Manufacturing Processes - Final Exam
Functional dimensions - Answer-They can over-define the part so need to determine which are critical
and have the rest determined by them.
Tolerance stacking - Answer--The tolerance field is the sum of the inaccuracies of the dimension that
determine it (the dimensions on which it is dependent)
-Tolerances are always added. You cannot reduce uncertainty by adding more uncertainty
-Tolerance stacking can also be used on complex assemblies. i.e. the looseness of motion of the output
of a set of gears which is influenced by many different tolerances on many different gears and
components
Root Sum Squared Method (RSS) - Answer--If we only need a specific accuracy on the specific functional
dimension, we can relax our requirements on the manufactured parts.
-We only care that our final assembly meet this requirement. we use the statistical method of combining
standard deviations to use parts that are rated to have a wider tolerance.
-The RSS tells us what the chances of having the parts at the extreme ends of the tolerance in the same
assembly
-Assuming the dimensions are completely independent then the covariance is zero
-We are assuming that all of the parts are normally distributed
What is machining? - Answer--Cutting, which generally involved single-point or multipoint cutting tools
and processes, such as turning, drilling, milling and sawing
-Abrasive processes, such as grinding, honing and lapping
-Advanced machining processes using electrical, chemical, thermal, hydrodynamic and optical sources of
energy
,Why machining? - Answer--Closer dimensional accuracy may be required than can be achieved by
metalworking or casting processes alone
-it may be more economical to machine the part than to make it by other processes (if number of parts
is small)
Machining model - Answer--The tool has a rake angle and a relief angle. -rake + relief = 90degrees.
-Metal chips are produced by a shearing mechanism
-Shearing takes place along the shear plane, which makes an angle with the workpiece surface, called
the shear angle
-Due to the relative movement, there is friction involved between the chip and the rake face of the tool.
Machining. Chip Thickness - Answer--The chip thickness is always greater than the depth of cut
-The chip compression ratio is always greater than unity
Machining. Shear strain - Answer-High shear strains are associated with low shear angles and low or
negative rake angles
Machining. Chip velocity and strain rate - Answer-Because the chip thickness is greater than the under-
formed chip thickness, the velocity of the chip must be lower than the cutting speed (since mass
continuity has to be maintained)
Machining. Types of chips - Answer--A chip has 2 surfaces: one surface has been in contact with the rake
face of the tool, and the other surface is the newly-generated surface of the workpiece.
-The tool side of the chip surface is shiny or burnished which is caused by rubbing of the chip as it climbs
up the tool face of the tool.
-The other surface of the chip has a jagged, step-like appearance which is due to the shearing
mechanism of the chip formation
,-As a result of strain hardening (caused by the shear strain to which it is subjected), a chip generally
becomes harder, stronger, and less ductile than the original workpiece material
Machining. Continuous chips - Answer--Generally formed with ductile materials, machined at high
cutting speeds and/or at high rake angles
-Generally undesirable. They tend to become entangles and interfere with the machining operation and
can become a safety hazard to the operator
Machining. Built Up Edge (BUE) - Answer--Generally undesirable, although a thin but stable BUE is
desirable because it protects the tool surface
-one factor that affects it is adhesion affinity of the workpiece material to the rake face of the tool
-Way to decrease/eliminate it: increase cutting speed, decrease depth of cut, increase rake angle,
decrease tip radius of the tool, use effective cutting fluid.
Machining. Discontinuous chips - Answer--If not sufficiently stiff, the machine tool may begin to vibrate
and chatter. Vibration adversely affects the surface finish and dimensional accuracy of the machined
component and may cause damage of the cutting tool and the machine tool.
-Happens when the workpiece material contains hard inclusions and impurities
-As depth cut increases, the probability of such defects being present in the cutting zone increases
-Happens when the depth of cut is too large or the rake angle is low
-Happens when there is a lack of an effective cutting fluid
Machining. Cutting Forces - Answer--Tau is the average shear stress on the shear plane
-It is possible to have an upward thrust force when friction at the tool-chip interface is low and/or when
the rake angle is high
-The thrust force in cutting is important because the toolholder and the machine tool must be
sufficiently stiff to support that force with minimal deflections
-Change in the direction and magnitude of the thrust force can lead to instability
, -The cutting force increases with increasing depth of cut, decreasing rake angle, and decreasing cutting
speed.
Machining. Rake angle - Answer--Positive rake angles make the tool sharper and more pointed (reducing
strength of the tool), reduces cutting forces and power requirements, helps in the formation of
continuous chips in ductile materials, helps avoid the formation of a built-up edge.
-Cutting tool with negative rake angle is stronger
Machining. Tool wear - Answer--Dull tools require higher power and result in higher forces
-Cutting speed, feed rate and depth of cut are of decreasing order of importance
Turning - Answer--Positive-rake angles improve the cutting operation by reducing forces and
temperatures; however, positive angles also have a small included angle of the tool tip.
-Depending on the toughness of the tool material, a small included angle may cause premature tool
chipping and failure
-The smaller the radius, the rougher the surface finish of the workpiece, and the lower the strength of
the tool; on the other hand, large nose radii can lead to tool chatter
Turning. Surface roughness - Answer-In machining, the usual procedure is to first take one or more
roughing cuts (typically at high feed rates and large depths of cut). The material-removal rates are high,
and there is little consideration for dimensional tolerance and surface roughness of the workpiece.
These cuts are then followed by a finishing cut, typically done at a lower deef and smaller depth of cut,
for a good surface finish.
Peripheral milling - Answer--Helical teeth are preferred over straight teeth (each tooth is always partially
engaged with the workpiece as the cutter rotated)
-The cutting force and the torque on the cutter are lower, resulting in a smoother milling operation and
reduced chatter
End milling - Answer-The cutter can remove material on both its end and on its cylindrical cutting edges
Functional dimensions - Answer-They can over-define the part so need to determine which are critical
and have the rest determined by them.
Tolerance stacking - Answer--The tolerance field is the sum of the inaccuracies of the dimension that
determine it (the dimensions on which it is dependent)
-Tolerances are always added. You cannot reduce uncertainty by adding more uncertainty
-Tolerance stacking can also be used on complex assemblies. i.e. the looseness of motion of the output
of a set of gears which is influenced by many different tolerances on many different gears and
components
Root Sum Squared Method (RSS) - Answer--If we only need a specific accuracy on the specific functional
dimension, we can relax our requirements on the manufactured parts.
-We only care that our final assembly meet this requirement. we use the statistical method of combining
standard deviations to use parts that are rated to have a wider tolerance.
-The RSS tells us what the chances of having the parts at the extreme ends of the tolerance in the same
assembly
-Assuming the dimensions are completely independent then the covariance is zero
-We are assuming that all of the parts are normally distributed
What is machining? - Answer--Cutting, which generally involved single-point or multipoint cutting tools
and processes, such as turning, drilling, milling and sawing
-Abrasive processes, such as grinding, honing and lapping
-Advanced machining processes using electrical, chemical, thermal, hydrodynamic and optical sources of
energy
,Why machining? - Answer--Closer dimensional accuracy may be required than can be achieved by
metalworking or casting processes alone
-it may be more economical to machine the part than to make it by other processes (if number of parts
is small)
Machining model - Answer--The tool has a rake angle and a relief angle. -rake + relief = 90degrees.
-Metal chips are produced by a shearing mechanism
-Shearing takes place along the shear plane, which makes an angle with the workpiece surface, called
the shear angle
-Due to the relative movement, there is friction involved between the chip and the rake face of the tool.
Machining. Chip Thickness - Answer--The chip thickness is always greater than the depth of cut
-The chip compression ratio is always greater than unity
Machining. Shear strain - Answer-High shear strains are associated with low shear angles and low or
negative rake angles
Machining. Chip velocity and strain rate - Answer-Because the chip thickness is greater than the under-
formed chip thickness, the velocity of the chip must be lower than the cutting speed (since mass
continuity has to be maintained)
Machining. Types of chips - Answer--A chip has 2 surfaces: one surface has been in contact with the rake
face of the tool, and the other surface is the newly-generated surface of the workpiece.
-The tool side of the chip surface is shiny or burnished which is caused by rubbing of the chip as it climbs
up the tool face of the tool.
-The other surface of the chip has a jagged, step-like appearance which is due to the shearing
mechanism of the chip formation
,-As a result of strain hardening (caused by the shear strain to which it is subjected), a chip generally
becomes harder, stronger, and less ductile than the original workpiece material
Machining. Continuous chips - Answer--Generally formed with ductile materials, machined at high
cutting speeds and/or at high rake angles
-Generally undesirable. They tend to become entangles and interfere with the machining operation and
can become a safety hazard to the operator
Machining. Built Up Edge (BUE) - Answer--Generally undesirable, although a thin but stable BUE is
desirable because it protects the tool surface
-one factor that affects it is adhesion affinity of the workpiece material to the rake face of the tool
-Way to decrease/eliminate it: increase cutting speed, decrease depth of cut, increase rake angle,
decrease tip radius of the tool, use effective cutting fluid.
Machining. Discontinuous chips - Answer--If not sufficiently stiff, the machine tool may begin to vibrate
and chatter. Vibration adversely affects the surface finish and dimensional accuracy of the machined
component and may cause damage of the cutting tool and the machine tool.
-Happens when the workpiece material contains hard inclusions and impurities
-As depth cut increases, the probability of such defects being present in the cutting zone increases
-Happens when the depth of cut is too large or the rake angle is low
-Happens when there is a lack of an effective cutting fluid
Machining. Cutting Forces - Answer--Tau is the average shear stress on the shear plane
-It is possible to have an upward thrust force when friction at the tool-chip interface is low and/or when
the rake angle is high
-The thrust force in cutting is important because the toolholder and the machine tool must be
sufficiently stiff to support that force with minimal deflections
-Change in the direction and magnitude of the thrust force can lead to instability
, -The cutting force increases with increasing depth of cut, decreasing rake angle, and decreasing cutting
speed.
Machining. Rake angle - Answer--Positive rake angles make the tool sharper and more pointed (reducing
strength of the tool), reduces cutting forces and power requirements, helps in the formation of
continuous chips in ductile materials, helps avoid the formation of a built-up edge.
-Cutting tool with negative rake angle is stronger
Machining. Tool wear - Answer--Dull tools require higher power and result in higher forces
-Cutting speed, feed rate and depth of cut are of decreasing order of importance
Turning - Answer--Positive-rake angles improve the cutting operation by reducing forces and
temperatures; however, positive angles also have a small included angle of the tool tip.
-Depending on the toughness of the tool material, a small included angle may cause premature tool
chipping and failure
-The smaller the radius, the rougher the surface finish of the workpiece, and the lower the strength of
the tool; on the other hand, large nose radii can lead to tool chatter
Turning. Surface roughness - Answer-In machining, the usual procedure is to first take one or more
roughing cuts (typically at high feed rates and large depths of cut). The material-removal rates are high,
and there is little consideration for dimensional tolerance and surface roughness of the workpiece.
These cuts are then followed by a finishing cut, typically done at a lower deef and smaller depth of cut,
for a good surface finish.
Peripheral milling - Answer--Helical teeth are preferred over straight teeth (each tooth is always partially
engaged with the workpiece as the cutter rotated)
-The cutting force and the torque on the cutter are lower, resulting in a smoother milling operation and
reduced chatter
End milling - Answer-The cutter can remove material on both its end and on its cylindrical cutting edges
