Bill Meiklejohn, director of business development, Kinetic Ceramics
In the field of ultra-precision manufacturing, the demands are extraordinary as regards the attainment of tolerances and accuracy, and technology solutions to cater for these demands are brought to market regularly. This article takes a look at one area of interest for companies looking to machine parts and components at sub-micron accuracy, namely fast tool servos (FTSs). The options in this area are numerous and often confusing, but some developments—especially in terms of piezo-driven FTSs—are opening up new ultra-precision manufacturing applications.
There is pan-industrial, global demand for miniature products and components as well as larger products with ever more exacting miniature features. Increasingly, microns might matter but nanometre tolerances and accuracy are often required. Technology suppliers are constantly pushed to meet the ever more exacting demands of their customers, who not only want to attain extreme accuracy but want it delivered in a timely and cost-effective way.
FTSs
A FTS is a system capable of rapidly and accurately positioning a single point diamond cutting tool in a lathe manufacturing operation, such that the cut surface can include features that are not symmetric with respect to part rotation. If required, FTSs can enable the manufacture of complicated surfaces with nanometre-scale resolution. So saying, manufacturers that use FTSs on their lathes can produce—with exacting precision and at speed—freeform and aspheric structures such as eyeglass lenses and moulds, contact lens moulds, films for brightness enhancement and controlled reflectivity, oval pistons for engines, sinewave ring mirrors used in CO2 resonators, fusion experiment targets, two-dimensional planer encoders, micro optical devices, and the list goes on.
(Kinetic-Ceramics-FTS-High-Travel-High-Speed): Kinetic Ceramics’ high-travel and high-speed FTSs are precision cutting tools that can be adapted or customised to integrate with any single point diamond turning lathe. They can be used to produce asymmetrical shapes, patterns and profiles with variable depths and lengths at extremely high speeds.
In order to accurately cut such contoured parts, the tool must move with high precision despite the high frequencies associated with the surface trajectory. This simultaneous requirement for precision and high dynamic performance makes the FTS area quite demanding and effectively places the technology at the intersection of precision engineering and mechatronics.
The use of piezoelectrics in FTSs
There are many different types of FTS, and there is much confusion about which is best for what application. One size most certainly does not fit all, and many FTS technologies cannot deliver sub-micron accuracy in a timely and cost-effective way, if at all. The main difference between commercially available FTSs today is the actuating mechanism and configuration, the key ones being voice coil, air bearing and piezo-driven mechanisms.
Of these options, piezoelectric actuated FTSs are generally regarded as the gold standard when it comes to accuracy and adaptability to a host of applications where sub-micron tolerances are required. The alternative actuating mechanisms can variously fall short on accuracy or suffer from limitations in stroke, signal bandwidth, acceleration and positioning noise level. With non-piezo-driven actuating mechanisms, precision or speed can often be sub-optimal.
Piezoelectric FTSs are characterised by accurate micron and sub-micron positioning, high speeds of responsiveness, high acceleration, and high stiffness. If ultra-precise machining accuracy and speed are the objectives, piezoelectric FTSs are invariably at the top of the list of technology solutions selected by manufacturers. They provide more stroke in less space, afford excellent reliability in an array of industrial applications and can achieve a poisition resolution approaching angstroms (an angstrom is 0.1 nm or one ten-billionth of a metre).
It is probably useful to explain what piezoelectrics (or rather piezoelectric materials) are, since it is their characteristics that afford the piezo-driven FTSs their outstanding functionality. In a nutshell, when electricity is applied to some materials, motion occurs, and when these materials are stressed, squeezed or compressed, they produce electricity. There are natural and synthetic materials that exhibit piezoelectricity, and industrial manufacturing is the largest market for piezoelectric devices.
Kinetic Ceramics and any other companies have made significant advances in piezoelectrics. There is, of course, a certain amount of competition in the ultra-precision arena, but these technology providers work collaboratively to an extent, learning from each other in order to increase the commercial use of piezoelectrics.
However, the piezoelectric FTS technology that is to become the focus of this article is based on Kinetic Ceramics’ proprietary lead zirconate titanate (PZT) material, a variation of one of the most commonly used ceramic piezoelectric materials. This material—known as PZWT-100—affords extremely high reliability as well as industrial strength, operating at five times that of conventional PZT materials.
(Kinetic-Ceramics-FTS-400-Cutting-Copper-Drum): The FTS-400 is a high travel FTS that achieves speeds up to 950 Hz and a total stroke of 400 μm at 100 Hz. It is shown above assisting in the cutting of a copper drum.
Kinetic Ceramics’ piezoelectric FTS technology
Kinetic Ceramics’ FTSs are high-travel and high-speed precision cutting tools that can be adapted or customised to integrate with any single point diamond turning lathe. The aforementioned piezoelectric FTS technology enables them to achieve nanometre surface finishes, off-axis spheres and other freeform shapes, patterns and profiles reliably at high speed and range.
The tool heads of the FTSs weigh less than 2.3 kg and can fit in the palm of the hand. Also, the entire control system—comprising amplifiers, power sources and cooling systems—requires less than 1 m2 of floor space. A small footprint is, at times, a hugely important factor, especially if retrofitting to existing lathes in established production environments.
Kinetic Ceramics’ FTSs are mostly used in highly confidential applications, but have been reported by some manufacturers to increase efficiences as well as enable machining of previously impossible asymmetrical shapes, patterns and profiles.
They allow for optical quality surface finishes down to 12 nm, meaning, for example, that lenses can be created with smoother finishes that are less prone to wear and eliminate the scattering of light, resulting in crisper images.
Furthermore, intelligent software enables manufacturers to create patterns using random signal generators, which is especially useful for thin films used in television and computer monitors. Manufacturers of thin films such as KDX and LG are embracing this capability for commercial applications.
Another key area for Kinetic Ceramics’ FTSs is piston manufacturing. The objective for all piston manufacturers is to minimise or eliminate cylinder-to-wall contact, thereby reducing friction. On this basis, the thrust side of a piston is ground to a different diameter and made oval, so that when at operating temperature—during the power stroke—the piston skirt against the cylinder optimally distributes the thrust load.
The creation of ovality is challenging in any manufacturing environment, however Kinetic Ceramics’ FTS-400S is being used to manufacture pistons with dual ovality for a tool manufacturer in a wet environment, both speedily and cost-effectively. The pistons are cut with a 50 mm diameter using 1.6 mn points of data in less than four seconds.
(Kinetic-Ceramics-High-Speed-FTS-20): The FTS-20 is a high-speed FTS that achieves speeds of 2,000–20,000 Hz and a total stroke of 10 μm at 20 kHz.
(Kinetic-Ceramics-High-Speed-FTS-20-Tool-Head): The FTS-20 tool head.
Conclusion
This article has attempted to partially illuminate a complicated but vitally important technology for those lathe manufacturing operations demanding sub-micron tolerances.
FTSs have been an important solution for the manufacture of products where tight accuracy and tolerance attainment is critical for more than 20 years. However, as mentioned at the outset, because industry is continually demanding smaller and smaller products or smaller and smaller features at sub-micron tolerances, technology suppliers are constantly refining and advancing technology options. Such is the case in the field of FTS technologies, and numerous companies now supply a host of different solutions.
The solution that is best depends to a large extent on where on the precision scale one resides. If micron tolerances are required, it may be that entry-level air-bearing or voice-coil actuated FTSs suffice. If, on the other hand, nanometer tolerances are required, review of piezo-driven FTSs is necessary.
The struggle has always been to combine attainment of accuracy in a way that is commercially viable in terms of cost and time. It is here that piezo-driven FTSs are opening up new ultra-precision manufacturing applications.
Kinematic Ceramics