– Ion implantation: the ions are accelerated in a vacuum to such an
extent that they penetrate the substrate to a depth of a few mm.
– Diffusion coating: a process in which an alloying element is
diffused into the surface of the substrate, thus altering its
properties. (carburizing, nitriding, boronizing)
• Electroplating
– The workpiece (cathode) is plated with a different metal (anode)
while both are suspended in a bath containing a water-base
electrolyte solution.
– Electroforming is a variation of electroplating. Metal is
elctrodeposited on a mandrel, which is then removed.
• Anodizing
– the workpiece is the anode in an electrolytic cell immersed in an
acid bath, resulting in chemical adsorption of oxygen from the
bath. The workpiece surface is covered with a hard and porous
oxide layer.
• Applications: aluminum furniture and utensils, architectural
shapes, automobile trim, picture frames, keys, sporting goods
• Diamond coating of metals, glass, ceramics, and plastics,
uses chemical and plasma-assisted vapor deposition
process and ion beam enhanced deposition techniques.
– Production of free-standing diamond films on the order of 1 mm
thick and up to 5” in diameter
• properties of diamond: hardness, wear resistance, high thermal
conductivity, and transparency to ultraviolet light and
microwave frequencies
• Used for scratchproof windows, sunglasses, cutting tools,
calipers, surgical knives, razors, electronic and infrared heat
sinkers and sensors, light emitting diodes, turbine blades, and
fuel injection nozzles.
• Cleaning surfaces
– Cleaning involves removal of solid, semisolid, or liquid
contaminants from a surface.
– Two types of cleaning methods
• Mechanical - physically disturbing the contaminants, as with
wire or fiber brushing, dry or wet abrasive blasting, tumbling,
and stream jet.
• Chemical - usually involves the removal of oil and grease from
surfaces. It consists of one or more of the following processes:
– solution - the soil dissolves in the cleaning solution
– saponification - a chemical reaction that converts animal
or vegetable oils into a soap that is soluble in water
– emulsification - the cleaning solution reacts with the soil
or lubricant deposits and forms an emulsion. The soil and
the emulsifier then become suspended in the emulsion
– dispersion - the concentration of soil on the surface is
decreased by surface-active materials in the cleaning
solution
– aggregation - lubricants are removed from the surface by
various agents in the cleaner and collect as large dirt
particles.
• Some common cleaning fluids are used in conjunction with
electrochemical processes for more effective cleaning. These
fluids include: alkaline solutions, emulsions, solvents, hot
vapors, acids, salts, and organic compound mixtures.
• Cleaning discrete parts having complex shapes can be difficult.
Design engineers should be aware of this difficulty and provide
alternative designs, such as avoiding deep blind holes or
making several smaller components instead of one large
component that may be difficult to clean, and provide
appropriate drain holes in the part to be cleaned.
Engineered Metrology and
Instrumentation
• Engineering metrology is defined as the measurement of
dimensions
– postprocess inspection
– in-process, online, real-time inspection
• Dimensional tolerances - the permissible variation in the
dimensions of a part
• Instruments
– Line-graduated (marked)
• steel rule, bar, or tape (1 mm or 1/64 in.)
• vernier caliper or caliper gage (25 mm or 0.001 in.)
• micrometer (2.5 mm or 0.0001) used for measuring the
thickness, and inside or outside diameters
• diffraction grating
• telescoping gage for indirect measurement of holes or cavities
• bevel protractor to measure angles
– Comparative length-measuring instruments
• Amplifies and measures variations or deviations in distance
between two or more surfaces
– dial indicators (1 mm or 40 min)
– electronic gages sense the movement of the contacting
pointer through changes in the electrical resistance of a
strain gage or through inductance or capacitance (linear
variable differential transformer - LVDT)
• Measuring straightness, flatness, roundness, and profile:
– straightness can be checked with straight edges or with dial
indicators
– flatness can be measured by using a surface plate and a dial
indicator
• or by interferometry using an optical flat
– roundness
• the round part is placed on a V-block or between centers and is
rotated, with the point of a dial indicator in contact with the
surface
• circular tracing, the part is placed on a platform, and its
roundness is measured by rotating the platform
– profile
• a surface is compared with a template or profile gage to check
shape conformity
• profile-tracing instruments
– Threads and gear teeth
• thread gages that compare the thread produced against a
standard thread
• optical projectors, called optical comparators
– Coordinate measuring and layout machines
• accuracy up to 0.25 mm or 10 min
• layout machines are used to measure parts with large
dimensions (+/- 0.04 mm or 0.0016 in)
• General characteristics of measuring instruments
– The characteristics and quality of measuring instruments are
generally described by certain specific terms. These terms, in
alphabetical order, are defined as follows:
• Accuracy - the degree of agreement of the measured dimension
with its true magnitude
• Amplification - see magnification
• Calibration - adjusting or setting an instrument to give readings
that are accurate within a reference standard
• Drift - see stability
• Linearity - the accuracy of the readings of an instrument over
is full working range
• Magnification - the ratio of instrument output to the input
dimension
• precision - degree to which an instrument gives repeated
measurement of the same standard
• repeat accuracy - same as accuracy, but repeated many times
• Resolution - smallest dimension that can be read on an
instrument
• Rule of 10 - an instrument or gage should be 10 times more
accurate than the dimensional tolerances of the part being
measured. Similarly, a factor of 4 is known as the Mil
Standard rule
• Sensitivity - smallest difference in dimension that an
instrument can distinguish or detect
• Speed of Response - how rapidly an instrument indicates the
measurement, particularly when a number of parts are
measured in rapid succession
• Stability - an instrument’s capability to maintain its calibration
over a period of time (drift)
• Automated measurement
– flexible manufacturing systems and manufacturing cells have led
to the adoption of advanced measurement techniques and systems.
– Automated inspection is based on various on-line sensor systems.
– What factors contribute to subsequent deviation in the dimension
of the same part produced by the same machine?
• Static and dynamic deflections of the machine because of
vibrations and fluctuating forces, cause by variations such as in
the properties and dimensions of the incoming material.
• Deformation of machine because of thermal effects.
• Wear of tolls and dies.
• Dimensional tolerances
– is defined as the permissible or acceptable variation in the
dimensions of a part.
– Tolerances become important only when a part is to be assembled
or mated with another part.
• Several terms are used to describe features of dimensional
relationships between mating parts. These terms, in
alphabetical order, are defined as follows:
– Allowance - the specified difference in dimensions between mating
parts; also called functional dimension or sum dimension.
– Basic size - Dimension from which limits of size are derived, using
tolerances and allowances.
– Bilateral tolerance - deviation (+/-) from the basic size
– Clearance - the space between mating parts.
– Datum - a theoretically exact axis, point, line, or plane.
– Feature - physically identifiable portion of a part, such as hole,
slot, pin, or chamfer.
– Geometric tolerancing - tolerances that involve shape features of
the part.
– Hole-basis system - tolerances based on a zero line on the hole;
also called standard hole practice, or basic hole system.
– Interference - negative clearance.
– Interference fit - a fit having limits of size so prescribed that an
interference always results when mating parts are assembled.
– International tolerance grade (IT) - a group of tolerances that vary
depending on the basic size, but provide that same relative level of
accuracy within a grade.
– Limit dimensions - the maximum and minimum dimensions of a
part; also called limits.
– Maximum material condition (MMC) - the condition where a
feature of size contains the maximum amount of material within
the stated limits of size.
– Nominal size - dimension that is used for the p urpose of general
identification.
– Positional tolerancing - a system of specifying the true position,
size, and form of the features of a part, including allowable
variations.
– Shaft-basis system - tolerances based on a zero line on the shaft;
also called standard shaft practice, or basic shaft system.
– Standard size - nominal size in integers and common subdivisions
of length.
– Transition fit - fit with small clearance or interference that allows
for accurate location of mating parts.
– Unilateral tolerancing - deviation in one direction only from the
nominal dimension.
– Zero line - reference line along the basic size from which a range
of tolerances and deviations are specified.
Testing and Inspection
• Nondestructive testing (NDT):
– is carried out in such a way that product integrity and surface
texture remain unchanged
– requires skill
– examples:
• liquid penetrant- fluids are applied to the surfaces of the part
and allowed to penetrate into the surface openings, including
cracks, seams, and porosity (width of 0.1 mm or 4 min)
– visible penetrants
– fluorescent penetrants
• magnetic particle - fine ferromagnetic particles are placed on
the surface. When the part is magnetized, a discontinuity on
the surface causes the particles to gather visibly around it.
•Liquid Penetrant Inspection:
•Principle: A liquid penetrant is drawn into surface flaws
by capillary action and subsequently revealed by developer
material in conjunction with visual inspection.
•Advantages: Simple, inexpensive, versatile, portable,
easily interpreted, and applicable to complex shapes.
•Limitations: Can only detect flaws that are open to the
surface; surfaces must be cleaned before and after
inspection; deformed surfaces and surface coatings may
prevent detection; and the penetrant may be wiped or
washed out of large defects.
•Material limitations: Must have nonporous surface.
•Geometrical limitations: Any size or shape permitting
accessibility of surfaces to be inspected.
•Permanent record: Photographs, videotapes, and
inspectors’ reports provide the most common records.
• Ultrasonic inspection - an ultrasonic beam travels through the
part. An internal defect, such as a crack, interrupts the beam
and reflects back a portion of the ultrasonic energy. The
amplitude of the energy reflected and the time required for
return indicates the presence and location of any flaws in the
workpiece.
• Acoustic emission - detects signals (high frequency stress
waves) generated by the workpiece during plastic deformation,
crack initiation and propogation, and sudden reorientation of
grain boundaries, friction and wear sliding interfaces.
• Acoustic impact - tapping the surface of an object and listening
to and analyzing the signals to detect discontinuities and flaws.
• Radiography - radiation (x-rays, etc.) is passed through the
sample and absorbed depending on thickness, material, and
flaws
• Eddy current testing - and electrically conductive workpiece is
brought bear an AC coil that produces a magnetic field,
therefore producing eddy currents in the workpiece, which in
turn produces a magnetic field interacting with the original.
This modifies the impedance of the coil. Defects in the
workpiece can affect the magnitude and direction of the
induced eddy currents.
– Radiography - involves x-ray inspection to detect internal flaws or
density and thickness variations in the part.
– Eddy current inspection - based on the principle of electromagnetic
induction.
• alternating current (60Hz to 6 MHz) flows through an electric
coil
• the current causes eddy current to flow in the part
• defects in the part impede and change the direction of the
current, causing changes in the magnetic field
• these changes affect the exciting coil, whose voltage is
monitored to determine the presence of flaws
– Thermal inspection - involves observing temperature changes by
contact or non-contact heat sensing devices (heat sensitive paints
and papers, liquid crystals, and other coatings).
• Automated inspection
– postprocess inspection - lacks flexibility, requires maintenance and
inventory, results in some defective parts going through the system
– automated inspection - uses a variety of sensor systems that
monitor the relevant parameters during the manufacturing process
(on-line inspection)
– using these measurements, the process automatically corrects itself
to produce acceptable parts
– it is flexible and responsive to product design change, less operator
skill is required, productivity is increased, parts have higher
quality, reliability, and dimensional accuracy
– Sensors for automated inspection
• sensors operate on the principle of strain gages, inductance,
capacitance, ultrasonics, acoustics, pneumatics, infrared
radiation, optics, lasers, and various electronic gages.
• Quality assurance
– It is the total effort by a manufacturer to ensure that its products
conform to a detailed set of specifications and standards
(dimensions, surface finish, tolerances, composition, color, and
mechanical, physical, and chemical properties).
• quality can not be inspected into a finished product
• 100% inspection is too costly
• several methods of inspecting smaller, statistically relevant
sample lots have been devised
– Inspection
• incoming materials
• individual components
• assembled product
• testing the product
• Statistical methods of quality control
– Sample size: The number of parts to be inspected in a sample,
whose properties are studies to gain information about the whole
population.
– Random sampling: Taking a sample from a population or lot in
which each item has an equal chance of being included in the
sample. Thus, when taking samples from a large bin, the inspector
does not take only those that happen to be within reach.
– Population: The totality of individual parts of the same design
from which samples are taken.
– Lot size: A subset of population. A lot or several lots can be
considered subsets of the population and may be treated as
representatives of the population.
– Sample is inspected for certain characteristics such as tolerances,
finish, and defects. These characteristics come in two types:
• quantitative (can be measured)
• qualitative (attributes)
• Statistical Process Control (SPC)
– developed in the early 1950’s
– it advises the operator to take certain measures and when to take
them in order to avoid producing further defective parts
– SPC consists of several elements
• control charts and setting control limits
• capabilities of the particular process
• characteristics of the machinery involved
Surface Treatments, Coatings, and
Cleaning
• Reasons for surface treatments:
–
–
–
–
–
–
–
–
–
Improve resistance to wear, erosion, and indentation
Control friction
Reduce adhesion
Improve lubrication
Improve corrosion and oxidation resistance
Improve stiffness and fatigue resistance
Rebuild surfaces on worn components
Improve surface roughness
Impart decorative features, color, or special texture
• Processes:
– Shot peening: the workpiece surface is hit repeatedly with a large
number of cast steel, glass, or ceramic shot (small balls), making
overlapping indentations on the surface. Because plastic
deformation is not uniform throughout a part’s thickness, shot
peening imparts compressive residual stresses on the surface, thus
inproving the fatigue life of the component.
– Roller burnishing: (surface rolling) the surface of the component
is cold worked by a hard and highly polished roller or series of
rollers. It is used to improve mechanical properties of surfaces, as
well as the shape and surface finish of components.
– Explosive hardening: large increase in surface hardness, very little
change (less than 5%) in the shape of the component.
– Cladding: metals are bonded with a thin layer of corrosionresistant metal by applying pressure with rolls or other means.
– Mechanical plating: fine metal particles are compacted over the
workpiece surfaces by impacting them with spherical glass,
ceramic, or porcelain beads.
– Case hardening: used to alter the surface properties of a part
(carburizing, carbonitriding, cyaniding, nitriding, flame hardening,
and induction hardening). Laser beams are used as a heat source in
surface-treatment.
– Hard facing: a relatively thick layer, edge, or point of wearresistant hard metal is deposited on the surface by one of the
welding techniques.
– Thermal spraying: metal in the form of a rod, wire, or powder is
melted in a stream of oxyacetylene flame, electric arc, or plasma
arc, and the droplets are sprayed on the preheated surface, at
speeds up to 100 m/s with compresses air spray gun. (cold spray 2 km/s)
• Thermal spray processes:
– plasma (8300 ºC)
– detonation gun
– high velocity oxyfuel (HVOF) gas spraying
– wire arc
– flame wire
– Surface texturing: etching using chemicals or sputtering, electric
arcs, or atomic oxygen which reacts with surfaces and produces
fine, conelike surface textures.
– Ceramic Coatings: for high temperature and electrical resistance
applications.
• Vapor deposition
– The substrate is subjected to chemical reactions by gases that
contain chemical compounds of the materials to be deposited.
– The deposited materials could be metals, alloys, carbides, nitrides,
borides, ceramics, or various oxides.
– The substrate may be metal, plastic, glass, or paper.
– Typical applications are coating tools, drills, reamers, milling
cutters, punches, dies, and wear surfaces.
– Two major deposition processes:
• physical vapor deposition
• chemical vapor deposition
• thermochemical