Measuring past a micron...

Infinity: That is the name of the new ultra-precision machine used by CERN's Metrology Service to measure the copper components for the CLIC accelerating structures. This project is the result of a collaboration between CLIC and the EN Department. Curious to find out more? Read on because there’ll be an opportunity for you to get a very close look at Infinity! 

Infinity, the new, ultra-precise, measuring machine, is currently in operation at the CERN Metrology Service.

The CLIC (Compact LInear Collider) radiofrequency structures will operate under very high electric fields (100 MV/m). They should be manufactured within minimal mechanical tolerances. To validate the quality of these components, they have to be measured with a precision that far exceeds the machining tolerances, i.e. 0.3 microns. No “ordinary” measuring machine can achieve this precision, but Infinity, the newly developed high-precision three-dimensional measuring machine in service at CERN since January can do just that.

Specially adapted by the manufacturer Leitz to meet CERN's requirements, this machine has several trump cards. To counteract vibrations transmitted through the ground, the 7.5 tonne Infinity sits on three vibration dampers. “The entire structure of the machine has been designed to minimise distortions: the base is made of granite, the gantry of steel and the measuring axis is ceramic,” explains Ahmed Cherif, responsible of the metrology service within the MME Group of the EN Department. “And the various mechanical parts move without making contact.”

Once a component has been placed on a support adapted to its geometry and after a series of tailor-made checks, it is measured by an arm tipped with a sapphire, silicon nitride or diamond ball called a “probe”. Attached to the machine's measuring head, the probe moves in three dimensions. “When it comes into contact with the component, it sends the precise coordinates of its point of contact via optical rulers,” explains Ahmed Cherif. These rulers are in vitroceramic and have an expansion coefficient close to zero and a resolution of 4 nm. They make a substantial contribution to Infinity’s exceptional performance.

One of the copper pieces from CLIC measured by Infinity.

Although the machine itself is one of Leitz’s standard products, its head is unique and is the result of a close collaboration between CERN and the manufacturer. “When the probe is in contact with a component, it exerts pressure on it. These pressure points leave marks which may damage the component and reduce its performance once it is installed in the accelerator,” explains Germana Riddone, who is in charge of the fabrication of radio-frequency structures for the CLIC project. To minimise such marks, which are of the order of a few nanometres, the core of the head floats in a bath of viscous oil, which dampens the slightest movement. The contact pressure of this new head is a factor 50 times lower than that of an “ordinary” measuring head.

In order to protect Infinity from any external disturbance, the EN Department has conceived a very special environment. Thus it has been installed in a room in which the humidity and the temperature are kept under close surveillance. As Ahmed Cherif further explains: “In this environment, the temperature variations do not exceed 0.2°C per hour, 0.4°C per day and 0.1°C per metre.” Installed in its closely monitored environment, Infinity will operate almost non-stop in the next few years because it will have to measure several hundred components produced by different European manufacturers. “With this machine, we shall be able to test various ultra-high precision machining techniques. It will help us to extend the collaboration between CERN, CLIC, and industry,” adds Said Atieh, who is in charge of machining the components.

We wish this record-breaking machine a long and successful career and invite those interested to come and see it at the metrology laboratory (see below). The Bulletin would like to thank the Metrology Service for this open invitation.

by Anaïs Schaeffer