## 4 Conclusions  
Micro-coordinate measuring machines (micro-CMMs) are being increasingly applied for accurate 3D measurements of micro/nano parts such as injection nozzles, turbine blades, microholes, and a variety of other small, tight tolerance features. Today, the developments of calibration standards as well as the metrology capabilities are crucially required for acceptance tests, along with the calibration and verification of these micro-CMMs. To satisfy these demands, a number of research activities are  
Figure 12: Photo of a workpiece-like microgear measurement standard (tip diameter: 20 mm, module 0.1 to 1 mm) developed at PTB. The design parameters of the standard are shown in the table.  
Figure 13: Measured lines on a spur gear tooth (module of 1 mm) of the standard, shown as: (**a**) profile; and (**b**) helix.  
being carried out at PTB. An overview of some recent developments concerning instrumentation and calibration standards is given in this paper.  
An ultra-precision nanopositioning and nanomeasuring machine (NMM) has been upgraded focusing on its mirror corner, interferometers and angle sensors, as well as its weight compensation, its electronic controller, its vibration damping stage and its instrument chamber. The upgrade has significantly reduced its positioning noise, e.g., from 1\(\sigma\) = 0.52 nm to 1\(\sigma\) = 0.13 nm for the \(z\)-axis.  
Further developments of several microprobes have been detailed, including the boss-membrane piezoresistive probe, the tactile-optical fibre probe and the AFM based probes. The 3D fibre probe has been significantly improved concerning its 3D measurement capability, isotropic probing stiffness and the design of dual-sphere probing styli. The development of a 3D-AFM and of assembled cantilever probes (ACPs) offers full 3D measurements of parts with sizes from a few micrometres to tens of nanometres. This is promising for filling the metrology gap between AFMs and micro-CMMs.  
Further developments of calibration standards for force, geometry, contour and microgears have been introduced. A reference spring artefact applicable for calibrating the 3D probing stiffness as well as for characterizing the probing adhesive force has been presented. An Aztec artefact, which applies micromachined micro-pyramidal marks for defining reference coordinates in 3D space, has also been presented. Compared to conventional calibration artefacts, it has advantages such as high surface quality, well-defined geometry and cost-effective manufacturing. A task-specific micro-contour calibration standard for ensuring the traceability, especially of high-precision optical measurements at microgeometries, has been introduced. A workpiece-like microgear standard embodying different gear geometries (modules ranging from 0.1 mm to 1 mm) has been presented.  
Unfortunately, despite the large development and research efforts, the take-up of the micro-coordinate measurement technique is currently quite low. The reasons for this are multifold, for instance, the high purchasing and running costs, low measurement throughput, and the lack of generally accepted physical and documentary standards. Therefore, more research and development work is expected in the future with a possible emphasis on more cost-effective instrumentation, better dynamics properties and measurement strategies for higher measurement throughput, multi-sensor and data fusion techniques to merge the advantages of different sensing techniques, as well as the further development of physical and documentary standards.  
The authors would like to thank [PERSON] of the m\({}_{2}\)c company and [PERSON] of the SIOS company for their kind help and excellent support.  
[PERSON] contributed the research results of SS 2.1, SS 2.4 and SS 3.1; [PERSON] and [PERSON] contributed that of SS 2.2 and SS 3.2; U.N.-R., [PERSON] and [PERSON] contributed that of SS 2.3, SS 3.3 and SS 3.4, respectively; [PERSON] wrote the paper with the supports from all co-authors.  
The authors declare no conflict of interest.