Drilling, tapping, counterboring, and countersinking are the usual operations done in sheet metals.
Drilling: Drilling is done in sheet metal only when piercing cannot deliver the accuracy required. For example, on a formed part, when holes on different features need to be coaxial, the accuracy obtained by machining may be required.
Tapping: Tapping can be done using cut threads or formed threads. Formed threads (thread rolling) is preferable for the following reasons:
- Thread rolling is faster than cutting.
- Fewer burrs are generated, so no clean up is required or risk of future hazards such as shorting with electronic components.
- Larger sized holes are required for thread rolling vs. tapping, resulting in improved tap life.
- Rolled threads are stronger due to cold working. Typically, rolled threads are 20% stronger than cut threads.
- For very thin stock, either threaded fasteners such as clinch nuts, or forming thread in extruded holes is recommended.
- The material is upset in the sheet metal hole to form one thread pitch.
Counterboring: Counterboring is often done to provide clearance and a bearing surface for the fastener's head.
Countersinking: Countersinking allows for flush mounting of flat head fasteners. Countersinking cannot always be done for very thin stock or for very large fasteners.