Mechanical Build

Body

When building our STM, we initially kept close to a reference design we had found on the web during our research (SXM Project). There are complete CAD drawings of the parts online. The reference design is based on a construction which contains two aluminium plates, one lying on top of the other with a three-point support. All three heights can be adjusted via fine approach screws. The scanning head is mounted between two of the screws, with an adjustable distance to the line connecting the two. This design makes the lever applied by the third screw at the back user-adjustable. The coarse approach of the tip is done using the screw at the back.
The design is definitely feasible, even without a lot of experience in crafting. After a short introduction to the school's workshop, we could manufacture all the parts, apart from the scanner head, on our own. The scanner head was produced on a CNC at the Paul Scherrer Institute. However, the design has several drawbacks. Its big size makes it more sensitive to vibrations and and thermal influences. Furthermore, in professional approaches, the alloy Invar is used as material due to its small thermal expansion.

Fig. 1: The body of our STM.

STMs operate on very small scales and are therefore very sensitive to vibration. Without any calculation, one can understand that a rigid design is the most important factor in vibration isolation. Vibrations do not influence a measurement if the whole STM vibrates. Nevertheless, damping is still required and even more with our big STM body. In a first attempt, we placed a 30kg steel plate on a tire. However, this provided only insufficient damping and we therefore decided for a new system. The metal plate is hung from two wooden stands using rubber strings. The strings are actually supposed for gymnastics. This approach performed much better so that it allowed us to conduct our experiments.

Fig. 2: The vibration damping system.