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Introductory Concepts of Machining Machining is basically removal of material, most often metal, from the workpiece, using one or more cutting tools to achieve the desired dimensions.

There are different machining processes, such as, turning, milling, boring etc.

In all these cases metal is removed by a shearing process, which occurs due to the relative motion between the workpiece and the tool.

Special Purpose MachinesNC MachinesFixed sequenceFlexible methodMass ProductionShort/BatchMachine require more time for settingWith no timeProgram is built in hardwareStored in softwareLess scope for variationLarge scope for variationV.Thulasikanth, AP, Mech Dept, SRM UNIV.Computer Numerical Control (CNC) Modern precision manufacturing demands extreme dimensional accuracy and surface finish. Such performance is very difficult to achieve manually, if not impossible, even with expert operators. In the 1940s when the U.S. Air Force perceived the need to manufacture complex parts for high-speed aircraft.

This led to the development of computer-based automatic machine tool controls also known as the Numerical Control (NC) systems.

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flexo writer

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What is Computer Numerical Control? Computer Numerically Controlled (CNC) machine tools, the modern versions of NC machines have an embedded system involving several microprocessors and related electronics as the Machine Control Unit (MCU). V.Thulasikanth, AP, Mech Dept, SRM UNIV.Over a period of time, several additional features were introduced, leading to increased machine utilisation and reduced operator intervention.

Some of these are: Tool/work monitoring: For enhanced quality, avoidance of breakdowns. (b) Automated tool magazine and palette management: For increased versatility and reduced operator intervention over long hours of operation

(c) Direct numerical control (DNC): Uses a computer interface to upload and download part programs in to the machine automatically. Advantages of a CNC Machine CNC machines offer the following advantages in manufacturing. Higher flexibility: This is essentially because of programmability, programmed control and facilities for multiple operations in one machining centre.

Increased productivity: Due to low cycle time achieved through higher material removal rates and low set up times achieved by faster tool positioning, changing, automated material handling etc. V.Thulasikanth, AP, Mech Dept, SRM UNIV.Improved quality: Due to accurate part dimensions and excellent surface finish that can be achieved due to precision motion control and improved thermal control by automatic control of coolant flow.

Reduced scrap rate: Use of Part programs that are developed using optimization procedures

Reliable and Safe operation: Advanced engineering practices for design and manufacturing, automated monitoring, improved maintenance and low human interaction Smaller footprint: Due to the fact that several machines are fused into one.

On the other hand, the main disadvantages of NC systems are

Relatively higher cost compared to manual versions More complicated maintenance due to the complex nature of the technologies Need for skilled part programmers. V.Thulasikanth, AP, Mech Dept, SRM UNIV.MACHINE TYPES

Group 1: -M/C Tools with rotating tool i.e. Milling M/C, Drilling M/C, Boring M/C , Tapping M/C

Group 2: -M/C Tool with Rotating work piece i.e. Lathe.

Group 3: -Non Rotating work Piece and non rotating tool i.e. Shaper, Planer, EDM, Wire cut.

Group: -Other than above Z Categories i.e. NC Drafting.V.Thulasikanth, AP, Mech Dept, SRM UNIV.

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V.Thulasikanth, AP, Mech Dept, SRM UNIV.Input Device

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V.Thulasikanth, AP, Mech Dept, SRM UNIV.The MCU is the heart of a CNC system. It accepts the information stored in the memory as part program.

This data is decoded and transformed into specific position control and velocity signals.

It also oversees the movement of the control axis or spindle and whenever this does not match with the programmed values, a corrective action as taken.

All the compensation required for machine acquires (like lead screw pitch error, tool wear out, backlashes.) are calculated by CPU depending upon the corresponding inputs made available to the system.

The same will be taken care of during the generation of control signals for the axis movement.

Also, some basic safety checks are built into the system through this unit and continuous necessary corrective actions will be provided by CPU unit.

Whenever the situation goes beyond control of the MCU, it takes the final action of shutting down the system and in turn the machine. Machine Control Unit V.Thulasikanth, AP, Mech Dept, SRM UNIV.

MCUV.Thulasikanth, AP, Mech Dept, SRM UNIV.

Polytetrafluoroethylene (PTFE)V.Thulasikanth, AP, Mech Dept, SRM UNIV.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Servo motors A servomotor is just a regular motor with a sensor installed, typically to measure angular position during operation.

Servomotors are used in applications such as robotics, CNC machinery or automated manufacturing.

A servomotor is a rotary actuator that allows for precise control of angular position, velocity and acceleration.

It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.MechanismAs the name suggests, a servomotor is a servomechanism.More specifically, it is a closed-loop servomechanism that uses position feedback to control its motion and final position. The input to its control is some signal, either analogue or digital, representing the position commanded for the output shaft.V.Thulasikanth, AP, Mech Dept, SRM UNIV.The motor is paired with some type of encoder to provide position and speed feedback. In the simplest case, only the position is measured.

The measured position of the output is compared to the command position, the external input to the controller.

If the output position differs from that required, an error signal is generated which then causes the motor to rotate in either direction, as needed to bring the output shaft to the appropriate position.

As the positions approach, the error signal reduces to zero and the motor stops.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.The back of the motor designates which type of feedback comes with the motor.

Red designates absolute encoder feedback and Yellow designates incremental as of late it would appear that absolute is all that is being sold, but repair and replacement for incremental is still everyday business.

AC ServomotorDC ServomotorAddition of an amplifier and a feedback device to normal one. V.Thulasikanth, AP, Mech Dept, SRM UNIV.Limitations and ConsiderationsSome of the following are limitations or considerations to AC Servo Motors:Needs a clean or sealed environment

Do not use force to align shafts and coupling

Electrical Technician is required for installation and wiring

Need to be mounted on solid part of machine

Susceptible to natural frequencies of machine and motor as a whole if motor gearing is not chosen wiselyNeeds a feedback loop for controllability

Subject to high heat and extreme cold

Best application in a controlled environment

Motor Selection is important depending on application

Power and feedback wiring needs to be run from stationary wall power to moving machineV.Thulasikanth, AP, Mech Dept, SRM UNIV.Stepper MotorWhat is a Stepper Motor? Motor that moves one step at a time A digital version of an electric motor Each step is defined by a Step Angle

As the name implies, the stepper motor moves in distinct steps during its rotation. Each of these steps is defined by a Step Angle. In the example above you may notice that there are 4 distinct steps for the rotor to make a complete 360 degree rotation. This defines the step angle at 90 degrees.Since this motor does move in a discreet fashion, we can say that a stepper motor is actually a digital motor. This characteristic makes it very suitable for digital interfaces such as with a microcontroller.V.Thulasikanth, AP, Mech Dept, SRM UNIV.21Why a Stepper Motor? Relatively inexpensive Ideal for open loop positioning control Can be implemented without feedback Minimizes sensing devices Just count the steps! Torque Holds its position firmly when not turning Eliminates mechanical brakes Produces better torque than DC motors at lower speeds Positioning applicationsV.Thulasikanth, AP, Mech Dept, SRM UNIV.More important is the fact that a stepper motor can actually be used without any type of feedback loop.

Since the motor moves in distinct steps as defined by a step angle, we need only count the number of steps to position the motor accordingly.

This doesnt mean you wouldnt use a feedback loop in some applications.

The unique torque characteristics of the stepper motor make it ideal for position applications.

In fact, stepper motors have been used for years in such applications as printers and machining equipment.

This type of motor will hold its position firmly at a given step providing a relatively high holding torque.

Other torque related benefits include the higher torque at lower revolutions per minute than your typical DC motor as well as no need for mechanical braking.V.Thulasikanth, AP, Mech Dept, SRM UNIV.Main Components

Consists of Stators Holds multiplewindings/phases A Rotor Magnetized Non-magnetizedEach stator will be wrapped with multiple windings or phases that will be energized using a voltage source, initiating current flow through the winding to produce a polarity on each end or pole of the stator.The rotor is the actual rotating component on the motor. This can either be magnetized, as shown here, or non-magnetizedV.Thulasikanth, AP, Mech Dept, SRM UNIV.

If we apply a voltage across the windings around a stator, current will flow through the winding.

Here we can see that each end of the stator is magnetized to opposite poles.

Magnetic flux will flow from North to South thereby continuing through the magnetic rotor to the opposite stator pole.

The flux will want to travel the path of least resistance or decrease the reluctance of the path.V.Thulasikanth, AP, Mech Dept, SRM UNIV.Basic Length Unit (BLU)

In motion control signals, each pulse activates a motion of one basic length-unit (BLU)

Each BLU unit corresponds to the position resolution of the axis of motion.

For example, 1 BLU = 0.0001" means that the axis will move 0.0001" for every one electrical pulse received by the motor. The BLU is also referred to as Bit (binary digit). Pulse = BLU = Bit The NC controller interprets instructions and then converts them into two types of control signals: motion control signals and miscellaneous control signals

Motion control signals are a series of electric pulse trains that are used to control the position and the speed of the machine table and spindle .

Each pulse activates a motion of one basic length-unit (BLU) which is the minimum increment size of the NC control systemV.Thulasikanth, AP, Mech Dept, SRM UNIV.Feed Back Devices

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V.Thulasikanth, AP, Mech Dept, SRM UNIV.Display UnitDisplay Unit serves as an interactive device between machine operator

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Types of CNC machines Based on Motion Type:Point-to-PointorContinuous path

Based on Control Loops:Open looporClosed loop

Based on Power Supply:ElectricorHydraulicorPneumatic

Based on Positioning SystemIncrementalorAbsoluteV.Thulasikanth, AP, Mech Dept, SRM UNIV.Classification of NC Systems

CNC machine tool systems can be classified in various ways such as :

1.Point-to-point or contouring : depending on whether the machine cuts metal while the workpiece moves relative to the tool

2. Incremental or absolute : depending on the type of coordinate system adopted to parameterise the motion commands

3. Open-loop or closed-loop : depending on the control system adopted for axis motion control V.Thulasikanth, AP, Mech Dept, SRM UNIV.V.Thulasikanth, AP, Mech Dept, SRM UNIV.Point-to-Point Tool MovementsPoint-to-point control systems cause the tool to move to a point on the part and execute an operation at that point only. The tool is not in continuous contact with the part while it is moving. Drilling, reaming, punching, boring and tapping are examples of point-to-point operations.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Continuous-Path Tool Movements

Continuous-path controllers cause the tool to maintain continuous contact with the part as the tool cuts a contour shape. These operations include milling along any lines at any angle, milling arcs and lathe turning.

Coordinate Systems

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Coordinate System and Machine MotionsCoordinate System -IThe purpose is to provide a means of locating the tool in relation to the work piece.Numerical control coordinate system is defined with respect to the machine tool table.Depending on the type of NC machine, the part programmer may have several options for specifying the location. One of these options: 1.*Fixed zero: the origin is always located at the same position on the machine table. All locations must be defined by x and y coordinates relative to that fixed origin. *Floating zero: Modern NC machines allow the machine operator to set the zero point at any position on the machine table.V.Thulasikanth, AP, Mech Dept, SRM UNIV.35

ABSOLUTE

INCREMENTALV.Thulasikanth, AP, Mech Dept, SRM UNIV.V.Thulasikanth, AP, Mech Dept, SRM UNIV.Loop Systems for Controlling Tool MovementOpen Loop SystemUses stepping motor to create movement. Motors rotate a fixed amount for each pulse received from the MCU. The motor sends a signal back indicating that the movement is completed. No feedback to check how close the actual machine movement comes to the exact movement programmed.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Loop Systems for Controlling Tool MovementClosed Loop SystemAC, DC, and hydraulic servo-motors are used. The speed of these motors are variable and controlled by the amount of current or fluid. The motors are connect to the spindle and the table. A position sensor continuously monitors the movement and sends back a single to Comparator to make adjustments.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Machining center is a machine tool capable of performing several different machining operations on a work part in one setup under program controlThe mach. center is capable of milling, drilling, reaming, tapping, boring, facing, and similar operations.Characterizations of an NC machining center:Automatic tool-changing capabilityAutomatic work part positioningPallet shuttleVertical mach centers for flat work, and horizontal mach. centers for cube-shaped parts.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.40Other machines and equipment utilizing NC-type controls:Electrical wire-wrap machinesComponent insertion machinesDrafting machinesCoordinate measuring machinesFlame cutting, plasma arc cutting, laser cutting, and similar machinesTube bendingCloth cuttingKnittingRivetingFilament winding

V.Thulasikanth, AP, Mech Dept, SRM UNIV.41V.Thulasikanth, AP, Mech Dept, SRM UNIV.CNC MachinesMachining Centers, equipped with automatic tool changers, are capable of changing 90 or more tools. Can perform milling, drilling, tapping, boring on many faces.

Three types of machining centresVertical Machining centreHorizontal Machining centreUniversal Machining centreMachining Centre - typesV.Thulasikanth, AP, Mech Dept, SRM UNIV.VerticalFor flat work pieces that require tool access from the top

Machining Centre - typesV.Thulasikanth, AP, Mech Dept, SRM UNIV.Vertical Machining Centre

V.Thulasikanth, AP, Mech Dept, SRM UNIV.HorizontalFor cube shaped parts, where access is required all sidesHas spindle on the horizontal axis

Machining Centre - types

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V.Thulasikanth, AP, Mech Dept, SRM UNIV.Spindle axis can be tilted from horizontal to verticalEquivalent to 5-axis machiningUniversal Machining Centre

V.Thulasikanth, AP, Mech Dept, SRM UNIV.V.Thulasikanth, AP, Mech Dept, SRM UNIV.CNC MachinesTurning Centers are capable of executing many different types of lathe cutting operations simultaneously on a rotating part.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.CNC ControllersThe NC controller is the brain of the NC system, it controls all functions of the machine. Motion control deals with the tool position, orientation and speed.Auxiliary control deals with spindle rpm, tool change, fixture clamping and coolant.Many different types of controllers are available in the market (GE, Fanuc, Seimens, Allen-Bradley, Okuma, Bendix, ).There are two basic types of control systems:point-to-point and continuous path. Configuration of CNC machine control unit

V.Thulasikanth, AP, Mech Dept, SRM UNIV.CNC Controllers

V.Thulasikanth, AP, Mech Dept, SRM UNIV.CNC controller interprets part program instructions and then converts them into two types of control signals: Motion control signals a series of electric pulse trains that control position and the speed of machine table and spindle. Each pulse activates a motion of one basic length-unit (BLU) which is the minimum increment size of the NC control systemnumber of pulses transmitted to each axis determines the incremental axis position; frequency of these pulses regulates the axis speed

CNC ControllersV.Thulasikanth, AP, Mech Dept, SRM UNIV.The control signals in CNC systems are in the form of binary words. Each word contains a fixed number of bits, 32 bits or 64 bits are commonly used.Each bit of data produces one BLU motion in the controlled axis. A 32-bit word could represent one of up to 232 = 4,294,967,296 different axial positions. If the system resolution is, for example, BLU = 0.0001 in., this number can represent up to 429,969 in. possible motions, which is more than enough for all types of applications

CNC ControllersV.Thulasikanth, AP, Mech Dept, SRM UNIV.Miscellaneous control signals-a set of on/off signals to implement the control of speed and direction of the spindle rotation, control of coolant supply, selection of cutting tool, automatic clamping and unclamping, etc.CNC ControllersV.Thulasikanth, AP, Mech Dept, SRM UNIV.Linear interpolationStraight line between two points in spaceCircular interpolationCircular arc defined by starting point, end point, center or radius, and directionHelical interpolationCircular plus linear motionParabolic interpolationCubic interpolationFree form curves using higher order equations

Types of interpolation

V.Thulasikanth, AP, Mech Dept, SRM UNIV. Circular Interpolation Methods

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the outside of the nominal curveV.Thulasikanth, AP, Mech Dept, SRM UNIV. Circular Interpolation Methods

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on both the inside and outside of the nominal curveV.Thulasikanth, AP, Mech Dept, SRM UNIV.Direct Numerical Control (DNC)Direct Numerical Control can be defined as a type of manufacturing system in which several NC or CNC machines are controlled remotely from a Host/Main frame computer or direct numerical control (DNC) control of multiple machine tools by a single (mainframe) computer through direct connection.

V.Thulasikanth, AP, Mech Dept, SRM UNIV.The basic DNC system requires following basic component are Main frame computer, Memory, Communication network, NC machine tool.

The communication network can be done either through connecting the remotely located computer, with lengthy cables to the individual machine control directly or connecting the main frame computer with a small computer at individual operators station known as satellite computer.

DNC system is expensive and is preferably used in large organizations. The combination of DNC/CNC makes possible to eliminate the use of programme as the input media for CNC machines.

The DNC computer downloads the program directly to the CNC computer memory.

This reduces the amount of communication required between the central computer and each machine tool. V.Thulasikanth, AP, Mech Dept, SRM UNIV.Distributed Numerical Control (DNC) In early 1980s, with advancement in computers and communication technologies, engineers realized that in a network of computers there must be a proper co-ordination for operations of a group of CNC machine tools.

Now, many CNC machines together with robots, programmable logic controllers, and other computer-based controllers have been integrated into DNC systems to make automated manufacturing systems possible

V.Thulasikanth, AP, Mech Dept, SRM UNIV.Voice Numerical Control (VNC)Voice Numerical Control (VNC) is similar to DNC machines but the programmer conveys the information needed to operate the machine by means of computer system. The programmer talks into the computer, and the memory receives the information using a wire. This information can be taken and used to run the machines.V.Thulasikanth, AP, Mech Dept, SRM UNIV.to adapt means to change a behavior to conform to new circumstances.

An adaptive controller a controller that can modify its behavior in response to the changes in dynamics of the processes and the disturbances acting on the process.

A self-correcting form of optimal controlAdaptive ControlV.Thulasikanth, AP, Mech Dept, SRM UNIV.In machining, it includes automatic adjustment of cutting parameters like speeds, feeds, depth of cut, etc.

Adaptive Control

General ConfigurationV.Thulasikanth, AP, Mech Dept, SRM UNIV.Adaptive controller performs 3 functionsIdentificationIdentifies the current value of performance indexFunctions continuously to be dynamicDecision- decide what changes have to be made to improve system performanceModification- implement the decisionAdaptive ControlV.Thulasikanth, AP, Mech Dept, SRM UNIV.Two types of Adaptive controlAdaptive Control with optimization (ACO)Adaptive Control with constraints (ACC)

ACO attempts to maximize IP index of performanceIP = MRR/ TWR MRR = Metal Removal Rate; TWR = Tool Wear Rateusually an economic index of performanceeg: Maximum production rate, minimum production costAdaptive ControlV.Thulasikanth, AP, Mech Dept, SRM UNIV.Adaptive Control with Constraints (ACC)Machining conditions are maximized within machine constraints eg: maximum force or torque or powerTwo sensors are employedTool vibration sensor accelerometer mounted on the housingSpindle torque sensor strain gauges mounted on the machine spindleAdaptive ControlV.Thulasikanth, AP, Mech Dept, SRM UNIV.

The three functions of adaptive control are: Identification function. Decision function. Modification function.V.Thulasikanth, AP, Mech Dept, SRM UNIV.