Hole preparation is often a forgotten consideration when threading a part. Depending on the parameters of the application, lack of attention to the size and condition of the hole can contribute greatly to thread size and finish, or the potential failure of the tap.
Holes to be tapped are straight or tapered. They can be cored, drilled, bored, punched, and extruded, all with or without reaming to finished size. Each one of these methods for producing a hole have inherent areas of concern.
In cored, or cast holes, "hard" spots may be