Drilling is the most common machining process whereby the operation involves making round holes in metallic and nonmetallic materials. Approximately 75% of all metal- cutting process is of the drilling operation.
Drills usually have a high length to diameter ratio that is capable of producing deep hole, however due to its flexibility, necessary precaution need to be taken to maintain accuracy and prevent drill from breaking.
Drilled holes can be either through holes or blind holes (see Figure 4.1). A through holes
is made when a drill exits the opposite side of the work; in blind hole the drill does not
exit the workpiece (Groover, 1996).
During the operation, chips that are produced within the workpiece must exit through the flutes to the outside of the tool. As the chip is formed and extracted towards the surface, it will generate friction. Friction subsequently heat is also generated when the drill bit touch the workpiece during the holemaking process. Therefore, chip disposal and cutting fluids are among the most important elements need to be consider during this process.
Normally, holes produced by drilling are bigger than the drill diameter and depending on its applications; the drilled holes will subjected to other operations such as reaming or honing to better surface finish and dimensional accuracy (Kalpakjian & Schmid, 2001).
There are also several apparatus needed during the drilling operation as shown below:
A)Drilling machine B)Center punch C)Hammer D) Center drill
E) Twist drills F)Coolant G)Vernier caliper
H) Two flute drill set:
i. Center drill
ii. Countersink drill
iii. Counter bore drill
iv. Drill various diameter
4.1.1 Types of Drills
A drill is a rotary-end cutting tool with one or more cutting edges or lips and one or more straight or helical grooves or flutes for the passage of chips and cutting fluids or coolants. The most common drills are as follows:
a) Step drills
to produce holes of two or more different diameters.
b) Core drills
to enlarge existing holes
c) Counterboring & Countersinking
to produce depression on the surface to accommodate heads of screws &
bolts.
d) Center drill
a short & stubby drill to produce holes so that work piece can be mounted
between lathe centers.
e) Spot drill
to start a hole
f) Spade drill
to remove large and deep holes
g) Crankshaft drill
good centering. Suitable for deep holes.
h) Gun drilling
deep hole making, length-to-diameter ratios up to 300 or higher, self-
centering, lubrication & coolant passage ~ trueness of holes
i) Trepanning
removal of disk-shaped piece.
j) Twist Drills
to remove the maximum volume of metals in a minimum period of time. It does not produce a precision hole: however, this can be achieved by a reaming operation.
Amongst the type of drills listed above, the twist drill is by far the most used cutting tools in the drilling operation. The twist drill is provided with two spiral grooves and two cutting edges. The chips produced are guided up through these
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MME1103
Workshop Technology Manual
Drilling
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spiral grooves. The grooves also serve as passage to the cutting fluid. In order that the cutting edges can cut off chips, two movements are required simultaneously; rotational speed and axial feed.
In twist drill, there are various angles to be considered (see Figure 4.2):
•Cutting angle (ca) or angle of
lip
:The two lips must be of same length and equal
angle. For ordinary work, the cutting angle is
59° and vary with metal to metal.
• If ca>= drill will not cut the metal easily and
will not hold its position centrally because of
being too flat.
• If ca<= more power is needed to turn the drill
and drill will cut at slower rate due to the
longer cutting edges.
• If ca different=one cutting edge will wear
quickly and hole will be larger than drill. Will result in wobbling of spindle and drill wears out quickly
•Lip clearance angle
:The cone shaped cutting end is the point from
the lips and varies from 12- 15° degrees. In drilling soft materials, the angle may be increased under heavy feeds. For hard materials, the recommended angle is 9° degrees. If reduced further the drill cannot cut into the metal and may break in the centre along the web.
•Rake angle
:It is the angle between the flute and the
workpiece that is usually 70-75° degrees. This helps to secure the lip over the correct space to curl the chips. If more there will be no edge for cutting and if less the cutting edge will be too thin and may break under strain.
Figure 4.2: Standard geometry of a twist drill (Groover, 1996).
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4.2 DRILLING MACHINES
Drilling can be done on a wide variety of machines, ranging from hand-held portable drills to multiple spindle heads capable of handling thousand of drills at the same time. For ordinary workshop purposes there are three main types of drilling machines in use shown in Table 4.1 below.
The sensitive drill is used for light drilling on small parts. The upright drill press is used for heavy duty drilling and finally the radial drill press is used for drilling large, heavy workpiece that are difficult to move. There are also other special purpose drilling machines ranging from microscopic drilling machine to deep-hole drilling and turret head drills (Kibbleat., 2002).
The one which is being used in the Workshop, Kulliyyah of Engineering is the pillar- drilling machine. The vertical feed is actuated by hand. The machinist can sense the how fast the drill is cutting and can control it according to the condition of the moment. These require some skills in judging the appropriate feed rate.
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The types of drilling machine can range from a simple bench type units used to drill small diameter holes, to large radial drills, which can accommodate large workpieces. The drill head of a universal drilling machine can be swiveled to drill holes at an angle. For high production rate operations, a multiple spindles or also known as gang drilling can be used. This type of machine is capable of drilling in one step as many as 50 holes of varying sizes, depth and locations.
Type of driller
Drill diam.
Mm
Spindle
speed
Change
method
Rev./min
Bench
Up to 5
3
Belt
450-1800
Sensitive
Round
column
5 - 12
3
Belt
500-1500
Pillar
12 - 30
9
Gear box
50-1020
Radial arm
30 - 90
18
Gear box
15-850
Table 4.1: The rules and principles of cutting speeds and RPM calculations in drilling
operations
Drilled holes are used to take up screws, bolts, shafts, electrical wiring, steam pipes, fitting of furniture and equipment and further more to pass through gases, fluids etc. It is clear that the production of holes is a major part of al engineering manufacture. The drilled holes either have through or blind holes and the machine tool used to produce holes is usually the drilling machine.
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Drills are classified by material, length, shape, number, and type of helix or flute, shank, point characteristics, and size series. Drills are produced from high-speed steel (HSS), solid carbide, or with carbide brazed inserts. Most drills are made for right-hand rotation. Right-hand drills, as viewed from their point, with the shank facing away from our view, are rotated in a counterclockwise direction in order to cut. Left-hand drills cut when rotated clockwise in a similar manner.
Because of the great force applied by the machine in drilling, it is essential to ensure the rigidity of both cutting tools and workpiece. The drill must be correctly held and the workpiece clamped to the table. There are three ways workpiece is usually held in drilling machine, by using: (a) vise, (b) fixtures and (c) jig. A vise is a general purpose workholding device possessing two jaws that grasp the workpiece. A fixture is a workholding device that is usually custom designed for the particular workpiece. A jig is a workholding device that is also specially design to the workpiece. The difference between a jig and a fixture is the jig provides a means of guiding the tool during drilling operation.
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4.3 BASIC FORMULA FOR DRILLING OPERATION
Cutting speeds and RPM calculations are those operations that are strictly drilling operations, but the drilling operation is also used to perform other operations such as reaming, tapping, countersinking and counterboring. The following are the basic formula that are commonly used in the drilling operation.
• Cutting speed is the peripheral speed of the cutting edge =π D N; where
D is the drill diameter
and
N is the rotational speed (rpm)
The cutting speed selection depends on both the workpiece and cutter materials;
N = V x 1000rpm, where
Ï€D
V is cutting speed
and
N is rotational speed (rpm)
* Note that V is in m/min and D in mm
•Feed (f) is the distance the drill penetrate per revolution (mm/rev), the share of each
cutting edge is = f/2
• Depth of cut is taken as half the diameter for drilling = D/2
• Drilling time (T) can be given by the equation;
T = L / f N; where
f is the feed (mm/rev)
N is the rotational speed (rpm)
L is the sum of hole depth, approach and over travel distances
*The approach is usually considered as 0.4D while over travel ranges from 1 to 3mm.
•Material removal rate (MRR) in drilling is the volume of material removed by the
drill per unit time.
MRR = (Ï€ D² / 4).(f).(N)
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4.3.1 Sample Questions
1) 20 mm diameter drill is on a drilling machine with cutting speed 25. If the
feed is 0.25 mm/rev, what is the MRR? What is the MRR is the drill diameter
is tripled?
2) The hole of a diameter of 44.7 mm is to be drilled to a depth of 60 mm and reamed to a diameter of 45 mm. The feed is 0.56 mm/rev for drilling. The cutting speed is 70.6 m/min. The reaming approach is 0.55 mm and over travel is 2 mm. Determine :-
a) The feed and spindle speed for reaming operation.
b) The drilling time
c) The reaming time
d) The MRR for the drilling operation.
4.4 DRILLING TECHNIQUES
In a standard drilling operation, the following steps are taken (Gupta, 1996):
a) Firstly, the workpiece is marked with a centre punch at the centre of the hole to
be drilled.
b) It is then held firmly in a vice or other suitable clamping device and place on the
table of the drilling machine.
c) The socket containing the drill is fitted in the machine spindle.
d) The spindle is lowered by the hand lever and it is ensured that the point of the
drill is in exact alignment with the previously marked centre of the hole.
e) The motor is now started and the rotating drill is gradually pressed into the
workpiece to produce the desired hole.
f) The pressure should be frequently relived during the drilling operations,
otherwise the cutting edges of the drill will be strained and the drill is damaged.
g) In order to avoid spoiling the cutting edge of the drill, coolant such as oil or soap
water should be used constantly during the drilling operation
4.4.1 Cutting Fluids
A large number of coolant and cutting oils are used in drilling operations. A good cutting fluid will cool the workpiece and tool and will also act as a lubricant between the chip and the tool to reduce friction and heat buildup. Table 4.2 lists some of the cutting fluids for different materials.
Type of cutting fluid
Materials
Dry
Soluble Oil Kerosene Sulfurised
Oil
Mineral
oil
Aluminum
x
x
Brass
x
x
Bronze
x
x
x
Cast iron
x
Steels
Low carbon
x
x
Alloy
x
x
Stainless
x
x
Others
Emulsifying or soluble oils
mixed in water
:
For main requirement to use inexpensive cooling medium
Animal or mineral oils with
added sulfur or chlorine
:
For operations that tend to create more friction and for
reaming, counterboring, countersinking and tapping
operations
Table 4.2 List of cutting fluids for different materials
4.5 GUIDELINES IN DESIGN
The general design guidelines recommended by Kalpakjianat. (2001) for drilling,
reaming and tapping operations are as follows:
a) Design should allow holes to be drilled on flat surfaces and perpendicular to
the drill motion. Exit surfaces for the drill should also be flat.
b) Interrupted hole surfaces should be avoided or minimized for improved
dimensional accuracy.
c) Hole bottom should if possible match standard drill point angles (Section
4.1.1). Flat bottoms or odd shapes should be avoided
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d) Through holes are preferred over blind holes. If holes with large diameters are
required, the parts should have a pre-existing hole.
e) Part should be designed so that all drilling can be done with a minimum of
fixturing and without repositioning the workpiece.
f) It may be difficult to ream blind or intersecting holes because of the
possibility of tool breakage. Extra hole depth should be provided.
g) Blind holes must be drilled deeper than subsequent reaming or tapping
operations that may be performed.
4.6 TROUBLESHOOTING
A general guideline to the probable causes of problems in drilling operations is listed
below Kalpakjian & Schmid, 2001:
Problem
Possible Causes
Drill breakage
:
Dull drill; seizing in hole because of chips clogging
flutes; feed too high; lip relief angle too small
Excessive drill wear
:
Cutting speed too high; ineffective cutting fluid; rake angle too high; drill burned and strength lost when sharpened
Tapered hole
:
Drill misaligned or bend; lips not equal; web not central
Oversized hole
:
Same as above; machine spindle loose; chisel edge not
central; side pressure on workpiece
Poor hole surface finish
:
Dull drill; ineffective cutting fluid; welding of workpiece material on drill margin; improperly ground drill; improper alignment
Table 4.3: List of problems and possible causes in drilling operation
Figure 4.4: Causes of oversize drilling: (a) drill
lips ground to unequal lengths, (b) drill lips
ground to unequal angles, and (c) unequal
angles and lengths
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4.7 SAFETY RULES
Poor work habits will lead to injuries. Chip flying into unprotected eyes, dropping heavy tools, getting hair or clothing caught in a rotating drill are among many hazards that can be avoided through safe work habits. The list below denotes a safety rules that student/ machinist need to be follow when doing the drilling operation:
a) Workpieces should always be secured with bolts and strap clams, C-clamps or
fixtures. A drill press vise should be used when drilling small parts. If a clamp
should come loose and a "merry go round" result, do not try to stop it from
turning with your hands. Turn of the machine quickly.
b) Ensure all parts are properly secured and safe to run before starting the
machine.
c) Never clean the taper in the spindle when the drill is running, since this could
result in broken fingers or worse injuries.
d) Always remove the chuck key immediately after using it. A key left in the
chuck will be thrown out at high velocity when the machine is turned on.
e) Never stop the drill press spindle with your hand after you have turned off the
machine, sharp chips often collect around the chuck or spindle. Do not reach
around, near or behind a revolving drill.
f) Do not leave the machine running unattended.
g) Remove and clear all metal chips in between machine parts before and after
usage.
h) Do not change gear when the machine is running.
i) Use a brush instead of your hands to clean chips off the machine. Never use
an air jet for removing chips as this will cause the chips to fly at a high
velocity and may lead to cuts and eye injuries. Do not clean chips or wipe oil
while the machine is running.
j) When moving the head or table on sensitive drill presses, make sure a safety clamps is set just below the table or head on the column; this will prevent the table from suddenly dropping if the column clamp is prematurely released.
4.8 PROJECT DRILLING
To be supplied by Sis Tuti
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