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High Accuracy CMM
Measurements at NIST
by
John Stoup
National Institute of Standards and Technology, USA
2007 CMM Users Meeting - Mexico
October 22, 2007
Today‟s Discussion
We will describe the equipment and processes used at NIST for making
world class CMM Measurements.
• Describe what is needed to make the best possible
measurements.
• Outline some techniques used to assess the CMM
environment‟s thermal performance.
• Discuss optimizing probe performance.
• Present machine performance using gauge data.
• Uncertainty calculations.
• Special measurement setup designs.
Repeatability vs. Time
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
0.450
0.500
0.550
0.600
0.650
0.700
0.750
0.800
0.850
0.900
0.950
1.000
1.050
5 minutes 5 days 5 years
Sta
nd
ard
De
via
tio
n (
mic
rom
ete
rs)
Probe limited performance
Sh
ort
te
rm
rep
ea
tab
ilit
y 25 millimeter dimension
repeatability data
1000 millimeter dimension
repeatability data
Thermal issues dominate
Mostly machine related issues
Cost of improvement
increases substantially as
you attempt to drop these
lines closer together
$
$
$$
$$$$
$$$$$$$$
$
$
$$
$$$$
$$$$$$$$
High Quality Industrial CMM in Good Lab
Repeatability vs. Time
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
0.450
0.500
0.550
0.600
0.650
0.700
0.750
0.800
0.850
5 minutes 5 days 5 years
Sta
nd
ard
De
via
tio
n (
mic
rom
ete
rs)
Probe limited
performance
High quality CMM in very good
laboratory
NIST PMM in very good lab
19.9˚ - 20.1˚C operating range
NIST Moore M48 CMM
What do we need to make very high
accuracy CMM Measurements?
• Extreme high quality lab space.
- gradient control most important.
• CMM capable of exceptional positioning repeatability.
- error mapping will take care of the rest.
• Probe with exceptional gauging repeatability.
• Data collection techniques.
- redundancy.
- test for stability during long data collection runs.
• Operators that strive for the highest accuracy result.
The NIST Advanced Measurement
Laboratory
• Large laboratory spaces.
• Airflow at the rate of 300 air changes/hour in CMM space.
• 20.00 ºC 0.01 ºC temperature stability.
• Improved power quality and mechanical reliability.
NIST M48 CMM in
AML laboratory
• Reflected room lights.
• Thermally controlled floor.
• Vibration isolation.
• Laser scales.
• All heating sources outside of room.
• Granite table added.
• 4 mm/s top speed!
M48 Motion
Mechanisms
• Roller bearing
twin V-ways.
• Lead screw
driven.
• All operation in
oil baths.
AML Thermal Performance – short term
19.98
19.982
19.984
19.986
19.988
19.99
19.992
19.994
19.996
19.998
20
20.002
20.004
20.006
20.008
20.01
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Hours
Av
era
ge
Te
mp
era
ture
(°
C )
AML Thermal Performance – long term
19.960
19.965
19.970
19.975
19.980
19.985
19.990
19.995
20.000
20.005
20.010
20.015
0 24
48
72
96
120
144
168
192
216
240
264
288
312
336
360
384
408
432
456
480
504
528
552
576
600
624
648
672
696
720
744
768
792
Hours
Averag
e T
em
peratu
re (
°C
)
Thermal Gradient Testing
• We need to find out if the moving parts of the CMM
maintain a constant temperature during operation.
• We need the temperature in the measuring volume of the
machine to be stable during operation.
Therefore, we must
• „Tune‟ the room to optimize these two requirements.
• Both axes – carriage and table motions.
Temperature sensor locations on M48
carriage
Right Side Left Side
Camera
Air sensor #1
Air sensor #4Air sensor #3
Air sensor #2
Metal Sensor
#5 & #6
Metal Sensor
#7 & #8
Metal Sensor
#9 & #10
Metal Sensor
#11 & #12
RAM
Thermistor difference data
Thermistor data - side to side differences
-0.14
-0.12
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
Time
Tem
pera
ture
Scale
(d
eg
C)
A3-A4
A1-A2
M5-M6
M7-M8
M9-M10
M11-M12
Tuning the Room
• Remove gradients from around machine by removing some ceiling tiles.
• Increased turbulent airflow with better air mixing around the machine.
• Differences reduced by ~80%
Thermistor data - side to side differences after airflow adjustments
-0.1
-0.09
-0.08
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
Time
Te
mp
era
ture
sc
ale
( °
C )
A3-A4
A1-A2
M5-M6
M11-M12
M7-M8
M9-M10
Error Mapping
Effort
• Error mapping the M48
takes about 2 months.
• Performed redundantly
over time to watch
warmup behavior.
• External laser used to
measure all rotational
errors directly.
• Full 21 component map
at 25mm intervals.
Y Axis Roll Map, Ryy
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850
Table position (mm)
Err
or
(ten
th m
icro
rad
ian
s) exisiting map
no warmup
1 hr warmup
2 hr warmup
3 hr warmup
CMM Probing – repeatability is key
• NIST uses a currently unavailable probe design.
• Stylus geometries mapped for optimum correction.
• Stem lengths kept as short as possible.
• Probe trigger design is important for dirt detection.
• Room airflow creates
vibrations in the probe.
• A cover is required for
highly repeatable results.
Do what it takes to get probe
repeatability!
• X axis repeatability ~ 9 nm.
• Y axis repeatability ~ 13 nm.
• Z axis repeatability ~ 7 nm.
Average Puck Repeatability – 1m Step Gage
Average Repeatability Standard Deviation - Step Gage Measurements
0.000000
0.000004
0.000008
0.000012
0.000016
0.000020
0.000024
0.000028
0.000032
0.000036
0.000040
Sta
nd
ard
Dev
iati
on
(m
m)
10/01 2/02 6/02 7/02 2/03 5/03 11/03 1/04 6/04 12/04 5/05 12/05 4/06 9/06 9/06
10/06
In AML
In Old Laboratory Environment
Move to AML July
2004
Step Gage Data – AML Comparison
Long Term Repeatability - Step Gage Data
all combined history vs. AML data
0.000000
0.000010
0.000020
0.000030
0.000040
0.000050
0.000060
0.000070
0.000080
0.000090
0.000100
0.000110
0.000120
0 40 80 120
160
200
240
280
320
360
400
440
480
520
560
600
640
680
720
760
800
840
880
920
960
1000
1040
Position (mm)
Sig
ma
(m
m)
AML Puck Side A
AML Puck Side B
History Puck Side A
History Puck Side B
Prior History Average Linear Fit
AML Average Linear Fit
~ 3 Day Length-Based Repeatability
Comparison – NIST M48 CMM
• Prior environment results: ulb = 0.035 + 0.022 L µm
• AML current results: ulb = 0.019 + 0.015 L µm
** A 45% improvement in performance with a better room!
This term is independent of error sources such as gage instability, inaccuracy of the CMM error map, fixturing effects, thermal gradient induced errors, and thermometer calibration.
But it does include CMM positioning, probing effects, error map stability and thermal stability of the machine space.
Table Setups Designed for Long Operation
• Measurements of ring and large plug gauges.
• Long gauge blocks, step gauges and end standards.
• Grid plates and scales.
• 30 % of artifacts we measure belong to NIST!
M48 CMM Uncertainty Components
Standard Deviations
Uncertainty Source μm ppm
• Machine Positioning Uncertainty 0.04
• Temperature difference in beam paths during calibration 0.01
• Laser Frequency Difference 0.02
• Measurement Reproducibility (probe effects are here) 0.04 0.04
• Edlén Equation 0.03
• Index of Refraction – Air Temperature 0.01
• Index of Refraction - Air Pressure 0.04
• Index of Refraction – Humidity 0.03
• Artifact Temperature Measurement Accuracy (4mK) 0.05
• Coefficient of Thermal Expansion (1ppm/˚C)( 0.05˚C) 0.05
• Contact Deformation 0.002
• Gage Surface Geometry 0.004
NIST M48 CMM Typical Uncertainty
Statements
• For 1D measurements:
Uc (k=2) = 0.11 + 0.2L µm (L is in meters)
• For 2D measurements:
Uc (k=2) = 0.13 + 0.2L µm (L is in meters)
Special Setups and Arrangements:
Silicon Spheres
• Required for even lower
uncertainties.
• Designed to reduce or
eliminate some
uncertainty components.
• Have achieved task
specific expanded
uncertainties (k = 2) of
about 0.03 micrometers.
Special Setups and Arrangements:
Double Corner Cube
• In one case, we created
better than a class 1000
cleanroom environment
around the machine.
• Designed for a 3D feature
measurement in a critical
component of a NASA
space interferometer to be
launched in the near
future.
Conclusions
• The NIST M48 CMM has state of the art performance.
• Everything is compromised or designed for the sake of accuracy and repeatability.
• For the highest accuracy you must have all the required elements as discussed earlier.
AND
• We are always making incremental improvements in its performance.
• Measurand definition becoming important due to surface imperfections of even the best of artifacts.
• We may become “probe-limited” soon.
Recommended