Drilling is one of the processes of cutting operations that drill bits, in order to cut or enlarge holes which are in circular cross section. These operations are performed on solid materials. A drill bit is used for the cutting operation; a drill bit is a rotating multipoint cutting tool. This drill bit rotates at the rate of hundreds of revolutions per minute and is pressed against the work piece. The force offered by the cutting edge of the tool on to the work piece will cut off the chips form the hole. Special shapes other than non-circular cross sections like square cross section are also possible.
Drill bit is a revolving tool that is fed into the material to drill the holes along its axis. The number of flutes, drill size, type of shank, geometry of the cutting tool etc. depends on the material used for the production of drill bit. It is generally made of round bar of tool material. This tool consists of three main parts as point, body and shank etc.
- Point: Point is a cone shaped end of this multi-point cutting tool. It consists of cutting elements.
- Axis: It is an imaginary line that forms the center line of the drill.
- Dead center: Dead center is at the extreme tip of the drill bit with sharp edge. This should always be at the center of the drill bit.
- Heel: Heel is a portion at the back of the cutting edge point.
- Shank: The drill is held and rotated by the shank. It is the portion that is clamped into the spindle. The shank may be straight or tapered. Straight shanks are used along with a chuck, whereas the tapered shank drills will have self-holding tapes that are suitable to the drill press spindle. Taper shank is also known as tang.
- Back taper: There is slight decrement provided in diameter from the point towards the shank in the dill body.
- Body: The portion between the point and shank is body. This body guides the drilling operation. The body has two helical grooves called flutes that assist the chip removal out of the drilled holes.
- Flutes: Flutes are the spiral grooves that run along the length of the drill body. The chip is curled tightly for an easy removal from the drilled holes. It helps the cutting edge of the drill bit point. Flutes act as channels through which the chips escape from the hole that is drilled. It allows the coolant or the lubricant to get down through the edge of cutting.
- Chisel edge: It is the edge at the end of the web connecting the cutting lips.
- Chisel edge angle: It is the angle between the chisel edge and the cutting edge (lips).
- Lip relief angle: It is the angle at the outer corner of the lip.
- Drill diameter: It is a diameter over the margins of the drill bit that is measured at the point.
- Margin: Margin is a narrow strip that is extended back of the entire length of the flute. It represents the full diameter of the drill bit.
- Body clearance: It refers to the reduced drill body in order to decrease the friction in between the hole that is drilled and the drill bit.
- WEB: It is the central portion of the body that joins the lands. A two flute drill forms a chisel edge at its end.
- Web Tapered: The thickness of the web increases from point to point of the shank in order to have an enhancement in the drill rigidity.
Types of drill bits:
- Twist drill bits
- Step drill bits
- Uni bit
- Hole saw
- Ejector bit
- Counter sink bit
- Gun drill bit
- Left hand bit
- Indexable drill bit
- Straightened flute bit
- Wooden drill bits
- Glass drill bits
- Diamond core bits
- Masonry drill bits
The drilled holes will have helical feed marks of the used drill bit. The chips that are present will remove the chips pint in the drill holes. The chips may be of the spiral, small flakes that depends on the work piece material. The machinability is an indicator of the type of chips formed. This process will affect the mechanical properties of the work piece that creates low residual stresses. A final finish operation is done in order to avoid the corrosion. Surface finish that is obtained through drilling is upto 32-500 micro inches. A cutting fluid is used in order to cool the drill bit that increases the tool life with an improvement in speeds and fees with good surface finish. This fluid will flood the work piece and which can be done even by the spray mist. Different types of drills are used with respect to different purposes as subland drill that drills up to holes of one diameter, spade drills that is used to drill large sized holes, indexable drills whixh are used to manage the chip formations in the drilling process.
Types of drilling:
1. Spot drilling:
Spot drilling operation is required to drill a hole that acts as a guide for drilling a final hole. It acts as beginner to the drilling process that guides the next processes of drilling.
2. Center drilling:
This drilling offers a center rotation to the drill bit on the work piece. This central drilling is performed with a special shape drill called center drill.
3. Deep hole drilling:
If the hole depth of drilled hole is ten times greater than diameter then the type of drilling is called deep hole drilling. These types of processes are to be provided with special equipment in order to maintain the straightness as well as tolerances. Deep drilling methods are achieved by few other tooling methods.
4. Gun drilling:
Gun drilling is special type of drilling, which is commonly used for drilling gun barrels and for drilling deeper diameter holes. The main feature of this method is it possesses self-centering bits. As a result this allows accurate deeper holes. The drill bit used has a similar motion to twist drill rotation. The drill bits are designed with special bearing pads that slide along the drill bit that is at the center. It generally takes place at low feed rates and high speeds.
This method is used to drill holes of larger diameter upto 1 meter. The drill bit is standard but not economical feasible. It helps the removal of desired diameter by cutting the solid material into disk i.e. it acts as a drafting compass. It is preferred to be performed on flat materials as metal sheets, plates, granite and other structural members as I-beams. This process is also used to make grooves especially for seal insertions etc.
6. Micro drilling:
The drilling operation refers to the holes that are less than 0.5mm i.e. 0.020 inches. Drilling of such holes may not be done easily as the process involves several problems such as requirement of high spindle speeds where the coolant fed drills cannot be used. Such problems somehow can be neglected with this micro drilling process.
7. Vibration drilling:
It is a technique that involves the use of vibrator and this induces oscillations to the drilling tool. It is carried out where deeper holes are required upto 20-30m. It does not involve any rotation of the tool. In general hard rock drilling is done with this process. Shallow holes productivity is increased with this type of drilling. This technology consists of fracturing chips that are controlled by the axial vibration of the drill. The flutes remove the small chips very easily.
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