MACHINE TOOLS - 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.
Operations performed by drilling machine/press:
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.