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Calibration of Coordinate Measuring Machines (CMM)

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This presentation is made in an effort to impart information regarding the techniques used for the calibration of coordinate measuring machines. These versatile machines are today being used for the inspection of very precise and accurate mechanical components manufactured by keeping in view advanced geometrical dimensioning and tolerancing techniques.

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Calibration of Coordinate Measuring Machines (CMM)

  1. 1. Calibration of CMMs Using E&R Test and Laser Interferometer By: Hassan Habib
  2. 2. Things we are going to learn • About Coordinate Measuring Machines (CMMs) • Measurements • What is a CMM? • How does a CMM work? • Calibration • What is calibration? • Calibration standards for CMM • Calibration procedure for CMM • E&R Test • Introduction • Definition of errors & Material Standard • Choice of Artifacts • Preliminary Setup • Measurements for error in length • Calculation of results • Probing test error
  3. 3. Things we are going to learn • Laser Interferometery • Introduction • Principle of Laser Interferometers • Widely used interferometers in the market • Components of Laser interferometer • Preparing the machine for calibration • Definition of geometrical deviations to be measured • Setting up the laser • Set up the measurements/optics • Collect the data and analyze according to international standards • Compensation of results • Some Post Checks
  4. 4. • Beauty is in the perfection of creation. Humans have been creating since their presence on earth • However, accuracy of these measurements has evolved over time and so has the beauty they have created • The earliest examples of accuracy can be found in the construction of Great Pyramids (The difference between height of two opposite corners at its base is 13 mm.) About Coordinate Measuring Machines (CMM) Measurements
  5. 5. About Coordinate Measuring Machines (CMM) Measurements Evolution of Measuring Instruments Cubit Micrometer Gauge Blocks Length Comparator Gauges and Dial Indicators Visual Inspection machines Universal Measuring Machines CMM
  6. 6. About Coordinate Measuring Machines (CMM) What is a CMM? • Modern machines used for very accurate and precise measurements • CMM works on the principle of Coordinate Measuring i.e. measurement based on collection of data points taken in a Cartesian Coordinate System • “The primary function of a CMM is to measure the actual shape of a work piece, compare it against the desired shape, and evaluate the metrological information such as size, form, location, and orientation.” [Ref: Read ‘Notes’]
  7. 7. About Coordinate Measuring Machines (CMM) What is a CMM? • CMM can measure complex geometrical tolerances and deviations on manufactured parts • The accuracy of these machines today are closer to 1µm • CMMs are capable of measuring point to point coordinates and they can also analyze continuous data points using advanced touch probes (SP600 and PH20) • CMMs are extensively used in Aerospace and automotive sectors where increased accuracy of measurement is required Features
  8. 8. About Coordinate Measuring Machines (CMM) How does CMM work?
  9. 9. About Coordinate Measuring Machines (CMM) How does CMM work? Types of CMMs
  10. 10. Calibration What is calibration? • It is the process of verifying and adjusting the accuracy of Measuring & Monitoring Equipment (MME) and machines by comparing them with standards of known accuracy. • The adjustment of instruments is performed by compensating the errors into the instrument. • OEM (Original Equipment Manufacturer) of the instrument usually defines calibration interval, environment of usage and tolerance limits within which the instrument will conform to its performance standard. • It is performed in regular intervals so as to ensure that the instrument is reliable.
  11. 11. Calibration What is calibration? • Calibration provides the confidence that their accuracy is as per the given specifications of OEM. • It ensures the repeatability of the measurements taken by the equipment. • The uncertainty is kept at minimum level further building the confidence of measurements. • It is performed in regular intervals so as to ensure that the instrument is reliable. • Calibrating instruments through certified bodies increases the confidence level of customers for your organization. Importance of Calibration
  12. 12. Calibration Calibration of CMMs • The calibration of CMM benefits as per the stated benefits of calibration. • For a CMM there can be number of sources of error that will remain undetected thereby nullifying usability. • The calibration of CMM is performed according to the guidelines provided in ISO 10360-2. • National Physics Laboratory (NPL) has also provided standard procedures for verification of performance level of CMMs by detailing guidelines. These guidelines are provided to perform E&R test on the machine using organizational standards used for calibration.
  13. 13. Calibration Calibration of CMMs Calibration Standards for CMMs
  14. 14. Calibration Calibration of CMMs • In the scope of this presentation we are going to study two standards of calibration of CMMs: • E&R test for verification of length measurements • Verification and Compensation of geometric errors with Laser Interferometry
  15. 15. Calibration Calibration Procedure for CMMs Choice of Artifacts Preliminary Setup Length Measurement Calculation of results Probing Test Procedure of verification for length measurement through E&R test
  16. 16. Calibration Calibration Procedure for CMMs Procedure of verification for geometric deviations through Laser Interferometer
  17. 17. E&R Test Introduction • The tests help in demonstrating traceability to national standards and estimating the accuracy of measurements made with three dimensional CMMs for maintaining confidence and reliability in the measurements. • The tests also indicate health of the machine that is necessary to perform maintenance.
  18. 18. E&R Test Introduction • Wear of components - the guide ways, the scales, the probe system and the qualification sphere; • Environment in which the CMM operates - the ambient temperature, temperature gradients, humidity and vibration; • The probing strategy used – the magnitude and direction of the probe force, the type of probe stylus used and the measuring speed of the probe; and • Characteristics of the workpiece – elasticity, surface roughness, hardness and the mass of the component Possible sources of error in CMM
  19. 19. E&R Test Definition of Errors & material standard • ISO 10360 strongly recommended that the material standard should be either a step gauge, end bar or a series of gauge blocks conforming to ISO 3650 • The material standard of size used for the tests must be calibrated. • The uncertainty of calibration must be taken into consideration and the calibrations must be traceable to the relevant national standard.
  20. 20. E&R Test • It is the term that specifies the length measuring accuracy of their CMM. EMPE,L is defined as the extreme value of the error of indication of a CMM for size measurement, permitted by specifications, regulations etc. • It is measured in one of the following ways: • a) EL,MPE = ± minimum of (A + L/K) and B • b) EL, MPE = ± (A + L/K) • c) EL, MPE = ± B where A is a positive constant, expressed in micrometres and supplied by the manufacturer; K is a dimensionless positive constant supplied by the manufacturer; L is the measured size, in millimetres; and B is the maximum permissible error a) b) c) Definition of Errors & material standard
  21. 21. E&R Test • The E&R test involves two types of measurement errors. • Volumetric length measuring error E • It applies to all measurements of distances, diameters, and positional tolerances. • Volumetric Probing Error P • It applies to all Form measurements of straightness, flatness, Cylindricity, roundness and free form tolerances. Definition of Errors & material standard
  22. 22. E&R Test Choice of artifacts • The choice of artifacts depend on the recommendations from the manufacturer and or the size of you CMM. Various types of artifacts are available for verification and re-verification, and interim check tests, some of them are: • Step Gauge • Length bar • Ball Plate • Hole Plate • Purpose made test piece
  23. 23. E&R Test Definition of Errors & material standard Comparison between different artifacts
  24. 24. E&R Test Preliminary Setup • Below are some of the pre-requisites for performing the E&R test: • CMM must be operated in accordance with the procedure stated in the instruction manual including machine start up, probe qualification and probe configuration. • Manufacturer supplied test sphere must be used. • Limits for permissible environmental conditions, such as temperature conditions, air humidity and vibration that influence the measurements are usually specified by the manufacturer • Cleaning of stylus tip; • Thermal stability of the probing system • Weight of stylus system and/or probing system; and location, type, number of thermal sensors
  25. 25. E&R Test • For the E test a set of 5 length gauges is measured three times in 7 spatial positions. • Total number of measurements: • 3 x 5 x 7 = 105 • 100 % of results must be in the specified limits • The seven spatial positions are: • Along x-axis • Along y-axis • Along z-axis • Along s-partial 1 (Diagonal in XY) • Along s-partial 2 (Diagonal in YZ) • Along s-partial 3 (Diagonal in XZ) • Along s-partial 4 (Diagonal in XYZ) Measurements for error in length E Test Sample positions
  26. 26. E&R Test Measurements for error in length E Test Measuring Lines for 7 spatial locations
  27. 27. E&R Test Calculation of results • For each of the 105 measurements the error of length measurement, EL is calculated. • Its value is the absolute value of the difference between the indicated value of the relevant test length and the true value of the material standard. • Sample results are shown:
  28. 28. E&R Test Calculation of results
  29. 29. E&R Test Calculation of results
  30. 30. E&R Test Calculation of results Graphical representation of the results
  31. 31. E&R Test Calculation of results • From the results it can be seen that some of the thirty- five test lengths have values of the error of length measurement • These values will have to be measured again ten times each at the relevant configuration Interpretation of the results
  32. 32. E&R Test Probing Test Error • This test of the CMM probing system is used to establish whether the CMM is capable of measuring within the manufacturer‘s stated value of PFTU, MPE by determining the range of values of the radial distance r when measuring a reference sphere. Where, P: associated with the probing system F: apparent Form error T: contact probing (that is to say Tactile) U: single (that is to say Unique) • It is advisable to carry out this test before an acceptance or re-verification test. Introduction
  33. 33. E&R Test Probing Test Error • The sphere supplied by the manufacturer for probe qualifying purposes (reference sphere) should not be used for the probing error test. Introduction
  34. 34. E&R Test Probing Test Error • The probing error is a positive constant, the value of which is supplied by the CMM manufacturer. • The test sphere should be between 10 mm and 50 mm diameter. • The test sphere should be mounted rigidly to overcome errors due to bending of the mounting stem. • Twenty-five points are measured and recorded. It is a requirement that the points are approximately evenly distributed over at least a hemisphere of the test sphere. • Their position is at the discretion of the user Procedure
  35. 35. E&R Test Probing Test Error • one point on the pole (defined by the direction of the stylus shaft) of the test sphere; • four points (equally spaced) 22.5° below the pole; • eight points (equally spaced) 45° below the pole and rotated 22.5° relative to the previous group; • four points (equally spaced) 67.5° below the pole and rotated 22.5° relative to the previous group; and • eight points (equally spaced) 90° below the pole (i.e., on the equator) and rotated 22.5° relative to the previous group. Measurements Measurement Pattern
  36. 36. E&R Test Probing Test Error Results
  37. 37. E&R Test Probing Test Error Graphical representation of results
  38. 38. E&R Test Probing Test Error Interpretation of Results • If the range rmax - rmin of the twenty-five radial distances (PFTU) is no greater than the manufacturer‘s stated value of PFTU, MPE when taking into account the measurement uncertainty, then the performance of the probing system is verified
  39. 39. Laser Interferomter Introduction • As stated before laser interferometer is a higher standard of measurement that is used for the calibration of CMM for its geometrical deviations. • The values from this calibration are also used as compensations for the deviations in the machine controller. • Once these compensations are provided to the controller all machine errors are compensated and the machine returns to the factory provided performance standard. • Laser interferometry has slowly evolved into easy to use equipment that can help to perform various tasks. • This type of calibrations all started with Michelson’s Interferometer
  40. 40. Laser Interferomter Principle • The working principle of laser interferometers today used, work on the principle of the Michelson’s Interferometer. • The Michelson interferometer is common configuration for optical interferometry and was invented by Albert Abraham Michelson. • Albert Michelson and Edward Morley performed their famous Michelson-Morley experiment in 1887. Edward Morley
  41. 41. Laser Interferomter Principle • Using a beam splitter, a light source is split into two arms. • Each of those is reflected back toward the beam splitter which then combines their amplitudes interferometrically. • The resulting interference pattern that is not directed back toward the source is typically directed to some type of photoelectric detector or camera. • Depending on the interferometer's particular application, the two paths may be of different lengths or include optical materials or components under test. Interference Patterns from an interferometer Michelson’s Interferometer
  42. 42. Laser Interferomter Principle Michelson’s Interferometer
  43. 43. Laser Interferomter Principle • M is partially reflective, so part of the light is transmitted through to point B while some is reflected in the direction of A. • Both beams recombine at point C' to produce an interference pattern incident on the detector at point E (or on the retina of a person's eye). • If there is a slight angle between the two returning beams, for instance, then an imaging detector will record a sinusoidal fringe pattern. • If there is perfect spatial alignment between the returning beams, then there will not be any such pattern but rather a constant intensity over the beam dependent on the differential path length. Michelson’s Interferometer
  44. 44. Laser Interferomter Principle Modern Laser Interferometer Example of laser measurement
  45. 45. Laser Interferomter Widely used interferometers in the market 5529A XL 80
  46. 46. Laser Interferomter Components of laser interferometer • Laser Head • Environment compensation unit • Material temperature sensors • Power Supply • Air sensors • Tripod stand with stage • Laptop with necessary software • Optics for different measurements Major Components
  47. 47. Laser Interferomter Preparing the machine for calibration Preliminary setup Check the air filters •Including machine filters and air dryer filters Check air tubing for replacement •Wet air would probably require replacement of tubing Check air bearings •They must have specified gap of air cushion Remove rubber pads from base •Removal makes foundation stiff Balance the machine bed •Use inclinometers Note: – You can use the master square to balance the z-axis – Balance the machine on three nodes and remove any redundant rests from the base – There should be no turbulence in air. Turbulence will cause laser error – The environment must be controlled as much as possible.
  48. 48. Laser Interferomter Definition of geometrical deviations to be measured Measurements needed for error mapping • Error mapping is done to calculate the 21 geometric deviations that can occur in an articulating machine including CMM and machining centers. • First these are measured and then compensated using the appropriate software to the controller of the machine. • The deviations are majorly of these types: • Linearity (3) • Straightness (6) • Rotation (9) • Squareness (3)
  49. 49. Laser Interferomter Definition of geometrical deviations to be measured Measurements needed for error mapping
  50. 50. Laser Interferomter Definition of geometrical deviations to be measured Geometrical compensation parameters • Usually the geometric deviations are termed as compensation parameters and are designated values such as: Rxx = Linear straightness in x-axis Ryy = Linear straightness in y-axis Rxy = Horizontal straightness of x-axis Rxz = Vertical straightness of x-axis Dxy = Yaw of x-axis Dxz = Pitch of x-axis . . . . etc.
  51. 51. Laser Interferomter Definition of geometrical deviations to be measured Geometrical compensation parameters • For the complete error map these measurements are to be compensated in the controller of the machine. • Software such as Geocomp are used to upload the valued of these deviations into the controller. • Two of these measurements can be taken from inclinometer. These are the straightness of x-axis and y- axis. • Rest of the measurements are taken from a laser interferometer • The linear straightness of the z-axis can be measured by using two Dzy measurements.
  52. 52. Laser Interferomter Setting up the laser Generic Procedure • Here we will outline a generic procedure to perform the linear measurement on ML10 laser interferometer of Renishaw. It will give us an overview of the procedure. It is similar to that of Agilent laser interferometer.
  53. 53. Laser Interferomter Setting up the laser Generic Procedure • First step is to setup the stage on the tripod stand. Secure and tight it on the stand. • Position the Laser head on the tripod stand with stage. • Arrange the laser interferometer according to the measurement you are about to take. • Switch on the laser. You have to wait for a specified time for the laser to get stabilized.
  54. 54. Laser Interferomter Setting up the laser Generic Procedure • Align and fix the optics for the measurement in our case it will be the linear interferometer that includes linear reflector and linear beam splitter as well as clamp blocks to fix it on the machine head. • Now you have to mount the interferometer on the machine bed and reflector has to be attached on machine spindle. Side surfaces of linear beam splitter and reflector have to be exactly parallel. Note: In our case the linear interferometer stays stationary while the splitter moves along the axis.
  55. 55. Laser Interferomter Setting up the laser Generic Procedure • The shutter of the laser head can be rotated. Rotate it so that the laser leaving the head has a reduced beam diameter. • Now adjust the tripod so as the spirit level of the tripod is at the central position. • Bring closer the reflector to the laser head and observe a white spot target on the front. Now move the machine in the x-axis until the beam hits the target. Shutter at closed position with no laser emitted
  56. 56. Laser Interferomter Setting up the laser Generic Procedure • Now remove white target and check if the beam hits the center of the laser head target on its shutter. If it doesn’t, keep on adjusting the position of the machine until it hits the target at the center. • Now adjust reflector and splitter as close as possible and align them together. • Make sure that the faces are parallel with the one another and with the machine axis.
  57. 57. Laser Interferomter Setting up the laser Generic Procedure • Now use a target at the input aperture with white spot at the top and translate machine axis vertically and horizontally so that the beam hits the target. • Now take away the target and check if the returned beam from the interferometer hits the center of the shutter. If it does not repeat the motion of the machine until it does hit the center.
  58. 58. Laser Interferomter Set up the measurements/optics Introduction • We have elaborated about the 21 measurements needed for complete calibration of CMM. • Majorly these measurements are divided into three groups: – Linearity – Straightness – Rotation and – Squareness • Now we will elaborate the different configurations of the optics that are used to take these measurements
  59. 59. Laser Interferomter Set up the measurements/optics Linearity • Linear measurements are made at multiple points along a machine’s travel path to measure linear displacement and velocity. • It is checked to improve positioning accuracy along an axis for any machine that requires positioning accuracy and velocity control.
  60. 60. Laser Interferomter Set up the measurements/optics Optics required • To make linear measurements following optics are required: – Beam Splitter – Linear Reflectors – Targets – Height adjustment fixtures Renishaw XL 80 Agilent 5529A
  61. 61. Laser Interferomter Set up the measurements/optics Principle
  62. 62. Laser Interferomter Set up the measurements/optics Configuration on machine
  63. 63. Laser Interferomter Set up the measurements/optics Straightness • Straightness measurements evaluate the unwanted side to-side or up-and-down motion of a machine tool’s travel in a specified direction.
  64. 64. Laser Interferomter Set up the measurements/optics Optics required • To make straightness measurements following optics are required: – Straightness reflector – Straightness interferometer – Targets – Height adjustment fixtures Renishaw XL 80 Agilent 5529A
  65. 65. Laser Interferomter Set up the measurements/optics Principle
  66. 66. Laser Interferomter Set up the measurements/optics Configuration on machine
  67. 67. Laser Interferomter Set up the measurements/optics Configuration on machine
  68. 68. Laser Interferomter Set up the measurements/optics Rotation • Angular measurements are made at multiple points along a machine’s travel path to test for rotation about an axis perpendicular to the axis of motion (roll, pitch and yaw) • Unwanted angular motion in a machine tool causes positioning errors that reduce the overall accuracy of your machine.
  69. 69. Laser Interferomter Set up the measurements/optics Optics required • To make straightness measurements following optics are required: – Angular reflector – Angular interferometer – Targets – Height adjustment fixtures Renishaw XL 80 Agilent 5529A
  70. 70. Laser Interferomter Set up the measurements/optics Principle
  71. 71. Laser Interferomter Set up the measurements/optics Configuration on machine
  72. 72. Laser Interferomter Set up the measurements/optics Configuration on machine
  73. 73. Laser Interferomter Set up the measurements/optics Squareness • Angular measurements are made at multiple points along a machine’s travel path to test for rotation about an axis perpendicular to the axis of motion (roll, pitch and yaw) • Unwanted angular motion in a machine tool causes positioning errors that reduce the overall accuracy of your machine.
  74. 74. Laser Interferomter Set up the measurements/optics Optics required • To make squareness measurements following optics are required: – Optical Square – Brackets – Targets – Height adjustment fixtures Renishaw XL 80 Agilent 5529A
  75. 75. Laser Interferomter Set up the measurements/optics Principle
  76. 76. Laser Interferomter Set up the measurements/optics Configuration on the machine
  77. 77. Laser Interferomter Collect the data and analyze according to international standards Parameters to be entered • After the required measurement optics have been setup and is ready to be measured. Usually the following information is entered in the software to begin taking measurements: – Start position for the measurement – End position (Length of the axis of your machine) – Interval (It can be 10, 50, 100) – No. of points – No. of cycles (For bidirectional checks)
  78. 78. Laser Interferomter Collect the data and analyze according to international standards Sample screen for entering parameters
  79. 79. Laser Interferomter Collect the data and analyze according to international standards Sample for measurement screen
  80. 80. Laser Interferomter Collect the data and analyze according to international standards Analyzing data • After the readings have been taken the software shows statistical chart that represents the points that are out of tolerance. • This graphical representation is according to the standard you have chosen. You choose the standard that your OEM has specified for the calibration of the machine.
  81. 81. Laser Interferomter Collect the data and analyze according to international standards Sample for analyzing data
  82. 82. Laser Interferomter Compensation of results • After the data has been analyzed according to a standard, software provides the compensation table for the measurement you have taken. • This information is then fed into the specific software of the machine used for this purpose. E.g Geocomp is used for compensation for Global Image CMM.
  83. 83. Laser Interferomter Compensation of results Example of compensation table
  84. 84. Laser Interferomter Some post checks Post Checks • After all the measurements have been compensated, machine is operated in dry run mode. That checks healthy operation. • E&R test elaborated before is also performed to check that all the measurements of the machine are in the tolerance defined by the macnufacturer
  85. 85. Calibration of CMMs References • Coordinate Measuring Machines and Systems by Robert J. Hocken & Paulo H. Pereira • Leitz 10360-2 guide • NPL Guidelines – Good Practice Guide No. 42 • Agilent 5529A Manual • Renishaw user guide for XL-80 • Wikipedia
  86. 86. Calibration of CMMs

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