Measuring the same part on 2 CMMs, Different Results

We have 2 4.5.4 SFs. I ran a part 5 times on CMM A and then ran the exact same part on CMM B. Got different and same results.

CMM A runs 2016 and CMM B runs 2017. Both use the same star probe build. Both checked the same features with the same parameter... number hits, prehit retract, etc..

One CMM seems to read a little larger than the other. CMM B measures one feature 0.002" smaller than CMM A and CMM A measure a feature 0.0006" smaller than CMM B. One feature was exact on both CMMS, exact nominal and deviation, 0.000.

Essentially I copied the program from CMM A to CMM B and just adjusted some movespeeds.

Is this something normal? Operators at night sometimes will fail parts on one cmm and then take them to the other cmm and it pass so they pass the part through. I know they CMM will not match perfectly but they should be closer than 0.002". I'm trying to build confidence in the CMMs for the operators as its only be a few years of having CMMS.

Any advice or ideas on this?

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John Riggins over 7 years ago

The 2 machines are repeating but there is as John stated .0005" variance between the 2
try recalibrating with a lower touch speed, this looks like a calibration variable is cause
CMM A is off 0.010mm ?? start there---that being said ring is 117.5 do you have a cert? what does that state. That would be comparison value.

are these machines in an environmentally different area of shop? Had an issue with a correlation after a vent was altered for a machinist and changed to get cool air at him which also happened to be at the SF CMM.....caused a bit of a shift

I level these machine when moved---no one said do it just seemed to make sense....just another thought

Rich makes a good point here.
Temperature can have a significant effect on results.
I am not talking about part temp or air temp (though both of those DO matter), I'm referring to temperature effects on the machines themselves. Is there cool air blowing on one area of one machine and not on the other?
Are these machines in the same room?
Do you have vibration issues?
There are many things which may be causing the problems you are seeing.
If I were seeing those kind of discrepancies between our two machines, I would determine which one was the most accurate and put all the critical stuff on that machine.
And I would keep working to find out what environmental issues are causing the discrepancies.
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John Riggins over 7 years ago

The 2 machines are repeating but there is as John stated .0005" variance between the 2
try recalibrating with a lower touch speed, this looks like a calibration variable is cause
CMM A is off 0.010mm ?? start there---that being said ring is 117.5 do you have a cert? what does that state. That would be comparison value.

are these machines in an environmentally different area of shop? Had an issue with a correlation after a vent was altered for a machinist and changed to get cool air at him which also happened to be at the SF CMM.....caused a bit of a shift

I level these machine when moved---no one said do it just seemed to make sense....just another thought

Rich makes a good point here.
Temperature can have a significant effect on results.
I am not talking about part temp or air temp (though both of those DO matter), I'm referring to temperature effects on the machines themselves. Is there cool air blowing on one area of one machine and not on the other?
Are these machines in the same room?
Do you have vibration issues?
There are many things which may be causing the problems you are seeing.
If I were seeing those kind of discrepancies between our two machines, I would determine which one was the most accurate and put all the critical stuff on that machine.
And I would keep working to find out what environmental issues are causing the discrepancies.
Cancel
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