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Transcript
Course Materials
(These materials are for non-commercial educational use only. If copied for that purpose, a
courtesy email to craig@mtu.edu is requested)
Because of file size, actual transparencies in a specified format are not included here. However,
the text can be copied into overheads for in-class use.
Precision Engineering and Practices
Cause and Effect Relationships
a precision engineer must be deterministic
“an automatic machine is always operating perfectly. It may not be doing what is
required, but that is because it is not suitably arranged.”
“random results are the consequences of random procedures”
“we see statistics as, at best, a distraction and, at worst, a cloak of respectability for bad
metrology” (processes)
What is Precision Engineering
working at the forefront of current technology
shooting after the next decimal place
those striving for the best possible product
engineering wherein the tolerances are 1 part in 10,000 or less of a feature/part size
an attitude wherein there is no such thing as randomness, all effects have a deterministic
cause
Very Brief History of Precision Engineering
precision engineering has its roots in astronomy and sailing
Hipparchus in 2nd century BC and Ptolemy in 150 AD used “graduated”
instruments
the angular diameter of Tycho’s Star in Casseopia (supernova in 1572 AD) was
measured to be from 4.5 to 39 arc minutes using the best instruments of the day
during the middle ages and industrial revolution, many improvements were made in
timekeeping
the founders of Browne and Sharpe and the maker of the first diffraction grating
were clock/watch makers
the first lathes and many other machine tools are rooted in clock/watch making
in modern times, precision engineering is pushed by defense programs
the laws of nature and physicist’s equations do not have provision for tolerances
Cutting Tool Variables
shape and size of cutting edge
accuracy of cutting edge
uniformity of cutting edge
diamond crystal orientation
sharpness of cutting edge
Machine Tool Variables
accuracy, resolution, repeatability
stiffness
spindle vibration
smoothness of motion
feedback errors
Definitions
accuracy - difference between desired position and actual position, for multiple data
points it is the difference between desired position and average of measured positions,
can be compensated if quantified
repeatability - ability to reproduce a given position from the same direction
(unidirectional repeatability) or from two directions (bi-directional repeatability), for
multiple data points it is the distribution about the mean (standard deviation), usually the
most important
resolution - the smallest increment of movement, all movements are integer multiples of
this amount
Feedback Errors
leadscrew non-linearity
static deflection
tool wear
axis orthogonality
flatness, straightness
resolution
Work Piece Variables
fixtures
material and heat treatment
coolant
cutting speed and feed rate
Environmental Variables
variations in room temperature
floor vibration
acoustic vibration
spindle windage
spindle warpage
Thermal Effects
virtually all the energy supplied to a machining process is converted to heat
the resulting temperature field and thermal distortion results in errors in the work being
machined or measured
thermal effects become dominant as the required accuracy increases
Total Thermal Error in Machining
errors come from conduction, convection, and radiation from
room environment, walls, heating/cooling, coupling to outdoors
coolants and lubricants
people, each person is “worth” 150 Watts in sensible and latent heat
the machine, electronics, friction
cutting process
Thermal Stabilization
thermal growth often represents the largest error in micro/precision machining
+/- 1.0C change causes 0.5 micrometers of change in 2.54 centimeters of aluminum
a 1C change causes a 1 wavelength change in 1 inch of aluminum
Advantages of Air Bearings
low friction and torque, low thermal influence
high stiffness at low speed, hydrostatic not hydrodynamic bearings
operable at wide temperature range so long as water does not condense
minimal effect on surroundings, environmentally friendly
zero wear
Disadvantages of Air Bearings
external pressurized source required, unless self feeding (not for high forces)
Joule-Thomson thermal distortion due to internal cooling
relatively high cost
low “crash” tolerance
requires pressure control
Sensitive Direction in Machining
each process has a direction where precision is most sensitive
in turning (lathe), the radial coordinates are more important than axial or vertical
in facing (lathe), axial coordinates are more important than radial
Causes of Surface Finish Errors
high feed rate
too deep of a cut
too shallow of a cut
dull tool
tool rake angle
tool nose radius
material
insufficient coolant
center anomalies (tool not at rotational axis of part)
Surface Characterization
Components of Topography
roughness - closely spaced, high frequency irregularities
waviness - widely spaced, low frequency irregularities
form - generally desired shape of the part or surface
flaws - discrete and infrequent irregularities (pits, cracks, scratches)
Surface Roughness Parameters
Rt - peak to valley - difference between highest and lowest points
Ra - average roughness - average deviation from the mean of the surface points
Rq - root-mean-square roughness - rms deviation from the mean of the surface points
Surface Texture
the power spectral density is the square of the surface Fourier transform (square of
Fourier coefficients for each increment of frequency)
spectral peaks at low frequencies indicates surface waviness
spectral peaks at higher frequencies indicates periodic roughness
there is no agreement where waviness ends and roughness begins
harmonics are mainly due to periodic effects such as machining marks, kinematic errors
Autocorrelation Function
although more mathematically complex, the autocorrelation function
indicates the overall periodicity of the surface
for a mathematically random surface, the value of the autocorrelation
quickly drops from maximum value to zero and is “noisy” about the zero value
for a purely periodic surface, the value drops more slowly and continues to
decay to zero while oscillating about zero
all real surfaces are a combination of these two extreme cases
autocorrelation will often show periodicity which is not obvious in the
power spectral density or surface roughness plots
Pseudocode to Calculate Peak to Valley Roughness (Rt)
open datafile of vertical Yvalues for each lateral Xvalue
read Yvalues save numeric maximum
read Yvalues save numeric minimum
Rt = Ymax - Ymin
close datafile
Pseudocode to Calculate Average Roughness (Ra)
open datafile of vertical Yvalues for each lateral Xvalue
save total number of data points; N
save smallest and largest Xvalues; minX, maxX
deltaX = (maxX - minX) / N
save smallest and largest Yvalues; minY, maxY
find area by trapezoidal rule and find mean line
meanline = minY + (area / (maxX - minX))
calculate sum of deviations from meanline for each Yvalue
sum = sum i..N {abs (Yvalue - meanline)}
calculate average roughness
Ra = sum / N
Pseudocode to Calculate Root-Mean-Square Roughness (Rq)
open datafile of vertical Yvalues for each lateral Xvalue
save total number of data points; N
save smallest and largest Xvalues; minX, maxX
deltaX = (maxX - minX) / N
save smallest and largest Yvalues; minY, maxY
find area by trapezoidal rule and find mean line
meanline = minY + (area / (maxX - minX))
calculate sum of squares of deviations from meanline for each Yvalue
sum = sum i..N {(Yvalue - meanline)^2}
calculate average roughness
Rq = sqrt{sum / N}
Diamond Machining
Characteristics of Diamond Machining
small material volume removal process for part form (if large volumes
need to be removed, use other process first)
high quality tool normally of single crystal natural diamond
high precision, low vibration machine with very smooth motions
high precision parts in metals, plastics, crystals
Properties of Diamond as a Tool Material
elastic modulus ~ 1000 GPa ( ~ 145 million psi, 5 times stiffer than steel)
high modulus means small distortion under the presence of cutting forces
shear modulus ~ 300 GPa
critical tensile cleavage strength 4 GPa ( ~ 580,000 psi)
no detectable plastic flow up to critical cleavage stress
highest thermal conductivity of any material at room temperature
chemically inert (but will be absorbed by “carbon-hungry” materials)
can be polished to a very sharp cutting edge (sub-micrometer scale)
Types of Diamond Tools
natural single crystal diamond
multiple crystal configurations
no control of impurities (research shows impurity content alters
tool life)
relatively expensive depending on size of diamond, accuracy of
cutting edge, etc (~ USD$-hundreds)
polycrystalline
diamond particles embedded in a metal binder (cermet type tool)
random orientations of cutting edges
machines more like very fine grinding wheel than single point tool
less expensive (~ USD$-less than 100 typically)
synthetic diamond
control of impurities
better machining properties
most expensive (~ USD$-hundreds to thousand)
Diamond Machinable Metals (not exhaustive but typical)
aluminum alloys (little if any built-up edge)
brass
gold
silver
tin
zinc
electroless nickel (not wrought or electroplated nickel)
fine chips of these materials should not be inhaled
Diamond Machinable Plastics (not exhaustive but typical)
polycarbonate
fluoroplastics
acrylics
styrene
propylene
fine chips of these materials should not be inhaled
Diamond Machinable Crystals (not exhaustive but typical)
silicon
germanium
lithium niobate
zinc sulfide
gallium arsenide
cadmium telluride
crystals must be machined in “ductile-mode” to reduce or eliminate the
creation or propagation of surface and sub-surface cracks (very small depth of cut
to reduce tensile stresses in material)
Difficult to Machine Metals (not exhaustive but typical)
wrought nickel alloys
beryllium and alloys
ferrous alloys including stainless steel
titanium and alloys
molybdenum and alloys
Diamond Machining Characteristics (also check Machining Data Handbook)
typically performed at or about (+/- 3 degrees) zero rake angle
roughing depth of cut hundreds of micrometers or less depending on
material and specific cutting energy
finishing depth of cut tens of micrometers or less depending on material
and specific cutting energy
typical machining speed depends on materials (acrylics for optics ~
hundreds of meters per minute, for example)
Handling Diamond Turned Surfaces
handle parts by edges only
skin oil and dry cotton can damage surface (metals such as aluminum and
copper are very soft and easily scratched, opposed to glass optics normally
encountered)
do not talk over a diamond machined surface, saliva is very
corrosive/caustic (aluminum particularly) and saliva marks are very difficult to
remove
never lay a diamond machined surface on anything, it will scratch
Cleaning Diamond Turned Surfaces
pure liquid soap (sodium lauryl sulfate) for heavy contamination
soaked cotton ball with very light pressure
rinse with distilled water
remove water residue with acetone (wear gloves and use ventilation) or
alcohol
remove meniscus with edge of tissue
some materials rapidly corrode in atmosphere, such as copper
Testing Diamond Turned Surfaces
non-contact techniques are preferred
stylus techniques can scratch (diamond machined surfaces can have
roughness of a few nanometers)
for qualitative analysis, use surface as reflector
find reflected image of overhead diffuse light
focus on mirror surface, not light (takes practice)
slowly tilt part to move reflected image across surface
look for waviness, diffraction, sharp changes in image
technique can detect defects into the sub-micrometer range
Micromilling
Classic Method for Deep X-ray Mask Fabrication
create optical lithography mask with pattern generator (expose quartz with
resist, develop, deposit chromium optical absorber layer)
use optical lithography and optical mask above to create thin (2-3
micrometers thick gold) intermediate x-ray mask
use intermediate mask above at synchrotron to create final x-ray mask
(10-15 micrometers thick gold)
use final x-ray mask to expose desired microstructures by deep x-ray
lithography
Alternate Method for Deep X-ray Mask Fabrication
requires 100 keV electron beam writer access
directly pattern resist to depth of approximately 10 micrometers (~ limit)
develop and plate with gold to make x-ray mask directly
electron beam writers cost ~ USD$ 1-million and typically require
full-time operator / technician
Micromilled X-ray Masks
can not obtain same results as lithography-produced masks due to tooling
size, machining forces, etc
for micromechanical (mesoscale) components, features are often 1-10
micrometers and part is millimeters in overall size
micromilling has been shown to produce 4-micrometer wide walls in
acrylic and aluminum very quickly - material removal rate is ~ 3,000
cubic-micrometers per second or greater
Characteristics of Focused Ion Beam Machine to Make Micromilling Tools
operates similarly to a scanning electron microscope except ions are used
tungsten needle wetted with gallium metal (~ liquid at high room
temperature)
6 kV extraction voltage used to ionize gallium
20 keV ion beam energy steered by 8-pole electrostatic filed
sub-micrometer spot diameter with nano-amp range current gives several
amps per square centimeter
typical work sputter yield is 3-5 atoms per each incident ion with total
removal rate of ~ < 1 cubic micrometer per second
Technique for Quantifying and Compensating Micromilling Tool Radial Runout
milling tools used with precision collet (not vee-block) will have chucking
error which causes them to rotate eccentrically
milling tool can be centered in collet to a precision of 3-5 micrometers of
remaining radial runout using a “tapping-in” procedure with a video microscope
the residual eccentricity will cause the tool to machine with an effective
diameter larger than its true diameter, if quantified the effective diameter can be
used to alter machining geometry
two trenches are milled with known center-center spacing (assumes
machine tool has adequate accuracy)
width of wall between trenches can be measured in scanning electron
microscope, interferometric microscope, etc
effective tool diameter = center-center spacing minus wall width
this is a fixed error (assuming zero tool wear) and can be compensated
it also sets the minimum possible width of a trench feature
Microdrilling
Microdrilling Practices
microdrills have a an aspect ratio (length / diameter) of 7 to 14 for
diameters larger than ~ 100 micrometers
microdrills have an aspect ratio of 4 to 7 for diameters ~ 25 micrometers
to ~ 100 micrometers
due to chip breakage, microdrills fail in use due to fluctuating torsional
loads (assuming gross breakage does not occur due to bending or column
buckling)
a microdrilled hole normally requires 10 - 30 peck cycles but ultimately
depends on the depth
peck cycles leaves a measurable artifact on the hole wall
drills should not be allowed to dwell on the bottom of the hole as work
hardening will likely occur
Microdrilling Practices
always a light lubricant to clear chips
a smaller point angle (118 degrees) normally results in a smaller hole due
to better centering during hole startup
holes in curved surfaces often requires the area to be flattened depending
on the local slope of the surface relative to the axis of the drill, drills have very
low bending stiffness and strength
Drills and Materials
drills are normally available with 118 degree and 135 degree include point
angles, 135 degree is for harder materials (stronger point structure)
drills are normally available in C-3 micrograin tungsten carbide or M42
cobalt high-speed steel
C-3 tungsten carbide is very hard with small grain structure and is
normally available in the larger aspect ratios
M42 is lower cost but compared to other steels it has high resistance to
wear and good resistance to cracking
Laser Micromachining
Laser Microfabrication Processes
ablation
etching
deposition
photopolymerization
lithography
microelectroforming
Advantages of Ultraviolet Laser Machining
high lateral resolution, shorter wavelength improves focusability
strong material interaction, shallow penetration
can be used to initiate photochemical reactions
Characteristics of Small UV Lasers
waveguide excimers, 193 nm to 351 nm wavelength, 10 to 100
microJoules per pulse, up to 2 kHz pulse repetition rate
small excimers, 193 nm to 351 nm wavelength, 1 to 5 milliJoules per
pulse, up to 100 Hz pulse repetition rate
YAG (yttrium-aluminum-garnet), harmonics at 266 nm and 351 nm,
energy per pulse in the range of microJoules to milliJoules, pulse repetition rate
up to 20 kHz
Typical Laser Operating Characteristics
laser photoablation
laser:
wavelength:
pulse energy:
pulse rate:
pulse duration: 70 ns
peak power:
average power:
beam diameter:
working gas:
Typical Laser Operating Characterisrics
laser photopolymerization
laser:
wavelength:
pulse energy:
pulse rate:
pulse duration: 100 ns
peak power:
average power:
beam diameter:
working gas:
Electrical Discharge Machining
excimer
248 nm
45 microJoules
2 kHz maximum
400 W
75 mW
< 10 micrometers
krypton-fluoride
excimer
351 nm
< 30 microJoules
2 kHz maximum
< 300 W
< 60 mW
variable to < 10 micrometers
xenon-fluoride
Electrical Discharge Machining Basics
material is removed from the conductive work piece by repetitive spark
discharges
the dielectric cools, flushes chips, and provides insulation in the gap
between the electrode and work piece
lower current and higher spark frequency results in smoother surface
finish but is accompanied by a slower machining time (microEDM uses very low
current sparks at high frequency)
the electrode wears as well as the work piece
EDM Parameter Tradeoff
situation 1
high material removal rate
large surface roughness
low frequency sparks
high spark energy
large gap between electrode and work
modest electrode wear
situation 2
low material removal rate
low surface roughness
high frequency sparks
low spark energy
small gap between electrode and work
low electrode wear
Typical EDM Parameters
open circuit voltage - 50 to 300 VDC
frequency - 50 Hz to 500 kHz
dielectric - hydrocarbon oil, silicone oil, de-ionized water, kerosene, dry
gas
electrode material - graphite, copper, brass, zinc-tin, tungsten
gap size - 1 to ~100 micrometers depending on other parameters
polarity - “standard” is positive on work piece
Wear Ratio
volumetric wear ratio is volume of work removed per volume of electrode
consumed
in ram EDM, linear wear ratio is depth of cut per length of electrode
consumed (constant cross section area so volume reduces to linear dimension)
Electrode Material and Properties
graphite - low cost, excellent machinability to create electrode shape,
linear wear ratio ~ 100:1, finish depends on graphite particle size and density
copper - moderate cost, good machinability, low wear ratio, good finish
but not good for high accuracy
steel - low cost, excellent machinability, good for thru-holes, not good on
carbide, leaves large heat affected zone, corrodes
tungsten - high cost, poor machinability, brittle, excellent for small holes
high rigidity, good wear ratio, used almost exclusively in microEDM
Dielectric Properties
hydrocarbon oils - widely used, cuts better after a few minutes of use,
inexpensive
de-ionized water - used in wire cutting, good dielectric strength (especially
electronic-grade DI water)
kerosene - good for fine finishes, very hazardous, spark must remain
submerged and fire suppression system normally required
silicone oil - can be very expensive
EDM Surface Characteristics
surface is a series of small overlapping craters
craters are larger with higher spark energy
recast and heat-affected layers are present in all materials
plastics can be EDMed if sufficiently conductive
Typical MicroEDM Characteristics
discharge energy is very small, typically less than 100 nanoJoules, E = 0.5
CV^2, where C is supply capacitance (Farads) and V is voltage across discharge
capacitance
to reduce spark energy, discharge capacitance typically less than 10 pF,
stray capacitance in the machine structure is a problem, ceramics widely used,
power supply and wires often exceed this capacitance, power supply should look
like a battery to the electrode
tungsten electrodes normally used because of high stiffness, but flow of
dielectric bends electrode
