Bore Micrometer v CMM

For what it's worth, back in the machine shop days we found that the best way to replicate a 2 point bore gage was to use maximum inscribed coupled with circularity. That would tend to show the maximum and minimum results you could find with a bore gage.

Sadly, we also found the best way to get the CMM to regularly agree with bore gages was to take every 3 point bore mic and lock them in a closet.
They are great tools, but they also hide flaws that are easily picked up with even a cheap bore gage. 

For what it's worth, back in the machine shop days we found that the best way to replicate a 2 point bore gage was to use maximum inscribed coupled with circularity. That would tend to show the maximum and minimum results you could find with a bore gage.

Sadly, we also found the best way to get the CMM to regularly agree with bore gages was to take every 3 point bore mic and lock them in a closet.
They are great tools, but they also hide flaws that are easily picked up with even a cheap bore gage. 

2-point checks have limitation also.  You can completely miss tri lobing for example, which exactly the type of error a 3 point bore mic is designed to detect.  To illustrate this point, back when I used to train customers, I liked to ask them to measure the form error of a 50 pence coin using a micrometre or calliper.  Since those are 2-point measurements, the results would indicate that the coin was almost perfectly round, whilst you can clearly see that it is not.