Drilling is the most common machining process. One estimate is that 75% of all
metal-cutting material removed comes from drilling operations. Drilling
involves the creation of holes that are right circular cylinders. This is
accomplished most typically by using a twist drill. The figure below
illustrates a cross section of a hole being cut by a common twist drill:
The characteri stics of drilling that set it apart from other powered metal
cutting operations are:
- The chips must exit out of the hole created by the cutting.
- Chip exit/ movement can cause problems when chips are large and/or continuous.
- The drill can wander upon entrance and for deep holes.
- For deep holes in large work pieces, coolant may need to be delivered through
the drill body to the cutting front.
- Of the powered metal cutting processes, drilling on a drill machine is the most
likely to be performed by someone who is not a machinist.
In any drilling operation, the following factors are important:
- Work must be kept clean and chips removed frequently, since dirt and chips act
as an abrasive to dull the drill.
- Drills must be carefully selected and correctly ground.
- Drills must be properly aligned and the work piece firmly supported/ clamped.
- A stream of cutting fluid must be properly directed at the hole.
- Drills should be chucked/ held for shortest drilling length to avoid
whipping or flexing, which may break drills or cause inaccurate work.
- Drill coatings, such as TiN, TiAlN, &l; TiCN, will increase the wear resistance
and consequently the drill life.
- Deep Holes: In case of deep holes (Depth of hole > 5 times the diameter) the
chips may clog and make the drilling difficult. A wood pecking approach is
required in this case. The general rule for HSS drills is to drill to a depth
of three to four times the diameter of the drill for the first bite/ cycle, one
or two diameters for the second bite/cycle and around one diameter for each of
the subsequent bites. This is not a good technique for carbide drills. Carbide
drills have a tendency to break on a peck operation. A groove ground parallel
to the cutting edge in the flute for chip clearance will allow deeper holes to
be drilled per bite, particularly with larger-size drills. The groove breaks up
the chip for easier removal.
- Drills should not generally be allowed to dwell during cutting. Dwelling glazes
the bottom of the hole due to rubbing action. Therefore, in case of deep holes
and to avoid chip clogging (to break the chip), drills should be retracted and
reinserted at full speed to avoid glazing.
- Drills with a non-cutting land at the point (i.e. without chisel edge) should
be avoided. This non-cutting land only pushes material away from the center and
will work harden the material. This action can cause hard spots in the center
of the material and lead to premature drill failure.
Parameters of drilling
- The material to be drilled (For Example- Mild steel, Alloy Steel, Cast Iron,
- The Drill material (Like HSS or Carbide)
- The selection of speed and feed also may depend upon the rigidity of the drilling machine, the motor power and the drill jig/ fixture.
- The combination of Speed and Feed is important in determining the rate of production.
- Point Angle: The most commonly used point angle is 118 degrees.
- The lip relief (clearance) angle should be between 9 degrees and 15 degrees, and the two cutting edges must be equal in length and angle.
- The web thickness at the point should be about 12.5% of the drill diameter,
or less. A thinner web reduces feed pressure, heat generation but weakens
- Center drilling
- Hole milling
- Spot facing
- Counter boring
- Counter sinking
- Flow drilling
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