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A Deterministic Microgrinding Process For Specialty Tool Steels

Steel/Molding

Grinding and polishing the specialty steels (Stavax® and Optimax®) used to manufacture plastic optics mold tool inserts is a laborious process. This paper describes a recent COM development that presents a simple and very cost effective deterministic microgrinding process for these difficult materials. The work was sponsored by the DARPA Technology Reinvestment Project.

The optical quality of a molded plastic lens surface is determined (mostly) by the optical quality of the molding tool insert and the machine/molding cycle parameters. The tool insert that produces the final molded optical surface must be able to withstand constant cycling at high temperature and pressure, as well as the corrosive nature of the plastic molding process. A poorly finished surface will result in lenses sticking in the mold, imperfect surface replication, or, in the worst case, premature insert failure. Since scratches and digs replicate onto the molded lens surface, achieving a very high quality finish on the insert's optical surface is important.

Stavax® ESR and Optimax® are through-hardening tool steels that are often used to produce mold tool inserts (for details go to: http://www.uddeholm.com). Along with its reasonable polishability, Stavax® has excellent corrosion and wear resistance properties. However, since it is hardened and tempered before final grinding and polishing, it is difficult to grind and even harder to polish. The manufacturer's suggested sequence requires time-consuming, iterative (slowly changing grit size) grinding and polishing steps, which are highly dependent on the skill, experience and technique of the operator.

ground on OptiPro SX50
Figure 2. Results achieved on Stavax® ground on
the OptiPro SX50.

COM has recently demonstrated a deterministic microgrinding process that replaces the iterative conventional grinding and polishing sequence suggested by the manufacturer. It may even eliminate the need for polishing in some applications. COM's deterministic microgrinding process is able to achieve optical quality surfaces on Stavax® in minutes instead of days.

An example of the process potential is presented in Figure 2. A 70 mm convex radius was microground onto each of several 28 mm diameter hardened and tempered Stavax® tool inserts provided by Opkor, Inc. The extraordinary finishes were achieved on the OptiPro SX50 using Lunzer Inc. metal-bonded CBN (cubic boron nitride) ring tools (32 mm inner diameter, 4 mm wall thickness). The three step process (rough grit: 65 µm, medium grit: 6-12 µm, fine grit: 2-4 µm), at a tool spindle speed of 9,560 rpm and a work spindle speed of 135 rpm, required a total process time of 30 minutes.

After microgrinding, the final surface finish quality was examined on the Zygo NewView® 200. As can be seen in Figure 2, the surface has a very uniform finish quality. The surface roughness for the test samples typically ran between 20-30 Å rms. The surface figure error was under 1 wave p-v.

As with any initial development, it is likely that the process could be further refined and that the results could be improved. In addition to saving significant time and grinding cost, the deterministic microgrinding technique will substantially reduce final polishing time and could potentially eliminate it. After processing, check to be certain that the surface hardness was not compromised.

Dennis Van Gee developed the initial process.
For more detail, please contact:

John Schoen
Phone: 585-275-8296
Fax: 585-275-7225
E-mail:schoen@lle.rochester.edu

Reprinted from the July/August 1998 issue of Convergence

 

 

 

 

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