High Accuracy CMM Measurements at NIST

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Alicia Scott
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1 High Accuracy CMM Measurements at NIST by John Stoup National Institute of Standards and Technology, USA 2007 CMM Users Meeting - Mexico October 22, 2007

2 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.

3 Standard Deviation (micrometers) Short term repeatability Repeatability vs. Time High Quality Industrial CMM in Good Lab Cost of improvement increases substantially as you attempt to drop these lines closer together $ $ $$ $$$$ $$$$$$$$ $ $ $$ $$$$ $$$$$$$$ 5 minutes 5 days 5 years 1000 millimeter dimension repeatability data Thermal issues dominate 25 millimeter dimension repeatability data Mostly machine related issues Probe limited performance

4 Standard Deviation (micrometers) Repeatability vs. Time High quality CMM in very good laboratory NIST PMM in very good lab C operating range minutes 5 days 5 years NIST Moore M48 CMM Probe limited performance

5 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.

6 The NIST Advanced Measurement Laboratory Large laboratory spaces. Airflow at the rate of 300 air changes/hour in CMM space ºC 0.01 ºC temperature stability. Improved power quality and mechanical reliability.

7 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!

8 M48 Motion Mechanisms Roller bearing twin V-ways. Lead screw driven. All operation in oil baths.

9 Average Temperature ( C ) AML Thermal Performance short term Hours

10 Average Temperature ( C) AML Thermal Performance long term Hours

11 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.

12 Temperature sensor locations on M48 carriage Right Side Metal Sensor #5 & #6 Left Side Air sensor #1 Air sensor #3 Metal Sensor #7 & #8 Metal Sensor #9 & #10 Air sensor #2 Air sensor #4 Metal Sensor #11 & #12 RAM Camera

13 Temperature Scale (deg C) Thermistor difference data Thermistor data - side to side differences A3-A4 A1-A2 M5-M6 M7-M8 M9-M10 M11-M Time

14 Temperature scale ( C ) 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 A3-A4 A1-A2 M5-M6 M11-M12 M7-M8 M9-M10 Time

Error (tenth microradians) Error Mapping Effort Error mapping the M48 takes about 2 months. Performed redundantly over time to watch warmup behavior.

15 Error (tenth microradians) 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, R yy exisiting map no warmup 1 hr warmup 2 hr warmup 3 hr warmup Table position (mm)

16 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.

17 Do what it takes to get probe Room airflow creates vibrations in the probe. A cover is required for highly repeatable results. repeatability! X axis repeatability ~ 9 nm. Y axis repeatability ~ 13 nm. Z axis repeatability ~ 7 nm.

18 Standard Deviation (mm) Average Puck Repeatability 1m Step Gage Average Repeatability Standard Deviation - Step Gage Measurements In Old Laboratory Environment Move to AML July 2004 In AML /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

19 Sigma (mm) Step Gage Data AML Comparison Long Term Repeatability - Step Gage Data all combined history vs. AML data 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 Position (mm)

20 ~ 3 Day Length-Based Repeatability Comparison NIST M48 CMM Prior environment results: u lb = L µm AML current results: u lb = 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.

21 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!

22 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) 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 Gage Surface Geometry 0.004

23 NIST M48 CMM Typical Uncertainty Statements For 1D measurements: U c (k=2) = L µm (L is in meters) For 2D measurements: U c (k=2) = L µm (L is in meters)

24 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.

25 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.

26 Other Special Arrangements

27 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.

Machine Positioning Uncertainty with Laser Interferometer Feedback

Machine Positioning Uncertainty with Laser Interferometer Feedback The purpose of this discussion is to explain the major contributors to machine positioning uncertainty in systems with laser interferometer