Machining Operations Related to Drilling

There are several operations related to drilling. Some of them are:

1. Reaming,
2. Tapping,
3. Counterboring,
4. Countersinking,
5. Center drilling, and
6. Spot facing

These are illustrated in Figure 1 and described in this section. Most operations follow the drilling process; a hole must be created first using the drilling method, and then the hole is modified by one of the other drilling-related operations. Centering and spot facing operations are exceptions to this rule. All of the operations use rotating tools.

Figure1: 1. reaming, 2. tapping, 3. counterboring, 4. countersinking, 5. center drilling, and 6. spot facing
Figure 1: Machining operations related to drilling

Reaming: The Reaming operation is used to slightly enlarge a hole, providing a better tolerance for its diameter and improving its surface finish. The tool is called a reamer, and it usually has straight flutes.
Tapping: The tapping operation is performed by a tap and is used to provide internal screw threads in an existing hole.
Counterboring: The counterboring operation provides a stepped hole, where a larger diameter follows a smaller diameter partially into the hole. A counterbored hole is used to seat bolt heads into a hole, so the heads do not protrude above the surface.
Countersinking: countersinking operation is similar to counterboring, except that the step in the hole is cone-shaped for flat head screws and bolts.
Centering: The centering operation, also known as center drilling, involves drilling a starting hole to accurately establish the location for subsequent drilling. The tool is known as a center drill.
Spot facing: Spot facing operation is similar to milling. This operation is used to provide a flat machined surface on the workpiece in a localized area.