Milling has been evolved into a method that machines a very broad range of operations. In addition to all the conventional applications, milling is a strong alternative for producing holes, threads, cavities and surfaces that used to be turned, drilled or tapped
Holes and Cavities (Pocketing)
Drilling is the traditional and fastest method for producing a hole, but chip breaking can be a challenge in some materials. It also lacks the flexibility to produce varying diameters and non-round shapes.
Widening Existing Hole
Boring, on the other hand, is a process of enlarging a hole that has already been made by another process. boring is a secondary finishing operation.The boring process is significantly different from the drilling process. Boring is accomplished using a heavy, stiff boring bar that has a single-point cutting tool at the end. This manufacturing process gives the machinist much more control of the diameter of the hole, allowing for a much tighter tolerance for diameter to be achieved. The goal of boring is to enlarge an existing hole, create a tapered hole from an existing hole, or achieve a desired surface finish inside an existing hole. Boring can be done on lathes, milling machines, or machining centers. There are also specialized machines for boring, such as boring mills or jig borers.
Suppose you need a hole with a diameter of 5 mm ± .2 mm. The best you can achieve with your drill press is 5 mm ± .5 mm. In that case, you would probably need to drill the hole slightly smaller than it needs to be, then use the boring process to achieve the tighter tolerance of ± .2 mm