Theoretical And Experimental Investigation Into Electrorheological Polishing Of WC Molding Dies

1. Introduction With the development of photoelectron communication technology, the size of every component for opo-electrical devices has been reduced in recent years. The demands for the micro-size optics glass lenses with lower surface roughness are considerable, and it has stimulated research into new methods for die machining for the production of micro-aspherical lenses. Conventional methods of polishing are based on the use of a small polishing pad, which is difficult in limiting the manufacturing the work area to a small range. The ER polishing is a new finishing technique for polishing of extremely fine three-dimensional micro or meso-scale devices.2. Physical model of the ER particle and interaction force1) Physical model of the ER particle and abrasive particle combined. Using the model of physics and interaction force on ER fluid, we can acquire the physical model of the ER particle and abrasive particle combined. Dielectric particles suspending in a dielectric fluid are polarized with the application of electric field, and they experience the dipolar interaction force each other and the translational force along the field. As a result of the calculations of exerted forces on ER particle and abrasive particle suspending in silicone oil, ER particle strongly attract the abrasive as well as abrasive particle which is adhere to ER particles, their structure is offered figure 1 as follow.2) Interaction force among ER particlesThe interaction force among ER particles include the interaction force between ER particle and abrasive particle, the interaction force among abrasive particle including two ER particle and the interaction force among other abrasive particle along ER chain,etc.a)The interaction force between ER particle and abrasive particleHere,ε₀ is the permittivity of free space,ε f is relative permittivity of ER, r1 is the diameter of ER particles, r₂ is the diameter of abrasive particles, D is the distance between two ER particles, Ri j is the distance between two polarized particles.b)The interaction force among abrasive particles included two ER particlesc)The interaction force among other abrasives particle along ER chainWhere,h is the distance between two abrasive along ER chain.3) Analysis on effective layer of the interaction force among particle From figure 2 can see that the effective layer of the interaction force between ER abrasive and abrasive particles is one layer and the interaction force among other abrasive particle along ER chain is one layer.3. The analysis on effective area and the amount of abrasive1) The analysis on effective area of ER fluid-assisted polishingThe electric field stiffened ER polishing fluid constitutes a small lap, which is used to polish optical element. The interaction force on abrasive particle which is adhere to ER particles will suffer the interaction force of other particle,dielectro- phoretic force and centrifugal force in polishing process, as shown in figure 3, thedielectrophoretic force facilitates abrasive particles collection at the tool tip, even while the tool is rotating. Howover, it is hard to persistently hold the abrasive particles around the target area due to exerted force on the abrasive particles which is not effective participate in polishing. When the abrasive particle is away from the tool tip, because of the interaction force between ER particle and abrasive particle is less than centrifugal force,and they will disengage from the ER chain. As a result, it will form effective polishing area around the tool tip.As shown in figure 3,the relation on the interaction force on abrasive particle.Where, F|-_{DF - ML} is the interaction force between ER particle and abrasive particle, F|-_{ML - ML} is the interaction force among abrasive particle, FD EF is dielectrophoretic force, FL is centrifugal force.We can acquire the theoretical effective polishing area from equation (3). Comparing figure 4(a) to figure 4(b), we can see that the theoretical effectivepolishing area is general the same as experimental effective polishing area. It indicates that the theory of effective polishing area can appropriate reflect the practical polishing area.Fig.5 Design of polishing path.According to the effective polishing area we can design the distance of polishing path which is equal to the diameter of effective polishing area in the theory as shown in figure 5, so that the polishing can avoid the disadvantage of the pitch oversize or undersize. In fact, we can effective control and optimize the polishing path to acquire the lower surface roughness of workpiece.2)The analysis on amount of abrasive for ER fluidFrom figure 6 can know when the amount of abrasive for ER fluid is too thick, some abrasive particle will dissociate around the abrasive particle which adhere to ER chain. As the interaction force between ER particle and abrasive particle less than centrifugal force, the abrasive particle will break away from the ER chain. So the amount of abrasive particle is decrease not to participate in polishing, that it is not profit for ER fluid polishing. Therefore, the capability of ER particle combine abrasive particle is limited. By the investigation on the amount of ER particle combine abrasive particle effectively, we can analysis the effect of the amount of abrasive for ER fluid on the surface roughnessCompar ing figure 7 to figure 8, we can see that the curvilinear of the capability of ER particle combine abrasive particle and the relationship between the amount of abrasive for ER fluid and surface roughness, which variety trend is consistent. By investigate on the amount of ER particle combine abrasive particle effectively, it make clear the effect of the amount of abrasive for ER fluid on the surface roughness of workpiece.Fig.7 Relationship between the amount of abrasive for ER fluid and the capability of ER particle combine abrasive particle.Fig.8 Relationship between the amount of abrasive for ER fluid and surface roughness.4. Material removal model of ER polishingThe Preston equation is used to determine the theoretical material removal function. ER polishing is a newly developed method for finishing optical component with the ER fluid by the electric. According to Preston equation, thestock removal volume R is calculated from the following equation. R = kpv (5)Here k is the coefficient of Preston equation, p is the pressure of workpiece surface in the polishing area, v is the speed of ER fluid. The pressure acting on workpiece surface consists of hydromechanical pressure pd , polarization pressure p j and flotage p g. p = pd + p j + pg (6)From figure 9 can see that the removal rate of material increases first, and then is descend. Comparing figure 9 to figure 10, we can know that the removal rate and the stock removal volume are consistent.