ABS vs INC and Ball Adapter

[N7AS-1forum]

I have a Harbor Freight Digital Micrometer which I think is the same as the Fowler 54-850-001.
It has an ABS and INC mode. Which should I use to measure a part as I am not clear to which to use? Can someone explain the function of each setting?

I know that the Ball Adapter is used to measure a curved surface such as measuring the wall thickness of tubing.
Does the Ball Adapter attach to the anvil or spindle?

 

[T Bredehoft]

Does the Ball Adapter attach to the anvil or spindle

The ball goes on the inside of the curved surface. Best on the anvil.

Run the mike closed (without the ball), just snug, don't wrench on it, set abs 0.0000
Then, if you use the ball, install it, touch with the spindle, set inc 0.0000

 

[RJSakowski]

If you zero a micrometer in ABS mode, it will reset the zero reference for all measurements. Resetting zero in INC mode should only affect that mode. Going back to ABS mode, your zero should not have changed. My micrometer is a Tormach branded but yours should work the same way

The ball should go on the fixed anvil rather than the spindle. You would not be able to make a wall measurement on a closed tube otherwise as the ball is always on the concave side.