Designing and Constructing a Sub-Micro-Inch Resolution Lathe

Rank Pneumo has been attempting to develop machine slides that can deliver a high level of precision and thus help enhance the performance of its diamond grinding and turning machines. Following successful completion of these machine slides, the company was able to design a lathe that has a feedback resolution of 0.05 microinches. This resulted in a design which was strikingly different from the design of the previous diamond turning machines developed by Rank Pneumo.

In fact, over the past few years, the company has been asked to design machines with sub micro inch or SMI resolution. Such resolutions offset both process and machine errors that are quite small to be visualized on the finish machined surface.

Through high resolution, the amplitude of the servo “hunting” can be reduced significantly, which occurs when a fixed position is being maintained by an axis, making it feasible to machine improved flat surfaces on two axis equipment. Users of diamond turning machines are careful when it comes to SMI machines. High resolution, together with Rank Pneumo’s newly designed spindle and enhanced hydrostatic slides, will aid in developing advanced diamond turning equipment that can deliver continuous performance, which is not possible in present generation of machines.

Resolution and Accuracy

Servo systems are integrated in the diamond turning machines. If the resolution of this servo system is improved, repeatability can also be improved to a large extent. However, increased resolution does not enhance the system precision. If a machinist’s scale is even slightly short, it will not be precise, irrespective of the fine design of the graduations. It is possible to achieve precision by controlling both the environment and machine geometry and by precisely knowing the length of the "yardstick". This yardstick is the wavelength of the light utilized in the interferometers. Here, SMI resolution makes it impossible to command the machine to shift in increments that are too small to be visible on the work piece surface.

Machine Design

The new lathe design is shown in Figure 1. In this figure, the spindle is placed on the X axis which in turn is positioned on risers so that the X axis covers the Z axis. This set up makes it possible to utilize the tough, non-telescoping way covers which in no way affect the precision of slide, and which tolerates a profuse flood of coolant. The slide tops have varying heights that provide sufficient room for a rotary third axis or for improved swing capacity. On a self leveling air suspension system, a huge granite block is mounted that serves as a supporting structure under the slides.

Figure 1. General design of the lathe

Using modular hydrostatic slides, Rank Pneumo has significantly improved the shape of its SMI lathe. These slides can be crushed to achieve straightness of travel that is better than 0.5 ppm without the necessity for manual hand lapping. This is better than the specifications given on Rank Pneumo’s present-generation of machines. Moreover, similar enhancements have been made in pitch, roll, and yaw. Rank Pneumo used a ball bearing intermediate slide, which was hydrostatically connected to the main slide, to increase the longtitudinal stiffness of the slide stiffness by a factor of four and to simultaneously introduce considerable damping into the system. The increased damping and stiffness play a major role in improving the machine dynamics and this in turn enhances the performance of the servo systems.

Slides

Slides can be used in a wide range of sizes and configurations to design a variety of machine types. These modular units use hydrostatic bearings in a constrained, box way design.

An intermediate slide separates the slides from the lead screws and translates on ball ways. A hydrostatic thrust bearing and the ball nut are carried by this intermediate slide. The former transfers the thrust force from the ball nut to the primary slide. Figure 2 shows a cross section of one of the slides, and figure 3 demonstrates how the intermediate slide is organized.

Figure 2. Cross section of slide

Figure 3. Arrangement of the intermediate slide

If the thrust bearing is designed properly, the right combination of damping and stiffness can be achieved so as to realize a responsive and highly damped system. The ball screws are driven by means of D. C. servomotors, which in turn are driven by linear power amplifiers. Non-contacting, limit switches are used to prevent the effect on slide straightness induced by the force required to actuate traditional switches.

Spindle

An advanced hydrostatic, air bearing spindle was designed for use on the machine. This spindle is motorized with the motor’s rotor adhered permanently on the spindle. At the rear of the front journal bearing, the spindle thrust bearing is placed directly to reduce the axial drifting of the spindle nose. In comparison to the earlier designs, the span of the journal bearings has been improved significantly. In order to render improved acceleration characteristics, the rotational moment of inertia has been considerably reduced. Figure 4 illustrates a section of the spindle and shows the design features.

Figure 4. A section of the spindle

Environment

The environment controls the machine’s ability to create high quality parts. An improved environment translates into better machine precision. To record the structure of an SMI lathe, a temperature-controlled environment is required within a few tenths of a degree Fahrenheit, while the relative humidity is maintained within a percentage point.

Wavelength of Light

The wavelength and velocity of light depend on the medium via which it travels. Although all of the interferometry can be enclosed in a vacuum, this technique is rather costly and also involves the maintenance of vacuum pump and its related accessories. Instead of using a vacuum, Rank Pneumo‘s design uses a refractometer to determine the refractive index of the air and rectifies the errors caused by locally changing conditions in real time. The refractometer is placed at the rear of the Z axis slide and proximal near to the surface of the granite machine base.

Interferometry

A number of new products were introduced into the market following the patent expiration of the Hewlett Packard laser technology. Among these was a product offering from Zygo that utilizes an acousto-optic modulator to produce a dual frequency laser beam with a 20 Mhz frequency split. This is an order of magnitude higher than the modulators used by Hewlett Packard and other producers of analogous products.

The outcome is that Zygo’s double pass interferometers exhibit a resolution of 0.05 micro inches and are capable of operating at 0.5 m/s. Also, the cost of the Zygo interferometer is similar to the cost of the Hewlett Packard system. The design of the interferometry on the machine is shown in Figure 5. Here, it can be observed that the X axis beam path is placed close to the back of the vacuum chuck and is at spindle centerline height. The Z axis includes two interferometers: one to determine the pitch and the other to determine the displacement of the slide. This setup aids in correcting the Abbe errors.

Figure 5. The layout of the interferometry on the machine

Closing the Servo Loops

A controller is required for accepting the 32 bit parallel information from the interferometers, processing the same at a high rate, and closing the servo loops. A computer will be able to produce the commands required to machine an optical surface, given only the equation that describes that surface as input. Such a device is produced by Cranfield Precision Systems (CPS), a division of CUPE. US and UK manufacturers of machine tools are using the CUPROC-16 controller. The cost of this system is competitive with the cost of the Allen-Bradley controller.

Conclusion

Improved feedback resolution is just one of the parameters that plays a role in making machines more capable. Care must be taken on the overall machine design, the secondary electronics which control it, and the environment in which it should function. Both controllers and lasers are now available to be integrated with Rank Pneumo’s advanced machine design to develop an excellent diamond turning equipment.

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

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