Post on 16-Jul-2015
Calibration of CMMs
Using E&R Test and Laser Interferometer
By: Hassan Habib
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
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
• Beauty is in the perfection of creation. Humans have beencreating since their presence on earth
• However, accuracy of these measurements has evolvedover time and so has the beauty they have created
• The earliest examples of accuracy can be found in theconstruction of Great Pyramids (The difference betweenheight of two opposite corners at its base is 13 mm.)
About Coordinate Measuring Machines (CMM)
Measurements
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
About Coordinate Measuring Machines (CMM)
What is a CMM?
• Modern machines used for very accurate and precisemeasurements
• CMM works on the principle of Coordinate Measuring i.e.measurement based on collection of data points taken in aCartesian Coordinate System
• “The primary function of a CMM is to measure the actualshape of a work piece, compare it against the desiredshape, and evaluate the metrological information such assize, form, location, and orientation.” [Ref: Read ‘Notes’]
About Coordinate Measuring Machines (CMM)
What is a CMM?
• CMM can measure complex geometrical tolerances anddeviations on manufactured parts
• The accuracy of these machines today are closer to 1µm
• CMMs are capable of measuring point to point coordinatesand they can also analyze continuous data points usingadvanced touch probes (SP600 and PH20)
• CMMs are extensively used in Aerospace and automotivesectors where increased accuracy of measurement isrequired
Features
About Coordinate Measuring Machines (CMM)
How does CMM work?
About Coordinate Measuring Machines (CMM)
How does CMM work?
Types of CMMs
Calibration
What is calibration?
• It is the process of verifying and adjusting the accuracyof Measuring & Monitoring Equipment (MME) andmachines by comparing them with standards of knownaccuracy.
• The adjustment of instruments is performed bycompensating the errors into the instrument.
• OEM (Original Equipment Manufacturer) of theinstrument usually defines calibration interval,environment of usage and tolerance limits within whichthe instrument will conform to its performance standard.
• It is performed in regular intervals so as to ensure thatthe instrument is reliable.
Calibration
What is calibration?
• Calibration provides the confidence that their accuracyis as per the given specifications of OEM.
• It ensures the repeatability of the measurements takenby the equipment.
• The uncertainty is kept at minimum level furtherbuilding the confidence of measurements.
• It is performed in regular intervals so as to ensure thatthe instrument is reliable.
• Calibrating instruments through certified bodiesincreases the confidence level of customers for yourorganization.
Importance of Calibration
Calibration
Calibration of CMMs
• The calibration of CMM benefits as per the statedbenefits of calibration.
• For a CMM there can be number of sources of error thatwill remain undetected thereby nullifying usability.
• The calibration of CMM is performed according to theguidelines provided in ISO 10360-2.
• National Physics Laboratory (NPL) has also providedstandard procedures for verification of performancelevel of CMMs by detailing guidelines. Theseguidelines are provided to perform E&R test on themachine using organizational standards used forcalibration.
Calibration
Calibration of CMMs
Calibration Standards for CMMs
Calibration
Calibration of CMMs
• In the scope of this presentation we are going to studytwo standards of calibration of CMMs:
• E&R test for verification of length measurements
• Verification and Compensation of geometric errorswith Laser Interferometry
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
Calibration
Calibration Procedure for CMMs
Procedure of verification for geometric deviations through Laser Interferometer
E&R Test
Introduction
• The tests help in demonstrating traceability to nationalstandards and estimating the accuracy of measurementsmade with three dimensional CMMs for maintainingconfidence and reliability in the measurements.
• The tests also indicate health of the machine that isnecessary to perform maintenance.
E&R Test
Introduction
• Wear of components - the guide ways, the scales, theprobe system and the qualification sphere;
• Environment in which the CMM operates - the ambienttemperature, temperature gradients, humidity andvibration;
• The probing strategy used – the magnitude and directionof the probe force, the type of probe stylus used and themeasuring speed of the probe; and
• Characteristics of the workpiece – elasticity, surfaceroughness, hardness and the mass of the component
Possible sources of error in CMM
E&R Test
Definition of Errors & material standard
• ISO 10360 strongly recommended that the materialstandard should be either a step gauge, end bar or aseries of gauge blocks conforming to ISO 3650
• The material standard of size used for the tests must becalibrated.
• The uncertainty of calibration must be taken intoconsideration and the calibrations must be traceable tothe relevant national standard.
E&R Test
• It is the term that specifies the length measuringaccuracy of their CMM. EMPE,L is defined as the extremevalue of the error of indication of a CMM for sizemeasurement, permitted by specifications, regulationsetc.
• 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 = ± BwhereA 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; andB is the maximum permissible error
a)
b)
c)
Definition of Errors & material standard
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 positionaltolerances.
• Volumetric Probing Error P• It applies to all Form measurements of straightness, flatness, Cylindricity,
roundness and free form tolerances.
Definition of Errors & material standard
E&R Test
Choice of artifacts
• The choice of artifacts depend on the recommendationsfrom the manufacturer and or the size of you CMM.Various types of artifacts are available for verificationand re-verification, and interim check tests, some ofthem are:• Step Gauge• Length bar• Ball Plate• Hole Plate• Purpose made test piece
E&R Test
Definition of Errors & material standard
Comparison between different artifacts
E&R Test
Preliminary Setup
• Below are some of the pre-requisites for performing theE&R test:• CMM must be operated in accordance with the
procedure stated in the instruction manualincluding machine start up, probe qualification andprobe configuration.
• Manufacturer supplied test sphere must be used.• Limits for permissible environmental conditions,
such as temperature conditions, air humidity andvibration that influence the measurements areusually 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
E&R Test
• For the E test a set of 5 length gauges is measured threetimes 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
E&R Test
Measurements for error in length
E Test Measuring Lines for 7 spatial locations
E&R Test
Calculation of results
• For each of the 105 measurements the error of lengthmeasurement, EL is calculated.
• Its value is the absolute value of the difference betweenthe indicated value of the relevant test length and thetrue value of the material standard.
• Sample results are shown:
E&R Test
Calculation of results
E&R Test
Calculation of results
E&R Test
Calculation of results
Graphical representation of the results
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 lengthmeasurement
• These values will have to be measured again ten timeseach at the relevant configuration
Interpretation of the results
E&R Test
Probing Test Error
• This test of the CMM probing system is used toestablish whether the CMM is capable of measuringwithin the manufacturer‘s stated value of PFTU, MPE bydetermining the range of values of the radial distance rwhen measuring a reference sphere.Where,
P: associated with the probing systemF: apparent Form errorT: contact probing (that is to say Tactile)U: single (that is to say Unique)
• It is advisable to carry out this test before an acceptanceor re-verification test.
Introduction
E&R Test
Probing Test Error
• The sphere supplied by the manufacturer for probequalifying purposes (reference sphere) should not beused for the probing error test.
Introduction
E&R Test
Probing Test Error
• The probing error is a positive constant, the value ofwhich is supplied by the CMM manufacturer.
• The test sphere should be between 10 mm and 50 mmdiameter.
• The test sphere should be mounted rigidly to overcomeerrors due to bending of the mounting stem.
• Twenty-five points are measured and recorded. It is arequirement that the points are approximately evenlydistributed over at least a hemisphere of the test sphere.
• Their position is at the discretion of the user
Procedure
E&R Test
Probing Test Error
• one point on the pole (defined by the direction of thestylus 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 previousgroup.
MeasurementsMeasurement Pattern
E&R Test
Probing Test Error
Results
E&R Test
Probing Test Error
Graphical representation of results
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 statedvalue of PFTU, MPE when taking into account themeasurement uncertainty, then the performance of theprobing system is verified
Laser Interferomter
Introduction
• As stated before laser interferometer is a higher standardof measurement that is used for the calibration of CMMfor its geometrical deviations.
• The values from this calibration are also used ascompensations for the deviations in the machinecontroller.
• Once these compensations are provided to the controllerall machine errors are compensated and the machinereturns to the factory provided performance standard.
• Laser interferometry has slowly evolved into easy to useequipment that can help to perform various tasks.
• This type of calibrations all started with Michelson’sInterferometer
Laser Interferomter
Principle
• The working principle of laser interferometers todayused, work on the principle of the Michelson’sInterferometer.
• The Michelson interferometer is common configurationfor optical interferometry and was invented by AlbertAbraham Michelson.
• Albert Michelson and Edward Morley performed theirfamous Michelson-Morley experiment in 1887.
Edward Morley
Laser Interferomter
Principle
• Using a beam splitter, a light source is split into twoarms.
• Each of those is reflected back toward the beam splitterwhich then combines their amplitudesinterferometrically.
• The resulting interference pattern that is not directedback toward the source is typically directed to some typeof photoelectric detector or camera.
• Depending on the interferometer's particularapplication, the two paths may be of different lengths orinclude optical materials or components under test.
Interference Patterns from an interferometer
Michelson’s Interferometer
Laser Interferomter
Principle
Michelson’s Interferometer
Laser Interferomter
Principle
• M is partially reflective, so part of the light istransmitted through to point B while some is reflected inthe direction of A.
• Both beams recombine at point C' to produce aninterference 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 returningbeams, for instance, then an imaging detector willrecord a sinusoidal fringe pattern.
• If there is perfect spatial alignment between thereturning beams, then there will not be any such patternbut rather a constant intensity over the beam dependenton the differential path length.
Michelson’s Interferometer
Laser Interferomter
Principle
Modern Laser Interferometer
Example of laser measurement
Laser Interferomter
Widely used interferometers in the market
5529A
XL 80
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
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.
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)
Laser Interferomter
Definition of geometrical deviations to be measured
Measurements needed for error mapping
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-axisRyy = Linear straightness in y-axisRxy = Horizontal straightness of x-axisRxz = Vertical straightness of x-axisDxy = Yaw of x-axisDxz = Pitch of x-axis....etc.
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.
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.
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.
Laser Interferomter
Setting up the laser
Generic Procedure
• Align and fix the optics for the measurement inour case it will be the linear interferometer thatincludes linear reflector and linear beam splitteras well as clamp blocks to fix it on the machinehead.
• Now you have to mount the interferometer on themachine bed and reflector has to be attached onmachine spindle. Side surfaces of linear beamsplitter and reflector have to be exactly parallel.
Note: In our case the linear interferometer stays stationary while thesplitter moves along the axis.
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
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.
Laser Interferomter
Setting up the laser
Generic Procedure
• Now use a target at the input aperture with white spotat the top and translate machine axis vertically andhorizontally so that the beam hits the target.
• Now take away the target and check if the returnedbeam from the interferometer hits the center of theshutter. If it does not repeat the motion of the machineuntil it does hit the center.
Laser Interferomter
Set up the measurements/optics
Introduction
• We have elaborated about the 21 measurements neededfor complete calibration of CMM.
• Majorly these measurements are divided into threegroups:
– Linearity– Straightness– Rotation and– Squareness
• Now we will elaborate the different configurations ofthe optics that are used to take these measurements
Laser Interferomter
Set up the measurements/optics
Linearity
• Linear measurements are made at multiple pointsalong a machine’s travel path to measure lineardisplacement and velocity.
• It is checked to improve positioning accuracy along anaxis for any machine that requires positioningaccuracy and velocity control.
Laser Interferomter
Set up the measurements/optics
Optics required
• To make linear measurements following optics arerequired:
– Beam Splitter– Linear Reflectors– Targets– Height adjustment fixtures
Renishaw XL 80
Agilent 5529A
Laser Interferomter
Set up the measurements/optics
Principle
Laser Interferomter
Set up the measurements/optics
Configuration on machine
Laser Interferomter
Set up the measurements/optics
Straightness
• Straightness measurements evaluate the unwanted sideto-side or up-and-down motion of a machine tool’stravel in a specified direction.
Laser Interferomter
Set up the measurements/optics
Optics required
• To make straightness measurements following opticsare required:
– Straightness reflector– Straightness interferometer– Targets– Height adjustment fixtures
Renishaw XL 80
Agilent 5529A
Laser Interferomter
Set up the measurements/optics
Principle
Laser Interferomter
Set up the measurements/optics
Configuration on machine
Laser Interferomter
Set up the measurements/optics
Configuration on machine
Laser Interferomter
Set up the measurements/optics
Rotation
• Angular measurements are made at multiple pointsalong a machine’s travel path to test for rotation aboutan axis perpendicular to the axis of motion (roll, pitchand yaw)
• Unwanted angular motion in a machine tool causespositioning errors that reduce the overall accuracy ofyour machine.
Laser Interferomter
Set up the measurements/optics
Optics required
• To make straightness measurements following opticsare required:
– Angular reflector– Angular interferometer– Targets– Height adjustment fixtures
Renishaw XL 80
Agilent 5529A
Laser Interferomter
Set up the measurements/optics
Principle
Laser Interferomter
Set up the measurements/optics
Configuration on machine
Laser Interferomter
Set up the measurements/optics
Configuration on machine
Laser Interferomter
Set up the measurements/optics
Squareness
• Angular measurements are made at multiple pointsalong a machine’s travel path to test for rotation aboutan axis perpendicular to the axis of motion (roll, pitchand yaw)
• Unwanted angular motion in a machine tool causespositioning errors that reduce the overall accuracy ofyour machine.
Laser Interferomter
Set up the measurements/optics
Optics required
• To make squareness measurements following opticsare required:
– Optical Square– Brackets– Targets– Height adjustment fixtures
Renishaw XL 80
Agilent 5529A
Laser Interferomter
Set up the measurements/optics
Principle
Laser Interferomter
Set up the measurements/optics
Configuration on the machine
Laser Interferomter
Collect the data and analyze according to international standards
Parameters to be entered
• After the required measurement optics have been setupand is ready to be measured. Usually the followinginformation is entered in the software to begin takingmeasurements:
– 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)
Laser Interferomter
Collect the data and analyze according to international standards
Sample screen for entering parameters
Laser Interferomter
Collect the data and analyze according to international standards
Sample for measurement screen
Laser Interferomter
Collect the data and analyze according to international standards
Analyzing data
• After the readings have been taken the software showsstatistical chart that represents the points that are out oftolerance.
• This graphical representation is according to thestandard you have chosen. You choose the standardthat your OEM has specified for the calibration of themachine.
Laser Interferomter
Collect the data and analyze according to international standards
Sample for analyzing data
Laser Interferomter
Compensation of results
• After the data has been analyzed according to astandard, software provides the compensation table forthe measurement you have taken.
• This information is then fed into the specific softwareof the machine used for this purpose. E.g Geocomp isused for compensation for Global Image CMM.
Laser Interferomter
Compensation of results
Example of compensation table
Laser Interferomter
Some post checks
Post Checks
• After all the measurements have been compensated,machine is operated in dry run mode. That checkshealthy operation.
• E&R test elaborated before is also performed to checkthat all the measurements of the machine are in thetolerance defined by the macnufacturer
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
Calibration of CMMs