Using a Modular Approach for Diamond Machining

Customized designed components are used for developing equipment that is used for machining diamond turned optical components. These components comprise slides and spindles that are produced particularly for the design at hand.

Although they have no other application, careful design can help in creating machine parts, which can be used in a variety of applications sans affecting the overall performance. In fact, the overall performance of these components can be enhanced by designing modular spindles and slides with an eye towards potential applications.

Rank Pneumo has designed a range of machine components, which can be integrated in various ways to produce grinding, flycutting, and diamond turning machines with a wide range of configurations. In addition, these components can be utilized as building blocks to develop highly accurate positioning or measuring systems.

Recent papers have focused on the design and construction of the two axis diamond turning machine that utilizes two modular slides in a staggered tee configuration. These components may be used to develop a lathe that can turn hardened steel components by means of ceramic cutting tools.

Slides

For experimental diamond turning lathe, the slides were designed in such a way that they can be created in three entirely different sizes and in several different standard lengths of travel. The way covers can take a number of different configurations based on the limitations of the specific machine design.

Since the slides are modular and can be positioned separately, it was easy to adjust the machine geometry. The lathe exhibits squareness between its slides of 0.2 arc seconds, an order of magnitude better than comparable machines developed with dedicated slides. It is possible to design straightness of travel into modular slides.

Given that the slides are standalone components, they are stiffer than most other designs and this aid in their geometric stability. The X axis of Precitech’s lathe spans a gap of 16 inches and yet travels straight within -5 microinches through its 12 inch travel.

Spindles

It takes more than a set of slides to create a machine tool. Besides a slide or slides, one more spindle is needed. Precitech’s set of modular components contains two spindles whose mounting dimensions are similar. These spindles were chosen to cover a range of load capacity and speed.

Air Bearing Spindle

The initial spindle is an air bearing device designed with a three horsepower motor (Figure 1). The spindle’s speed can range from 400 and 7200rpm. The thrust bearing is placed behind the front journal bearing in order to reduce the axial motion of the spindle nose with temperature and at the same time improve the radial stiffness.

The AC motor is liquid cooled and thermally separated from the spindle housing by a glass-epoxy thermal break. Vacuum can be transmitted to the chuck coaxially via a non-contacting seal at the back of the housing.

In order to control the centerline height as the spindle warms at increased speed, the housing is made to mount to the machine at the spindle centerline. The bolts are arranged so that the spindle housing will extend axlally from a point in line with the thrust bearing.

Air bearing spindle

Figure 1. Air bearing spindle

Oil Hydrostatic Spindle

The second spindle exhibits oil hydrostatic bearings and is driven by an eight horsepower motor (Figure 2). This spindle has a maximum speed of is 1800rpm. Likewise, this design also places the thrust, bearing at the front, although in this case to reduce heating, the thrust bearing covered the front Journal bearing.

Similar to the air bearing design, the motor is liquid cooled and thermally separated from the spindle housing. This spindle needs a hydraulic unit to supply the oil to the bearings at 250psi. A heat exchanger is also required to allow control of the temperature of the oil.

Besides spindles and slides, some kind of position feedback system is required. Here, the entire range of available technology can be used, from encoders and resolvers through laser displacement measuring instruments to linear scales. Resolution can be anything from 100 mlcroinches to 0.05 microinches based on the application.

Oil hydrostatic spindle

Figure 2. Oil hydrostatic spindle

Machine Bases

Bases required for these modular components can be designed to suit specific applications. Eepoxy or granite concrete are suitable for high precision applications, but steel fabrications or iron castings can also be considered if the job is not too complex.

Application Examples

Facing lathe

Facing lathe (Figure 3) is a simple tool that utilizes a 6” travel version of the mid-sized slide. Here, air bearing spindle was chosen. The base is natural granite, positioned on an air isolation system. The isolation system does not have self-leveling capability.

The spindle is supported by an epoxy concrete riser, which is mounted on its side because, for the application being considered, there is no need to cut to the center of the workpiece and hence control of the spindle center height was not a major concern. A spray mist system supplied the coolant, while a small, solid state programmable unit served as a controller.

Facing lathe

Figure 3. Facing lathe

Drum Lathe

Similar to the facing lathe, drum lathe (Figure 4) uses the same epoxy concrete riser. Here, the riser is turned 900 and integrated with an 18” travel version of the mid-sized slide. This machine does not need high work speed and turns heavier parts.

It utilizes the oil hydrostatic spindle. In this case, a natural granite base was employed but this time with a self-leveling isolation system. The air bearing tailstock is the only new component here. This machine, like the facing lathe, did not need position feedback and hence the controls were analogous to the facing lathe.

Drum lathe

Figure 4. Drum lathe

Horizontal Spindle Flycutter

A horizontal spindle flycuttlng machine is shown in Figure 5. This example of a machine dedicated for designing a single component uses the same 18” travel slide as the drum lathe, but this time integrated with the 6” travel -slide from the facing lathe and the air bearing spindle to construct a unique flycutter.

The spindle is mounted traditionally in its cradle shaped support. Since this machine has to be used with an oil shower system for extended temperature control, an epoxy concrete base on self-leveling isolation mounts was selected. This option enables channels for coolant to be cast into the base. Only the base and the fixturing were designed from scratch for this machine.

Horizontal spindle flycutter

Figure 5. Horizontal spindle flycutter

Two Axis Diamond Turning Machine

The Two axis diamond turning machine is designed to be a test bed for the latest technology in diamond turning (Figure 6). It utilizes two similar 12” stroke slides. The spindle is the 7200rpm air bearing design. Both position and Z axis pitch data is fed back to the machine controller by the laser interferometry which has a resolution of 0.05 microinches.

The machine’s base is natural granite, mounted on self-leveling air isolation. The control system is a full-featured CNC system that is commercially available in the market.

Two axis diamond turning machine

Figure 6. Two axis diamond turning machine

Two Axis Grinder for Optical Components

In this design, the staggered tee configuration is utilized again to produce aspheric optical surfaces. Here, stiffness is more critical than a large swing, so the wheel-spindle is mounted vertically on the upper slide and the workspindle is mounted on the lower of the two slides.

This configuration places the grinding wheel just Inches from the X axis guide ways, thus improving the stiffness of the system. Since coolant is needed, for effective grinding of glass, the base for this machine is an epoxy concrete structure made to handle a profuse flow of cutting fluid.

This application does not need the use of lasers to provide position resolution so feedback is achieved from glass scales with a 4 microinch resolution. Figure 7 shows the two axis grinder.

Two axis grinder

Figure 7. Two axis grinder

Lathe for Turning Hardened Steel Parts

Similar to the lens grinder, this production lathe is also developed with excellent rigidity (Figure 8). The slides utilized are the same slides as those employed on the diamond turning lathe.

The workspindle is the hydrostatic design and is mounted to the lower Z axis leaving the X axis to carry the tooling which is mounted as low on the slide as possible. Air isolation is not required for this application. The base is epoxy concrete, the controller is a commercial CNG unit, and the feedback is achieved from glass scales.

Hard turning lathe

Figure 8. Hard turning lathe

Conclusion

By employing pre-designed parts, new machine designs can be produced in less time than required, provided all of the components were designed from scratch. The reduced engineering effort can lead to lower overall costs and improved designs.

These components reduce the risk associated with a new machine design, as in most cases the design has already been shaken down.

Moreover, production methods can be further improved if series of similar components are produced together as is often possible when designs have a striking similarity.

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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