Jaws of Vernier Caliper
The “inside jaws” are a bit tougher to use than outside.
first they have to be in the hole both straight and at the widest point (for a round hole). I usually wiggle mine back and forth and let them “find” the maximum diameter of a round hole.
If a square shape I measure at 1 end to maintain the position or measure a specific distance away from the end on both edges to make the measurement. In this case I am looking for the smallest distance.
So for round I am looking for the largest distance and square the smallest.
This tells me my measurement is true as long as the holes are round or square in the methods outlined.
To clean calipers a piece of paper is usually enough clamp on the paper and slide out was the way a machinist taught me to clean the outside jaws. I haven’t had a problem with the inside jaws because they are close to knife edges I guess.