Hole degree of deviation is very bad.
Workpiece not positioned or fixed well.
Drill guide is not suitable, big space between the drill guide and gun drill、
No good support for the gun drill shank.
Workpiece structure is not good, such as workpiece wall thickness is too big, and material issue such as not even.
Hole roughness too bad
Main spindle rotating speed, feeding rate is too slow.
Cutting oil not suitable;Pressure low, fow low, oil temperature is too ...