Wegener also noted that the continents move up and down to maintain equilibrium in a process called isostasy. As an example he cited the sinking of Northern Hemisphere lands under the weight of continental ice sheets in the last ice age, and their rise since the ice melted some 10,000 years ago.
Drill bits are cutting tools used to create cylindrical holes, almost always of circular cross-section. Drill bits come in many sizes and have many uses. Bits are usually connected to a mechanism, often simply referred to as a drill, which rotates them and provides torque and axial force to create the hole.
This type of drill machine is used for very light works. Fig.1 illustrates the sketch of sensitive drilling machine. The vertical column carries a swiveling table the height of which can be adjusted according to the work piece height.The table can also be swung to any desired position.he drilling operation can also be accomplished in lathe, in which the drill is held in tailstock and the work is held by the chuck. The most common drill used is the twist drill. Drilling Machine. It is the simplest and accurate machine used in production shop.