Designing a Machine Tool for Spherical Lens Production

In the early 1990s, the Opticam® SM machine was developed for use in the analysis of microgrinding at the Center for Optics Manufacturing, University of Rochester. In mid-1992, a multi-axis flexible machine was delivered to the University and is now situated at the Center's site in Rochester. This SM machine signified a milestone in the design and development of machine tools for spherical lens production.

Built on the experience gained from the use of the original SM machine, a design is now being modeled for an innovative machine tool for production of general lens. This article illustrates the design of the latest machine tool, the Microform®SM.

Machine Orientation

The most obvious difference between the Opticam®SM and the Microform®SM is machine orientation. The earlier design SM is a horizontal machine tool, that is, both the work spindles and wheel are horizontal, while the new SM machine features a vertical orientation and its wheel spindle's axis always remains vertical.

This orientation change has allowed the machine design to achieve a number of goals. For instance, it offers a more perceptible grinding environment for operators and allows better accessibility for part loading, whether with automated robotics or manually.

A vertical orientation achieves this by reducing the base footprint of the machine. This allows the operator to move closer to the grinding theater and also makes manual tool loading easier. The machine orientation simplifies spray guarding considerations by allowing gravity to aid in coolant delivery and collection.

Innovative Arrangement of Machine Elements

A new arrangement of machine elements has been developed to achieve all the preferred operations in a vertical orientation. Figure 1 depicts a schematic arrangement of the rotary, spindle and translational machine elements in this novel design.

Microform®SM - schematic arrangement

Figure 1. Microform®SM - schematic arrangement

Foremost in this design is the separation of these elements into upper and lower sections. The upper section includes stacked translational slides, which support a vertical wheel spindle. The lower section includes a rotary positioning axis carrying the work spindle. Through this arrangement, certain benefits in the modular structure of the machine tool are realized.

Rotary Positioning Axis

The rotary positioning axis is positioned in the lower section and represents an important part in the machine design. This axis is positioned symmetrically from two bearing supports on the base of the machine. Its feedback and drive components are outside the supports. These components are kept out of the immediate grinding theater to protect them from coolant splash.

The symmetrical mounting arrangement not only secures the machine together via this axis, but also strengthens the overall structure of the machine. Moreover, thermal growth issues in the machine structure are brought down significantly through this symmetry.

By expanding the axis' range of motion to 900, the machine can carry out lens segmentation. This option of segmentation needs an angular positioning work holding spindle, but eliminates the need for an extra slide. The 900 flexibility also provides a number of benefits in certain automatic loading conditions.

The upper section of the machine is mounted on top of the machine base to serve as a superstructure, and its lowest translational slide travels horizontally towards and away from the operator. A manual translating adjustment is supported on this slide which can be utilized to bring the axes of the wheel and work spindles into intersection. A bracket located on top of this adjustment supports a vertical translation slide.

Another manual adjustment is located between the grinding spindle and the slide that can be utilized to laterally slant the grinding spindle. These manual adjustments should be applied during machine installation and as a field service adjustment when a grinding spindle requires replacement. This stacked slide arrangement aids in reducing the overall grinding theater sans reducing the exact grinding volume. The slides also provide motions that prove practical in automatic wheel changing.

This innovative arrangement of machine elements needs an extra level of sophistication in the machine tool control system. In contrast to the original SM machine that infeeds the work into the wheel with a single moving axis, the new SM machine needs both translational axes to shift. The axes’ motions are coordinated to maintain the edge of the grinding wheel placed along the middle line of the lens axis.

A virtual axis is produced in the numerical control software so as to make this coordinated motion easy for the machine tool operator. This virtual axis allows the operator to utilize simple instructions and commands to produce the preferred part geometries with the numerical control system of the machine to ensure the required geometrical transformations.

Conclusion

The design of the Microform SM machine tool is now in prototype development. This machine was anticipated to manufacture its first lens during the third quarter of 1994. This SM machine represents a milestone in the design of machine tools for spherical lens production.

About Precitech

Precitech began operations in 1992, but continues the rich history of ultra-precision machine tool building dating back to 1962, when Pneumo Precision was founded. In October of 1997, the Pneumo ultra-precision machine tool division of Taylor Hobson (formerly Rank Taylor Hobson / Rank Pneumo) was merged with Precitech. The Precitech name was retained for this corporate entity and all offices and manufacturing facilities are now located at 44 Blackbrook Road in Keene, New Hampshire.

Our facility staffs approximately 100 talented individuals in a recently designed 60,000 Sq. Ft. building.

Precitech is a member of AMT (The Association of Manufacturing Technology) and has corporate affiliations with several professional societies and academic institutions such as Germany’s Research Community for Ultra Precision Technology at the Fraunhofer Institute, ASPE the American Society for Precision Engineering, and EUSPEN the European Society for Precision Engineering and Nanotechnology.

This information has been sourced, reviewed and adapted from materials provided by Precitech.

For more information on this source, please visit Precitech.

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