Measurement Variance Between Two Probes

[[Pa...]]

Dear Expertise, 

I need your support regarding an issue I am currently facing. There is a significant measurement variance between two different probes:

1. Short probe – Ø3 length 85 mm
2. Long probe – Ø5 length 125 mm

The measurement differences are quite large even though the alignment remains the same.
We are using an RDS with a VAST XXT rotating probe system.

Could anyone please advise how to eliminate this error?

As below measurement table for your referance.

Feature        Circle3 (short)    Circle9 (long)     Difference (short - long)

X actual (mm)     -0.02173           -0.01202             0.00971

Z actual (mm)     -0.02633           -0.01052            0.01685

[[Ma...]]

I would start with calibration. If there is same problem even after calibration, then i don't know.

[[RP...]]

Cleaning and Re-qualifying would be what I would start with.  Are both scans being done with the same scanning settings?

[[Da...]]

Circle3 and Circle9 were not measured at the same Y position. Therefore, they cannot be compared.

[[Pa...]]

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we are doing with probing 6 points all same angle. 

[[Ch...]]

Measure a gage ring and then measure a gage block with both probes.  Be sure to measure in the exact same spots.  This will help you determine if you have a equipment issue such as a bad probe or cmm problem. 

If your gage ring/block test passes, then you will know its something with your measurement strategy.  Like David said, it would be a difference in strategy like taking points at different locations. 

[[Ke...]]

The pictures you have attached show that you are measuring with two different features, which makes diagnosis extremely difficult. As there is a 7mm difference in measuring location, it could be that the difference may come from the part itself either from manufacturing variance or cleanliness,  the difference in location and probe may be causing some sort of interference/shanking, or there may be different settings used in the measurement (speed, filtering, outlier settings,number of points, point locations, angle range, etc). Additionally, there could be variance from the probes, whether this is from the calibration or the probe itself such as a loose joint or cracked extensions. The only thing that can be done is to start isolating and testing these individually until the root cause is found. All of the above suggestions are good ways of testing various items on the list, such as redoing calibration after cleaning to check the quality of the calibration and ensure it is current, using both probes to check a master such as a ring gage to identify if the issue is in the probes, and I will add that instead of testing 2 different features, measure one of the features then change that feature to use the other probe and remeasure, to eliminate any feature settings as the issue.

[[Ma...]]

I would try to change configuration: I if i am not wrong with XXT max stylus lenght on horizzontal reach should not be more than 65 mm; so in both cases you have to accept probability to have some uncertanty on form and sigma.