The particle size distribution of aqueous metal-oxide slurries can evolve during the polishing of optical glass in response to changes in mechanical and chemical process factors. The size-evolution phenomenon and its consequences were systematically studied in a planar continuous-polishing process. The concurrent application of electrokinetic techniques to characterize common optical shop materials has contributed new insight into the nature of silicate glass polishing by demonstrating the pivotal role of fluid chemistry, particularly pH, in maintaining electrokinetically favorable conditions for a well-dispersed polishing agent. According to the proposed slurry-charge-control effect, a well-dispersed polishing agent is the key to obtaining the smoothest possible glass surfaces, especially when a recirculated slurry is used.

Because there are no practical commercially available bound abrasive polishing media, we are developing a bound abrasive polisher for deterministic finishing of optical glasses. Several in- house formulated polishing pellets, molded laps, and ring tools have been studied. Two experimental test beds were employed. The first involved the polishing of flat optical glass parts on single pellet and molded pellet laps. The tests were conducted on a single spindle machine. The performance of in-house manufactured laps was compared to experimental and commercial formulations obtained from industry. Compositions which polished the glass below 20 angstrom rms surface roughness were selected for additional testing. The second test bed for these formulations was the Opticam SM. Materials were molded into a ring tool geometry. Although the tools polished effectively, more work is required to control surface figure during final finishing.

Magnetorheological (MR) fluids are multicomponent systems consisting of a non-colloidal magnetic-dispersed phase in a carrier liquid, which undergo rapid, sharp and reversible changes of their internal structure in an external magnetic field. As a consequence, their rheological properties, such as viscosity, plasticity and elasticity are controllably changed (for example, see Figure 1).

The role of surface charge effects is a subject of increasing interest in optical finishing [1 - 3]. This represents an intersection of the empirically driven world of the optician with the more fundamentally oriented field of colloid science. In this paper, we examine the effect of slurry fluid chemistry on the performance of a planar continuous glass polishing process using the electrokinetic methods of colloid science to characterize the properties of optical glass and polishing agents. The goal is to understand any consequences of the chemically modulated development of electric charge on the surfaces of metal-oxide polishing agents and glass surfaces during aqueous polishing. Our discussion here is confined to experimental work using nine combinations of commercially important silicate optical glass types (Schott BK7 borosilicate crown and SF6 dense flint and Corning 7940 fused silica) and metal-oxide polishing agents (CeO 2 [4], monoclinic ZrO 2 and nanocrystalline AI 2 O 3 [5]). A more extensive study of the polishing process is the subject of the first author’s doctoral dissertation [6].

Metal bonded diamond tools have been used at the Center for Optics Manufacturing (COM) for the deterministic microgrinding of glass. With use of metal-bonded 2-4 micron single crystal diamonds in the final grinding step, very smooth surfaces with only a few microns of subsurface damage can be consistently manufactured. While these results are very positive, we are currently seeking means to further improve performance. In particular we are examining the effects of changing the diamond on tool performance.

A pre-prototype magnetorheological (MR) finishing machine has been constructed at the Center for Optics Manufacturing. It consists of an electromagnet, a trough for MR fluid containment and a work spindle (see Figures 1 and 2). A glass part is mounted on the spindle, positioned within the trough and above the magnet pole pieces. Polishing occurs on the surface of the glass as a function of the movement of polishing abrasives through a zone of high pressure, created by the action of the magnetic field on the MR suspension[1]. Polishing slurry in the zone of high pressure is continually refreshed by the rotation of the trough. By rotating the work spindle, an annular ring is polished out on the part (see Figure 3). The entire lens surface is polished out by adjusting spindle tilt (theta, in Figure 2) and dwell time.

The objective of this research is to develop a ring tool polisher for deterministic finishing of optical glass parts on Opticam® machining centers at the Center for Optics Manufacturing (COM). To be successful, the ring tool must reduce rms roughness from 100-200Å to 10-20Å, remove 2-4μm of residual subsurface damage, and preserve the surface figure of the glass part. The medium used to fabricate the polisher must satisfy several criteria. It must contain and be able to release polishing agent to the glass surface in an aqueous environment. It must be capable of being formed or machined into a ring (solid or segmented) with a sharp edge. It must maintain its physical integrity at high tool velocities without exhibiting catastrophic failure under load, and it must not show excessive wear during use. It must not require the assistance of additional polishing slurry. These are formidable requirements.

There are a number of commercial coolants available to the metal cutting industry. These coolants have been optimized for the various properties of specific metal types, and instructions are available for their use [1,2]. Recently, coolants advertised to be designed especially for the optical industry have become available. A grinding study to evaluate the performance of several metal and glass grinding coolants is being conducted at the Center for Optics Manufacturing. This paper presents some recent results for K7 optical glass.