The Factors Affecting Surface Finish in the Diamond Turning Process

In diamond-turned surfaces, a number of factors play an important role in their final finish. However, the interactions of these factors are complex and create considerable amount of confusion.

Workpiece Material

The most important factor to consider is the material being machined. The material should have tool wear and sufficient ductility in order to enable clean cutting with reduced surface damage.

Contamination in the material and its grain structure also tends to limit surface finish. This makes it complex to obtain better than a 3nm Ra finish on most aluminum alloys.

Typical performance achieved with SPDT of electroless nickel plated mold pin.

Figure 1. Typical performance achieved with SPDT of electroless nickel plated mold pin.

In contrast, an electroless nickel plating with high phosphorus content is amorphous and can be deposited in a very pure form (Figure 1). This is a perfect material when there is a need for ultimate surface finish.

With diamond turning electroless nickel, it has been possible to obtain better than 0.6nm Ra finish. In certain materials, the depth of cut and the surface speed during cutting can also affect tool wear and surface finish.

Diamond Tool

A diamond tool must be very sharp and free of chips on the nanometer scale. It is necessary to ensure that the tool rake angle, and at times, the tool radius are optimized for a specified material and workpiece to obtain the best finish (Figure 2).

Cutting edge geometry: Rake angle α and clearance angle β.

Figure 2. Cutting edge geometry: Rake angle α and clearance angle β.

Good surfaces can also be realized through proper tool lubrication. One common issue is that chips drag on the newly cut surface. The forces produced during the cutting process can stimulate vibrations both in the workpiece and the diamond tool.

For this purpose, the diamond must be firmly attached to a tool shank, which in turn must be clamped to a tool holder. If the workpiece is thin, it must be properly supported and must be firmly clamped to the diamond turning machine.

Considering a suitable tool, material, and machine, a theoretical limit still exist to the achievable finish in diamond turning. This limitation is due to the cusps which are left behind subsequent to feeding a circular-shaped tool across the workpiece.

It is possible to reduce the theoretical finish by reducing the feed rate. However, in production, the feed rate is a major factor that determines the cost of diamond turning.

Cutting Forces and Dynamic Stiffness

The above factors are independent of the properties of the diamond turning machine itself. During most finishing operations, cutting forces are so small that they can only stimulate major vibrations in the small inertias of the workpiece and tool.

However, other dynamic forces acting on a diamond turning machine are much greater than the cutting forces. Figure 3 shows a boxslide regarding dynamic stiffness.

Dovetail/boxslide in relation to dynamic stiffness

Figure 3. Dovetail/boxslide in relation to dynamic stiffness

Spindle

Spindle is one of the largest dynamic forces that act on a diamond turning lathe. Since these forces repeat every revolution of the spindle, and often have insignificant components greater than 12 times per revolution, they only influence surface finish within 2 or 3mm of the middle of rotation.

Beyond this center region, the errors induced by spindle fall within the form regime. However, this is a standard misconception. Sometimes, the spindle applies forces that have an asynchronous component. Since these forces do not repeat every revolution, they contribute to the surface finish.

Environment and Peripheral Devices

Sound pressure due to fans, loudspeakers, etc can stimulate machine vibrations. Measures can be adopted to separate these environmental influences, but they are only required when the machine exhibits a low dynamic stiffness.

However, even when environmental influences are eliminated, a machine requires dynamic stiffness to oppose other forces on board the machine. If the machine is dynamically compliant, all these can contribute to surface finish.

Positioning System

Another major factor that affects surface finish is the machine’s motion control system. This system begins with actuation and bearings mechanisms that are free of slip or stick. Also, timing errors in the controller influence surface finish, as well as the point spacing and the number of digits utilized to program the tool path. The amount of structure between the workpiece and sensor, and between the tool tip and position sensor also has an effect.

Tool contact and edge geometry: Radius and waviness of tool contour are to the order of 10nm (rms)

Figure 4. Tool contact and edge geometry: Radius and waviness of tool contour are to the order of 10nm (rms)

Dynamic forces can distort the structures in the tool-sensor- workpiece loop, such as the workpiece, sensors and their mounts, the spindle, the tool, the base, and the slides (Figure 4).

This deformation makes the control system to assume that the position of the tool to the workpiece is different than it really is. One typical example of this effect is vibration in the sensor mounting.

Sensor Resolution

The slide of a diamond turning machine can hold and sense position. As a result, the position sensing system must exhibit low noise. With new advancements in sensor technology, very fine resolutions out to very high frequencies can be achieved.

On the other hand, high levels of interpolation between grating lines on a scale are needed to do this. These interpolation methods are not perfect and can induce other finish problems.

There is a common misconception that finer sensor resolution provides better position control and hence better surface finish. However, when resolution reaches less than 5nm, its contribution to surface finish is imperceptible when compared to the other factors discussed here.

Earlier, the sensor noise was considerably large and made a significant contribution to the overall precision of the machine. This is yet another misconception regarding diamond turning machines. Since the sensor resolution/noise is so small, it impacts only the surface finish in diamond turning, and even in this case only has a slight effect.

Averaging

The above-mentioned factors can play a major role in surface finish; however, they cannot be combined directly as some of these factors will partly cancel others. One suitable method is to utilize the root-sum-square of the normal deviation of each error source.

This will help in estimating the RMS surface finish, also called as Rq. However, this estimate will be larger than in practice because the diamond turning process can have an averaging effect on finish errors caused by machine.

If vibration is present and the feed rate is relatively slow, the diamond tool will not be able to cut every revolution and will only cut at the low end of each vibration cycle. At the high end each vibration cycle, the tool will cut air as it is close to the pass when the tool was low. The averaging effect will increase with higher frequencies of vibration and slower feed rates.

Conclusion

Several factors play an important role in creating optimized surface finish during the diamond turning process. In order to achieve the best surface finish, a measure of the noise floor of the machine system in tandem with all the other factors should be considered.

The best machine for a specific use does not come from the newest controller, lowest resolution, or stiffest spindle, but would be the one where different parts have been assembled together in a seamless package and where all components function properly. This way, high quality optical components can be developed quickly during 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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