Petróleo_Drilling

7/29/2019 Petrleo_Drilling

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Prepared by: Syed Sajid Hussain Page1 of 7

Drilling and Drills:

Holes are generally used either for assembly with fasteners, such as bolt, screws and

rivets, or to provide access to the inside of a part. Holemaking is among the most

important operations in manufacturing. In automotive engine production, the cost of

holemaking is one of the largest machining costs. Drilling is a major and common

holemaking process.

Drills:

Because drills usually have a high length-to-diameter ratio, they are capable of

producing relatively deep holes. They are somewhat flexible, however, depending ontheir diameter, and should be used with care in order to drill holes accurately and to

prevent the drill from breaking. Furthermore, the chips that are produced within the

workpiece have to move in the direction opposite to the axial movement of the drill.

Consequently, chip disposal and the effectiveness of cutting fluids can present

significantdifficulties in drilling.

Generally, the hole diameter produced by drilling are slightly larger than the drill

diameter (oversize). The amount of oversize depends on the quality of the drill and of

the equipment used, as well as on the practice employed.

Depending on their thermal properties, some metals and nonmetallic materials expand

significantly due to the heat produced by drilling. So the final hole could be smaller than

the drill diameter.

Twist Drill:

The most common drill is the conventional standard-point twist drill (Fig.), the main

features of which are the point angle, lip-relief angle, chisel-edge angle, and the helixangle.


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Prepared by: Syed Sajid Hussain Page2of 7

Generally, two spiral grooves (flutes) run the length of the drill, and the chips produced

are guided upward through these grooves. The grooves also serve as passageways to

enable the cutting fluid to reach the cutting edge. Some drills have internal longitudinal

holes through which cutting fluids are forced, improving lubrication and cooling as well

as washing away the chips.Drills are available with a chip-breaker feature ground along the cutting edges. This

feature is important in drilling with automated machinery where disposal of long chips

with out operator assistance is necessary.

Drill Point Geometries:

Small changes in drill geometry can have a significant effect on the drill's performance,

particularlyin the chisel edge region.

Too small a lip relief angle increases the thrust force, generates excessive heat,and increase wear.

Too large a lip relief angle can cause chipping or breaking of the cutting edge.


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Other Types of Drills:

Several types of drills are shown in Fig.

1. A step drill produces holes of two or more different diameter.2. A core drill is used to make an existing hole larger.3. Counterboring and countersinking drills produce depressions on the surface to

accommodate the heads of screws and bolts.

4. A center drill is short and is used to produce the hole at the end of a piece ofstock so that it may be mounted between centers in a lathe (between theheadstock and the tailstock).

5. A spot drill is used to spot (to start) a hole at the desired location on a surface.6. Spade drills have removable tips or bits and are used to produce large and deep

holes.

7. Crankshaft drills have good centering ability, and because chips tend to break upeasily, these drills are suitable for producing deep holes.

8. Gun drilling, developed originally for drilling gun barrels, gun drilling is usedfor drilling deep holes and requires a special drill (Fig.). The depth-to-diameter

rations of holes produced can be 300:1 or even higher. Cutting speeds in gun

drilling are usually high and feeds are low. The cutting fluid is forced under high

pressure through a longitudinal hole in the body of the drill.


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9. Trepanning: In trepanning, the cutting tool (Fig.) produces a hole by removing adisk-shaped piece (core), usually from flat plates. A hole is produced without

reducing all the materials removed to chips, as in case of drilling. Trepanning can

be done on lathes, drill presses, or other machines, using single-point or

multipoint tools.


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Drill Reconditioning:

Drills are reconditioned by grinding them either manually or using fixtures. Proper

reconditioning of drills is important, particularly with automated manufacturing on

CNC machines. Hand grinding is difficult and requires considerable skill in order to

produce symmetric cutting edged.

Measuring Drill Life:

The resharpening or replacement of dull drills is important, particularly in automated

production. The use of dull drills increases forces and power; causes surface damage,

and produce inaccurate holes. The life of drills is usually measured by the number of

holes drilled before they become dull. The test procedure consists of clamping a block of

material in suitable dynamometer or force transducer and drilling a number of holes

while recording the torque or force during each successive operation. After a number of

holes have been drilled, the torque and force begins to increase because the tool is

becoming dull. (Fig.)

Drill life is defined as the number of holes drilled until this transition begins.


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Material Removing rate in drilling:

The material removal rate (MRR) in drilling is the volume of material removed

by per unit time. For a drill with a diameter D, the cross sectional area of the

drilled hole is x D2 / 4 . The velocity of the drill perpendicular to the

workpiece is the product of the feed, f (the distance drill penetrates per unit

revolution), and the rotational speed N, Where N = V / D. Thus

MRR = {

x D2

/ 4} x f x N


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Problem : A hole is being drilled in a block of magnesium alloy with a 10 mm

drill bit at a feed of 0.2 mm/rev., and with the spindle running at N = 800 rpm.

Calculate the material removal rate and the torque on the drill.? (Unit power for

magnesium alloy is 0.5 W . s/ mm3

MRR = { x D2 / 4} x f x N

= ( x 10 x 10 / 4) x 0.2 x 800

= 12,570 mm3

/ min.= 210 mm3/ sec.

Power = 210 x 0.5 = 105 W

Torque = Power / rotational speed

Rotational speed = 800 x 2 x / 60= 83.8 radians per sec.

Hence,

Torque = 105/83.8

= 1.25 N.m

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