Incomplete threads or poor thread quality are often created by using a dull drill or a reground drill that is not perfectly concentric.
Such drills create holes that may be rough and torn or crooked and bent or oval-shaped making tapping difficult.Ensure that the drill is perfectly concentric with the cutting lips of equal length and the same angles.
Holes should never be undesized and should not be tapered since threading requires straight walls.
Ensure that the material has not shrunk after drilling resulting in smaller holes. Holes drilled on surfaces that have hardened due to high temperatures or welding cannot be threaded effectively.
By Raj Kumar Singh
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