Polishing Machine
Polishing machines are basically much the same as lapping machines with respect to the progress of the work enhanced by polisher movement. Theory-based analysis and controls of the wear and deformation of polishers become important in order to secure accuracy and a mirror surface.
Lapping and polishing machines are sorted out in Table 6.3. With a motor as their power source, the majority of workpieces and tools move by revolution.
Table 6.3.
Classification of Lapping and Polishing Machines
Work Lap or pad (polisher) Notes
Single-sided machine Flat •
Force rotation
•
Upword •
Disk plate
•
Downward Lens lapping and polishing machine
•
Forced rotation
•
Conditioning ring
•
Downward •
Swiss and rotation
•
Ring (disk) plate
•
Upword Conditioning ring and type lapping polishing machine
•
Followed rotaion
•
Conditioning ring
•
Downward
•
Forced rotation •
Forced rotation
•
Ring plate
•
Upward
•
Forced rotation
•
(Conditioning ring)
•
Downward
•
Rotation and swing •
Ring palte
•
Upward
•
Forced rotation
Motion-free Oscillation
Spherical Lens lapping and polishing machine
Conditioning-type lapping and polishing machine
•
•
Upward
•
Forced rotation •
Ring (plate)
•
Downward
•
Swing and rotation
•
•
Sideward
•
Forced rotation •
Ring (plate)
•
Sideward
•
Swing and rotation Advanced curve generator
Aspherical Lens lapping and polishing machine
•
•
Upward
•
Computer control rotation •
Aspheric tool
•
Downward
•
Swing and synchronous
•
rotation
•
•
Upward
•
Computer control rotation •
Small tool
•
Downward
•
Circular swing Computer-controlled lapping and polishing machine
•
•
Sideward
•
Forced rotation •
Small tool
•
Sideward
•
Forced rotaion and cam drive
Double-sided machine Flat and Parallel •
Supported by carrier
•
Rotation on carrier
and tool center •
Lower and upper ring plates
•
Fixed Two motion–type double-sided
lapping machine
•
Supported by carrier
•
Rotation on carrier and tool center •
Lower and upper ring plates
•
Either rotation Three motion–type double-sided lapping machine
•
Supported by carrier
•
Rotation on carrier and tool center •
Lower and upper ring plates
•
Opposite rotation Four motion–type double-sided lapping machine
•
Supported by carrier
•
Circular swing of carrier •
Lower and upper disk plates
•
Fixed
•
•
Sideward
•
Followed rotation by roll supporting •
Disk plate
•
Sideward
•
Both forced rotation Magnetic memory substrate double-sided lapping machine
Table options
In the theoretical analysis of polishing, the proportional constants of Preston’s law are used for the calculation of stock removal as well as the wear amount. These constants represent specific stock removal and wear amounts of the polisher; their dimensions are the same as that of the specific wear amount in the friction and wear. These values are influences by the characteristics of workpieces, abrasives, slurries, and polishers and become one of the polishing conditions in the theoretical analysis [21].
Polishing speed and time are given by kinematic analysis between the work and polisher. In this case, diameters, revolutions, and true contact ratio are critically important factors. Moreover, if polishing starts under such initial polishing conditions that the shape of both surfaces are expressed in a mathematical formula, the relationship between the behaviors of both surfaces and pressure distributions can be obtained with the following assumptions:
1.
The abrasives and machined traces are considered to be in close contact when they are distributed uniformly across both surfaces.
2.
The pressure distributed on the work surface is given by the elastic deformation amount of the polisher.
3.
The inelastic behavior of the polisher should be included in the wear.
4.
No negative pressure occurs across the entire surface.
Stock removal hw (μm) and wear amount of polisher hp (μm) at a random point on the work or polisher surface are expressed as follows:
equation(6.1)
Turn MathJaxon
where, Δt (= t/m, min) is a short interval; η(μmkm-1/Pa) stands for the specific stock removal or specific wear amount of polisher; (v) (m/min) or (p) (Pa) is the average velocity or pressure on machined traces of every random point; and α is a ratio of actual processing time at random points against apparent polishing time. It is important for the development of theoretical analysis that the sum of pressures at each point corresponds to the loading weight values on work. Figure 6.18 shows a relation between the work and polisher. Figure 6.19 shows calculation results of stock removal, wear amount of the tool, and flatness when obtained. Pitch polishing was performed for glass work from its initial conditions as given inFigure 6.18.
Figure 6.18.
Relationship between work and polisher (pad): (a) relation between disk work type polisher; (b) distributed pressure model taking into account the figure of a random point on work and polisher [21]
Figure options
Figure 6.19.
Calculation results of stock removal, wear amount of tool, and variation of flatness [21]
Figure options
According to numerical calculations, as polishing advances, the flatness markedly deteriorates with a large specific wear amount of polisher. The degree of flatness deterioration seems rather small when elastic deformation constants are large [21].
Advanced Polishing Methods
Recently, various polishing methods originating from the conventional lapping and polishing methods were newly proposed for device fabrications; these methods are indicated in Table 6.4.
Table 6.4.
Advanced Polishing Methods
________________________________________ Property of processing Application
[Improvement of lapping]
Ultraprecision Mechanically stock removing with abrasive stone and oil or water-tyoe solution Block gauge
Low-speed agrinding End of scale
Ultraprecision lapping By using fine abrasives and metal or ceramics lap, and filtered oil or water-type solution Plate
Conventional lapping
Conventional polishing By using Al2O3 or SiC abrasives and cast iron lap, or CeO2 powders and pitch or plastic polisher, and lens lapping and polishing machine and conditioning ring-type lapping machine Glass lens or prism
[Improvement of optical polishing]
Mech. Ultraprecision polishing Suppressing mechanical action and dust influence by adopting fine abrasive powder, soft polisher and (ultra) pure water Quarts plate
Laster rod
Optoelectronic device
X-ray mirror
Large telescope mirror
Bowl feed polishing Work and polisher in slurry for suppressing mechanical action or impact and temperature rise
Complex Computer-aided polishing Making a small polisher to travel on work surface by using computer
[Using flow effect of slurry]
EEM Mild stock removing action with abrasives in hydrodynamic flow of slurry around ball tool X-ray mirror
Optical grade dies
Float polishing Stock removing action with abrasives in hydrodynamic flow of slurry on flat polisher Glass Plate
Ferrite
[Removing chemical reactive products]
Mechanochemical polishing(wet) Growing and removing reactive products as hydration film under wet condition Si wafer
GaAs wafer
Chem. (dry) Peeling off reactive product film with 0.01 μm SiO2 powder under dry friction condition Sapphere
Si3N4 ceramic
Hydration polishing Growing hydration film under 50-200°C water vapor condition and wiping off with wood or carbon tool Sapphere
ZnSe lens
[Using chemical removal]
Disk-type chamical polishing Chemically removing with friction by using non-abrasive chemical solution and non-woven fiber sheet GaAs wafer
InP wafer
Hydration polishing Chemically removing with non-abrasive chemical solution layer between work and polisher FdTe wafer
ZnSe wafer
P-MAC polishing Chemically removing by using non-abrasive chemical solution and fluorocarbon foam polisher Working conditions change as polishing advances
Table options
Improvement of Optical Polishing
In the manufacturing of advanced optics, it is necessary to avoid dust and to adopt fine abrasive powders, soft polishers, ultra pure water (DIW), and a clean environment.
In bowl feed polishing, the workpieces and polishers are soaked in slurry while polishing. High quality and high accuracy were obtained under such special polishing conditions that are excellent in classifying fine abrasives, cooling them, and in shock absorption by fluid. A superior surface quality was obtained with a pitch or fluorocarbon-coated polisher [25] and [26].
As often pointed out, a great deal of skill is required to accomplish high-grade polishing, and therefore, a rapid shift to computer-controlled polishing methods has been anticipated. For instance, a correcting polishing method for flat or spherical surfaces uses a small-size polisher that performs small circular motions while traveling across the work surface with the aid of a computer as shown in Figure 6.20. This polishing method is supported by the measuring technology. [26] and [28]
Figure 6.20.
Small tool used in computer-aided polishing [27,28]