DRILLING
MACHINE
The
drilling machine or drill press is one of the most common and useful machine
employed in industry for producing forming and finishing holes in a workpiece.
The unit essentially consists of:
1.
A spindle which turns the tool (called drill) which can be advanced in the
workpiece either automatically or by hand.
2.
A work table which holds the workpiece rigidly in position.
Working principle:
The rotating edge of the drill exerts a large force on the workpiece and the
hole is generated. The removal of metal in a drilling operation is by shearing
and extrusion.
Types of Drilling
Machines
A
wide variety of drilling machines are available ranging from the simple
portable to highly complex automatic and numerically controlled machines are as
follows:
1.
Portable drilling machine: It is a
small light weight, compact and self-contained unit that can drill holes upto
12.5 rnrn diameter. The machine is driven by a small electric motor operating
at high speed. The machine is capable of drilling holes in the workpieces in
any position.
2.
Sensitive drill machine/press: This
is a light weight, high speed machine designed for drilling small holes in
light jobs. Generally the machine has the capacity to rotate drills of 1.5 to
15.5 rnrn at high speed of 20,000 rev/min.
Construction
The
machine has only a hand feed mechanism for feeding the tool into the workpiece.
This enables the operator to feel how the drill is cutting and accordingly he
can control the down feed pressure. Sensitive drill presses are manufactured in
bench or floor models, i.e., the base of machine may be mounted on a bench or
floor. The main operating parts of a sensitive machine/drill press are Base,
Column, Table, and Drill Head.
1.
Base: The base is a heavy casting
that supports the machine structure; it provides rigid mounting for the column
and stability for the machine. The base is usually provided with holes and
slots which help to Bolt the base to a table or bench and allow the work-holding
device or the workpiece to be fastened to the base.
2.
Column: The column is a vertical
post that Column holds the worktable and the head containing the driving
mechanism. The column may be of round or box section.
3. Table: The table, either rectangular
or round. Drill machine/press in shape supports the workpiece and is carried by
the vertical column. The surface of the table is 90-degree to the column and it
can be raised, lowered and swiveled around it. The table can be clamp/hold the
required the workpiece. Slots are provided in most tables to allow the jigs,
fixtures or large workpieces to be securely fixed directly to the table.
4.
Drilling Head: The drilling head,
mounted close to the top of the column, houses the driving arrangement and
variable speed pulleys. These units transmit rotary motion at different speeds
to the drill spindle. The hand feed lever is used to control the vertical
movement of the spindle sleeve and the cutting tool. The system is called the
sensitive drilling machine/press as the operator is able to sense the progress
of drill with hand-faced.
Operations
performed by drilling machine/press as follows:
1. Drilling:
Drilling is the operation of producing a hole by removing metal from a solid
mass by the rotating edge of a cutting tool known as drill.
2. Spot facing:
Spot facing is the operation of smoothing and squaring the surface around and
at the end of a hole so as to provide a smooth seat for a nut or for the head
of a cap screw. Spot facing is generally done on castings and forgings.
3. Tapping:
Tapping is the operation for making internal threads in a hole by means of a
tool called tap. The tap is essentially a bolt with threads cut on it.
4. Boring:
Boring is the operation of truing and enlarging a previously drilled hole by
means of a single point cutting tool. Boring is done on drilling machine to
perform the following tasks on a hole already drilled.
5. Reaming:
The holes that are produced by drilling are rarely straight and cylindrical in
form. To produce accurate and smooth holes, the drilled holes are reamed by a
tool called reamer. The reamer is a cutting tool having several cutting edges
in straight or helix shape.
6. Counter
boring: Counter boring is the operation of enlarging one end of an existing hole
concentric with the original hole with square bottom. It is done to accommodate
the heads of bolts, studs and pins. The cutting edges of the counter-bore (tool
used for counter boring) may have straight or spiral teeth.
7. Counter
sinking: is the operation of making a cone shaped enlargement at the end of a
hole to provide recess for a flat head screw or a countersunk rivet. The
counter-sunks (tools used for counter sinking) carry included angles of 60°,
82° or 90° and the cutting edges of the tool are formed at the conical surface.
