What is the origin of the idiom "an ax to grind" and when to use it?

The idiom "an ax to grind" originates from a 19th-century story written by Charles Miner, an American writer. In the story, a young boy tricks an unsuspecting farmer into allowing him to grind his ax. The farmer is initially hesitant but eventually agrees. However, the young boy intentionally grinds the ax poorly, making it less effective. When confronted by the farmer, the boy reveals that he had an ulterior motive – he wanted to borrow the farmer's plow. The idiom came to symbolize having a hidden agenda or a personal motive behind one's actions, often with the intention of benefiting oneself at the expense of others. It is often used when someone is suspected of pursuing their personal interests, seeking revenge, or trying to gain an advantage while appearing to be focused on a different goal.