Drilling is one of the most fundamental machining technologies and is moving toward high precision/high speed applications for productivity enhancement. One notable drilling technology, micro-hole drilling, is becoming increasingly more prominent in a variety of precision industries, such as the production of automotive fuel injection nozzles, watch and camera parts, medical needles, air bearing, etc. Especially, its applications in the electronics and computer industries are rapidly expanding. It is mainly used in machining of printed circuit boards (PCB) and IC masking. The increase in the degree of integration demands improved technologies for the manufacture of smaller holes with larger aspect ratios for higher density circuit boards. Furthermore, the increasing competition in micro part developments puts an additional impetus on micro-hole manufacturing technologies. Drilling of holes at this small diameter presents greater problems since coolant fed drills cannot be used and high spindle speeds are required. Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool. It cuts by applying pressure and rotation to the work piece, which forms chips at the cutting edge. For fluted drill bits, any chips are removed via the flutes. Chips may be long spirals or small flakes, depending on the material, and process parameters. The type of chips formed can be an indicator of the machinability of the material, with long gummy chips reducing machinability [25]
As the name implies this type of drilling machine can be operated with ease anywhere in the workshop and is used for drilling holes in work pieces in any position which cannot be drilled in a standard drilling machine. Some of the portable machines are operated by hand power, but most of the machines are driven by individual motor. The entire drilling mechanism including the motor is compact and small in size. The motor is, usually of universal type which may be driven by both A.C. and D.C. The maximum size of the drill that it can accommodate is not more than 12 to 18 mm. The machine is operated at high speed as smaller size drills are only used. Some of the portable machines are driven by pneumatic power. [25]
Fig. 1.1 Portable Drilling Machine [25]
The sensitive drilling machine is a small machine designed for drilling a small hole at high speed in light jobs; the base of the machine may be mounted on a bench or on the floor. It consists of vertical column, a horizontal table, a head supporting the motor and driving mechanism, and a vertical spindle for driving and rotating the drill. There is no arrangement for any automatic feed of the drill spindle. The drill is fed into the work by purely hand control. High speed and hand feed are necessary for drilling small holes. High speeds are necessary to attain required cutting speed by small, diameter drill. Hand feed permits the operator to feel or sense the progress of the drill into the work, so that if the drill becomes worn out or jams on any account, the pressure the drill may be released immediately to prevent it from breaking. As the operator senses the cutting action, at any instant, it is called sensitive drilling machine. Sensitive drilling machines are capable of rotating drills of diameter from 1.5 to 15.5 mm. Super sensitive drilling machines are designed to drill holes as small as 0.35 mm in diameter and the machine is rotated at a high speed of 20,000 r.p.m. or above. Fig.1.2 illustrates a sensitive drilling machine.
Fig.1.2 Sensitive Drilling Machine [26]
The upright drilling machine is designed for handling medium sized work pieces. In construction the machine is very similar to a sensitive drilling machine for having a vertical column mounted upon the base. But this is larger and heavier than a sensitive drill and is supplied with power feed arrangement. In an upright drilling machine a large number of spindle speeds and feeds may be available for drilling different types of work. The table can move vertically and radially of the machine have different types of adjustments.
There are two general classes of upright drilling machine
Fig.1.3 Up-Right Drilling Machine [23]
The round column section upright drilling machine or pillar drilling machine consists of a round column that rises from the base which rests on the floor, an arm and a round table assembly, and a drill head assembly. The arm and the table have three adjustments for locating work pieces under the spindle. The arm and the table may be moved up and down on the column for accommodating work pieces of different heights. The table and the arm may be moved in an arc up to 180° around the column and may be clamped at any position. This permits setting of the work below the spindle. Moreover, heavy and odd size work may be supported directly on the base of the machine and drilled after the arm is swung out of the way. The table may be rotated 360°about its own centre independent of the position of the arm for locating work pieces under the spindle. The construction of the machine being not very rigid and the table being supported on a horizontal arm, this is particularly intended for lighter work. The maximum size of holes that the machine can drill is not more than 50 mm.