8.

DRILLING MACHINES & DRILLING OPERATIONS

The drilling machine (drill press) is a single purpose machine for the production of holes.
Drilling is generally the best method of producing holes. The drill is a cylindrical bar with
helical flutes and radial cutting edges at
one end. The drilling operation simply
consists of rotating the drill and feeding
it into the work piece being drilled.

The process is simple and reasonably

accurate and the drill is easily controlled
both in cutting speed and feed rate. The
drill is probably one of the original
machining processes and is the most
widely used.

Drilling machine -important features/dimensions

Drilling Machines
Normal drilling machines (Drill Press) are specified basically by the size of hole the machine
can drill in Mild Steel i.e. a 16mm machine can drill holes up to and including 16mm diameter
in mild steel. The speed range of a drilling machine is related to the size e.g. machines for
small holes down to 1mm can have speed ranges up to 8000rpm. Larger drilling machines
more suited for drilling holes. up to 25mm will have a more limited range. A machine which is
used to drill larger holes (>15mm) is not generally suitable for drilling small diameter holes
(<1 mm). Smaller machines are provided with permanent chucks whilst larger machines
generally include Morse tapers for fixing the drills.

Most drilling machines are manually fed using a rotating lever driving the vertical motion of
the spindle. Larger machines can have power drives feeds.

A belt driven spindle is often a convenient low cost option but there is a tendency in modern
times to use geared drives.

When drilling holes in a material, a number of factors should be considered including

• Machine to be used
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• Material being drilled

• Hole size
• Speed /Feed required
• Depth of hole
• Need for coolant
• Method of work holding. Hand held, vice, clamped
• Swarf control

TYPES OF DRILLING MACHINES

Bench Drill
The most common form of drilling machine is the
bench drill. As the name implies this machine is
normally bolted down to a bench. The work piece
can be clamped onto the worktable or onto the
base. Tee slots are normally provided for this
function. The worktable can be moved up and
down the vertical column. The worktable can be
clamped at the selected height. The drill is
normally located in a three jaw chuck which is
rotated by the drive system. The figure below
shows a belt drive. Modern bench drills are driven
by more sophisticated arrangements. The chuck is
moved up and down by a feed handle which drives
rotating spindle via a rack and pinion mechanism.

Pillar Drill

The pillar drill has the same features as the

bench drill and is of a far heavier construction
able to take larger drills and hold larger
components which can be mounted directly
onto the base. The larger drills normally have
taper shanks which are located within a taper
bore in the spindle end. These tapers are
standardized as Morse tapers.
The drilling head holds two gearboxes, one for
the spindle speed and the other for the feed
rate.
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This machine also has a coolant tank that will provide continuous flow to keep workpiece and
drill bit cool.
Radial Arm Drill

The radial drill is a free standing and the work

piece is clamped in position on the base. The
drill Arm is positioned using motorized drives.

The arm can rotate around the column and the

drilling head slides along the arm hence
making it easier to locate any position on the
workpiece to be drilled

Also has a coolant tank to provide continuous

flow of coolant.

Types of Drills Bits

There are two common types of

twist drills, high-speed steel drills,
and carbide-tipped drills. The most
common type used for normal
workshop practice is the high-speed
steel twist drill because of its low
cost. Carbide-tipped metal drills
are used in production work where
the drill must remain sharp for
extended periods, such as in a
numerically controlled drilling
machine. Other types of drills
available include solid carbide drills,
TiN coated drills, diamond drills etc.
etc.

Twist drills shanks are either straight shank or tapered shank (Morse taper). Straight
shank twist drills are usually 12mm or smaller and are gripped in the drill chucks. Tapered
shank drills are usually for the larger drills that need more strength which is provided by
the taper socket chucks.

Drilling Feeds and Speeds

The notes below relate to HSS drills. For drills manufactured with more exotic material
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combinations much higher feed and speed rates are viable

Material
Aluminium 35-65
Brass / Bronze 37-75
Copper 30-60
Grey Cast Iron 24-30
Mild Steel 20-30
Medium Carbon Steels 14-20
Stainless Steels 6-15

Cutting Conditions In Drilling:

v
N =
πD
N= rotational speed, rev/min
V = cutting speed, in/min (mm/min)
D= drill diameter, in (mm)

Drill angles
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Drilling Technique
Example: When a customer specifies that two or more holes need to be drilled on a
component and they specify that one hole has tight tolerances (±0.012 mm) and needs
to be eg. 50 ±0.3 mm apart, then extreme care must be taken to ensure you meet the
specifications of the customer.
1. The diagram is the steps
that must be followed to
ensure specifications.
2. A punch mark does not
provide sufficient control
as it is very shallow. A drill
bit could still drift off the
punched mark as the
cutting edges is still
exposed above the surface,
hence a centre drill must be used to ensure that the drill bit diameter is below the
surface when bottoming.
3. As the hole must be reamed, a hole of 0.3 mm smaller than the specified size must
drilled to allow the reamer to smooth out the hole.
4. Chamfering should be done before reaming as the chamfering tool produces a burr on
the inside of the hole then allowing the reamer to remove the burr.
5. The reamer produces a hole according to specifications as well as smoothing out the
inside of the hole and produces a perfectly round hole compared to a drill bit that
cannot produce these.

Other processes related to drilling

Reaming
Used to slightly enlarge a hole to provide a better tolerance on its perfectly round diameter
and to improve its surface finish. Before reaming takes place, a hole 0.3mm smaller than
final size must be drilled as a reamer has cutting edges on the sides.

Tapping
Performed by a tap, used to provide
internal screw threads on an existing hole

Counterboring:
Used to seat Allen cap screws below the
surface of the component. Mainly
done to prevent interference with other
components and prevent injury.
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A hole is first drilled followed by the

counter boring tool
Countersinking:
Similar to counterbore but it is
cone shaped to accommodate
countersunk screws

Centering
Also called center drilling. The tool is also called as center drill. Used to start the
subsequent drilling accurately

Spotfacing:
Similar to milling operation, provides flat
working surface on the workpart.
Bolts require a flat seating to prevent
unnecessary stresses in the bolt when
tightened

LOCATING POSITIONS ON A COMPONENT

Quite often users think that
they should move the workpiece
continuously to locate different
positions when a component
needs to be drilled. They also
believe that if a radial arm
drilling machine is available, it
would be much easier and
quicker.
However, the pedestal drilling
machine can be used as a radial
arm machine.
On the pedestal drilling
machine, the table is held in
position onto a bracket by
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means of a clamp which when released allows the table to rotate on its own axis. Secondly,
the bracket is clamped to the column, which when
released allows the bracket to swing around the
column. This movement to the bracket and the
table allows any position to be located on a
workpiece without the need to loosen the
workpiece from the vice or table as the spindle is
a fixed position in the drilling head.
After the position has been located, the bracket
and the table must be clamped to ensure no movement occurs during drilling. Repeat the
process to drill the next hole.

Reasons why drill bits break

1. Drill point ground incorrectly. All bits must have a chisel edge
2. Heavy feed. No need to force the bit to drill quicker
3. Blunt drill bit. If its blunt, it will not cut the work-piece
4. Clogging of spiral grooves. Swarf must exit the hole when drilling
5. Work-piece not clamped correctly.

OPERATING PROCEDURE - DRILLING MACHINE

1. Be aware where the emergency stop button is located.
2. Calculate the speed according to the diameter of the drill bit.
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3. Set the calculated speed on the drilling machine.

4. Choose the correct drill bit diameter.
5. Ensure that the drill bit is sharp.
6. Secure the drill bit in the chuck.
7. Always clean the vice jaws before locating the work piece.
8. Secure the work piece in the drill vice.
9. Align the drill bit with the punched center mark on the work piece.
10. Secure the table of the drilling machine.
11. Use safety specs to protect your eyes.
12. Use the correct coolant for the work piece.

NOTES
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