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Ultra-precise joints under the microscope

Groundbreaking investigations on the dimensional and thermal stability of different joint technologies

PTB-News 3.2015
01.10.2015
Especially interesting for

mechanical engineering

precision engineering

Within the scope of the European re-search project “Thermal design and dimensional drift” (T3D), various joining technologies have been investigated at PTB for the first time with regard to their long-term behavior and to their reaction to thermal influences with a precision of approx. 1 nm.

A gauge block screwed onto an end plate (top photo in the figure). Phase topography allows very accurate measurement of topographic changes within one wavelength. In this repre-sentation, the phase shifts occurring around integral multiples of the wavelength at the transitions between the base plate and the gauge block or in the vicinity of the screws have not been corrected so as to make the actual deflection visible. It can be clearly seen that both the gauge block surface (central area) and the base plate (area at the top and at the bottom) exhibit deformations that are due to the forces applied when tightening the joint.

“Conventional” technologies (such as screwings or soldering) are frequently used even for the assembly of ultra-precise instruments. In assemblies in machine tools, in optical instruments, in scanning force microscopes or in semiconductor production facilities, this may lead to changes in length that are so small that they cannot be detected by means of conventional measuring devices. These changes often occur due to structural changes inside the construction materials, but are also due to external influences such as thermal or mechanical stress.

PTB and the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF – Jena, Germany) have manufactured diverse joint variants (screwed, glued and soldered connections as well as silicate bonding), especially for the T3D project. Due to the expected changes in length in the nanometer range, gauge blocks were used as base elements. As a reference, the traditionally used wringing technique without additional connection material was chosen. To measure the dimensional stability both vertically and laterally to the connected surfaces, the gauge blocks were connected with each other either by their end face or by their side face. In addition, also the orientation of the surfaces was determined by analyzing the interference phase topography. The measurements were carried out using PTB's high-precision interferometers. The temporal stability of the connections was checked over a period of one year. The thermal behavior from 10 °C to 30 °C was investigated within the same time interval.

With regard to length and orientation, screw joints did not exhibit any detectable change. Glued joints, in contrast, may behave very differently, depending on the hardening and on the moisture absorption of the glue or on the parallelism of the adhesive joint. In the case of soldered and bonded connections, the behavior mainly depends on the thickness of the joint layer: the thinner the layer, the more constant the length and orientation.

From these findings, a “Good Practice Guide” has been elaborated for the users of joint techniques in ultra-precise mechanical engineering which also contains guidance from EURAMET Technical Committee L's experts group. This document as well as an online tutorial on the interferometric measurement of the stability of joints are available on the T3D website (Opens external link in new windowhttp://projects.npl.co.uk/T3D/publications.html).

Contact

René Schödel
Department 5.4 Interferometry on Material Measures
Phone: +49 (0)531 592-5400
E-mail: rene.schoedel(at)ptb.de

Scientific publication

H. Lorenz, E. Beckert, R. Schödel: Phase topography-based characteri-zation of thermal effects on materials and joining techniques. Applied Optics 54, 2046–2056 (2015);
DOI 10.1038/NCOMMS9287