Basic NC and CNC Dr. J. Ramkumar Professor, Department of Mechanical Engineering Micro machining Lab, I.I.T. Kanpur Micro machining Lab, I.I.T. Kanpur
Outline 1. Introduction to CNC machine 2. Component and Function of CNC 3. Coordinate System
1.Introduction to CNC machine CNC = Computerized Numerical Control
History and Development of Technology Conventional M/C NC M/C (1948 US Air force, MIT 21 months ) 1997 1 st commercial NC m/c CINCINNATIC HYDROTEL VERTICAL-SPINDLE MACHINE CNC M/C
History and Development of Technology
Conventional vs. CNC machine Machine Structure The CNC machine tools are basically built in the same way as conventional machine tools. The difference lies in the fact that the machine components relevant for turning and milling processes are controlled by computers.
Conventional vs. CNC machine Function
Conventional vs. CNC machine
Conventional vs. CNC machine Conventional machine eyes, hands, brain, skill CNC machine Program Control unit Motor Motion Measuring & Reflection Unit No skill is required for operating CNC m/c.
Conventional machine CNC machine
Difference between Conventional M/C & CNC M/C Item Conventional machine CNC machine 1. Movement Acme screw Ball screw 2. Feed Manual Motor 3. Measurement Manual Linear scale
Advantages of CNC Flexible, high accuracy Short production time Complex shapes Short setting time No skill requirement Short inspection time/ high quality product Low cost
Disadvantages of CNC High machine cost Complicated maintenance Skill & training are required for programming and maintenance. Parts are imported from aboard. High tooling cost Temperature, humidity & dust must be controlled.
Why CNC? HUMAN LIMITATION
Components of Traditional NC Systems
Direct Numerical Control (DNC)
Direct Numerical Control (DNC)
Component and Function of CNC Feed drive Measuring system Direct / Indirect Work spindle hydraulic Cooling system reduce heat Tool turret
Feed drive
Measuring System
Tool change facilities
Coordinate System
Axes on a CNC lathe
Axes on a CNC milling machine
Zero and reference points on CNC
Zero Point of machine on a CNC lathe
Machine Zero Point and Work part zero point on CNC milling machine
Classification based on the motion type
Classification based on the motion type
Classification based on the motion type
Classification based on the control loops
Classification based on the control loops
Classification based on the control loops
Classification based on the control loops
Classification based on the number of axes
Classification based on the number of axes
Classification based on the number of axes
Classification based on the number of axes
Classification based on the power supply
Driving System The requirement is that the driving system has to response accurately according to the programmed instructions. The motor is coupled either directly or through a gear box to the machine lead screw to moves the machine slide or the spindle. Three types of electrical motors are commonly used: 1. Stepping motor 2. DC Servo motor 3. AC Servo moto
1. Stepping Motor The stepper motor is known by its property to convert a train of input pulses (typically square wave pulses) into a precisely defined increment in the shaft position. Each pulse moves the shaft through a fixed angle. Multiple "toothed" electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external driver circuit or a micro controller. In that way, the motor can be turned by a precise angle.
What does Stepper means? To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear's teeth. When the gear's teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one. From there the process is repeated. Each of those rotations is called a "step", with an integer number of steps making a full rotation.
Stepper Motor / Electro magnet
2 1 S N 1 2 Rotor Stator Outside Casing Coils Stator Rotor Internal components of a Stepper Motor
2 1 S N N S 1 2 Cross Section of a Stepper Motor Stators Rotor
Full Step Operation Four Steps per revolution i.e. 90 deg. steps.
Half Step Operation Eight steps per. revolution i.e. 45 deg. steps.
2 1 N N N 1 S S b a S N 1 S 2 Winding number 1 2 a 6 pole rotor Winding number 2 b One step
Six pole rotor, two electro magnets How many steps are required for one complete revolution?
Practical Stepper motor operation The top electromagnet (1) is turned on, attracting the nearest teeth of a gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from electromagnet 2 The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the nearest teeth slightly to the right. This results in a rotation of 3.6 in this example.
The bottom electromagnet (3) is energized; another 3.6 rotation occurs. The left electromagnet (4) is enabled, rotating again by 3.6. When the top electromagnet (1) is again enabled, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example.
Stepper motor applications Stepping Motor to move read-write head
Stepper motor applications Paper feeder on printers Stepper motors CNC lathes
Stator coils Rotor CNC Stepping Motor
Control sequence to turn a stepper motor + CW Step 1 0 0 1 1 Step 2 1 0 1 0 Step 3 1 1 0 0 Step 4 0 1 0 1 CCW
Advantages / Disadvantages Advantages:- Low cost for control achieved Ruggedness Simplicity of construction Can operate in an open loop control system Low maintenance Less likely to stall or slip Will work in any environment Disadvantages:- Require a dedicated control circuit Use more current than D.C. motors High torque output achieved at low speeds
Open Loop Positioning Systems Stepper Motor calculations It uses a stepper motor to rotate the lead screw. A stepper motor is driven by series of electrical pulses generated by MCU. For each pulse the motor rotates a fraction of revolution called Step Angle, it is given by Where, n s = Number of step angles for the motor (an integer). If n p is the pulses received by the motor then angle through which motor rotates is
Stepper Motor calculations Lead Screw is connected to the motor shaft through a gear box. Angle of the lead screw rotation taking the gear ratio into account is given by r g = Gear ratio = A m /A= N m / N N m = RPM of motor, N= RPM of lead Screw The linear movement of worktable is given by p = pitch of lead screw
Stepper Motor calculations Total number of pulses required to achieve a specified x-position increment is calculated by: Where,n s = 360/ α Control pulses are transmitted from pulse generator at a certain frequency which drives the work table at the corresponding velocity. The rotational speed of lead screw depends on the frequency of the pulse train Equation (1) N = RPM of lead screw, f p = frequency of pulse train (Hz, Pulses/sec)
Stepper Motor calculations The table travel speed in the direction of lead screw axis is determined by: Equation (2) Where, V t = Table travel speed (mm/min) f r = Table feed rate (mm/min) p= Lead screw pitch (mm/rev) The required pulse train frequency to drive the table at a specified linear travel rate by combining equations (1) and (2):
2. DC Servo Motor The principle of operation is based on the rotation of an armature winding in a permanently energized magnetic field. The armature winding is connected to a commutator, which is a cylinder of insulated copper segments mounted on the shaft. DC current is passed to the commutator through carbon brushes, which are connected to the machine terminals.
Servo Motor Detail Actuator Reduction gear Position feedback + 5V Potentiometer (closed loop system) Small electric DC motor
3. AC Servo Motor In an AC servomotor, the rotor is a permanent magnet while the stator is equipped with 3-phase windings. The speed of the rotor is equal to the rotational frequency of the magnetic field of the stator, which is regulated by the frequency converter.
CNC Programming Programming consists of a series of instructions in form of letter codes Preparatory Codes: G codes- Initial machining setup and establishing operating conditions N codes- specify program line number to executed by the MCU Axis Codes: X,Y,Z - Used to specify motion of the slide along X, Y, Z direction Feed and Speed Codes: F and S- Specify feed and spindle speed Tool codes: T specify tool number Miscellaneous codes M codes For coolant control and other activities
Programming Key Letters O - Program number (Used for program identification) N - Sequence number (Used for line identification) G - Preparatory function X - X axis designation Y - Y axis designation Z - Z axis designation R - Radius designation F Feed rate designation S - Spindle speed designation H - Tool length offset designation D - Tool radius offset designation T - Tool Designation M - Miscellaneous function
Table of Important G Codes G codes are instructions describing machine tool movement G00: Rapid Transverse G01: Linear Interpolation G02: Circular Interpolation, CW G03: Circular Interpolation, CCW G17: XY Plane, G18: XZ Plane,G19: YZ Plane G20/G70: Inch units G21/G71: Metric Units G40: Cutter compensation cancel G41: Cutter compensation left G42: Cutter compensation right
G43: Tool length compensation (plus) G44: Tool length compensation (minus) G49: Tool length compensation cancel G80: Cancel canned cycles G81: Drilling cycle G82: Counter boring cycle G83: Deep hole drilling cycle G90: Absolute positioning G91: Incremental positioning
Table of Important M codes M Codes are instructions describing miscellaneous functions like calling the tool, spindle rotation, coolant on/off etc.,
Basic CNC program for turning operation % N10 T104 M06 N20 G97 S2000 G95 F0.1 M03 N30 G00 X18 Z2 M08 N40 G01 Z-22N50 G01 X26 N60 G00 X200 Z200 M09 N70 M30
Basic CNC program for turning operation O0001 N5 M12 N10 T0101 N15 G90 N20 G0 X50 Z50 N25 M3 S600 N30 M8 N35 G1 X50 Z0 F600 N40 Z-30 F200 N45 X100 Z-50 F150 N50 G0 X50 Z50 N55 T0100 N60 M5 N65 M9 N70 M13 N75 M30 N80 %
Basic CNC program for milling operation N5 G90 G71 N10 T1 M6 N15 X-100 Y86 Z95 N20 G00 X0 Y0 S2500 M3 N25 Z12.5 N30 G01 Z-12.5 F150 N35 X-20 Y30 N40 G02 X10 Y100 R80 N45 G01 X140 Y60 N50 G02 X150 Y0 R50 N55 G01 X0 Y0 N60 G00 Z12.5 N65 G91 G28 Z0 M5 N70 G91 G28 X0 Y0 N75 M30
Basic CNC program for milling operation: Circular interpolation N2 G17 G71 G90 G94 G54 N4 T1 L90 N6 G00 Z5 D5 M3 S500 X20 Y90 N8 G01 Z-2 F50 N10 G02 X60 Y50 I0 J-40 or, N10 G02 X60 Y50 R40 N12 G03 X100 Y50 I20 J0 or, N12 G03 X100 Y50 R20 N14 G00 Z100 N16 M02
Basic CNC program for drilling operation N1 T16 M06 N2 G90 G54 G00 X0.5 Y-0.5 N3 S1450 M03 N4 G01 Z-0.375 F9 N5 G00Z 5 N6G00 X 1.5Y-0.5 N7G01Z-0.375 F9 N8 G00Z 5 N9 G00 X1.5 Y-1.5 N10Z-0.375F9 N11 G00Z 5 N12 G00X0.5Y-1.5 N13 Z-0.375F9 N14 G00 Z5 X0Y0 N15 M30
Sample problem: Milling G90 Absolute X Y G91 Increment X Y P0 X0 Y0 P0 X0 Y0 P0-P1 X-20 Y-20 P0-P1 X-20 Y-20 P0-P2 X12 Y-20 P1-P2 X32 Y0 P0-P3 X20 Y-12 P2-P3 X8 Y8 P0-P4 X20 Y14 P3-P4 X0 Y26 P0-P5 X12 Y20 P4-P5 X-8 Y6 P0-P6 X-20 Y20 P5-P6 X-32 Y0
Exercise: Write CNC program for the following sequence of milling operation
Thank You