Drilling

Drilling, Parts of drill, Specification of drilling machine, Types of Drilling Machine, Sensitive Drilling Machine, Radial Drilling Machine, Tool Holding Devices, Types of Operations in Drilling, Types of Drill, Twist drill nomenclature, Drill angles

Introduction:

          A drill is a rotary end cutting tool with one or more cutting lips and usually one or more flutes for the passage of chips and the admission of cutting fluid. A drilling machine is a machine tool designed for drilling holes in metals. It is one of the most important and versatile machine tools in a workshop.

Drilling Principle:

          Drilling is the process of producing hole on the work piece by using a rotating cutter called drill. Drilling is the process to removes solid metal from the job to produce a circular hole. Before drilling, the hole is located by drawing two lines at right angle and a center punch is used to make an indentation for the drill point at the center to help the drill in getting started. A suitable drill is held in the drill machine and the drill machine is adjusted to operate at the correct cutting speed. The drill machine is started and the drill starts rotating. Cutting fluid is made to flow liberally and the cut is started. The rotating drill is made to feed into the job. The hole, depending upon its length, may be drilled in one or more steps. After the drilling operation is complete, the drill is removed from the hole and the power is turned off. Besides drilling round holes, many other operations can also be performed on the drilling machine such as;

• Counter-boring

• Counter-sinking

• Honing

• Reaming

• Lapping.

Parts of drill:

1. Point:

The cone shaped end which does the cutting is called the point. It consists of a dead centre, lips or cutting edges, and a heel.

2. Shank:

This is the driving end of the drill which is fitted on the machine. Shanks are of two types. Taper shank used for larger diameter drills, and straight shank used for smaller diameter drills.

3. Tang:

This is a part of the taper shank drill which fits into the slot of the drilling machine spindle.

4. Body:

The portion between the point and the shank is called the hotly of a drill. The parts of the hotly are flute, land/margin, body clearance and web.

5. Flutes:

Flutes are the spiral grooves which run to the length of the drill. The flutes help,

• To form the cutting edges

• To curl the chips and allow to come out

• To flow the coolant to the cutting edge.

6. Land/margin:

The land/margin is the narrow strip which extends to the entire length of the flutes. The diameter of the drill is measured across the land/ margin.

7. Body clearance:

Body clearance is the part of the body which is reduced in diameter to cut down the friction between the drill and the hole being drilled.

8. Web:

Web is the metal column which separates the flutes. It gradually increases in thickness towards the shank.

Specification of drill:

• Drills are generally specified by the diameter of hole it is intended to produce.

• The size of the drill is marked on the shank just above the flutes.

Specification of drilling machine:

• The specification of drilling machines depends on the type of machine.

• The portable drilling machines are specified by the maximum diameter of drills they can hold.

• Sensitive machines are specified by the diameter of the largest work piece that can be drilled.

• In a 500 mm size upright drilling machine, the distance between spindle and front face of column is slightly greater than 250 mm.

• The radial drilling machine is specified by the length of the arm and diameter of column.

Types of Drilling Machine:

1. Portable Drilling Machine

2. Sensitive Drilling Machine

3. Radial Drilling Machine

4. Upright Drilling Machine or Pedestal Drilling Machine

5. Gang Drilling Machine.

1. Portable Drilling Machine:

● It is light in weight, smaller, and compact machine.

● It can be easily taken to required place.

● It is used for drilling small hole up to 18 mm in large work piece.

2. Sensitive Drilling Machine:

● It is light in Weight, High Speed Machines

● Feed is given by hand only.

● The operator can travel of the drill.

● It is divided into two types, they are,

(i) Bench mounting & (ii) Floor mounting.

3. Radial Drilling Machine:

● It is used for heavy and large work pieces.

4. Upright Drilling Machine or Pedestal Drilling Machine:

● It is a heavy duty drilling machine mounted on the floor.

● It classified into two types:

(i) Round Column Section & (ii) Box Column Section.

5. Gang Drilling Machine:

● A number of single spindle drilling machine columns is placed side by side on one

base and has a common work table.

● Each spindle head has individual motor.

● It is used to make one or many holes at a time.

Sensitive Drilling Machine

          It is usually placed on Workbenches, but can also be placed on the floor. This machine enables the operator to sense or feel the cutting action and apply the required pressure while drilling, hence it is called Sensitive drilling machine. Holes up to 15 mm can be drilled in these machines.

Parts of Sensitive drilling machine:

1. Base:

Heavy casting made of cast iron. Supports the column and other parts of the machine.

2. Vertical column:

It is mounted on the base. It carries a moving head. It contains the speed head and driving mechanism.

3. Table:

It supports the workpiece and the work holding devices.

4. Drive mechanism:

Power transmission to the main spindle is done through the stepped cone pulley drives & gearing systems.

Working of Sensitive Drilling Machine:

          Super sensitive drilling machines are designed to drill holes of minimum diameter 0.35mm. These machines rotate with speed of 20,000 rpm and above. This machine uses hand feed which is provided by principle of rack and pinion. Vertical column carries a swiveling table with table clamp to position the table at required height. This work table can be rotated to 360° along the column axis. A driving unit is mounted on the top of the column to drive the spindle. Power from the driving unit is transmitted to spindle through the belts drives. The driving unit drive the drill against the work piece the cutting edge of the drill exerts a large force on the work piece. This forces causes the shearing of material and generates the hole. These machines have capacity of rotating the drills of diameter from 1.5 mm to 15.5 mm.

Sensitive drilling machines are used for;

● Drilling

● Reaming

● Boring

● Counter boring

● Other similar operations.

Radial Drilling Machine

          This type of machine is mounted on the floor and suitable for drilling medium to large and heavy work pieces. It consists of a vertical column with an arrangement to raise or lower and to revolve the ram. This arrangement helps to accommodate jobs of different heights and radial arm can swing to any position above the bed. The major components of the radial drilling machine are:

1. Base:

          This is a heavy rectangular casting. The top of the base is machined to hold the column and the work table.

2. Column:

          It is a cylindrical casting placed vertically on the base. The radial arm is arranged on the column to slide up and down.

3. Radial arm:

          Radial arm can be moved along the column manually as well as by the power operation. This is a large casting with guideways, exactly machined to carry the drill head.

4. Drill head:

          This block is mounted on the radial arm and consists the drill spindle. Gear Mechanism is used to drive the drill spindle at different speeds and also feed the drill.

5. Spindle:

          Spindle is the main part on the drilling machine. Spindle is connected to electric motor shaft which fixed vertically on the radial arm. The chuck is attached to the spindle to hold the drill.

6. Feed Mechanism:

          This radial drilling machine uses the hand feed mechanism. This hand feed is given by rotating the feed hand wheel. This feed mechanism has rack and pinion mechanism to convert the wheel rotation into spindles linear motion.

Classification of radial drilling machine:

Radial Drilling Machine may be classified as following:

1. Plain Radial Drilling Machine:

In a Plain Radial Drilling machine has three axis movements,

(i) Vertical movement of the arm on the column,

(ii) Horizontal movement of the drill head along the arm, and

(iii) Circular movement of the arm in horizontal plane about the vertical column.

2. Semi Universal Radial Drilling Machine:

In a Semi Universal Drilling Machine, head can also swing about a horizontal axis perpendicular to the arm.

3. Universal Radial Drilling Machine:

In Universal Machine, the head can swing on both vertical and horizontal axis to the arm.

Advantages:

● Larger and heavier work may be machined.

● Drilling head can be raised or lowered and can be moved around the column.

● Drilling head moves rapidly to any desired position while work piece remains clamped.

● Suitable to drill large diameter holes and deep holes.

Tool Holding Devices

          Drill chuck are used on the drilling machine for holding the straight shank drills. The drill chuck consist of an arbor having morse taper and is inserted into the spindle under pressure. The drill is inserted into the drill chuck and fitted with a drill chuck key to grip the drill rigidly. It consist of three holes that are used for tightening the drill. Drill chuck are available in various design. According to number of jaws used the chuck can be classified as following;

● Two jaw drill chuck,

● Three jaw drill chuck.

Drilling Machine Spindle:

          Drilling machine spindle is a vertical shaft which holds and rotates the drill. It gets power from the motor through the bevel gear or cone pulley. The spindle rotates within a non rotating sleeve called quill. The rack teeth are cut on the outer surface of the sleeve may be moved up and down by rotating pinion of the vertical movements of the spindle. The lower end of the spindle carries the taper bore to hold the collet and the drill.

Drill Sleeve and Socket:

1. Sleeve:

          It gets power from the motor through the bevel gear or cone puiley. The hole inside the sleeve is tapered to fit the tapered shank twist drill. The outside of the sleeve has a morse taper t0 fit the sleeve in the machine spindle. The tangent end fits into the slot in the spindle. The tapered shank drill is removed from the sleeve with help of drill drift and hammer.

2. Socket:

          A drill socket is used to hold the twist drill which is too large to fit into either the drill spindle or sleeve. It has a tanged end to fit into the slot in the drill spindle. It has a tanged end to fit into the slot in the drill spindle.

Work Holding Devices

          Work piece to be drilled should be properly hold or clamped to prevent them from rotating along with the drill. Improperly secured work is not only a danger to the operator but can also causes inaccurate work and breakage to the drill. Various devices used to ensure proper holding of Work piece are;

● Drill vices

● Parallel bars

● Step blocks

● Angle plates

● V—blocks

● Clamps and T-bolts

● Drill jigs

Drill Vice:

          Drill vice is mechanical screw apparatus used for holding or clamping a work piece to allow work to perform operations. Drill Vices usually have one fixed jaw and another parallel jaw which is moved towards or away from the fixed jaw by the screw. The fixed jaw is molded with the vice body and movable jaw is mounted on a screw. Rotation of screw causes the movable jaw to open and close. This jaw may include a dog hole to hold a bench dog. This vise are made up of Cast Steel or Malleable Cast Iron for high strength.

Types of Operations in Drilling

Drilling:

● Drilling is the operation of making a circular hole by removing of metal from the work piece by rotating cutting tool called drill.

● Drilling is done to removes solid metal from the work piece to produce a circular hole.

● Producing a cylindrical hole by rotating a fluted cutting tool known as drill, is called drilling.

● The drill point is pressed against the job and feed is given to produce the hole.

Reaming:

● Reaming is the operation of sizing and finishing a hole already made by a drill by a tool called reamer.

● A hole produced on the drilling operation has less accurate.

● For accurate finishing, the reamer tool is inserted into the machine spindle and feed the tool into drilled hole as in drilling.

● This reaming operation removes only a small amount of materials.

● This reaming tool may be classified as solid reamer and adjustable reamer.

Boring:

● Boring is the operation of enlarging the already drilled holes.

● On a drill machine it is carried out by holding the boring tool.

● This bore tool is a single point cutting tool.

● Generally this boring operation is performed when a suitable size of drill is not available for drilling.

● Boring is done on a drilling machine to increase the size of an already drilled hole.

● When a suitable sized drill is not available, initially a hole is drilled to the nearest size and using a single point cutting tool, the diameter of the hole is enlarged.

Counter Boring:

● It is the operation of enlarging the end of a hole cylindrically.

● The tool used counter boring is known as counter bore.

● It is to increase the size of a hole at one end only to a small depth.

● The counter boring forms a larger sized recess or a shoulder to the existing hole.

● The counter bore tool enters the work piece axially and enlarges the top portion of an existing hole to the diameter of the tool.

● Counter boring is often performed after drilling to provide space for the head of a fastener, such as a bolt, to sit flush with the work piece surface.

● The counter boring too] has a pilot on the end to guide it straight into the existing hole.

Tapping:

● It is the operation of cutting internal threads by using a tool called tap.

● A tap is similar to a bolt with accurate threads cut on it.

● To perform the tapping operation, a tap is screwed into the hole by hand or by machine.

● It is cutting internal threads using a cutting tool known as tap.

● Tap is similar to a bolt having hardened and ground cutting edges in the threads.

● Generally a set of three taps are used one after another for threading.

● The tap removes metal and cuts internal threads, which will fit into external thread of the same size.

● For all materials except cast iron, a little lubricating oil is applied to improve the action.

● The tap is not turned continuously, but after every half turn, it should be reversed slightly to clear the threads.

Counter sinking:

● This is the operation of making a cone shaped enlargement of the mouth of a hole.

● A counter sink tool enters the work piece axially and enlarges the top portion of an existing hole to a cone shaped opening.

● It is the operation of making the end of a hole into a conical shape. It is done using a counter sinking tool.

● The counter sinking process may also be employed for deburring the holes.

● Counter sinking is often performed after drilling to provide space for the head of a fastener, such as a screw, to sit flush with the work piece surface.

● Common included angles for a countersink include 60, 82, 90, 100, 118, and 120 degrees.

Spot facing:

● Spot facing is an operation of removing enough material to provide a flat surface around already drilled hole.

● Spot facing accommodate the head of a bolt or a nut.

● Spot facing tool is very nearly similar to the counter bore.

Trepanning:

● Trepanning is the operation of producing large holes by specially designed tool.

● The tool has cutting edges at one end and a solid shank at the other end to fit into the drill spindle.

● It is rotated in a drilling machine to cut out large holes from solid metal by cutting a narrow groove along the circumference.

Types of Drill:

1. Flat drill

2. Straight fluted drill

3. Twist drill:

(i) Parallel shank twist drill

(ii) Taper shank twist drill.

1. Flat drill:

          Is the simple drill used to produce hole in soft materials like wood and plastics. Cutting angle ranges from 90 to 120 degrees.

2. Straight fluted drill:

          Has two cutting edges and two straight flutes. ls used for drilling non ferrous metals. Best suited for drilling on sheets.

3. Twist drill:

          Most commonly used in work shops. Flutes facilitate easy removal of chips during drilling.

(i) Parallel shank twist drill:

Parallel shank drills are held in drill chuck, depending upon the length of the drill. These are sub-divided into three series;

● Short series twist drill

● Stub series twist drill.

● Long series drill.

          In jobber drill, the diameter ranges from 0.2 mm to 16 mm. The long series with dia ranging from 1.5 mm to 26 mm and stub series diameter ranging 0.5 mm to 40mm are used for special jobs.

(ii) Taper shank twist drill:

These drills fit into the internal taper of the drilling machine spindle. The shank for these drills conform to Morse tapers.

Deep hole drilling machine:

          Very deep holes of L/D ratio 6 to even 30, required for rifle barrels, long spindles, oil holes in shafts, bearings, connecting rods etc, are very difficult to make for slenderness of the drills and difficulties in cutting fluid application and chip removal. Such drilling cannot be done in ordinary drilling machines and by using ordinary drills. It needs machines like deep hole drilling machine such as gun drilling machines with horizontal axis or vertical axis. These machines are provided with;

● High spindle speed

● High rigidity

● Tool guide

● Pressurised cutting oil for effective cooling, chip removal and lubrication at the drill tip.

Deep hole drilling machines are available with both hard automation and CNC system.

Twist drill nomenclature:

          The various parts and angles of a twist drill are shown;

Drill angles:

All cutting tool and drills are provided with certain angles for efficiency in drilling.

Angles:

There are different angles for different purpose. They are listed below.

1. Point angle.

2. Helix angle.

3. Rake angle.

4. Clearance angle.

5. Chisel edge angle.

1. Point angle/Cutting angle:

The point angle of a general purpose [standard] drill is 118°. This is the angle between the cutting edges [lips]. The angle varies according to the hardness of the material to be drilled.

2. Helix angle:

Twist drills are made with different helix angles. The helix angle determines the rake angle at the cutting edge of the twist drill. The helix angles vary according to the material being drilled. According to Indian Standard, three types of drills are used for drilling various materials.

● Type N - for normal low carbon steel.

● Type H - for hard and tenacious materials

● Type S - for soft and tough materials.

The type of drill used for general purpose drilling work is type N.

3. Rake angle:

Rake angle is the angle of flute [Helix angle].

4. Clearance angle:

Clearance angle is to prevent the friction of the tool behind the cutting edge. This will help in the penetration of the cutting edges into the material. If the clearance angle is too much, the cutting edges will be weak, and if it is too small, the drill will not cut.

5. Chisel edge angle/Web angle:

This is the angle between the chisel edge and the cutting lip.

6. Designation of drills:

Twist drills are designated by the,

● Diameter.

● Tool type

● Material.

 Cutting Parameters

          Cutting speed is the speed at which the cutting edge passes over the material while cutting, and is expressed in metres per minute. Cutting speed is also sometimes stated as surface speed or peripheral speed. The selection of the recommended cutting speed for drilling depends on the materials to be drilled, and the tool material. Tool manufacturers usually provide a table of cutting speeds required for different materials. The recommended cutting speeds for different materials are given in the table. Based on the cutting speed recommended. The r.p.m, at which a drill has to be driven, is determined. Calculating r.p.m.;

Cutting parameters:

Feed:

          The feed of a drill is the distance the drill moves into the work at each revolution of the spindle. It is expressed in millimeter. The feed may also be expressed as feed per minute. The feed per minute may be defined as the axial distance moved by the drill into the work per minute. The feed per minute may be calculated as;

Depth of cut:

          The depth of cut in drilling is equal to one half of the drill diameter. Thus if ‘d’ be the diameter of the drill, the depth of cut [t] may be expressed as;

Machining time in drilling:

          Machining time in drilling is determined by the formula;

Where,

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