2. CUTTING OF METALS
• Cutting of metals implies severing or removal of metal.
Cutting is the process of separating metals, i.e., a metal piece
is separated or split into two parts. Cutting of metals is an
everyday practice in industry.
• It is employed for the following purposes:
(i) Cutting desired lengths and shapes of (rolled) metal pieces
for assembly and other processing operations to be carried
out on different machine tools and presses. Many times it is
required to cut a gear blank from a plate or a blank is cut
from a plate for subsequent forming operations.
(ii) For preparing the edges of plates for welding them
together.
(iii) For cutting gates and risers from the castings.
(iv) For salvage work.
3. FLAME CUTTING
• Also known as Oxy-Fuel Gas Cutting
• It is an oxygen cutting process wherein the severing of
metals is effected by means of the chemical reaction of
oxygen with the base metal at elevated temperatures, the
necessary temperature being maintained by means of a gas
flame obtained from the combustion of a fuel gas (such as
acetylene, hydrogen, propane, etc.) and oxygen.
4. Principle of operation
The oxy acetylene flame cutting process makes use of a cutting
torch. The mixes the acetylene and the oxygen in correct
proportions to produce a preheating flame and also the torch
supplies a uniformly concentrated stream of high purity oxygen
(known as cutting oxygen) to the reaction zone.
After a spot area along the line of cut is heated to ignition
temperature (900°C), i.e., reddish yellow colour by the preheat
flames, keeping the flame cones 1.5 to 3 mm above the surface of
material, to be cut, a thin jet of high purity oxygen is then directed
or shot at this heated spot.
The jet quickly penetrates through the steel. The iron and oxygen
combine to form iron oxide. The oxygen jet blows the reaction
products from the joint and thus the torch moves progressively
forward over the metal surface, cutting a narrow slot or kerf along
the desired line of severance.
In actual practice, the top surface of the material is frequently
covered with mill scale or rust. They must be melted away by the
preheating flame to expose a clean metal surface to the oxygen jet.
The cutting action is self progressing provided the (cutting) oxygen
jet is sustained and the pre heating flame is maintained on the top
surface of the metal object being cut.
5. Summary
(i) Oxy acetylene flame preheats the metal to the ignition
point at the place to be cut. It also provides a protective
shield between the cutting oxygen stream and the
atmosphere.
(ii) Cutting oxygen combines with iron to form iron oxide.
(iii) Cutting oxygen jet blows away molten iron and iron
oxide thereby cutting a narrow slit or kerf in the metal
object.
6. CUTTING TORCH
A cutting torch is made up of a yellow brass body. It:
(a) Mixes acetylene and oxygen and carries the gas mixture to
the orifices where it is ignited to produce preheating flames.
(b) Carries cutting oxygen to the central orifice from where as it
emerges, it oxidizes the metal and blows the same away to
form an open slot (kerf).
There are two types of cutting torches:
(a) The injector type, in which the acetylene is delivered to the
torch at pressures below 1 psig. The acetylene is drawn into the
torch with the help of an injector. Acetylene and oxygen for the
preheating flames are mixed in the tip of the cutting torch.
(b) The equal pressure type, in which oxygen and acetylene are
delivered at pressures above 1 psig. The mixing of the gases
takes place within the torch (and not in the tip).
8. GAS PRESSURE REGULATORS
Gas pressure regulators, etc., are similar in design to those
used for oxy acetylene welding.
9. USES AND APPLICATIONS
Uses/Applications of Oxy-Fuel Gas Cutting:
(i) To prepare edges of plates for bevel and groove weld joint
designs.
(ii) To cut small sized work pieces from bigger plates for
further processing.
(iii) To cut rivets gates and risers from castings.
(iv) To cut many layers of thin sheets at the same time (stack
cutting) to reduce both time and cost for production work.
(v) To pierce holes and slots in steel plates.
(vi) For salvage work
10. ADVANTAGES AND
DISADVANTGES
Advantages
(i) Shapes and sizes difficult to be machined by mechanical methods
can be easily cut by flame cutting.
(ii) The process is faster than mechanical cutting methods.
(iii) The cost of flame cutting is low as compared to that on a
machine tool, i.e. mechanical cutting machine.
(iv) Flame cutting equipment being portable also, can be used for the
field work.
(v) Multitorch machines can cut a number of pieces simultaneously.
Disadvantages
(i) Flame cutting is limited to the cutting of steels and cast iron.
(ii) As compared to mechanical cutting, the dimensional tolerances
are poor.
(iii) The place of cutting needs adequate ventilation and proper fume
control.
(iv) The expelled red hot slag and other particles present fire and
burn hazards to plant and workers.
11. METALS CAN BE CUT USING
FLAME CUTTING
1. Plain Carbon Steel.
2. Low Alloy Steel.
3. Manganese Steel.
4. Low-content Chromium Steel.
12. METALS CANNOT BE CUT BY
FLAME CUTTING
1. Stainless Steel.
2. Carbon Steel.
3. Aluminium.
4. Non-ferrous Metals.
13. PLASMA ARC CUTTING
It is an arc cutting process wherein the severing of the metal is
obtained by melting a localized area with a constricted arc and
removing the molten material with a high velocity jet of hot,
ionized gas issuing from the orifice.
14. PRINCIPLE OF OPERATION
Plasma arc cutting makes use of DCSP (electrode negative) with
a constricted transferred arc* struck between a tungsten
electrode situated within (and not protruding) the torch and
the workpiece to be cut.
The cutting arc between the electrode and the workpiece is
initiated by a pilot arc established between electrode and
nozzle. The nozzle is connected to ground (+) through a
current limiting resistor and a pilot arc relay contact.
The pilot arc is initiated by a high frequency generator
connected to the electrode and nozzle. Ionized orifice gas
from the pilot arc is blown through the constricting nozzle
orifice. This forms a low resistance path to ignite the main arc
between the electrode and the workpiece. Once the main arc is
ignited the pilot arc goes off.
15. A high-frequency electric arc thus established is passed
through a stream of inert gas (usually nitrogen) the latter is
ionized. Both the ionized gas column and the arc are forced
through a small orifice in the torch nozzle. The nozzle, having
a relatively small orifice, constricts the arc and thus increases
current density and arc temperature. This high temperature
arc is localized and concentrated upon a small area of the plate
where its intense heat melts the metal to the cut.
The gas which is heated by the arc cannot expand due to the
constriction of the nozzle orifice and it emerges in the form of
a supersonic jet. The base metal continuously melted by the
intense heat of the arc is removed by the jet-like gas stream
(issuing from the torch nozzle) to form a narrow kerf and
smooth surface. The combined heat and force of the arc
stream produce a high quality, saw-like cut.
16. GASES USED IN PLASMA ARC
The gases that are used in plasma-arc cutting:
1. Nitrogen
2. Nitrogen + hydrogen
3. Nitrogen + argon
4. Compressed air
17. ADVANTAGES AND
DISADVANTAGES OF PLASMA
ARC CUTTING
Advantages
(i) It cuts carbon steel up to 10 times faster than oxy-fuel
cutting, with equal quality more economically.
(ii) It leaves a narrower kerf.
(iii) Plasma cutting being primarily a melting process can cut
any metal.
(iv)Arc plasma torches give the highest temperature available
from many practicable sources. The energy seems to be
unlimited in this method.
Disadvantages
High initial cost of the equipment
18. APPLICATION OF PLASMA ARC
1. Plasma cutting is used to cut particularly those nonferrous
and stainless metals that cannot be cut by the usual rapid
oxidation induced by ordinary flame torches.
2. Plasma cutting can be used for stack cutting, plate
bevelling, and shape cutting and piercing.
3. With some modifications, plasma arc cutting can be used
under water.
4. Plasma arc cutting finds applications in many industries
such as shipyard, chemical, nuclear and pressure vessel.
5. It is used for removing gates and risers in foundry.
6. It cuts hot extrusions to desired length.
7. It is used to cut any desired pipe contour.
8. It is also employed for gouging applications.
9. It finds use in the manufacture of automotive and railroad
components.
