Elements of Cnc Machine

UNIT –II

Elements of CNC Machines

Karthikeyan.N, A.P.,SRM Univetsity.

The method by which a control system calculates the intermediate points and the speed of the motor is called ‘interpolation’

Interpolation

1. Linear interpolation– Straight line between two points in space

2. Circular interpolation– Circular arc defined by starting point, end point, center or

radius, and direction3. Helical interpolation

– Circular plus linear motion4. Parabolic interpolation5. Cubic interpolation

– Free form curves using higher order equations

Types of interpolation

Circular Interpolation Methods

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the inside of the nominal curve


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 curve


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 curve

Based on Feedback Control 1. Open loop

2. Closed loop

3. Semi-closed loop

• Servomechanism : A group of electro- mechanical, pneumatic or hydraulic or hydraulic components that control the position or velocity of a m/c slide.

Classification of CNC machines

- Where the final value is not directly measured and checked against the desired value.

- no feedback device to measure actual position of tool or work table.

Open Loop Control

• Applications– Light duty machinery <5KW & torque 200 Nm– Economy constraints– Less accuracy required– Less maintenance required

• Drawbacks– Backlash errors in leadscrew due to wear and tear– Periodical adjustments required to compensate for tool

geometry or drive mechanism– Less accurate slide movement – a few mm short

Open Loop Control-Applications & Drawbacks

Closed loop control

(Differential Analyzer)

Transducer

• Uses feedback measurements to confirm that the final position of the worktable is the location specified in the program

• Used for higher degree of accuracy

• Gives feedback for displacement, so ‘closed loop’• ‘Open’ as it depends on the accuracy of the lead screw

or gear arrangement• Reduces bulkiness

Semi-closed loop systems

CNC terminology

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Servo Controller

Counter Comparator

Encoder A/C Motor

Input (converted from analog to digital value)

TableLeadscrew

CNC terminology BLU: basic length unit

smallest programmable move of each axis.

Controller: (Machine Control Unit, MCU) Electronic and computerized interface between

operator and m/c

Controller components:

1. Data Processing Unit (DPU)

2. Control-Loops Unit (CLU)

CNC terminology Data Processing Unit:

Input device [RS-232 port/ Tape Reader/ Punched Tape Reader]

Data Reading Circuits and Parity Checking Circuits

Decoders to distribute data to the axes controllers.

Control Loops Unit:

Interpolator to supply machine-motion commands between data points

Position control loop hardware for each axis of motion

Configuration of CNC machine control unit

CNC Controllers

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 system- number of pulses transmitted to each axis determines the incremental axis

position; frequency of these pulses regulates the axis speed

CNC Controllers

• 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 Controllers

Miscellaneous control signals

-a set of on/off signals to implement the control of

1. speed and direction of the spindle rotation,

2. control of coolant supply,

3. selection of cutting tool,

4. automatic clamping and unclamping, etc.

CNC Controllers

Advantages of CNC

• Increased productivity• Reduced production costs• Facilitation of complex machining operations• Improved production planning and control• Facilitation of flexible automation• High accuracy and repeatability• Reduced indirect operating costs• Greater flexibility• Lower operator skill requirement

• High initial cost– Equipment and installation

• High maintenance cost• Skilled CNC personnel required• Have to be installed in air-conditioned places• Not suitable for long run applications

Disadvantages of CNC

• Direct numerical control (DNC) – control of multiple machine tools by a single (mainframe) computer through direct connection and in real time– 1960s technology– Two way communication

• Distributed numerical control (DNC) – network consisting of central computer connected to machine tool MCUs, which are CNC– Present technology– Two way communication

Direct Numeric Control (DNC)

- Bypasses the need for a MCU- Processing of part programs are done on a centralized

computer- Program transmitted to the MCU, one block at a time (BTR)

Basic components• Mainframe computer

– Connected via satellite computers or lengthy cables

• Bulk memory• Communications network• NC machine tools


Advantages:• Higher reliability than hardwired MCUs• Elimination or error prone tape and tape-reader• Control of multiple machines• Improved computational capability for

interpolations• Part programs stored in a central location• Computers located in an environmentally agreeable

location


Types of DNC

- based on communication link between machine tool and computer

1. Dedicated MC- Similar to a CNC as MCU is hardwired

2. Behind the tape reader (BTR) interface- cheaper to use

- easily implemented

- receives only the program of the part to be manufactured



Connection to MCU is behind the tape reader (BTR). In distributed NC, entire programs are downloaded to each MCU, which is CNC rather than conventional NC

• Similar to CNC computers• Improved data collection

Distributed Numeric Control (DNC)

DNC –switching network configurationLimitations

1. No of machines

2. Frequency of service required for each machine

Direct Numeric Control (DNC)-LAN configuration

• “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 control

Adaptive Control

• In machining, it includes automatic adjustment of cutting parameters like speeds, feeds, depth of cut, etc.

Adaptive Control

General Configuration

Adaptive controller performs 3 functions

1. Identification- Identifies the current value of performance index- Functions continuously to be dynamic

2. Decision

- decide what changes have to be made to improve system performance

3. Modification

- implement the decision

Adaptive Control

Two types of Adaptive control• Adaptive Control with optimization (ACO)• Adaptive Control with constraints (ACC)

• ACO – attempts to maximize IP – index of performance

IP = MRR/ TWR

MRR = Metal Removal Rate; TWR = Tool Wear Rate– usually an economic index of performance

• eg: Maximum production rate, minimum production cost

Adaptive Control

• Adaptive Control with Constraints (ACC)• Machining conditions are maximized within machine

constraints eg: maximum force or torque or power

• Two sensors are employed– Tool vibration sensor – accelerometer mounted on the

housing– Spindle torque sensor – strain gauges mounted on the

machine spindle

Adaptive Control

The three functions of adaptive control are: 1. Identification function. 2. Decision function. 3. Modification function.

Elements of Cnc Machine

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