MT-284
MANUFACTURING PROCESSES
INSTRUCTOR:
SHAMRAIZ AHMAD
MS-Design and Manufacturing
Engineering
Shamraiz_88@yahoo.com
Topic:
Turning and Surface Finish Processes
Today’s Lecture
• Forces in Turning process
• Friction and heat sources in cutting
• Surface Finishing processes
– Electroplating
– Honing
– Lapping
– Super finishing
– Polishing
– Buffing
TURNING OPERATION
• Turning operation : the work piece is rotated and a
single edge cutting tool removes a layer of material
as it moves parallel to axis of rotation. Turning is
traditionally carried out at Lathe machine.
• The starting material is
generally a work piece
generated by other processes
such as casting and extrusion
etc
CUTTING FORCES
•
The force acting on the tool in orthogonal
cutting are shown in fig.
•
The cutting force Fc acts in the direction of
the cutting speed, V, and supplies the
energy
required
for
the machining
operation.
•
The thrust force Ft acts in the direction
normal to the cutting velocity.
•
The two forces produces the resultant force
R, which can be resolved into two
components acting on tool (Friction force F
resisting the flow of chip on rake face, and
Normal force N perpendicular to F)
•
The Resultant force R of F and N is
oriented at an angle β, called the friction
angle.
Another force Fs, shear force that causes
shear deformation and Fn normal to Fs.
The shear stress is t=Fs/As.
•
• The cutting force is shown to be:
• The ratio of F to N is the coefficient of friction
µ, at the tool chip interface. The angle is
known as the friction angle β. The coefficient
of friction is shown as:
SHEAR ANGLE RELATIONSHIPS
• M.E Merchant relationship is based upon:
– The shear angle adjust itself so that is cutting force is
minimum
– Maximum shear stress occurs in shear plane.
• As the rake angle decreases and/or as the friction at
the tool chip interface increases, the shear angle ᶲ
decreases and thus the chip become thicker.
TEMPERATURES IN CUTTING
•
In cutting, nearly all of energy ( approx 98%) dissipated
in plastic deformation is converted into heat that in turn
raises the temperature in the cutting zone.
–
There are severe temperature gradients in the cutting
zone.
CAUSES:



FIGURE Typical temperature distribution in
the cutting zone.
Temperature generated in shear plane is a function of the specific energy
for shear and the specific heat of the material.
The temperature rise at the tool-chip interface is also a function of the
coefficient of friction.
Flank wear is an additional source of heat, caused by rubbing of the tool
on the machine surface.
Temperature Effect
• During cutting rise in temperature is very
important, because:
– Adversely affect the strength, hardness and wear
resistance of the tool.
– Cause dimensional changes in the part being
machined, making control of dimensional
accuracy difficult.
– Can induce thermal damage to the machined
surface, adversely affecting its properties and
service life
– The machine tool itself may be subjected to
temperature gradient.
Temperature in Cutting
The temperature in metal cutting can be
reduced by:
• application of cutting fluids (coolants).
• change in the cutting conditions by reduction
of cutting speed and/or feed;
• selection of proper cutting tool geometry
(positive tool orthogonal rake angle).
SURFACE FINISHING
• “ Surface finishing may be defined as any process
that alters the surface of a material or aesthetic or
functional purposes.”
Why Surface Finishing Processes:
• Surface finishing can hide any number of faults in a
casting.
• Surface finishing can improve the aesthetic appeal
of a casting by changing its gloss, shininess and
color which increases sales value.
• It can improve corrosion resistance and tailor
surface properties.
1. ELECTROPLATING
• First use reported almost 200 years ago.
“It may be defined as the process wherein an electric
current is carried across an electrolyte and in which a
substance is deposited at one of the electrodes. “
• Electrolysis is possible because solvents, water in
particular, have the ability to ionize substances dissolved
in it.
• These ions are electrically charged and are attracted to
oppositely charged electrodes where they are neutralized
by the charges on these electrodes.
• The cathode product is the deposition of metal and the
anodic product most often is the dissolution of metal.
Electroplating
 This is a process by which a thin layer of metal is
deposited on the surface of an electrically
conducting part.
 The part is used as a cathode, and the depositing
material forms the anode.
 The electrodes are dipped in a solution of the
appropriate salt, such that on application of
voltage, the metal from the anode is dissolved
into the solution, and deposited on the cathode.
 A simple example of this process is copper plating
using CuSO4 solution, using Copper anodes.
Electroplating
Electroplating Process
Uses of Electroplating
• Electroplating is one of the means available to
the surface finisher to apply a metal coating to a
metallic
• Electroplating is used to deposit a very wide
range of pure metals and alloys for se in
decorative, functional and jewellery applications.
• Nickel/chromium composites (usually called
“chrome” plating), copper, brass an alloy), bronze
(an alloy) and zinc are used for decorative
applications.
2. HONING
• “Honing is a abrasive finishing process performed by
a honing tool, which contains a set of three to a
dozen and more bonded abrasive sticks.”
• The sticks are equally spaced about the periphery of
the honing tool. They are held against the work
surface with controlled light pressure, usually
exercised by small springs.
• The honing tool is given a complex rotational and
oscillatory axial motion, which combine to produce a
crosshatched lay pattern of very low surface
roughness
Honing
• It produces surface finish of about 0.1 μm with
crosshatched surface characteristics which
improves the function and service life.
• Honing is low-speed operation, 25 to 95 m/min
while grinding is high speed operation.
• A cutting fluid must be used in honing to cool and
lubricate the tool and to help remove the chips.
• A common application of honing is to finish the
holes. Typical examples include bores of internal
combustion
engines,
bearings,
hydraulic
cylinders, and gun barrels.
3. Lapping
• “In lapping, instead of a bonded abrasive tool, oilbased fluid suspension of very small free abrasive
grains (aluminum oxide and silicon carbide, with
typical grit sizes between 300 and 600) called a
lapping compound is applied between the work
piece and the lapping tool.”
• The lapping tool is called a lap, which is made of soft
materials like copper, lead or wood. The lap has the
reverse of the desired shape of the work part.
• To accomplish the process, the lap is pressed against
the work and moved back and forth over the
surface. Lapping is sometimes performed by hand,
but lapping machines accomplish the process with
greater consistency and efficiency.
• Lapping is used lo produce optical lenses, metallic bearing surfaces,
gages, and other parts requiring very good finishes and extreme accuracy.
Lapping
•
Abrading process used to remove
minute amounts of metal from internal
or external surfaces surface
Reasons for lapping
•
–
•
•
•
Increase wear life of part
Improve accuracy and surface finish
Improve surface flatness
Provide better seals and eliminate need for
gaskets
20
4. SUPERFINISHING
• Super finishing is a finishing operation similar
to honing, but it involves the use of a single
abrasive stick. The reciprocating motion of the
stick is performed at higher frequency.
• Also, the grit size and pressures applied on the
abrasive stick are smaller.
• A cutting fluid is used to cool the work surface
and wash away chips.
Super finishing
• In super finishing, the cutting action terminates by
itself when a lubricant film is built up between the
tool and work surface.
• Thus, super finishing is capable only of improving
the surface finish but not dimensional accuracy.
• The result of these operating conditions is mirror
like finishes with surface roughness values around
0.01 μm.
• Super finishing can be used to finish flat and
external cylindrical surfaces.
5. POLISHING
• Polishing is a finishing operation to improve the
surface finish by means of a polishing wheel
made of fabrics or leather and rotating at high
speed. The abrasive grains are glued to the
outside periphery of the polishing wheel.
Polishing operations are often accomplished
manually.
• Polishing is often used to remove contamination
of instruments, remove oxidation
• Polishing and Buffing terms are generally used
together.
6. BUFFING
• Buffing is a finishing operation similar to
polishing, in which abrasive grains are not glued
to the wheel but are contained in a buffing
compound that is pressed into the outside
surface of the buffing wheel while it rotates.
• As in polishing, the abrasive particles must be
periodically replenished.
• As in polishing, buffing is usually done manually,
although machines have been designed to
perform the process automatically.
• Polishing is used to remove scratches and burrs and to
smooth rough surfaces while buffing is used to provide
attractive surfaces with high luster.
Thank you