Drilling is the operation of cutting a round hole by a rotating tool called drill. Before the process of drilling, the center of the hole is positioned on the work piece. Two lines at right angles to each other are drawn. A center punch is used to mark the center point at the meeting of two lines. The rotating drill is pressed at the center point scribed on the work piece to produce the hole.
Drilling does not produce a precise hole. Only rough internal surface will be produced by the drilling process. The hole is lightly bigger than the size of the drill bit used due to the vibration of the drill.
the process of sizing and finishing the already drilled hole is called as Reaming. The tool used for reaming is known as a reamer. It is a cylindrical tool having many cutting edges. Reamer cannot drill a hole. It simply follows the path of an previously drilled hole. It removes a very small volume of metal. The volume of metal removed in reaming is approximately 0.375mm. In this process the spindle speed is half that of drilling process.
Boring is a process of increasing a hole with the help of a single point cutting tool. Boring is done where right size drill is not available. If the size of the hole is very large the hole cannot be drilled. Then to enlarge the hole boring is done. By boring, the hole is finished accurately to the required size. The internal surface of a hole in a casting is machined by this boring process. The cutter is held in a boring bar which has a tapped shank to fit into spindle hole.
The operation of enlarging the end of a hole cylindrically is known as a counter boring. The operation of making a cone-shaped enlargement of the end of a hole is known as countersinking.
The operation of squaring and smoothing the surface around hole is known as spot facing. Figure illustrates the process of spot facing.
The operation of cutting internal threads in a hole by using a cutting tool is called as tapping. A tap has cutting edges in the shape of threads. the tap is screwed into the hole which will remove metal and cut internal threads.
Tap drill size = 0.8 x Outside diameter of the thread.
The operation of manufacturing a large hole by removing metal along the circumference of a hollow cutting tool is known as trepanning. There is a pilot inside the trepanning tool which pass in the small previously drilled hole to produce the larger. It is used for the diameter more than size of particular machine and where hole depth is much more in comparison with normal work.
The operation of increasing the dimension of thehole at any point between its ends is known as undercutting.
The operation of removing large amount of stock with respect to lapping and honing operations from hardened material and super finishing a surface accuracy upto ±0.0025mm by means of a grinding tool wheel is known as grinding.
The operation of sizing hardened holes and extremely limited in stock removal is known as lapping.
The operation of finishing large holes such as automobile cylinders by means of slow moving abrasives is known as honing.