signals are interpolated or digitized differently in a direct manner. One of these signals is always used by the accompanying digital readout or numerical control to determine and establish home position on the linear machine axis in case of a power interruption or for workpiece referencing. Overall, there are two physical versions of a linear scale: exposed or enclosed as shown in the figure 24.3. With an enclosed or "sealed" scale, the scanning unit is mounted on a small carriage guided by ball bearings along the glass scale; the carriage is connected to the machine slide by a backlash-free coupling that compensates for alignment errors between the scale and the machine tool guide ways. A set of sealing lips protects the scale from contamination. The typical applications for the enclosed linear encoders are primarily machine tools. Exposed linear encoders also consist of a glass scale and scanning unit, but the two components are physically separated. The typical advantages of the non-contact system are easier mounting and higher traversing speeds since no contact or friction between the scanning unit and scale exists. Exposed linear scales can be found in coordinate measuring machines, translation stages, and material handling equipment. Another version of the scale and scanning unit arrangement is one that uses a metal base rather than glass for the scale. With a metal scale, the line grating is a deposit of highly reflective material such as gold that reflects light back to the scanning unit onto the photovoltaic cells. The advantage of this type of scale is that it can be manufactured in extremely great lengths, up to 30 meters, for larger machines. Glass scales are limited in length, typically three meters. There are several mechanical considerations that need to be understood when discussing linear encoders. It is not a simple matter to select an encoder based just on length or dimensional profile and install the encoder onto a machine. These characteristic considerations include permissible traversing speeds, accuracy and resolution requirements, thermal behaviour and mounting guidelines.

Figure 24.4: Principle of rotary and linear encoders

( 3 ) CNC Controller

There are two types of CNC controllers, namely closed loop and open loop controllers. These have been discussed in details in section 22.2.

( 3.1 ) Controller Architecture:

Most of the CNC machine tools were built around proprietary architecture and could not be changed or updated without an expensive company upgrade. This method of protecting their market share worked well for many years when the control technology enjoyed a four-to-five year life cycle. Now a day the controller life cycle is only eight-to-twelve months. So CNC manufacturers are forced to find better and less expensive ways of upgrading their controllers. Open architecture is the less costly than the alternatives. GE Fanuc and other manufacturers introduced control architecture with PC connectivity to allow users to take advantage of the new information technologies that were slowly gaining acceptance on the shop floor. They created an open platform that could easily communicate with other devices over commercially available MS Windows operating system, while maintaining the performance and reliability of the CNC machine tool.

CNC SYSTEMS - MECHANICAL COMPONENTS

The drive units of the carriages in NC machine tools are generally the screw & the nut mechanism. There are different types of screws and nuts used on NC machine tools which provide low wear, higher efficiency, low friction and better reliability.

(1) Recirculating ball screw The recirculating ball screw assembly shown in figure 25.1 has the flanged nut attached to the moving chamber and the screw to the fixed casting. Thus the moving member will move during rotational movement of the screw. These recirculating ball screw designs can have ball gages of internal or external return, but all of them are based upon the "Ogival" or "Gothic arc". In these types of screws, balls rotate between the screw and nut and convert the sliding friction (as in conventional nut & screw) to the rolling friction. As a consequence wear will be reduced and reliability of the system will be increased. The traditional ACME thread used in conventional machine tool has efficiency ranging from 20% to 30% whereas the efficiency of ball screws may reach up to 90%.

Figure 25.1: Recirculating ball screw assembly

Figure 25.2: Preloaded recirculating ball screw

There are two types of ball screws. In the first type, balls are returned through an external tube after few threads. In another type, the balls are returned to the start through a channel inside the nut after only one thread. To make the carriage movement bidirectional, backlash between the screw and nut should be minimum. One of the methods to achieve zero backlash is by fitting two nuts. The nuts are preloaded by an amount which exceeds the maximum operating load. These nuts are either forced apart or squeezed together, so that the balls in one of the nuts contact the opposite side of the threads. These ball screws have the problem that minimum diameter of the ball (60 to 70% of the lead screw) must be used, limiting the rate of movement of the screw.

(2) Roller screw

Figure 25.3: Roller screw

These types of screws provide backlash-free movement and their efficiency is same as that of ball screws. These are capable of providing more accurate position control. Cost of the roller screws are more compared to ball screws. The thread form is triangular with an included angle of 90 degrees. There are two types of roller screws: planetary and recirculating screws.

Planetary roller screws: Planetary roller screws are shown in figure 25.3. The rollers are threaded with a single start thread. Teeth are cut at the ends of the roller, which meshes with the internal tooth cut inside the nut. The rollers are equally spaced around and are retained in their positions by spigots or spacer rings. There is no axial movement of the rollers relative to the nut and they are capable of transmitting high load at fast speed.

Recirculating roller screws: The rollers in this case are not threaded and are provided with a circular groove and are positioned circumferentially by a cage. There is some axial movement of the rollers relative to the nut. Each roller moves by a distance equal to the pitch of the screw for each rotation of the screw or nut and moves into an axial recess cut inside the nut and disengage from the threads on the screw and the nut and the other roller provides the driving power. Rollers in the recess are moved back by an edge cam in the nut. Recirculating roller screws are slower in operation, but are capable of transmitting high loads with greater accuracy.

(1) Tool changing arrangements

There are two types of tool changing arrangements: manual and automatic. Machining centres incorporate automatic tool changer (ATC). It is the automatic tool changing capability that distinguishes CNC machining centres from CNC milling machines.

(1.1) Manual tool changing arrangement: Tool changing time belongs to non-productive time. So, it should be kept as minimum as possible. Also the tool must be located rigidly and accurately in the spindle to assure proper machining and should maintain the same relation with the work piece each time. This is known as the repeatability of the tool. CNC milling machines have some type of quick tool changing systems, which generally comprises of a quick release chuck. The chuck is a different tool holding mechanism that will be inside the spindle and is operated either hydraulically or pneumatically. The tool holder which fits into the chuck can be released by pressing a button which releases the hydraulically operated chuck. The advantage of manual tool changing is that each tool can be checked manually before loading the tools and there will be no limitation on the number of tools from which selection can be made.

(1.2) Automatic tool changing arrangement  Tooling used with an automatic tool changer should be easy to center in the spindle, each for the tool changer to grab the tool holder and the tool changer should safely disengage the tool holder after it is secured properly. Figure 27.1 shows a tool holder used with ATC. The tool changer grips the tool at point A and places it in a position aligned with the spindle. The tool changer will then insert the tool holder into the spindle. A split bushing in the spindle will enclose the portion B. Tool changer releases the tool holder. Tool holder is drawn inside the spindle and is tightened.

Figure 27.1: Tool holder

( 2) Tool turrets

An advantage of using tool turrets is that the time taken for tool changing will be only the time taken for indexing the turret. Only limited number of tools can be held in the turret. Tool turrets shown in figure 27.2 a, b & c are generally used in lathes. The entire turret can be removed from the machine for setting up of tools.

Figure 27.2(a): Six station tool turret

Figure 27.2(b): Eight station tool turret

Figure 27.2(c): Twelve station tool turret

( 3 ) Tool magazines

Tool magazines are generally found on drilling and milling machines. When compared to tool turrets, tool magazines can hold more number of tools and also more problems regarding the tool management. Duplication of the tools is possible and a new tool of same type may be selected when ever a particular tool has been worn off. Though a larger tool magazine can accommodate more number of tools, but the power required to move the tool magazine will be more. Hence, a magazine with optimum number of tool holders must be used. The following types of tool magazines exist: circular, chain and box type. 

( 3.1 ) Chain magazine: These magazines can hold large number of tools and may hold even up to 100 tools. Figures 27.3 a & b show chain magazines holding 80 and 120 tools respectively. In these chain magazines, tools will be identified either by their location in the tool holder or by means of some coding on the tool holder. In the former it is followed for identifying the tool and then the tool must be exactly placed in its location. The positioning of the magazine for the next tool transfer will take place during the machining operation.

Figure 27.3 (a) 80-tool chain magazine

Figure 27.3 (b) 120-tool chain magazine

( 3.2) Circular magazine: Circular magazines shown in figure 27.4 will be similar to tool turrets, but in the former the tools will be transferred from the magazine to the spindle nose. Generally these will be holding about 30 tools. The identification of the tool will be made either by its location in the tool magazine or by means of some code on the tool holder. The most common type of circular magazine is known as carousel, which is similar to a flat disc holding one row of tools around the periphery. Geneva mechanism is used for changing the tools.

Figuure 27.4: Circular magazine

( 3.3 ) Box magazine: In these magazines, the tools are stored in open ended compartments. The tool holder must be removed from the spindle before loading the new tool holder. Also the spindle should move to the tool storage location rather than the tool to the spindle. Hence, more time will be consumed in tool changing. Box magazines are of limited use as compared to circular and chain type of tool magazines.

( 4 ) Automatic tool changers :

Whenever controller encounters a tool change code, a signal will be sent to the control unit so that the appropriate tool holder in the magazine comes to the transfer position. The tool holder will then be transferred from the tool magazine to the spindle nose. This can be done by various mechanisms. One such mechanism is a rotating arm mechanism. 

Rotating arm mechanism: Movement of the tool magazine to place the appropriate tool in the transfer position will take place during the machining operation. The rotating arms with grippers at both the ends rotate to grip the tool holders in the magazine and the spindle simultaneously. Then the tool holder clamping mechanism will be released and the arm moves axially to remove the tool holder from the spindle. Then the arm will be rotated through 180 degrees and the arm will then move axially inwards to place the new tool holder into the spindle and will clamped. Now the new tool holder is placed in the spindle and the other in the magazine. Figure 27.5 and 27.6 show various stages during tool change with a rotating arm mechanism.

Figure 27.5: Rotating arm mechanism

Figure 27.6: Rotating arm mechanism

( 5 ) Tool wear monitoring :

Most of the modern CNC machines now incorporate the facility of on-line tool wear monitoring systems, whose purpose is to keep a continuous track of the amount of tool wear in real time. These systems may reduce the tool replacement costs and the production delays. It is based on the principle that the power required for machining increases as the cutting edge gets worn off. Extreme limits for the spindle can be set up and whenever it is reached, a sub-program can be called to change the tool. Following figures show some typical tool wear monitoring systems.

Figure 27.7: ON-line tool wear monitoring system

Figure 27.8 : Graphical display of tool wear monitoring system

28. CNC WORK HOLDING DEVICES
With the advent of CNC technology, machining cycle times were drastically reduced and the desire to combine greater accuracy with higher productivity has led to the reappraisal of work holding technology. Loading or unloading of the work will be the non-productive time which needs to be minimized. So the work is usually loaded on a special work holder away from the machine and then transferred it to the machine table. The work should be located precisely and secured properly and should be well supported.

28.1 Turning center work holding methods:
Machining operations on turning centers or CNC lathes are carried out mostly for axi-symmetrical components. Surfaces are generated by the simultaneous motions of X and Z axes. For any work holding device used on a turning centre there is a direct "trade off" between part accuracy and the flexibility of work holding device used.

Work holding methods	Advantages	Disadvantages
Automatic Jaw & chuck changing	Adaptable for a range of work-piece shapes and sizes	High cost of jaw/chuck changing automation. Resulting in a more complex & higher cost machine tool
Indexing chucks Figure 28.1	Very quick loading and unloading of the workpiece can be achieved. Reasonable range of work piece sizes can be loaded automatically	Expensive optional equipment. Bar-feeders cannot be incorporated. Short/medium length parts only can be incorporated. Heavy chucks.
Pneumatic/Magnetic chucks Figure 28.3	Simple in design and relatively inexpensive. Part automation is possible. No part distortion is caused due to clamping force	Limited to a range of flat parts with little overhang. Bar-feeders cannot be incorporated. Parts on magnetic chucks must be ferrous. Heavy cuts must be avoided.
Automatic Chucks with soft jaws	Adaptable to automation. Heavy cuts can be taken. Individual parts can be small or large in diameter	Jaws must be changed manually & bared, so slow part change-overs. A range of jaw blanks required.
Expanding mandrels & collets Figure 28.2	Long & short parts of reasonably large size accommodated. Automation can be incorporated. Clamping forces do not distort part. Simple in design	Limitation on part shape. Heavy cuts should be avoided.
Dedicated Chucks	Excellent restraint & location of a wide range of individual & irregular -shaped parts can be obtained.	Expensive & can only be financially justified with either large runs or when extremely complex & accurate parts are required. Tool making facilities required. Large storage space.

( 2) Work holding for Machining Centres:

Workholding methods	Advantages	Disadvantages
Modular Fixtures Figure 28.6	Highly adaptable. Can be purchased in stages to increase its sophistication. Reasonable accuracy. Speedily assembled. Small stores area is required. Can be set-up to a machine more than one part. Proven technology	Costly for a complete system. Difficult to automate. Skills required in kit assembly
Automatic Vices	Relatively inexpensive. Can be operated by mechanical, pneumatic, or by hydraulic control. Quick to operate with ease of set-up. Reasonable accuracy. Easily automated. Simplicity of design. Using multi-vices allows many parts to be machined. Proven Technology	Work holding limitations. Clamping force limitations. Jaws can become strained. Work location problems. Limitations on part size.
Pneumatic/Magnetic Work holding devices	Relatively inexpensive. Reasonable accuracy. Can machine large areas of the work piece. Quick setups. Easily automated. Simplicity of design. Many parts can be machined at one set up.	Large surface area is required. Swarf can be a problem. Nonferrous material limitation on magnetic devices.
4/5 axis CNC work holding devices	Allows complex geometric shapes to be machined. High accuracy. Opportunity for "one hit" machining. Easily automated.	Costly & limited part geometry clamping. Part size limitations. Usually only one part can be machined. Cannot be fitted to all machines.
Dedicated Fixturing	Large & small parts are easily accommodated. High accuracy of part location. Easily automated. Simplicity of design. Proven technology. Many parts can be machine at one setup good vibration damping capacity	Large storage space required. No part flexibility. Heavy fixtures. Tool making facilities required.

Figure 28.1: Indexing chucks Figure 28.2: Mandrels Figure 28.3: Magnetic chucks   Figure 28.4: Vise

Figure 28.5(a): Pallets    Figure 28.5(b) Figure 28.5(c)

Figure 28.6 : Modular fixture

Figure 28.7 : Chucks

Module F (1) : CNC Part Programming I

(1)  Programming fundamentals

Machining involves an important aspect of relative movement between cutting tool and workpiece. In machine tools this is accomplished by either moving the tool with respect to workpiece or vice versa. In order to define relative motion of two objects, reference directions are required to be defined. These reference directions depend on type of machine tool and are defined by considering an imaginary coordinate system on the machine tool. A program defining motion of tool / workpiece in this coordinate system is known as a part program. Lathe and Milling machines are taken for case study but other machine tools like CNC grinding, CNC Hobbing, CNC filament winding machine, etc. can also be dealt with in the same manner.

·         (1.1) Reference Points Part programming requires establishment of some reference points. Three reference points are either set by manufacturer or user. a) Machine Origin The machine origin is a fixed point set by the machine tool builder. Usually it cannot be changed. Any tool movement is measured from this point. The controller always remembers tool distance from the machine origin. ·           b) Program Origin It is also called home position of the tool. Program origin is point from where the tool starts for its motion while executing a program and returns back at the end of the cycle. This can be any point within the workspace of the tool which is sufficiently away from the part. In case of CNC lathe it is a point where tool change is carried out. ·           c) Part Origin The part origin can be set at any point inside the machine's electronic grid system. Establishing the part origin is also known as zero shift, work shift, floating zero or datum. Usually part origin needs to be defined for each new setup. Zero shifting allows the relocation of the part. Sometimes the part accuracy is affected by the location of the part origin. Figure 29.1 and 29.2 shows the reference points on a lathe and milling machine.

Figure 29.1. Reference points and axis on a lathe

Figure 29.2. Reference points and axis on a Milling Machine

(1.2 ) Axis Designation 

An object in space can have six degrees of freedom with respect to an imaginary Cartesian coordinate system. Three of them are liner movements and other three are rotary. Machining of simple part does not require all degrees of freedom. With the increase in degrees of freedom, complexity of hardware and programming increases. Number of degree of freedom defines axis of machine.
Axes interpolation means simultaneous movement of two or more different axes to generate required contour.
For typical lathe machine degree of freedom is 2 and so it called 2 axis machines. For typical milling machine degree of freedom is   , which means that two axes can be interpolated at a time and third remains independent. Typical direction for the lathe and milling machine is as shown in figure 12 and figure 13.