Introduction :
➤ The drilling machine is a very important machine tool in the industry.
➤ Drilling is an operation through which holes are produced in a solid metal by using a revolving tool which is called as drill.
➤ In a drilling machine holes are drilled quickly and at a low cost.
➤ With the help of drilling operation, perfectly true holes are not produced hence it is also considered as a roughing operation.
➤ Therefore, for accurate holes, drilling is followed by another operation which is called as reaming.
➤ In reaming operation, a multi-tooth revolving tool is used which is termed as reamer.
➤ With the help of reaming operation, the required dimensional accuracy and fine surface finish is obtained.
➤ Sometimes, for enlarging an existing hole size, one more operation is done which is known as boring.
➤ The operation of drilling, boring and reaming can be done by hand feed as well as power feed on various machines such as centre lathe, capstan and turret laths, drilling machine, boring machine, portable machines and special purpose machines.
Working Principle of Drilling Machine :
➤ In a drilling machine, the work-piece is clamped firmly on the work table by using nuts and bolts.
➤ The used to produce holes in solid material called as drill is press fitted in a drill chuck.
➤ The rotating drill is made of harder material than that of the work-piece and it is fed against the stationary work-piece by hand feed as well as power feed.
➤ The rotating drill is made of harder material than that of the work-piece and it is fed against the stationary work-piece by hand feed as well as power feed.
➤ During the operation, the material is removed in the form of chips.
➤ While drilling, the feed handle is rotated in anticlockwise direction for providing the feed to the drill and at the end of operation the feed handle is rotated in clockwise direction to remove the drill from the work-piece.
➤ It is important to note that, during the process high amount of heat is generated, hence continuous supply of coolant is required.
Classification :
The tool used to originate or enlarge a hole in a solid material is called as drill. Drills are manufactured in several different forms and sizes. Some of the commonly used drills are as follows :
( 1 ) According to the type of shank :
➤ Parallel shank
➤ Taper shank
( 2 ) According to the type of flutes :
➤ Flat drills
➤ Twist drills
( 3 ) According to length of drill :
➤ Short series drills
➤ Stub series drills
➤ Long series drills
( 4 ) According to the applications :
➤ Core drills
➤ Centre drills
➤ Masonary drills
( 5 ) According to the material of tool :
➤ High speed steel drills
➤ Carbide tipped drills
Types of Drilling Machines :
Drilling machines are manufactured in various sizes and varieties to suit different types of work-piece. They are broadly classified as follows :
( 1 ) Portable drilling machine
( 2 ) Sensitive drilling machine
➤ Bench mounting machine
➤ Floor mounting machine
( 3 ) Upright drilling machine
➤ Round column machine
➤ Box column machine
( 4 ) Radial drilling machine
➤ Plain machine
➤ Universal machine
➤ Semi-universal machine
( 5 ) Deep hole drilling machine
➤ Vertical machine
➤ Horizontal machine
( 6 ) Gang drilling machin
( 7 ) Multiple spindle drilling machine
( 8 ) Automatic drilling machine
Portable Drilling Machine :
➤ Portable drilling machine is very small, compact and self contained unit carrying a small electric motor inside it.
➤ It is generally used for drilling holes in components which cannot be transported to the workshop, because of their size or weight.
➤ In such cases, the drilling operation is performed on the site by using portable electric drill.
➤ Portable drills are light in weight and manufactured in different sizes and capacities.
➤ Usually, they are made to hold drills of sizes 12 mm to 18 mm diameter.
➤ One of the major advantages of portable drills is that, the holes can be drilled at any desired inclination.
➤ As small size of drill is used, the machine is operated at high speed.
➤ The motor used in machine is universal type which may be driven by AC or DC supply.
Radial Drilling Machine :
➤ Radial drilling machine is designed for drilling medium to large and heavy workpieces.
➤ It consists of a heavy, round and vertical column mounted on a large base.
➤ To accommodate workpieces of different heights, the column supporting a radial arm can be raised or lowered.
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| Radial drilling machine |
➤ The arm may be swung around to any required position over the work bed.
➤ The drill head is mounted on a radial arm and can be moved horizontally on the guide-ways.
➤ These three movements in a radial drilling machine when combined together, allows the drill to be located at any required point on a large workpiece for drilling the hole.
➤ When several holes are to be drilled on a large workpiece, the position of the arm and drill head is changed, so that the drill spindle may be moved from one position to another position after drilling the hole without changing the setting of the workpiece.
➤ Based on the type and number of movements possible, the radial drilling machine can be grouped as :
➤ Plain radial drilling machine :
➤ Semi universal radial drilling machine :
➤ Universal radial drilling machine :
Radial drilling machine parts :
( 1 ) Base
( 2 ) Column
( 3 ) Radial arm
( 4 ) Drill head
( 5 ) Spindle speed and feed mechanism
Automatic Drilling Machine :
➤ These machines are arranged in series to perform a number of different operations in sequence at successive work stations.
➤ After completion of one operation, the workpiece is transferred to the next station automatically for further operation, hence it works as a transfer line.
➤ According to the cutting speed and feed, the operation sequence are arranged and synchronised such that, once the workpiece is loaded at the first station, it is automatically transferred to next position and finally unloaded.
➤ As per the requirement, the spindle heads may be single spindle, multispindle or combination of these.
➤ These types of machines are suitable for production purposes and used for milling, grinding and similar operations in addition to drilling and tapping.
Drilling Machine Operations :
The different operations that can be performed on a drilling machine are as follows :
1. Drilling
2. Reaming
3. Boring
4. Counter-boring
5. Counter-sinking
6. Spot facing
7. Tapping
8. Trepanning
2. Reaming
3. Boring
4. Counter-boring
5. Counter-sinking
6. Spot facing
7. Tapping
8. Trepanning
Now,,,,,
( 1 ) Drilling :
➤ Drilling is the operation of producing circular hole in a solid metal by using revolving tool which is called as drill.
➤ Before drilling, the centre of the hole is locating on the work-piece by drawing two lines at right angle to each other and centre punch is used to produce an indentation at the centre.
( 2 ) Reaming :
➤ Reaming is the operation of finishing a hole which has been previously drilled and has a coarse surface finish.
➤ The operation is performed by using a multi-tooth tool called as reamer.
➤ Reamer cannot produce a hole, it only follows the path which has been previously drilled and removes a very small amount of material.
( 3 ) Boring :
➤ Boring is the process of enlarging a drilled hole by using an adjustable cutting tool which is called as boring tool.
➤ The boring operation is used for the following purposes.
⟹ To finish a drilled hole accurately and boring it to the required size.
⟹ To correct the roundness of a hole.
➤ It is important to note that, the boring tool is having only one cutting edge.
( 4 ) Counter-boring :
➤ The operation which is used for enlarging only limited portion of the hole is called as counter-boring and the tool used for this purpose is called as counter-bore.
➤ Counter-boring operation in which the enlarged hole forms a square shoulder with the original hole.
➤ The cutting edges of counter-bore may have straight or spiral teeth.
( 5 ) Counter-sinking :
➤ Countersinking operation is used for enlarging the end of a hole and to give it a conical shape for a short distance.
➤ Counter-sinking operation in which the tool used is called as countersunk.
➤ The standard countersinks have the included angles of 60ᣞ, 82ᣞ or 90ᣞ and cutting edges of tool are at the conical surface.
( 6 ) Spot facing :
➤ It is operation of smoothing and squaring the surface around a hole for the seat of a nut or the head of a screw.
➤ The hole may be spot faced below the rough surface or above it.
➤ For spot facing, counter-bore or special spot facing tool is used.
( 7 ) Tapping :
➤ Tapping is an operation of cutting internal threads by using a cutting tool called as tap.
➤ For tapping purpose, the machine should be equipped with a reversible motor or some other reversing mechanism.
➤ A tap is considered as a bolt with accurate threads cut on it and the threads act as cutting edges which are hardened and ground.
➤ Tapping operation in which the tap is screwed into the hole and metal is removed.
( 8 ) Trepanning :
➤ When large diameter holes are required in a sheet metal, then trepanning operation is carried out.
➤ It is carried out by using a trepanning tool.
➤ A small hole, to suit the pilot is drilled at the centre of the required position.
➤ The adjustable arm is extended so that the edge of the high speed-steel cutting tool produces the required hole size.
➤ The pilot is located in the pilot hole and the tool rotates as it is fed through the work-piece.
➤ Drill sockets are much longer in size than the drill sleeves.
➤ It consists of a solid shank attached to the end of a cylindrical body.
➤ The taper shank of the socket confirms to the taper of the drill spindle and fits into it.
➤ The socket body has a tapered hole larger than the drill spindle taper into which the taper shank of any tool may be fitted.
➤ The tang of the tool fits into the slot provided in the socket and the tang of the socket fits into the spindle slot.
Drill chucks :
➤ The chucks are especially made for holding smaller sized drills.
➤ A sleeve or socket can hold only one size of tool shank, but a chuck is used to hold different sizes of tool shanks within a certain limit.
➤ The drill chucks have tapered shanks which fit into the machine spindle.
➤ The most commonly used chucks are :
⟹ Quick change chuck
⟹ Three jaw self-centering chuck
Types of Reamers :
The various type of reamers which are used in the industry are as follows :
1. Hand reamers
2. Chucking or machine reamers
3. Adjustable reamers
4. Expansion reamers
5. Taper reamers
6. Taper pin reamers
7. Shell reamers
8. Carbide tipped reamers
Hand Reamers :
➤ Generally, hand reamers are provided with a parallel shank and square tang.
➤ The flutes of hand reamer may be straight or spiral.
➤ In order to have an unrestricted entry into the previously drilled hole, the reamer carries a taper towards the end of its flutes.
➤ In any case, these reamers are not used for machine reaming.
Tool Holding Devices :
Different cutting tools for different operations can be held on a revolving spindle of a drilling machine. The various methods used for holding tools in a drill spindle are as follows :
1. By directly fitting in the spindle
2. By a sleeve
3. By a socket
4. By chucks
Directly holding the tool :
➤ All drilling machines have the spindle bored out to a standard taper for receiving the taper shank of the tool.
➤ The taper used in a drill spindle is Morse standard taper, which is generally 1:20.
➤ During the tool fitting, the shank is forced into the tapered hole and the tool is gripped by friction.
➤ The tool may be rotated with the spindle by friction between the tapered surface and the spindle, but to ensure a positive drive, the tang of the tool fits into a slot at the end of the tapered hole.
➤ To remove the tool, a tapered wedge called as drift is inserted into the slotted hole of the spindle.
Sleeve :
➤ The main limitation of drill spindle is that, it is suitable only for holding one size of shank.
➤ Hence, if the taper shank of the tool is smaller than the taper in the spindle hole, then a taper sleeve is used.
➤ The outside taper of the sleeve confirms to the drill spindle taper and the inside taper holds the shanks of smaller size tools.
➤ The sleeve fits into the taper hole of the spindle and holds tool shank of smaller sizes.
➤ By forcing a drift within the slot of the spindle, the sleeve with the tool can be removed by using similar process the tool can be separated from the sleeve.
➤ By using different sizes of sleeve, different sizes of tool shank may be held in the spindle.
Socket :
➤ Drill sockets are used to hold the tool, when the tapered tool shank is larger than the spindle taper.➤ Drill sockets are much longer in size than the drill sleeves.
➤ It consists of a solid shank attached to the end of a cylindrical body.
➤ The taper shank of the socket confirms to the taper of the drill spindle and fits into it.
➤ The socket body has a tapered hole larger than the drill spindle taper into which the taper shank of any tool may be fitted.
➤ The tang of the tool fits into the slot provided in the socket and the tang of the socket fits into the spindle slot.
Drill chucks :
➤ The chucks are especially made for holding smaller sized drills.
➤ A sleeve or socket can hold only one size of tool shank, but a chuck is used to hold different sizes of tool shanks within a certain limit.
➤ The drill chucks have tapered shanks which fit into the machine spindle.
➤ The most commonly used chucks are :
⟹ Quick change chuck
⟹ Three jaw self-centering chuck
Cutting Speed, Feed and Depth of Cut :
The cutting speed, feed and depth of cut in a drilling operation depends upon the following factors :
➤ Material to be cut ( Work-piece )
➤ Material of the tool
➤ Surface finish required
➤ Types of coolant used
➤ Machine capacity and type
➤ Life of the too, etc.
Cutting Speed :
➤ In drilling machine, cutting speed refers to the peripheral speed of a point on the surface of the drill in contact with the work-piece.
➤ Generally, cutting speed is expressed in meters per minute.
➤ Cutting speed is calculated by using following relation :
Cutting speed = ã„«dN / 1000 m / min.
Where, d = Diameter of drill in mm,
N = Spindle speed in r.p.m.
➤ From the above relation, it is clear that for the same cutting speed, the smaller the drill diameter more will be the rotational speed of the spindle.
➤ It means smaller drill rotates at high speed than a larger drill, to remain the same cutting speed.
➤ Also, the cutting speed is maximum at the periphery and zero at the centre of the drill.
Feed :
➤ Feed is the distance that the drill moves, parallel to its axis and into the work-piece in each revolution of the spindle.
➤ Generally, it is expressed in mm per revolution.
➤ If the distance moved by the drill in one minute into the work-piece is considered, then it is expressed in mm per minute.
➤ To convert mm / rev. in mm / min. following relation is used :
Feed in mm / min. = Feed in mm / rev. * N
Depth of Cut :
➤ Depth of cut in drilling operation is measured at right angles to the axis of the drill.
➤ Generally, depth of cut is one-half of the drill diameter.
➤ It is given by,
Depth of Cut = Drill diameter / 2 mm
Machining Time :
➤ In drilling operation the machining time is given by :
T = L / N*f , minutes
Where, T = Machining time in minutes,
L = Length of axial travel of drill in mm,
f = Feed per revolution in mm,
N = Drill speed in r.p.m.
➤ The above equation is used only to drill a single hole.
Alignment Tests on Drilling Machine :
( 1 ) Flatness of the machine table and the base :
➤ In this test, the straight edge is placed on the table and base of the machine with the help of two gauge blocks at various positions.
➤ The error is noted down by using the feeler gauges and the permissible error should not exceed 0.1 mm.
( 2 ) Perpendicularity of drill head with table and base :
This test can be carried out in the following two ways :
➤ Vertical plane passing through axes of spindle and column.
➤ Plane perpendicular to vertical plane.
➤ The frame level is provided on the column and base plate. The error is calculated by the difference in the level of both the frame level.
( 3 ) True running of spindle taper :
➤ In this test, the mandrel is fixed in the tapered hole oh the spindle and the dial indicator is fixed on the table.
➤ The plunger of the dial gauge pressed against the test mandrel.
➤ The spindle is then rotated slowly and the deviations in the dial gauge are noted down.
➤ The permissible error is up to 0.03 to 0.04 mm.
( 4 ) Perpendicularity of spindle axis with table :
➤ In this test, the straight edge is placed in a position xx' and yy'.
➤ The dial gauge is fitted in the spindle and its plunger is pressed against the straight edge. The first reading at x is noted down.
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| Perpendicularity of spindle axis with table |
➤ Spindle is then rotated by 180ᣞ and the another reading at x' is noted down. The difference between these two readings gives the error in the perpendicularity.
➤ Similar process is repeated for yy'.
➤ The permissible error is up to 0.05 to 0.08 mm.
