calibration of coordinate measuring machines (cmm)

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


Measurements

Evolution of Measuring Instruments

Cubit

Micrometer

Gauge Blocks

Length Comparator

Gauges and Dial Indicators

Visual Inspection machines

Universal 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’]


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


How does CMM work?


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)


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.


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


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


• 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


E&R Test


Graphical representation of the results

E&R Test


• 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


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.


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


Measurements needed for error mapping

Laser Interferomter


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


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


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


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


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


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


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


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


Optics required

• To make linear measurements following optics arerequired:

– Beam Splitter– Linear Reflectors– Targets– Height adjustment fixtures

Renishaw XL 80

Agilent 5529A

Laser Interferomter


Principle

Laser Interferomter


Configuration on machine

Laser Interferomter


Straightness

• Straightness measurements evaluate the unwanted sideto-side or up-and-down motion of a machine tool’stravel in a specified direction.

Laser Interferomter


Optics required

• To make straightness measurements following opticsare required:

– Straightness reflector– Straightness interferometer– Targets– Height adjustment fixtures

Renishaw XL 80

Agilent 5529A

Laser Interferomter


Principle

Laser Interferomter



Laser Interferomter


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


Optics required

• To make straightness measurements following opticsare required:

– Angular reflector– Angular interferometer– Targets– Height adjustment fixtures

Renishaw XL 80

Agilent 5529A

Laser Interferomter


Principle

Laser Interferomter



Laser Interferomter


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


Optics required

• To make squareness measurements following opticsare required:

– Optical Square– Brackets– Targets– Height adjustment fixtures

Renishaw XL 80

Agilent 5529A

Laser Interferomter


Principle

Laser Interferomter


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


Sample screen for entering parameters

Laser Interferomter


Sample for measurement screen

Laser Interferomter


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


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

calibration of coordinate measuring machines (cmm)

Engineering

modern machines

principle of coordinate

capable of measuring

cmm work

pereira http

hocken paulo

laser interferometerby

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