Measuring the same part on 2 CMMs, Different Results

We have 2 4.5.4 SFs.

checked a master ring to both CMMs. CMM A was measuring -0.0003 under nominal and CMM B was measuring 0.0001" over nominal. This this be a cause for concern on a individual cmm basis?

The 454 SF is an inexpensive shop-floor workhorse CMM, not a high-precision lab-grade CMM. The published first term of uncertainty is 3.1 microns, which is 0.000122”. The Rule of Tens applies: always use a measurement device whose uncertainty is a tenth or less of the total part tolerance. I would not choose a 454 SF to chase tolerance levels under .0012”.

I would like to know what the actual calibrated uncertainty is from these two machines’ latest Hexagon Service calibration is (and how many crashes ago were the calibrations done, hmmm?)
In any case, we can add to that the probe calibration Standard deviation, then we can add a few more microns for differences in touch-probe module health (any leaking oil?), add a few more and finally there may be a few more rounding errors in Qual Sphere size definition. Also long styli add more microns, plus changing wrist angles adds a lot more microns and those get multiplied by long styli.
That will get us close to or even over the 4 to 6 ten-thousanths inch differences we see here, but I agree there may be room for improvement to get the machines to match better.

I would run some MM-units tests on a certified ring gauge, not a part(!), swapping this equipment between machines:

• Qual sphere (created anew in the MM-units test program which needs to have 6 decimal places set)
  
• Probe body
  
• brand-new Light Force probe module
  
• brand-new large-dia short-shaft Stylus tip, like a 5x20
  
• probe qual done with 25 points over 4 levels, touchspeed set to 5
  
• Ring gage measurement with touchspeed set to 5.
  

Please report back with this data. We may be able to arrange having our Service Engineers get your twins to correlate a little better.


FYI… if you need to hold 0.0001” with long styli and changing wrist angles, then your employer needs to spend waaayyyy more money on high-accuracy air-bearing CMMs with analog wrist probes or even fixed-head analog probes.

We have 2 4.5.4 SFs.

checked a master ring to both CMMs. CMM A was measuring -0.0003 under nominal and CMM B was measuring 0.0001" over nominal. This this be a cause for concern on a individual cmm basis?

The 454 SF is an inexpensive shop-floor workhorse CMM, not a high-precision lab-grade CMM. The published first term of uncertainty is 3.1 microns, which is 0.000122”. The Rule of Tens applies: always use a measurement device whose uncertainty is a tenth or less of the total part tolerance. I would not choose a 454 SF to chase tolerance levels under .0012”.

I would like to know what the actual calibrated uncertainty is from these two machines’ latest Hexagon Service calibration is (and how many crashes ago were the calibrations done, hmmm?)
In any case, we can add to that the probe calibration Standard deviation, then we can add a few more microns for differences in touch-probe module health (any leaking oil?), add a few more and finally there may be a few more rounding errors in Qual Sphere size definition. Also long styli add more microns, plus changing wrist angles adds a lot more microns and those get multiplied by long styli.
That will get us close to or even over the 4 to 6 ten-thousanths inch differences we see here, but I agree there may be room for improvement to get the machines to match better.

I would run some MM-units tests on a certified ring gauge, not a part(!), swapping this equipment between machines:

• Qual sphere (created anew in the MM-units test program which needs to have 6 decimal places set)
  
• Probe body
  
• brand-new Light Force probe module
  
• brand-new large-dia short-shaft Stylus tip, like a 5x20
  
• probe qual done with 25 points over 4 levels, touchspeed set to 5
  
• Ring gage measurement with touchspeed set to 5.
  

Please report back with this data. We may be able to arrange having our Service Engineers get your twins to correlate a little better.


FYI… if you need to hold 0.0001” with long styli and changing wrist angles, then your employer needs to spend waaayyyy more money on high-accuracy air-bearing CMMs with analog wrist probes or even fixed-head analog probes.