Designing CNC Grinding Machines for the Deterministic Micro Grinding of Glass Optical Lenses

Back in the early 1990s, Rank Pneumo designed and built the Opticam SM lens grinding machine according to the specifications laid down by the Center for Optics Manufacturing (COM) in Rochester, New York.

Since then, Rank Pneumo has undertaken the design of the Microform® SM, a second generation and production oriented lens grinding machine (Figure 1). This innovative machine is smaller and less costly than that of the original version, and yet grinds glass surfaces to the same specifications as the original one.

The Microform SM spherical/aspheric CNC deterministic micro grinding machine.

Figure 1. The Microform SM spherical/aspheric CNC deterministic micro grinding machine.

Newly designed from the ground up, the Microform SM machine does not resemble the earlier design of the Opticam SM machine. Its design features constructive inputs from Opticam customers, operators, and COM technicians.

The orientation of the work spindle was changed to be vertical to allow access to view the component. The wheel spindle is then positioned over the work piece for grinding. Three CNC axes (X, Z, and B) are needed to shift the spindles into the proper orientation for grinding a sphere by chordal generation methods (Figure 2).

Isometric view of the work area showing the X, Z, and B CNC axes, and the Y and A manual adjustment axes.

Figure 2. Isometric view of the work area showing the X, Z, and B CNC axes, and the Y and A manual adjustment axes.

Machine Configuration

First, the wheel spindle was tilted so that the machine resembles the existing lens generators. This way, customer’s acceptance can be increased. Nevertheless, this left the work spindle on the X and Z stacked axes, in a part of the machine that was heavily splashed by coolant and did not present a satisfactory solution.

In order to close the CNC axes from coolant splash, there was an option of designing all three axes in a stacked configuration, wherein the work spindle is portioned directly to the base, whilst the wheel spindle shifts in X, Z, and B.

Another options was to tilt the work spindle in B and stack the wheel spindle on X and Z, beyond the heavy splash zone. Here, the latter design was selected, for serviceability and also to improve the structural loop rigidness of the machine.

However, this presents another difficulty, that of feeding the wheel into the work. In order to feed in the direction of the depth of cut, there is a need for a coordinated motion of X and Z along a virtual axis, that turns around with the B angle position. This is rendered by the controller and is not seen by the operator during standard usage of the machine.

Reduced Size

The Microform SM machine just needs one half of the floor space of an Opticam SM and retains the same work piece capacity of 150mm diameter.

The base of the earlier design Opticam SM machine weighs approximately 5000kg and is supported on five air isolators, while the base of the Microform SM weighs about 645kg and is supported on passive isolators on a steel frame to increase the working height to a suitable level.

Machine Alignment

On the older Opticam SM machine, it was difficult to align the two spindles to intersect precisely. This alignment is necessary to grind an accurate sphere. Flexible elements were added to the Microform SM so that spindles can be shifted into intersection sans the additional cost of a slide way.

The flexing elements were designed by means of finite element analysis so that motion is provided without over stressing the flexures and at the same time the preferred vertical stiffness of 180 N/micron is also maintained.

Thermal drifting issues were also observed in the older Opticam machine during warm up and use. These errors are harmful when the drift shifted the spindles out of intersecting alignment. In contrast, the Microform SM machine has been designed with symmetry about this intersecting alignment plane to reduce the thermal drift in this direction.

Another source of drift for the earlier design Opticam machine is the high-speed, water-cooled wheel spindle. Even when it is switched off, the spindle chiller still operates, cooling the spindle yet causing the wheel position to drift. On the new Microform SM machine, a coolant bypass valve was added to reduce the cooling rate, and drift, when the spindle is off.

Work Piece and Wheel Fixturing

The Opticam SM machine includes grinding wheels that were positioned in a collet integral with an SK-30 taper spindle adapter, but the design was altered to secure the wheels directly to a HSK-32 taper spindle adapter (Figure 3).

This considerably enhanced the balance quality and improved the rigidness, by having a fewer number of joints, and eliminating the neck down to the collet holding diameter in the previous holders. The work piece clamping mechanism was also modified to a three jaw chuck. This enables component fixtures to be easily made on a lathe.

Work piece and wheel fixturing

Figure 3. Work piece and wheel fixturing

Coolant

The Opticam SM machine, as a result of clearance with the tool changer, needs the coolant nozzles to be placed a foot away from the wheel and work. A large box is also needed around the grinding zone to accommodate the coolant splash.

On the Microform SM machine, the wheel spindle is backed away from the coolant box for changing the tools. Consequently, the coolant splash box is relatively smaller and it is possible to place nozzles sans these limitations. The box is made of stainless steel to protect from corrosion.

Access is provided to the component by sliding back two covers. With the help of the dual canister filter system, the operator can easily change the filter in use by operating a manual valve. When one filter is being used, the other is sealed from the circuit, enabling a contaminated filter element to be changed while the other filter is being used by the machine.

Controller

On previous machinery, grinding programs (software) presented certain limitations. These aspects forced the grinding cycle to have, for instance, one rough, one medium, and one finish pass.

Now, it is possible to rearrange grinding sequences or can be added as required. Part programs can hold almost any number of beveling, edging, sagging, convex, and concave surface grinds in a single mounting of the lens.

Additionally, coordination of medium, rough, and fine stock removal are made easy. The spherical radius can also be tuned for each grind so that an equal depth of cut is obtained from the entire surface with each wheel.

Perceptibly, grinding wheels tend to wear away during usage. In existing equipment, stored wheel data contains one diameter and length. This requires the wheel to exhibit a sharp cutting edge to ensure that the wheel grit zone remains in the same position for any head angle.

Precitech’s new Nanopath® CNC control software monitors the wheel wear in a map of the wheel grit area for each wheel. This prevents wheel dressing to restore a sharp edge. Moreover, the control software rectifies any workpiece error ensuing from wheel wear.

If the operator incorrectly enters the data for a surface ground, the controller updates the wheel grit map to offset the error. This way, the successive component to be ground is properly surfaced and the wheel grit area map is kept up to date.

Aspherics

When compared to spherical optics, aspheres are more difficult to produce and polish. Nevertheless, the CNC controlled axes on the Microform SM enable an aspheric surface to be ground through a peripheral style grinding wheel having continuous path contouring of the X, Z, and B axes concurrently.

This makes the aspheric surface closer to the final form than otherwise found when utilizing the best fit sphere. Additional operations such as beveling, edging, and sagging can still be integrated on aspheric lenses, which help simplify the alignment of the optical axis while assembling the lens within the system.

Process

As lenses can be created in a semi-polished state, the loose abrasive grinding or lapping procedure that usually follows the spherical generation is prevented. This also avoids the cost relating to the development of cast iron tools, which were needed for each different radius processed by this provisional lapping process.

Optional Features of the Machine

A centrifuge is provided with an optional coolant system to remove swarf from the waste coolant stream. The Opticam SM had coolant pump seal failures which may be due to the remaining levels of contaminants after the coolant had been cleaned with vortex type cyclonic filters.

The centrifuge utilized on the Microform SM machine continuously cleans the coolant as it passes through it, and a wash nozzle keeps the coolant flowing through the centrifuge constantly.

A toolchanger is also available for the automatic exchange of grinding wheels with an eight station magazine, and a mist extractor is utilized to remove the grinding coolant mist. Such mist extractors prolong the lifespan of the machine.

Conclusion

Standard finish ground surfaces are sufficiently smooth and hence can be viewed directly on an interferometer for surface form errors. The first Microform SM machine was installed at a customer's facility in March 1996.

To demonstrate the machine’s capability, a test part lens of BK-7 was ground on all sides. The part was edged, beveled, and sagged, and on the SM machine.

P-V form error (irregularity) of 0. 14µm (0.5 fringe) (Zygo)

Figure 4. P-V form error (irregularity) of 0. 14µm (0.5 fringe) (Zygo)

The following precisions were obtained:

  • Radius size error of .003mm (electronic spherometer)
  • P-V form error of 0.14µm (0.5 fringe) (Zygo) (Figure 4)
  • Surface roughness of 75 Angstroms Ra (Wyco) and 120 Angstroms Rq (RTH Talysurf)
  • Surface wedge following grinding was calculated to be 0.004mm +/- .001mm

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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