The document discusses drilling operations and drilling machines. It provides details on the construction of drilling machines including their base, column, drill head, table, and spindle drive and feed mechanisms. It also describes various tool and work holding devices used in drilling like drills, chucks, v-blocks, and drilling jigs. Finally, it outlines different types of drilling operations such as drilling, boring, counterboring, tapping, and reaming.

1. DRILLING
Introduction:
Making holes in the materials is one of the most common operations. This operation
is called Drilling, which consists of originating holes in variety materials, by rotary and axial
movements of tool called Drill. The machines used for this operation are called Drilling
Machines. Drilling tools are available in varying dimensions and may either be twist drill or
straight drill. Usually, in drilling operation the drill rotates and is fed axially into the work
held on the machine.
Construction of a Drilling Machine:
Drilling is probably the most economic method of producing holes into the solids.
The machines, which are capable of rotating the drills, feed the drills into the work to
produce or to enlarge the holes, consist of the following important parts.
a) Base; b) Column; c) Drill Head; d) Table; and e) Spindle drive and feed mechanism.
(a)Base :- It is a cast iron heavy solid body or little bit hallow body carrying bolts to be
grouted on the floor. It carries a vertical column and a motor on the side of the base. In the
heavy machines, the base and table may become the same. In such cases, the base and table
are heavy and rigid and usually used for heavy work. The base may also support pulleys
and motor for spindle driving.
(b)Column :- It is a vertical column of cast iron (round or box section) and supports the table
and head or arm. It is very heavy (little bit hollow) and rigid and should be able to
withstand the entire load of cutting process. The head or arm can move vertically up and
down along the column on rack teeth cut on the face of the column with the help of hand
wheel. The box section column had got the accurate guideways on which the table can go
up and down for vertical adjustment.
(c)Drill Head :- Usually, the head is mounted on the column and is housing the spindle
driving and feeding mechanisms. In universal and radial drilling machines, the head is
mounted on a radial arm extending over the base and can move along the guideways
provided on the arm. The head or arm can move up or down according to the job, and can
be locked at desired position.
(d)Table:- It is heavy casting of round or rectangular shape. For quick and accurate clamping,
T-slots are cut on the table face. Table can move up and down to accommodate variety of
work sizes. Further , other work holding devices ( vices and jigs ) can also be clamped on
the table. The table movement can be locked in any position.
(e)Spindle drive and feed mechanism- These mechanisms are housed in the drill head.
Basically, it consists of (I) spindle, (ii) spindle rotation and (iii) feed mechanism.
Spindle is a vertical shaft which can hold the drill (tool) firmly. The tower end of the
spindle carries a morse taper hole with a slot at its end for accommodating taper shank drill
and tang of the drill, with the help of a key, the drill can be pushed down through the slot,
to take the drill out . Sometimes for holding smaller diameter drills, Morse taper socket or
chuck is provided on the spindle. The spindle gets the motion from motor or pulleys
through bevel gears and is able to rotate at desired speeds. The spindle rotates in a non-
rotating sleeve carrying rack- teeth at its outer surface. The sleeve can be moved up and
down by rotating a pinion with the help of hand wheel which meshes the rack teeth.

2. The spindle is provided with multiple speed driving mechanism. This mechanism
may be constituting of step cone pulley drive; step cone pulley drive with one or more back
gears; or complete gear driving mechanism. Modern heavy duty machines are generally
carrying motor mounted on the frame. The multiple speeds may be obtained by sliding gears
and sliding clutch.
The feed mechanism consists of vertical movement of the drill into the work, and This
movement may be controlled by hand or power, the automatic feed is applied while drilling
lager diameter holes, so that the cutting pressure can be maintained uniform, cross slot
TOOL HOLDING DEVICES:
The basic purpose of all tool- holders is to make the tool run true with the tapered hole in the
machine spindle. The different methods. Used for holding tools in a drill spindle are:
Directly holding drill - Morse standard taper (1:20) is bored into the spindle and can receive
the taper shank of the drill. The shank is always forced into the standard tapered hole and is
gripped by friction. To ensure positive drive, a cross slot is provided in the tapered hole in
which the tang of the drill shank is fitted. If the tapered shank is smaller than the tapered
hole, a taper shank socket or sleeve with inside and outside tapers is used. A wedge is to be
used to push the tool out of the socket. Usually, the standard taper can receive only one size
of shank. Drill chucks - The chucks are usually used to hod smaller size of drills. The sleeve
and socket are there, but they can hold only one size of shank in it, whereas the chuck can
hold a wide variety of drill sizes. One can use three jaw chuck of smaller size, but quick
change chuck is also available and is used for production work. This chuck is used for
locating a series of tools one after another without stopping the spindle. This reduces the
machining time considerably.
Floating driver - When a reamer or a tap is to be used in the previously drilled hole, then a
floating tool holder is used to compensate for out of alignment of drill spindle with work hole
and allows self alignment of the tool with the work hole. The two ends of alignment.
WORK HOLDING DEVICES:
In the drilling process, the work-piece must be clamped rigidly to the table by bolts and
traps, otherwise the drill will rotate the work also because the process exerts severe torque on
the work. This may ultimately cause injuries to the operator. The most common types of
work holding devices used are discussed here. Most of the students are advised to consult
figures 15.6 chapter 15 for details.
T- Bolts and Clamps- T-Bolts and clamps are the most common ways of clamping the
work. T- Bolts can easily suit the T-slots on the table of the machine.
V-Blocks- V-Blocks make good and stable support to the round workpieces. However, one
can use two or three v-blocks of similar type for locating and supporting the long round jobs.
C-clamps are used to clamp the work in the v-blocks.
Vises — These are the most commonly used devices for holding the work between the jaws.
These vises are usually bolted in the T-slots provided in the table. This arrangement gives a
very firm and rigid hold to the work of any shape and size. Sometimes, the universal vise is
also used, where the base of vise can also be swiveled at any angle. While clamping the
work, some support must be provided under the work.

3. Chucks - Three jaw chuck or four jaw chuck used on lathe machine can also be mounted on
the work table base of the machine with the jaws up to hold the work for drilling in proper
position.
Positioning Table - It is a table mounted on a saddle of the base of a drill press or radial drill
to position the workpiece under the spindle. The table and saddle can be located by
leadscrew, and dials to any desired position under the spindle.
Angle Plates - They are usually used when two holes are to be made perpendicular to each
other and the work would maintain accuracy.
Drilling Jigs - These are not only hold and locate the work piece, but also guide the drilling
tool, when the work piece is located and clamped in a jig, it means the work piece is fully
restricted from all six degrees of freedom (A freedom in space has six degrees of freedom
three axes and three by rotation). Such a restriction is obtained if the work piece is held
against three points in one plane; two points in second plane; and one in a third plane, with the
planes mutually perpendicular to each other. But full restriction may not be required always.
The important feature of a jig is that it provides the locating surfaces or points against which
all work pieces of one type can precisely be located. For guiding drills or reamers, bushes or
removable bushings properly hardered and tempered are used, jigs are always economical for
locating holes to be made in work pieces in large quantities.
DRILLING OPERTIONS
Different types of operations can be performed by the rotating tool and feeding it sown against
the work and they are listed here:
i) Drilling ; ii) Core drilling ; iii) step - drilling ; iv) Boring ;
v) Counter boring ; vi) Counter sinking ; vii) Reaming ; viii) Tapping.
Drilling :- It is the easiest way to produce or to enlarge a cylindrical hole into the solids. The
cylindrical holes are produced by removing metal by rotating edges of a cutting tool (drill). To
carry out this operation, the centre of hole is located first on the work piece, and then with the
help of centre punch, the centre is indented. The drill point should coincide with the indented
centre. In normal practice the drill does not produce an accurate cylindrical hole in the work -
piece and also may not make hole exactly on the centre. The internal surface of the so
produced hole is also comparatively rough. The hole is always oversized. The reasons may be
the vibrations caused in the machine spindle and it is very difficult to locate the drill at the
hole centre since does not have a point but an edge.
Core Drilling - Sometimes the drilled holes are to be enlarged and this is a common practice.
A hole of large diameter is never drilled at the first instance. A hole of 25mm dia should be
made after drilling hole of 10 - 15 mm dia. So enlarging the drilled hole is called core drilling
or counter drilling.
Step drilling - when a hole of two or more dia meters is made by rotating drill, then this
operation is called step drilling.
Boring- Basically, boring is an operation of enlarging a cylindrical hole, sometimes with the
implication of producing a more accurate hole from size, surface and location view points,
than by drilling. Boring operation can be performed on boring machine, lathe machine,
milling machine and drilling machine. On drilling machine, the drilled hole is enlarged by

4. means of an adjustable cutting tool having only one cutting edge. Sometimes, it becomes
essential because the drill of that size is not available or the hole size to be obtained is not a
standard one and availability of a drill is out of quastion. Drilling with one cutting edge
becomes essential under such circumstances.
This operation brings the hole into a more accurate size with good internal surface
finish. Roundness of the hole is improved and the location of the hole is more »T
accurate.
The tool consists of a boring bar having tappered shank to fit into the drill
spindle. A single point too! is inserted into the boring bar and the diameter of boring
bar with tool is adjusted to the required size. Thus the boring bar with tool point is
allowed to rotate to cut into a previously undersized drilled hole to bring the hole into
the desired accurate size.
Counter - boring : - It is an operation of enlarging a hole to a limited depth. If the
depth is shallow for the seat of a nut head or a screw head, stud head etc., then such a
counter boring is called spot facing. It is essential for a counter bore (tool for counter
boring) to provided with a pilot to guide it into position and keep it steady during
cutting. The pilot is an extension beyond the end of the cutting edges. The
counterbores are made with straight or tapered shank to fit in the drill spindle. The
cutting edges may be straight or spiral teeth. Usually counterboring speed is always
kept less than that of drilling operation.
Counter - sinking : - It is the operation of making a cone shaped enlargement at the
mouth of a drilledhole, for the purpose of providing a seat for the head of a
countersunk screw (flat head screw), a bearing surface in the work for a lathe centre or
a recess in which the end of a rivet may be spread. For large holes a countersink
provided with a pilot to fit the previously drilled hole is necessary in order to keep the
drill true and to prevent chatter. This operation may loosely be called as chamfering.
Reaming : - It is an operation to produce an already drilled hole into an accurate sized
hole with good surface finish. The tool used for this operation is called reamer which
has multiple cutting edges. Reamer by itself can not cut a hole. The reamer is fitted
into the drilling machine spindle which runs at almost half the speed of drilling
operation and the reamer should be fed automatically. The stock removal is very small.
Tapping : - It is an internal thread cutting operation. The tool employed is called tap which
can be mounted on the drilling machine spindle and should run at very slow speed. However,
this operation can also be performed by hand. A tap is like a bolt having very accurate threads
hardened and ground. To perform this operation, an undersized hole is to be drilled and then
the tap is screwed into the hole where it removes metal and cuts internal threads. For a tap
size, a hole of 0.8 times the tap size should be drilled into the work and then screw the tap.
This is a thumb rule which is reasonably accurate for most of the cases.
Tap drill size = out side diameter of the tap X 0.8
There are some additional operations also that can also be performed on this machine such as
lapping, grinding etc. both the operations are finishing operations and performed on already
drilled holes which are hardened (heat treatment operation to make a surface hardened). Such
already drilled holes can be brought to accurate size with very good finish.