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 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

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 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’]

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

How does CMM work?

How does CMM work?
Types of CMMs

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.

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

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.

Calibration
Calibration of CMMs
Calibration Standards for CMMs

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

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 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.

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

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.

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)

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.

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

E&R Test
Comparison between different artifacts

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

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

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 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:

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 length
measurement
• These values will have to be measured again ten times
each at the relevant configuration
Interpretation of the results

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

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

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

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

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 stated
value of PFTU, MPE when taking into account the
measurement uncertainty, then the performance of the
probing system is verified

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

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

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

Laser Interferomter
Principle

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.

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-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.

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 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.

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 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.

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

Laser Interferomter
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.

Laser Interferomter
Optics required
• To make linear measurements following optics are
required:
– 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 side
to-side or up-and-down motion of a machine tool’s
travel in a specified direction.

Laser Interferomter
Optics required
• To make straightness measurements following optics
are required:
– Straightness reflector
– Straightness interferometer
– Targets
Renishaw XL 80
Agilent 5529A

Laser Interferomter
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.

Laser Interferomter
Optics required
• To make straightness measurements following optics
are required:
– Angular reflector
– Angular interferometer
– Targets
Renishaw XL 80
Agilent 5529A

Laser Interferomter
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.

Laser Interferomter
Optics required
• To make squareness measurements following optics
are required:
– Optical Square
– Brackets
– Targets
Renishaw XL 80
Agilent 5529A

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 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)

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 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.

Laser Interferomter
Sample for analyzing data

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.

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 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

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 Coordinate Measuring Machines (CMM)

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