Often taps get the blame for poor thread quality or rejected threads and it is natural to look to the tap itself as the culprit. Actually, the tap is often the victim of a badly drilled hole. You can’t produce a great thread out of a bad hole! Following are some issues and possible resolutions.
- A dull drill will create a very rough torn hole. Expect poor or incomplete threads.
- A reground drill must be perfectly concentric. The cutting lips must be of equal length and be ground to the same angles. Failure to create a concentric point will cause the drill to cut on one side more than the other and a crooked, bent hole will result making an attempt to tap that hole very difficult. This can also produce an oval egg shaped hole.
- Castings sometimes have a tapered hole so the part will release easily. Threading requires straight walls and tapered walls are impossible to thread correctly.
- Undersized holes are difficult or impossible to thread.
- Holes that have had the surface work hardened by too high a temperature in the drilling process can become too hard to thread effectively.
- Materials that shrink or close-in after drilling are undersized for tapping.
- Holes too near welding or flame cut areas can become hardened and difficult to thread.
Bottom line, consider good hole quality as essential in producing quality holes and if you are having difficulty, dont forget to investigate the drilling process in addition to the tap.