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

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5 minutes 5 days 5 years

Sta

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De

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n (

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rom

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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

$

$

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High Quality Industrial CMM in Good Lab

Repeatability vs. Time

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5 minutes 5 days 5 years

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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

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0.000024

0.000028

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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

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0.000120

0 40 80 120

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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.

Other Special Arrangements

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.