COMPUTER NUMERICAL CONTROL OF MACHINE TOOLS

Transcription

1 COMPUTER NUMERICAL CONTROL OF MACHINE TOOLS Department of Mechanical Engineering and Aeronautics University of Patras, Greece Dr. Dimitris Mourtzis Associate professor Patras,

2 Chapter 2: Numerical Control Systems 2.2

3 Table of Contents Chapter 2: Numerical Control Systems Types of Control Systems in use on NC Equipment Drive systems used on CNC machinery The Cartesian Coordinate System Motion Directions of a CNC Milling Machine Absolute & Incremental Positioning Setting the Machine Origin Dimensioning Methods

4 Objectives of Chapter 2 Describe the two types of control systems in use on NC equipment Name the four types of drive motors used on NC machinery Describe the two types of loop systems used Describe the Cartesian coordinate system Define a machine axis Describe the motion directions on a three-axis milling machine Describe the difference between absolute and incremental positioning Describe the difference between datum and delta dimensioning 2.4

5 CNC Components A CNC machine consists of two major components: CNC components The Machine-Tool Controller Machine Control Unit (MCU) MCU is an on-board computer MCU and Machine Tool may be manufactured by the same company (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.5

6 Controllers CNC controllers manufacturers Fanuc Bridgeport Haas Cincinnati Milacron Mitsubishi Siemens Figure 2-1(a): A Fanuc CNC Controller (Source: 2.6

7 Controllers CNC controllers manufacturers market share Haas 14% Figure 2-1(b): CNC controllers manufacturers market share survey, FANUC owns the largest share 26% (Source: cnccookbook.com) 2.7

8 Controllers Each MCU is manufactured with a standard set of build in codes Other codes are added by the machine tool builders Program codes vary somewhat from machine to machine Every CNC machine is a collection of systems coordinated by the controller 2.8

9 Types of Control Systems There are two types of control systems used on CNC machines: Control Systems Point to Point Systems Continuous Path Systems 2.9

10 Point to Point machines: I. Move in straight lines Types of Control Systems II. They are limited in practical sense to hole operations: Drilling Reaming Boring etc III. IV. Straight milling cuts parallel to a machine axis When making an axis move all affected drive motors run at the same speed Cutting of 45 o angles is possible BUT not angles or arcs other than 45 o angles (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.10

11 Point to Point machines example Move to (X1, Y1) Move to (X2, Y1) Move to (X3, Y3) where X3 < X3 Move to (X3, Y3) Move to (X4, Y4 ) and move to (X4, Y4) Types of Control Systems (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.11

12 Types of Control Systems Point to Point machines: Angle and arc segments must be programmed as a series of straight line cuts Figure 2-2:Point to point angles and arcs (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.12

13 Types of Control Systems Movement of Tools example Point-to-point: The drill bit drills a hole at position 1, then is retracted and moved to position 2 and so on Figure 2-4: Point to Point tool movement (Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid) 2.13

14 Types of Control Systems Continuous - Path machines: Have the ability to move the drive motors at varying rates of speed while positioning the machine The cutting of arc segments and any angle can be easily accomplished Figure 2-3: Continuous path angles and arcs (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.14

15 Types of Control Systems Movement of Tools example Continuous path by a milling cutter Figure 2-5:Continuous Path tool movement (Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid) 2.15

16 Types of Control Systems Point to Point Machines where common Their electronics where less expensive to produce The machine tools where less expensive to acquire Technological advancements have narrowed the cost difference between point to point and continuous path machines Most CNC machines now manufactured are of continuous path type 2.16

17 Servomechanisms The drive systems used on NC machinery: STEPPER motors DC (Direct Current) servos AC (Alternating Current) servos Hydraulic servos Figure: The machine control sends a motion signal, to a servomotor attached to each machine axis. This causes the servomotor to rotate a ball screw attached to the table or column, causing it to move. ( 2.17

18 STEPPER Motors Move a set amount of rotation (a step) every time the motor receives an electronic pulse DC and AC servos Widely used variable-speed motors on small & medium continuous path machines A servo does not move a set distance When current is applied the motor starts to turn and when the current is removed the motor stops turning The AC motor can create more power than a DC motor used on CNC Machining Centers HYDRAULIC servos Are variable-speed motors Servomechanisms Produce much more power than an electric motor They are used on large CNC machinery with electronic or pneumatic system attached 2.18

19 Controllers Virtual Model of Trajectory Generation and Axes Control CL :"Cutter Location file "Jerk" is the time rate of change of acceleration. Jerk ramps the acceleration to smooth the velocity Article From: 9/9/2005 Modern Machine Shop, Norman Bleier, Siemens engineering manager (Moriwaki T., Multi-Functional Machine Tool,2008) 2.19

20 Loop Systems Loop systems are electronic feedback systems that send and receive electronic information from the drive motors Loop Systems Open Loop Closed Loop The type of system used affects the overall accuracy of the machine Open Loop use Stepper Motors Closed Loop usually use Hydraulic, AC and DC Servos 2.20

21 Loop Systems For Controlling Tool Movement Input Media Machine Control Unit Open Loop System: The machine receives its information from the reader and stores it in the storage device Reader Drive Motor Storage Memory Motor Controller When the information is needed it is sent to the drive motor (s) After the motor has completed its move a signal is sent back to the storage device telling it that the move has been completed and the next instruction may be received There is no process to correct for error induced by the drive system Figure 2-7: An Open Loop system 2.21

22 Loop Systems For Controlling Tool Movement Open Loop System MCU Figure 2-6: An open-loop control system for a numerical-control machine (Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid) 2.22

23 Loop Systems For Controlling Tool Movement Open Loop System An open loop system utilizes stepping motors to create machine movements. These motors rotate a fixed amount, usually 1.8, for each pulse received. Stepping motors are driven by electrical signals coming from the MCU. The motors are connected to the machine table ball-nut lead screw and spindle Upon receiving a signal, they move the table and/or spindle a fixed amount. The motor controller sends signals back indicating the motors have completed the motion The feedback, however, is not used to check how close the actual machine movement comes to the exact movement programmed 2.23

24 Loop Systems For Controlling Tool Movement Figure 2-9: A Closed Loop system Closed Loop System: The machine receives its information from the reader and stores it in the storage device When the information is sent to drive motor the motor s position is monitored by the system and compared to what was sent If an error is detected the necessary correction is sent to the drive system If the error is large the machine may stop executing the program for correcting the inaccuracy Most errors produced by the drive motors are eliminated Advanced Stepper Motors make possible extremely accurate Open Loop Systems and less HW 2.24

25 Loop Systems For Controlling Tool Movement Closed Loop System DAC : digital-to-analog converter MCU Figure 2-8: A closed-loop control system for a numerical-control machine (Source: Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid) 2.25

26 Loop Systems For Controlling Tool Movement Closed Loop System Special motors called servos are used for executing machine movements in closed loop systems Motor types include AC servos, DC servos, and hydraulic servos. Hydraulic servos, being the most powerful, are used on large CNC machines. AC servos are next in strength and are found on many machining centers A servo does not operate like a pulse counting stepping motor. The speed of an AC or DC servo is variable and depends upon the amount of current passing through it The speed of a hydraulic servo depends upon the amount of fluid passing through it. The strength of current coming from the MCU determines the speed at which a servo rotates (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.26

27 Coordinate Systems Geometrical means of communication between the operator and digitally driven machine-tool Univocal characterization of a point in the plane or in space relative to a fixed point Absolute coordinates Relative coordinates Figure 2-10:The simplest example of a coordinate system (en.wikipedia.org/) 2.27

28 The Cartesian Coordinate System The Cartesian Coordinate System is defined by two orthogonal axes which intersect at a point X Axis: Abscissa axis Y Axis: Ordinate axis Figure 2-11 : The Cartesian Coordinate System definition, X&Y axis (Modern methods of material processing andprogramming with PC, D. Mourtzis et al.) 2.28

29 The Cartesian Coordinate System Z Axis: Applicate axis Figure 2-12 : The Cartesian Coordinate System definition, Z axis (Modern methods of material processing and programming with PC, D. Mourtzis et al.) 2.29

30 The Cartesian Coordinate System Cartesian Coordinate System: Is divided into quarters (quadrants): I, II, III, IV in counter-clockwise direction This is the universal way of labelling axis quadrants The signs of X and Y change when moving from quadrant to quadrant Figure 2-13: Cartesian coordinate quadrants (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.30

31 The Cartesian Coordinate System Cartesian Coordinate System: Points on a two-axis Cartesian system: Each of the points can be defined as a set of coordinates (X, Y) In mathematics this set of points is called an ordered pair In NC programming the points are referred as coordinates Cartesian coordinates will be used in writing NC programs Figure 2-14: Cartesian coordinates 2.31

32 Quadrants The Cartesian Coordinate System Quadrants in the plane Quadrants in the space Figure 2-15:Positive and negative quadrants on a plane and in the space (Modern methods of material processing and programming with PC, D. Mourtzis et al.) 2.32

33 The Cartesian Coordinate System Rule of right hand rotation Figure 2-16: The Electronic Industries Association & Aerospace Industries of America established the right hand rule of coordinates. The right hand rule determines positive direction of each axis from the base of the finger to the tip 2.33

34 The Cartesian Coordinate System Transferring coordinates Scheduling cutting, based on the coordinate system with axes starting at the zero point of the piece W Transfer coordinates in the coordinate system of the machine-tool axes beginning from the zero point of the machine M Need to define the coordinate system of the user in connection with the zero point or the reference point of the machine Figure 2-17:Coordinate Systems Transfer (Modern methods of material processing and programming with PC, D. Mourtzis et al.) 2.34

35 The Cartesian Coordinate System Rotation of a Coordinate System Rotation can be made on any axis If the rotation is made on more than one axis, or with different angles of the 90 o, 180 o and 270 o using functions transformation of coordinates is required Scheme 1 Scheme 2 Figure 2-18: Rotation around the Z axis by -90 o (Modern methods of material processing and programming with PC, D. Mourtzis et al.) 2.35

36 The Cartesian Coordinate System The Cartesian Coordinate System in machines The basis for all machine movement is the Cartesian Coordinate system On a machine tool an axis is a direction of movement In a Two Axis Milling Machine: X is the direction of the Table travel Y is the direction of the Cross travel (W. S. Seames Computer Numerical Control: Concepts and Programming) Figure 2-19: Directions of movement on a machine ( 2.36

37 The Cartesian Coordinate System The Cartesian Coordinate System in machines Three Axis Milling machine: In a Three Axis Vertical Milling Machine: X is the direction of the Table travel Y is the direction of the Cross travel Z the Spindle travel up down Figure 2-20: Three Axis vertical mill 2.37

38 The Cartesian Coordinate System The Cartesian Coordinate System in machines Six Axis Milling machine: In a Six Axis Vertical Milling Machine: X is the direction of the Table travel Y is the direction of the Cross travel Z the Spindle travel up down A is the rotation around X axis B is the rotation around Y axis C is the rotation Z axis (spindle) Figure 2-21: Six Axis machine layout 2.38

39 Positive and Negative Movement Machine axis direction is defined in terms of spindle movement On some axes the machine slides actually move; on other axes the spindle travels For standardization the positive and negative direction for each axis is always defined as if the spindle did the travelling The arrows saw the positive and negative direction of spindle movement along axes Example To make a move in the +X direction (spindle right) the table would move to the left To make a move in the +Y direction (spindle toward the column) the saddle would move away the column The Z-axis movement is always positive (+Z) when the spindle moves towards the machine head and negative ( Z) when it moves toward the workpiece 2.39

40 The Polar Coordinate System The polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a fixed point and an angle from a fixed direction sin cos Arc tan Figure 2-23 :The Polar coordinate system (Modern methods of material processing and programming with PC, D. Mourtzis et al.) 2.40

41 Positioning Systems There are two ways that machines position themselves with respect to their coordinate systems : Positioning Systems Absolute Positioning Incremental Positioning 2.41

42 Positioning Systems Absolute Positioning: All machine locations are taken from one fixed zero point All positions on the part are taken from the (X0, Y0) point at the lower left corner of the part The 1 st hole will have coordinates of (X1.000, Y1.000) The 2 nd hole will have coordinates of (X2.000, Y1.000) The 3 rd hole will have coordinates of (X3.000, Y1.000) ZERO REFERENCE POINT FOR A MOVE TO ANY LOCATION Figure 2-24: Absolute positioning Every time the machine moves the controller references the lower left corner of the part (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.42

43 Positioning Systems ZERO LOCATION FOR A MOVE FROM HOLE #1 TO HOLE #2 ZERO LOCATION FOR A MOVE FROM HERE TO HOLE #1 Figure 2-25: Incremental positioning ZERO LOCATION FOR A MOVE FROM HOLE #2 TO HOLE #3 Incremental Positioning: The (X0, Y0) point moves with the machine spindle Each position is specified in relation to the previous one The 1 st hole coordinates are: (X1.000, Y1.000) The 2 nd hole coordinates are (X1.000, Y0) The 3 rd hole coordinates are (X1.000, Y0) After each machine move the current location is reset to (X0, Y0) for the next move The coordinate system moves with the location and the machine controller does not reference any common zero point 2.43

44 Positioning Systems THIS POINT IS ZERO FOR A MOVE FROM HOLE #1 TO #2 THIS POINT IS ZERO FOR A MOVE FROM HOLE #2 TO #3 ZERO POINT FOR A MOVE TO ANY LOCATION INITIAL ZERO POINT, (THIS POINT IS ZERO FOR A MOVE FROM HERE TO HOLE #1) Figure 2-26(a):Relationship of the Cartesian Coordinate system to the part when using absolute positioning Figure 2-26(b):Relationship of the Cartesian coordinate system to the part when using incremental positioning 2.44

45 Dimensioning Methods In conjunction with NC machinery there are two types of dimensioning practices used on blueprints : Dimensioning Methods Datum Dimensioning Delta Dimensioning These two dimensioning methods are related to absolute and incremental positioning 2.45

46 Dimensioning Methods Datum Dimensioning All dimensions on a drawing are placed in reference to one fixed zero point Is ideally suited to absolute positioning equipment All dimensions are taken from the corner of the part Figure 2-28: A datum dimensioned drawing 2.46

47 Dimensioning Methods Delta Dimensioning Dimensions placed on a Delta Dimensioned drawing are chainlinked Each location is dimensioned from the previous one Delta drawings are suited for programming incremental positioning machines Figure 2-29: A delta dimensioned drawing It is not uncommon to find the two methods mixed on one drawing 2.47

48 Dimensioning Methods Example of the two methods mixed on one drawing Delta Dimensioning Datum Dimensioning Figure 2-30: I-phone drawing (osxdaily.com) 2.48

49 Setting the Machine Origin Machine Coordinate System Most CNC machinery have a default coordinate system assumed during power-up the Machine Coordinate System The origin of this system is called the Machine Origin or Home Zero Location Home Zero is usually located at the Tool Change position of a Machining Center ( 2.49

50 Programmer Coordinate System Setting the Machine Origin A part is programmed independently of the Machine Coordinate System The programmer can pick a location on the part or fixture becoming the origin of the coordinate system for that part The programmer s coordinate system is called the Local or Part Coordinate System The Machine and Part Coordinate System will almost never coincide Prior running the part program the coordinate system must be transferred from the machine system to part system, known as setting ZERO POINT ( 2.50

51 Setting the Machine Origin There are three ways a ZERO POINT can be set on CNC machines ZERO POINT Setting Manual Setting Work Coordinates Absolute Zero Shift 2.51

52 Setting the Machine Origin Manual Setting The set-up person (technician) positions the spindle over the desired part zero Zero out the coordinate system on the Machine Control Unit (MCU) console The actual coding for accomplishing zero out varies depending on the Machine Control Unit (MCU) 2.52

53 Setting the Machine Origin Absolute Zero Shift An Absolute Zero Shift is a transfer of the coordinate system inside the NC program First: the programmer commands the spindle to the Home Zero Location Next: a command is given that tells the MCU how far from the Home Zero Location the Coordinate System Origin is to be located An Absolute Zero Shift is given as follows: (Send the spindle to home zero) N010 G28 X0 Y0 Z0 (Set the current spindle position) (To X5.000 Y6.000 Z7.000) N020 G92 X5.000 Y6.000 Z7.000 Line 0N10: Spindle moves to Home Zero Line 0N20: The location of the spindle became X5.0, Y6.0, Z7.0, for MCU The machine will now reference the Part Coordinate System G28 - Return to reference point G92 Program absolute zero point If more than a fixture is to be used on a machine, the programmer will use more than one part coordinate system send spindle back to home zero G28 X0, Y0, Z0 then G92 Line (W. S. Seames Computer Numerical Control: Concepts and Programming) 2.53

54 Setting the Machine Origin Work Coordinates A work coordinate is a modification of the absolute zero shift Work coordinates are registers in which the distance from home zero to the part zero can be stored The part coordinate system does not take effect until the work coordinate is commanded in the NC program When using G92 zero shifts, the coordinate system changes to the part coordinate system When using work coordinates a register can be set at one place in the program and called at another If more than one fixture is used a second part zero can be entered in a second work coordinate and called up when needed The work coordinate registers can be set manually by the operator or by the NC programmer without having to send the spindle to the home zero location This saves program cycle time by eliminating the moves to home zero 2.54

55 Setting the Machine Origin Work Coordinates Example Figure 2-27 shows a part gripped in a vise The outside dimensions of the part have already been milled to size on a manual machine before being set on the CNC machine The CNC is used to make the holes, pockets, and slot in this part The Work Coordinates are located in the upper-left corner of the block Figure 2-27 :A part gripped in a vise (Adapted by

56 Setting the Machine Origin Work Coordinates Are set and called in a program by commands called G-codes: G54, G55 and G56 G10 L2 offsets the origin of the axes (Set Coordinate System Command) An example of using the Work Coordinate System (WCS): (Set work coordinate P1-which is G54) (and work coordinate P2-which is G55) N010 G10 L2 P1 X5.000 Y6.000 Z7.000 N020 G10 L2 P2 X Y3.000 Z (Call work coordinate G54 and move) (To X1.000 Y1.000 Z0.500) N100 G54 X1.000 Y1.000 Z.500 (Call work coordinate G55 and move) (To X2.000 Y2.000 Z3.000) N110 G55 X2.000 Y2.000 Z3.000 Line N010: the G54 WCS is set to X5.0, Y6.0, Z7.0, from the zero home location Line N20: the G55 WCS is set to X10.0, Y3.0, Z15.0 from home zero Line N100: the G54 WCS is called activating the part coordinate system moving the spindle to X1.0, Y1.0, Z0.5 as referenced from the part activated part coordinate system Line N110: the G55 WCS is called activating the second part coordinate system moving the spindle to X2.0, Y2.0, Z3.0 as referenced from the part activated part coordinate system Work coordinates remain active until cancelled by another work coordinate 2.56

57 Summary 1/2 The two types of NC control systems are point-to-point and continuous path The four types of drive motors used on NC equipment are stepper motors, AC servos, and hydraulic servos Loop systems are electronic feedback systems used to help control machine positioning. There are two types of loop systems: open and closed. Closed-loop systems can correct errors induced by the drive system; open loop system cannot The basic of machine movement is the Cartesian Coordinate system. Any point on the Cartesian coordinate system may be defined by X/Y or X/Y/Z coordinates An absolute positioning system locates machine coordinates relative to a fixed datum reference point In an incremental positioning system, each coordinate location is referenced to the previous one 2.57

58 Summary 2/2 The machine coordinate system can be transferred to the part coordinate system manually, by an absolute zero shift, or by use of work coordinates The positive or negative direction of an axis movement is always thought of as spindle movement Machine movements occur along axes that correspond to the direction of travel of the various machine slides. On a vertical mill, the Z axis of a machine is always the spindle axis. The X and Y axes of a machine are perpendicular to the Z axis, with X being the axis of longer travel There are two dimensioning systems used on part drawings intended for numerical control: datum and delta. Datum dimensioning references each dimension to a fixed set of reference points; delta dimensioning references each dimension to the previous one 2.58

59 Vocabulary Introduced in this Chapter Absolute positioning Absolute zero shift Cartesian coordinate system Closed-loop system Continuous-path systems Datum dimensioning Delta dimensioning Incremental positioning Machine Control Unit (MCU) Point-to-point systems Open-loop system Work coordinates 2.59

60 References 1. Ann Mazakas, Manager of Technical Communications, DP Technology, The Art of Automation, courses materials of ESPTRIT World Conference on CNC 2. Chryssolouris G., «Manufacturing Systems: Theory and Practice», 2nd Edition, 2006, Springer-Verlag Kalpakjian S., «Manufacturing Engineering and Technology», 2nd Edition, 1992, Addison-Wesley Publishing company 10. Manufacturing, Engineering & Technology, Fifth Edition,S. Kalpakjian and S. R. Schmid 11. Mattson M., CNC Programming, Principles and Applications, Delmar, Moriwaki T., Multi-Functional Machine Tool, CIRP Annals Manufacturing Technology, Vol. 57/2, 2008, pp

61 References 13. Seams W., Computer Numerical Control, Concepts & Programming, 4th Edition, Delmar, Norman Bleier, Siemens engineering manager Article From: 9/9/2005 Modern Machine Shop, 15. Γ. Χρυσολούρης, «Συστήματα Παραγωγής Θεωρία και Πράξη» Μέρος Ι και ΙΙ, Εκπαιδευτικές Σημειώσεις, Πανεπιστήμιο Πατρών, 2001, 16. Γ. Χρυσολούρης, Δ. Μούρτζης, Κ. Τσίρμπας, Σ. Καραγιάννης, Ορθογωνική Κοπή, Εκπαιδευτικές Σημειώσεις, Πανεπιστήμιο Πατρών, Γ. Χρυσολούρης, Δ. Μούρτζης, και άλλοι, Εργαστήρια Μηχανουργικής Τεχνολογίας Ι και ΙI», Εκπαιδευτικές Σημειώσεις για το εργαστήριο του αντιστοίχου μαθήματος, Πανεπιστήμιο Πατρών, 2008 (4η Έκδοση) 18. Δ. Μούρτζης, Αριθμητικός Έλεγχος Εργαλειομηχανών Εκπαιδευτικές Σημειώσεις, Πανεπιστήμιο Πατρών, 2011 (3η Έκδοση) 19. Πετρόπουλου Π.Γ., «Μηχανουργική Τεχνολογία ΙΙ. Τεχνολογία κατεργασιών κοπής των μετάλλων», 1998, Εκδόσεις Ζήτη 20. Σύγχρονες μέθοδοι κατεργασίας υλικών και προγραμματισμός με Ηλεκτρονικό Υπολογιστή (Η/Υ),Δ. Μούρτζης,Κ. Σαλωνίτης 2.61