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Course Materials (These materials are for non-commercial educational use only. If copied for that purpose, a courtesy email to [email protected] 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
