Drilling

Drilling is the operation of producing circular hole in the work-piece by using a rotating cutter called DRILL.

• The machine used for drilling is called drilling machine.

 

• The drilling operation can also be accomplished in lathe, in which the drill is held in tailstock and the work is held by the chuck.

• The most common drill used is the twist drill.

 

Drilling Machine

• It is the simplest and accurate machine used in production shop.

 

• The work piece is held stationary ie. Clamped in position and the drill rotates to make a hole.

 

Types

            1) Based on construction:
Portable,
Sensitive,
Radial,
up-right,
Gang,
Multi-spindle

            2) Based on Feed:
Hand driven
Power driven

Components of drilling machine

Spindle
The spindle holds the drill or cutting tools and revolves in a fixed position in a sleeve.

Sleeve
The sleeve or quill assembly does not revolve but may slide in its bearing in a direction parallel to its axis. When the sleeve carrying the spindle with a cutting tool is lowered, the cutting tool is fed into the work: and when it’s moved upward, the cutting tool is withdrawn from the work. Feed pressure applied to the sleeve by hand or power causes the revolving drill to cut its way into the work a fraction of an mm per revolution.

Column
The column is cylindrical in shape and built rugged and solid. The column supports the head and the sleeve or quill assembly.

Head
The head of the drilling machine is composed of the sleeve, a spindle, an electric motor and feed mechanism. The head is bolted to the column.

Worktable
The worktable is supported on an arm mounted to the column. The worktable can be adjusted vertically to accommodate different heights of work or it can be swung completely out of the way. It may be tilted up to 90 degree in either direction, to allow long pieces to be end or angle drilled.
Base
The base of the drilling machine supports the entire machine and when bolted to the floor, provides for vibration-free operation and best machining accuracy. The top of the base is similar to the worktable and may be equipped with t- slot for mounting work too larger for the table.

Hand Feed

The hand- feed drilling machines are the simplest and most common type of drilling machines in use today. These are light duty machine that are operated by the operator, using a feed handled, so that the operator is able to “feel” the action of the cutting tool as it cuts through the work piece. These drilling machines can be bench or floor mounted.

Power feed

The power feed drilling machine are usually larger and heavier than the hand feed ones they are equipped with the ability to feed the cutting tool in to the work automatically, at preset depth of cut per revolution of the spindle these machines are used in maintenance for medium duty work or the work that uses large drills that require power feed larger work pieces are usually clamped directly to the table or base using t –bolts and clamps by a small work places are held in a vise. A depth –stop mechanism is located on the head, near the spindle, to aid in drilling to a precise depth.

Sensitive or Bench Drilling Machine

• This type of drill machine is used for very light works. Fig.1 illustrates the sketch of sensitive drilling machine.
• The vertical column carries a swiveling table the height of which can be adjusted according to the work piece height.
• The table can also be swung to any desired position.
• At the top of the column there are two pulleys connected by a belt, one pulley is mounted on the motor shaft and other on the machine spindle.
• Vertical movement to the spindle is given by the feed handle by the operator.
• Operator senses the cutting action so sensitive drilling machine.
• Drill holes from 1.5 to 15mm

 

 

Fig.1. Sensitive Drilling Machine

Up-Right Drilling Machine

• These are medium heavy duty machines.
• It specifically differs from sensitive drill in its weight, rigidity, application of power feed and wider range of spindle speed. Fig.2 shows the line sketch of up-right drilling machine.
• This machine usually has a gear driven mechanism for different spindle speed and an automatic or power feed device.
• Table can move vertically and radially.
• Drill holes up to 50mm

 

Fig.2 Up-Right Drilling Machine

Radial Drilling Machine

• It the largest and most versatile used for drilling medium to large and heavy work pieces.
• Radial drilling machine belong to power feed type.
• The column and radial drilling machine supports the radial arm, drill head and motor. Fig.3 shows the line sketch of radial drilling machine.

Fig. 3 Radial Drilling Machine

• The radial arm slides up and down on the column with the help of elevating screw provided on the side of the column, which is driven by a motor.
• The drill head is mounted on the radial arm and moves on the guide ways provided the radial arm can also be swiveled around the column.
• The drill head is equipped with a separate motor to drive the spindle, which carries the drill bit. A drill head may be moved on the arm manually or by power.
• Feed can be either manual or automatic with reversal mechanism.  

 

Drill Materials

The two most common types are
1. HSS drill
- Low cost
2. Carbide- tipped drills
- high production and in CNC machines

Other types are
Solid Carbide drill, TiN coated drills, carbide coated masonry drills, parabolic drills, split point drill. Fig.4 shows various types of drills

 

 

 

Fig. 4 Various types of drill

 

 

 

Drill fixed to the spindle

Fig. 5 Drill fixed to a spindle

 

Tool Nomenclature

 

Fig. 6 Nomenclature of twist drill

 

Tool holding devices

Fig.7 and Fig.8 shows the different work holding and drill drift device. The different methods used for holding drill in a drill spindle are

• By directly fitting in the spindle hole.
• By using drill sleeve
• By using drill socket
• By using drill chuck

 

 

Drilling operations

Operations that can be performed in a drilling machine are

•          Drilling
•          Reaming
•          Boring
•          Counter boring
•          Countersinking
•          Tapping

 

Drilling:

It is an operation by which holes are produced in solid metal by means of revolving tool called ‘Drill’. Fig. 9 shows the various operations on drilling machine.

Reaming:
Reaming is accurate way of sizing and finishing the pre-existing hole.
Multi tooth cutting tool. Accuracy of ±0.005mm can be achieved

 

Boring:
Boring is a process of enlarging an existing hole by a single point cutting tool. Boring operation is often preferred because we can correct hole size, or alignment and can produce smooth finish. Boring tool is held in the boring bar which has the shank. Accuracy of ±0.005mm can be achieved.

 

.
Fig. 9 Various operations on drilling machine

Counter Bore :-

            This operation uses a pilot to guide the cutting action to accommodate the heads of bolts. Fig. 10 illustrates the counter boring, countersunk and spot facing processes.

Countersink:-

            Special angled cone shaped enlargement at the end of the hole to accommodate the screws. Cone angles of 60°, 82°, 90°, 100°, 110°, 120°

 

 

 

Fig. 10 Counter boring, countersunk and spot facing

Tapping:-

            Tapping is the process by which internal threads are formed. It is performed either by hand or by machine. Minor diameter of the thread is drilled and then tapping is done. Fig. 11 show the tapping processes.

 

Fig. 11 Hand taps and tapping  process using tap wrench

 

Fig. 12 Various operations performed on drilling machine

 

Work Holding Devices

1. Machine Table Vice

The machine vice is equipped with jaws which clamps the work piece. The vice can be bolted to the drilling table or the tail can be swung around swung around. Fig. 13 shows the standard and swivel vice.
The swivel vice is a machine wise that can be swivel through 360° on a horizontal plane.

 

Fig. 13 Machine Table vice.

2. Step Blocks

These are built to allow height adjustment for mounting the drilling jobs and are used with strap clamps and long T-slot bolts.

3. Clamps

These are small, portable vises , which bears against the work piece and holding devices. Common types of clamps are C-clamp, Parallel clamp, machine strap clamp, U-clamp etc.. Fig. 14 shows the correct and incorrect methods of mounting the work piece.

Fig. 14 Correct and incorrect methods of clamping the work piece.

4. V-Blocks

These are designed to hold round work pieces.

5. Angles

Angle plates are made in a 90°angle with slots and bolt holes for securing work to the table.

6. Jigs

The jig guides the drill through a bushing to locate and drill holes accurately.

7. T- Slots Bolt

These are special bolts which has a T shaped head, which slides into the T slots of drilling machine work table.

 

 

 

Definitions

1. Cutting Speed (v):-

            It’s the peripheral speed of the drill.The cutting speed depends upon the properties of the material being drilled, drill material, drill diameter, rate of speed, coolant used etc…
v = P*D*N  where
D =  dia of the drill in m
N = Speed of rotation in rpm

 

2. Feed Rate (f):-

            It’s the movement of drill along the axis (rpm)

 

3. Depth of Cut (d):-

The distance from the machined surface to the drill axis.
d = D / 2

As the depth of hole increases,  the chip ejection becomes more difficult and the fresh cutting fluid is not able to cutting zone. Hence for machining the lengthy hole special type of drill called ‘gun drill’ is used.

 

4. Material Removal Rate:-

            It’s the volume of material removed by the drill per unit time
MRR = (P D2 / 4) * f * N  mm3 / min

5. Machining Time (T) :-

            It depends upon the length (l) of the hole to be drilled , to the Speed (N) and feed (f) of the drill
t = L / f N min

 

Precautions for Drilling machine

• Lubrication is important to remove heat and friction.
• Machines should be cleaned after use
• Chips should be removed using brush.
• T-slots, grooves, spindles sleeves, belts, and pulley should be cleaned.
• Machines should be lightly oiled to prevent from rusting

 

Safety Precautions

• Do not support the work piece by hand – use work holding device.
• Use brush to clean the chip
• No adjustments while the machine is operating
• Ensure for the cutting tools running straight before starting the operation.
• Never place tools on the drilling table
• Avoid loose clothing and protect the eyes.
• Ease the feed if drill breaks inside the work piece.

 

Problems

1. Calculate the speed of the  drill bit to drill a hole of dia 20mm where the cutting speed is 25mts/min.

N [rpm] =  (π * D *N)/1000
=  (25 * 1000) / (π * 20)
= 397.8rpm

2. The dia of one end of a taper plug is 150mm and dia of the other end is 80mm and the length is 300mm.  Calculate its taper.
Taper per mm = (D-d)/L
= (150-80)/ 300
= 0.233mm
3. . The dia of one end of a taper plug is 150mm and dia of the other end is 80mm and the length is 300mm.  Calculate its taper angle.
Tan α/2= (D-d)/2 L
= (150-80)/ 2*300
= 6.65°

Review Questions:

Part – A

1. What is meant by drilling?
2. Which tool is commonly used for drilling?
3. Name the different types of drilling machine?
4. What is meant by hand feed?
5. What is meant by power feed?
6. What are the salient features of radial drilling machine?
7. What are the different ways to mount the drilling tool?
8. Name the different types of drilling operations?
9. What is meant by reaming?
10. What is boring?
11. What is the difference between reaming and boring?
12. What is counter boring?
13. What is countersinking?
14. What is the purpose of tapping operation?
15. Name some work holding devices?

 

Part – B

1. Explain with neat sketch the salient features of radial drilling machine?
2. Draw and explain the working principle of upright drilling machine?
3. With neat sketch describe the nomenclature of twist drill?
4. Discuss is detail with diagram the various operations that can be performed in drilling machine