notes on cnc

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http://183.83.200.113:3333/NITWARANGAL/CNC_machines/overview/notes.htm[13-05-2011 00:19:32]

Historical Perspective

The development of numerical control owes much tothe United States air force, which recognized the needfor developing more efficient manufacturing methodsfor modern aircraft. Following World War II, thecomponents used to fabricate jet aircraft became morecomplex and required more machining. The concept ofNC was proposed in the late 1940s by John Parson’s ofTraverse City. Michigan. Parson recommended amethod of automatic machine control that would guidea milling cutter to produce a “thru axis curve” in orderto generate smooth profiles on the work-pieces. In1949, the U.S Air Force awarded Parsons a contract todevelop new type of machine tool that would be able tospeed up production methods. Parsons sub-contractedthe Massachusetts Institute of Technology (MIT) todevelop a practical implementation of his concept.Scientists and engineers at M.I.T built a control systemfor a two axis milling machine that used a perforatedpaper tape as the input media. This prototype wasproduced by retrofitting a conventional tracer mill withnumerical control servomechanisms for the three axesof the machine. By 1955, these machines wereavailable to industries with some small modifications.

The machine tool builders gradually began developingtheir own projects to introduce commercial NC units.Also, certain industry users, especially airframebuilders, worked to devise numerical control machinesto satisfy their own particular production needs. TheAir force continued its encouragement of NCdevelopment by sponsoring additional research at MITto design a part programming language that could beused in controlling N.C. machines.

In a short period of time, all the major machine toolmanufacturers were producing some machines withNC, but it was not until late 1970s that computer-basedNC became widely used. NC matured as an automationtechnology when electronics industry developed newproducts. At first, miniature electronic tubes weredeveloped, but the controls were big, bulky, and notvery reliable. Then solid-state circuitry and eventuallymodular or integrated circuits were developed. Thecontrol unit became smaller, more reliable, and lessexpensive.

Introduction to CNC

Computer Numerical Control

Computer numerical control (CNC) is the numericalcontrol system in which a dedicated computer is built

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into the control to perform basic and advanced NCfunctions. CNC controls are also referred to as soft-wired NC systems because most of their controlfunctions are implemented by the control softwareprograms. CNC is a computer assisted process tocontrol general purpose machines from instructionsgenerated by a processor and stored in a memorysystem. It is a specific form of control system whereposition is the principal controlled variable. Allnumerical control machines manufactured since theseventies are of CNC type. The computer allows for thefollowing: storage of additional programs, programediting, running of program from memory, machineand control diagnostics, special routines, inch/metric,incremental/absolute switchability.

CNC machines can be used as stand alone units or in anetwork of machines such as flexible machine centres.The controller uses a permanent resident programcalled an executive program to process the codes intothe electrical pulses that control the machine. In anyCNC machine, executive program resides in ROM andall the NC codes in RAM. The information in ROM iswritten into the electronic chips and cannot be erasedand they become active whenever the machine is on.The contents in RAM are lost when the controller isturned off. Some use special type of RAM calledCMOS memory, which retains its contents even whenthe power is turned off.

CNC milling machine

1.2. Direct Numerical Control

In a Direct Numerical Control system (DNC) system, amainframe computer is used to coordinate thesimultaneous operations of a number NC machines asshown in the figures 2 & 3. The main tasks performed

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by the computer are to program and edit part programsas well as download part programs to NC machines.Machine tool controllers have limited memory and apart program may contain few thousands of blocks.Sothe program is stored in a separate computer and sentdirectly to the machine, one block at a time.

First DNC system developed was Molins System 24 in1967 by Cincinnati Milacron and General Electric.They are now referred to as flexible manufacturingsystems (FMS). The computers that were used at thosetimes were quite expensive.

DNC system

DNC system

Advantages & Disadvantages of CNC machine tools

Manually operatedmilling milling

machine Computer controlled machine

milling machine

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Some of the dominant advantages of the CNCmachines are:

CNC machines can be used continuously andonly need to be switched off for occasionalmaintenance. These machines require less skilled people tooperate unlike manual lathes / milling machinesetc.CNC machines can be updated by improving thesoftware used to drive the machinesTraining for the use of CNC machines can bedone through the use of ‘virtual software’. The manufacturing process can be simulatedvirtually and no need to make a prototype or amodel. This saves time and money.Once programmed, these machines can be leftand do not require any human intervention,except for work loading and unloading.These machines can manufacture severalcomponents to the required accuracy without anyfatigue as in the case of manually operatedmachines. Savings in time that could be achieved with theCNC machines are quite significant.

Some of the disadvantages of the CNC machines are:

CNC machines are generally more expensivethan manually operated machines.The CNC machine operator only needs basictraining and skills, enough to supervise severalmachines. Increase in electrical maintenance, high initialinvestment and high per hour operating coststhan the traditional systems.Fewer workers are required to operate CNCmachines compared to manually operatedmachines. Investment in CNC machines can leadto unemployment.

Applications of NC/CNC machine tools

CNC was initially applied to metal working machinery:Mills, Drills, boring machines, punch presses etc andnow expanded to robotics, grinders, weldingmachinery, EDM's, flame cutters and also forinspection equipment etc. The machines controlled byCNC can be classified into the following categories:CNC mills and machining centres.

CNC lathes and turning centers CNC EDMCNC grinding machines

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CNC cutting machines (laser, plasma, electron,or flame) CNC fabrication machines (sheet metal punchpress, bending machine, or press brake) CNC welding machines CNC coordinate measuring machines

CNC Coordinate Measuring Machines:

A coordinate measuring machine is a dimensionalmeasuring device, designed to move the measuringprobe to determine the coordinates along the surface ofthe work piece. Apart from dimensional measurement,these machines are also used for profile measurement,angularity, digitizing or imaging. A CMM consists offour main components: the machine, measuring probe,control system and the measuring software. The controlsystem in a CMM performs the function of a liveinteraction between various machine drives,displacement transducers, probing systems and theperipheral devices. Control systems can be classifiedaccording to the following groups of CMMs.

1. Manually driven CMMs2. Motorized CMMs with automatic probing

systems3. Direct computer controlled (DCC) CMMs4. CMMs linked with CAD, CAM and FMS etc.

The first two methods are very common and selfexplanatory. In the case of DCC CMMs, the computercontrol is responsible for the movement of the slides,readout from displacement transducers and datacommunication. CMM are of different configurations—fixed bridge, moving bridge, cantilever arm figure21.5(a), horizontal arm and gantry type CMM as shownin figure

Cantilever type CMM Gantry type CMM

CNC welding machines:

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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 rotarytables.Weld moves, welding feed rate, wire feed, torchheights & welding current can be programmed.CNC welding machines are used for laserwelding, welding of plastics, submerged arcwelding, wire welding machines, butt welding,flash butt welding etc.These machines are generally used in automobilework shopsCost of these machines will be twice than theconventional welding machines.

CNC EDM & WEDM machines:

EDM is a nontraditional machining method primarilyused to machine hard metals that could not bemachined by traditional machining methods. Materialremoval will be taking place by a series of electric arcsdischarging across the gap between the electrode andthe work piece. There are two main types- ram EDM &wire cut EDM. In wire-cut EDM, a thin wire is fedthrough the work piece and is constantly fed from aspool and is held between upper and lower guides.These guides move in the x-y plane and are preciselycontrolled by the CNC. Wire feed rate is also controlledby the CNC.

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Ram EDM Wire cut EDM

Classification Of CNC Machine Tools

2.1.Based on the motion type à Point-to-point &Contouring systems There are two main types of machine tools and thecontrol systems required for use with them differbecause of the basic differences in the functions of themachines to be controlled. They are known as point-to-point and contouring controls.

Point-to-point systems

Some machine tools for example drilling, boring andtapping machines etc, require that the cutter and thework piece be placed at a certain fixed relativepositions at which they must remain while the cutterdoes its work. These machines are known as point-to-point machines as shown in figure 1 (a) and the controlequipment for use with them are known as point-to-point control equipment. Feed rates need not beprogrammed. In theses machines tools, each axis isdriven separately. In a point-to-point control system,the dimensional information that must be given to themachine tool will be a series of required position of thetwo slides. Servo systems can be used to move theslides and no attempt is made to move the slide untilthe cutter has been retracted back.

Contouring systems (Continuous path systems)

Other type of machine tools involve motion of workpiece with respective to the cutter while cuttingoperation is taking place. These machine tools includemilling, routing machines etc and are known ascontouring machines as in figure 1 (b) and the controlsrequired for their control are known as contouringcontrol. Contouring machines can also be used as point-to-point machines, but it will be uneconomical to usethem unless the work piece also requires having a

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contouring operation to be performed on it. Thesemachines require simultaneous control of axes. Incontouring machines, relative positions of the workpiece and the tool should be continuously controlled.The control system must be able to accept informationregarding velocities and positions of the machinesslides. Feed rates should be programmed.

Point-to-point system Contouring systems

Contouring systems

Based on the control loops à Open loop & Closedloop systems

Open loop systems:

Programmed instructions are fed into the controllerthrough an input device. These instructions are thenconverted to electrical pulses (signals) by the controllerand sent to the servo amplifier to energize the servomotors. The primary drawback of the open-loopsystem is that there is no feedback system to checkwhether the program position and velocity has beenachieved. If the system performance is affected by load,temperature, humidity, or lubrication then the actualoutput could deviate from the desired output. For thesereasons the open -loop system is generally used inpoint-to-point systems where the accuracyrequirements are not critical. Very few continuous-pathsystems utilize open-loop control.

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Open loop controlsystem Closed loop control system

Open loop system

Closed loop systems:

The closed-loop system has a feedback subsystem tomonitor the actual output and correct any discrepancyfrom the programmed input. These systems useposition and velocity feed back. The feedback systemcould be either analog or digital. The analog systemsmeasure the variation of physical variables such asposition and velocity in terms of voltage levels. Digitalsystems monitor output variations by means ofelectrical pulses. To control the dynamic behavior andthe final position of the machine slides, a variety ofposition transducers are employed. Majority of CNCsystems operate on servo mechanism, a closed loopprinciple. If a discrepancy is revealed between wherethe machine element should be and where it actually is,the sensing device signals the driving unit to make anadjustment, bringing the movable component to therequired location.

Closed-loop systems are very powerful and accuratebecause they are capable of monitoring operatingconditions through feedback subsystems andautomatically compensating for any variations in real-time.

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Closed loop system

Based on the number of axes à 2, 3, 4 & 5 axes CNCmachines.

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 thesaddle (along X-axis). In 3-axes machines, there willbe one more axis, perpendicular to the above two axes.By the simultaneous control of all the 3 axes, complexsurfaces can be machined.

4 & 5 axes CNC machines:

4 and 5 axes CNC machines provide multi-axismachining capabilities beyond the standard 3-axis CNCtool path movements. A 5-axis milling centre includesthe three X, Y, Z axes, the A axis which is rotarytilting of the spindle and the B-axis, which can be arotary index table.

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Importance of higher axes machining :

Reduced cycle time by machining complexcomponents using a single setup. In addition totime savings, improved accuracy can also beachieved as positioning errors between setups areeliminated.Improved surface finish and tool life by tiltingthe tool to maintain optimum tool to part contactall the times.Improved access to under cuts and deep pockets.By tilting the tool, the tool can be made normalto the work surface and the errors may bereduced as the major component of cutting forcewill be along the tool axis.Higher axes machining has been widely used formachining sculptures surfaces in aerospace andautomobile industry.

Turning centre:

Traditional centre lathes have horizontal beds. Thesaddle moves longitudinally and the cross slide movestransversely. Although the tools can be clearly seen,the operator must lean over the tool post to positionthem accurately. Concentration of chips may becreating a heat source and there may be temperaturegradients in the machine tool. Keeping the abovepoints in view, developments in the structure of theturning centres lead to the positioning the saddle andthe cross slide behind the spindle on a slant bed asshown in the figure.4. Chips fall freely because of slantbed configuration and is more ergonomicallyacceptable also from operators point of view.

Slant bed turning centre

Based on the power supply à Electric, Hydraulic &Pneumatic systems

Mechanical power unit refers to a device whichtransforms some form of energy to mechanical powerwhich may be used for driving slides, saddles organtries forming a part of machine tool. The inputpower may be of electrical, hydraulic or pneumatic.

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Electric systems:

Electric motors may be used for controlling bothpositioning and contouring machines. They may beeither a.c. or d.c. motors and the torque and directionof rotation needs to be controlled. The speed control ofa d.c. motor can be done by varying either the field orthe armature supply. The clutch-controlled motor caneither be an a.c. or d.c. motor. They are generally usedfor small machine tools because of heat losses in theclutches. Split field motors are the simplest form ofmotors and can be controlled in a manner according tothe machine tool. These are small and generally run athigh maximum speeds and so require reduction gearsof high ratio. Separately excited motors are used withcontrol systems for driving the slides of large machinetools.

Hydraulic systems:

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

Major Components Of A CNC System

Construction of CNC machine tools

Any CNC machine tool will essentially consists of thefollowing parts:

Part program:A part program is a series of coded instructionsrequired to produce a part. It controls the movement ofthe machine tool and on/off control of auxiliary

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functions such as spindle rotation and coolant. Thecoded instructions are composed of letters, numbersand symbols.

Program input device:The program input device is the means for partprogram to be entered into the CNC control. Threecommonly used program input devices are punch tapereader, magnetic tape reader, and computer via RS-232-C communication.

Machine Control Unit:The machine control unit (MCU) is the heart of a CNCsystem. It is used to perform the following functions:

Reads the coded instructions.Decodes the coded instructions.Implement interpolations (linear, circular, andhelical) to generate axis motion commands.Feeds the axis motion commands to the amplifiercircuits for driving the axis mechanisms.Receives the feedback signals of position andspeed for each drive axis.

Implement auxiliary control functions such as coolantor spindle on/off and tool change.

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

Machine Tool:CNC controls are used to control various types ofmachine tools. Regardless of which type of machinetool is controlled, it always has a slide table and aspindle to control of position and speed. The machinetable is controlled in the X and Y axes, while thespindle runs along the Z axis.

Feed Back System:The feedback system is also referred to as themeasuring system. It uses position and speedtransducers to continuously monitor the position atwhich the cutting tool is located at any particularinstant. The MCU uses the difference betweenreference signals and feedback signals to generate thecontrol signals for correcting position and speed errors.

Machine axes designationMachine axes are designated according to the "right-hand rule", When the thumb of right hand points in the

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direction of the positive X axis, the index finger pointstoward the positive Y axis, and the middle fingertoward the positive Z axis. Figure 10 shows the right-hand rule applied to vertical machines, while Figure 3.1applies to horizontal machines.

Right hand rule for vertical and horizontal machine

CNC Systems – Mechanical Components

The drive units of the carriages for tool or work foundon numerically controlled machine tool s are generallythe screw & the nut mechanism. The following are thetypes of screws and nuts used on NC machine toolswhich provide low wear, higher efficiency, reducedfriction and better reliability.

Recirculating ball screw The recirculating ball screw assembly shown in figure4.1 has the flanged nut attached to the moving chamberand to the screw to the fixed casting. Thus anyrotational movement of the screw will displace themoving member and the screw to the fixed casting.These recirculating ball screw designs can have ballgages of internal or external return, but all of them arebased upon the Ogival or “Gothic arc”

In these types of screws, balls rotate in between thescrew and nut and convert the sliding friction (as inconventional nut & screw) to the rolling friction andhence reduce wear and increase the reliability of thesystem. The traditional ACME thread used onconventional machine tool has efficiencies rangingfrom 20% to 30% whereas the efficiencies of ballscrews may reach up to 90%.

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Recirculating ball screw assembly

Preloaded recirculating ball screw

There are two types of ball screws. In the first type,balls are returned through an external tube after fewthreads. In another type, the balls are returned to thestart through a channel inside the nut after only onethread. To make the carriage movement bidirectional,backlash between the screw and nut should beminimum. One of the methods to achieve zerobacklashes is by fitting two nuts. The nuts arepreloaded by an amount which exceeds the maximumoperating load. These nuts are either forced apart orsqueezed together, so that the balls in one of the nutscontact the opposite side of the threads.

These ball screws have the problem that minimumdiameter of the ball (60 to 70% of the lead screw) mustbe used, limiting the rate of movement of the screw.

4.2 Roller screw

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These types of screws provide backlash-free movementand their efficiency is same as that of ball screws.These are capable of providing more accurate positioncontrol. Roller screws costs more when compared toball screws. The thread form is triangular with anincluded angle of 90 degrees. There are two types ofroller screws: planetary and recirculating screws.

Planetary roller screws: Planetary roller screws areshown in figure 4.3. The rollers are threaded with asingle start thread. Teeth are cut at the ends of theroller, which meshes with the internal tooth cut insidethe nut. The rollers are equally spaced around and areretained in their positions by spigots or spacer rings.There is no axial movement of the rollers relative to thenut and are capable of transmitting high loads at fastspeeds.

Recirculating roller screws: The rollers in this caseare not threaded and are provided with a circulargroove and are positioned circumferentially by a cage.There is some axial movement of the rollers relative tothe nut. Each roller moves by a distance equal to thepitch of the screw for each rotation of the screw or nutand moves into an axial recess cut inside the nut anddisengage from the threads on the screw and the nutand the other roller provides the driving power. Rollersin the recess are moved back by an edge cam in thenut.

CNC Controllers

Different types of controllers:

There are two types of CNC controllers, namely closedloop and open loop controllers.

Controller Architecture:

Most of the CNC machine tools were built aroundproprietary architecture and could not be changed orupdated without an expensive company upgrade. Thismethod of protecting their market share worked wellfor many years when the control technology enjoyed a

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four-to-five year life cycle. Now a day the controllerlife cycle is only eight-to-twelve months. So CNCmanufacturers are forced to find better and lessexpensive ways of upgrading their controllers. Openarchitecture is the less costly than the alternatives. GEFanuc and other manufacturers introduced controlarchitecture with PC connectivity to allow users to takeadvantage of the new information technologies thatwere slowly gaining acceptance on the shop floor.They created an open platform that could easilycommunicate with other devices over commerciallyavailable MS Windows operating system, whilemaintaining the performance and reliability of the CNCmachine tool.

CNC Tooling

Tool changing arrangements

There are two types of tool changing arrangements:manual and automatic tool changing arrangements.Machining centres incorporate automatic tool change(ATC). It is the automatic tool changing capability thatdistinguishes CNC machining centres from CNCmilling machines.

Manual tool changing:

Tool changing is the non-productive time. So, it shouldbe kept as minimum as possible. Also the tool must berigidly located in the spindle. The tool must beaccurately located in the spindle so as to assure propermachining and should maintain the same relation withthe work piece each time it is inserted in the spindle.This is known as the repeatability of the tool. CNCmilling machines have some type of quick toolchanging systems, which generally comprises of aquick release chuck. The chuck is a different toolholding mechanism that will be inside the spindle andis operated either hydraulically or pneumatically. Thetool holder which fits into the chuck can be released bypressing a button which releases the hydraulicallyoperated chuck. The advantage of manual toolchanging is that each tool can be checked manuallybefore loading the tools and there will be no limitationon the number of tools from which selection can bemade.

Automatic tool changing:

Tooling used with an automatic tool changer should beeasy to center in the spindle, each for the tool changerto grab the tool holder and the tool changer shouldsafely disengage the tool holder after it is secured

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properly. Figure 6.1 shows a tool holder used withATC. The tool changer grips the tool at point A andplaces it in a position aligned with the spindle. The toolchanger will then insert the tool holder into the spindle.A split bushing in the spindle will enclose the portionB. Tool changer releases the tool holder. Tool holder isdrawn inside the spindle and is tightened.

Tool holder

Tool turrets

An advantage of using tool turrets is that the time takenfor tool changing will be only the time taken forindexing the turret. Only limited number of tools canbe held in the turret. Tool turrets are generally used inlathes. The entire turret can be removed from themachine for setting up of tools. Figure 6.2 a, b & cshow.

Six station tool turret Eight station tool turret

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Twelve station tool turret

Tool magazines

Tool magazines are generally found on drilling andmilling machines. When compared to tool turrets, toolmagazines can hold more number of tools and alsomore problems regarding the tool management.Duplication of the tools is possible and a new tool ofsame type may be selected when ever a particular toolhas been worn off. Though a larger tool magazine canaccommodate more number of tools, but the powerrequired to move the tool magazine will be more.Hence, a magazine with optimum number of toolholders must be used. The following types of toolmagazines exist: circular, chain and box type.

Chain magazine:

These magazines can hold large number of tools andmay hold even up to 100 tools. Figures 6.3 a & b showchain magazines holding 80 and 120 tools respectively.In these chain magazines, tools will be identified eitherby their location in the tool holder or by means of somecoding on the tool holder. In the former is followed foridentifying the tool, then the tool must be exactlyplaced in its location. The positioning of the magazinefor the next tool transfer will take place during themachining operation.

80-tool chain magazine 120-tool chain magazine

Circular magazine: Circular magazines as shown in figure 6.4 will besimilar to tool turrets, but in the former the tools willbe transferred from the magazine to the spindle nose.Generally these will be holding about 30 tools. Theidentification of the tool will be made either by itslocation in the tool magazine or by means of somecode on the tool holder. The most common type ofcircular magazine is known as carousel, which issimilar to a flat disc holding one row of tools aroundthe periphery. Geneva mechanism is used for changingthe tools.

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Circular magazine

Box magazine: In these magazines, the tools are stored in open endedcompartments. The tool holder must be removed fromthe spindle before loading the new tool holder. Also thespindle should move to the tool storage location ratherthan the tool to the spindle. Hence, more time will beconsumed in tool changing. Box magazines are oflimited use as compared to circular and chain type oftool magazines.

Automatic tool changers

When ever controller encounters a tool change code, asignal will be sent to the control unit so that theappropriate tool holder in the magazine comes to thetransfer position. The tool holder will then betransferred from the tool magazine to the spindle nose.This can be done by various mechanisms. One suchmechanism is a rotating arm mechanism.

Rotating arm mechanism:

Movement of the tool magazine to place theappropriate tool in the transfer position will take placeduring the machining operation. The rotating arms withgrippers at both the ends rotate to grip the tool holdersin the magazine and the spindle simultaneously. Thenthe tool holder clamping mechanism will be releasedand the arm moves axially to remove the tool holderfrom the spindle. Then the arm will be rotated through180 degrees and the arm will then move axiallyinwards to place the new tool holder into the spindleand will clamped. Now the new tool holder is placed inthe spindle and the other in the magazine. Figures 6.5and 6.6 shows various stages during tool change with arotating arm mechanism.

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Rotating arm mechanism

Rotating arm mechanism

Tool wear monitoring

Most of the modern CNC machines now incorporatethe facility of on-line tool wear monitoring systems,whose purpose is to keep a continuous track of theamount of tool wear in real time. These systems mayreduce the tool replacement costs and the productiondelays. It is based on the principle that the powerrequired for machining increases as the cutting edgegets worn off. Extreme limits for the spindle can be setup and when ever it is reached, a sub-program can becalled to change the tool. Following figures show toolwear monitoring systems.

ON-line tool wearmonitoring system Graphical display of tool

wear Monitoring system

CNC Work Holding Devices

With the advent of CNC technology, machining cycletimes were drastically reduced and the desire tocombine greater accuracy with higher productivity hasled to the reappraisal of work holding technology.Loading or unloading of the work will be the non-productive time and hence needs to be minimized. Sothe work is usually loaded on a special work holderaway from the machine and then transferred it to themachine table. The work should be located preciselyand secured quickly and should be well supported.

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Turning center work holding methods:

Machining operations on turning centers or CNC lathesare carried out mostly for axi-symmetrical components.Surfaces are generated by the simultaneous motions ofX and Z axes. For any work holding device used on aturning centre there is a direct “trade off” between partaccuracy and the flexibility of work holding deviceused.

Work holdingmethods

Advantages Disadvantages

Automatic Jaw &chuck changing

Adaptable for arange of work-piece shapesand sizes

High cost ofjaw/chuckchangingautomation.Resulting in amore complex& higher costmachine tool

Indexing chucksFigure7.1

Very quickloading andunloading ofthe workpiececan beachieved.

Expensiveoptionalequipment.Bar-feederscannot beincorporated.

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Reasonablerange of workpiece sizes canbe loaded automatically

Short/mediumlength partsonly can beincorporated.Heavy chucks

Pneumatic/Magneticchucks

Figure 7.3

Simple indesign andrelativelyinexpensive.Partautomation ispossible. Nopart distortionis caused dueto clampingforce.

Limited to arange of flatparts with littleoverhang. Bar-feeders cannotbeincorporated.Parts onmagneticchucks mustbe ferrous.Heavy cutsmust beavoided.

Automatic Chuckswith soft jaws

Adaptable toautomation.Heavy cuts canbe taken.Individualparts can besmall or largein diameter.

Jaws must bechangedmanually &bared, so slowpart change-overs. A rangeof jaw blanksrequired.

Expanding mandrels& collets

Figure 7.2

Long & shortparts ofreasonablylarge sizeaccommodated.Automationcan beincorporated.Clampingforces do notdistort part.Simple indesign

Limitation onpart shape.Heavy cutsshould beavoided.

Dedicated Chucks Excellentrestraint &location of awide range ofindividual &irregular –shaped partscan beobtained.

Expensive &can only befinanciallyjustified witheither largeruns or whenextremelycomplex &accurate partsare required.Tool makingfacilities

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required.Large storagespace.

Work holding for Machining Centres:

Workholdingmethods

Advantages Disadvantages

Modular Fixtures Highlyadaptable. Canbe purchasedin stages toincrease itssophistication.Reasonableaccuracy.Speedilyassembled.Small storesarea isrequired. Canbe set-up to amachine morethan one part.Proven

Costly for acompletesystem.Difficult toautomate.Skills requiredin kitassembly.

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technology

Automatic Vices Relativelyinexpensive.Can beoperated bymechanical,pneumatic, orby hydrauliccontrol. Quickto operate withease of set-up.Reasonableaccuracy.Easilyautomated.Simplicity ofdesign. Usingmulti-vicesallows manyparts to bemachined.ProvenTechnology.

Work holdinglimitations.Clampingforcelimitations.Jaws canbecomestrained. Worklocationproblems.Limitations onpart size.

Pneumatic/MagneticWork holdingdevices

Relativelyinexpensive.Reasonableaccuracy. Canmachine largeareas of thework piece.Quick setups.Easilyautomated.Simplicity ofdesign. Manyparts can bemachined atone set up.

Large surfacearea isrequired.Swarf can be aproblem.Nonferrousmateriallimitation onmagneticdevices.

4/5 axis CNC workholding devices

Allowscomplexgeometricshapes to bemachined.High accuracy.Opportunityfor “one hit”machining.Easilyautomated.

Costly &limited partgeometryclamping. Partsizelimitations.Usually onlyone part canbe machined.Cannot befitted to allmachines.

Dedicated Fixturing Large & smallparts are easily

Large storagespace required.

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accommodated.High accuracyof partlocation. Easilyautomated.Simplicity ofdesign. Proventechnology.Many parts canbe machine atone setup goodvibrationdampingcapacity.

No partflexibility.Heavy fixtures.Tool makingfacilitiesrequired.

Indexing chucks Mandrels

Magnetic chucks Vise

Pallets

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notes on cnc

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