After mounting again the lathe apron we tested it and find a slight chatter when turning the hand wheel in one direction. Some time of careful inspection convinced us that the the source of the noise is one of the gears of the train.
We didn’t check all the train gears before because to disassemble this section of the train we need a couple of special wrenches. Now it is mandatory to build them: a pin wrench socket for the shaft of the noisy gear and a special screwdriver to extract the rod that supports the switch mechanism.
After making these tools we disassembled this section of the apron. This confirmed that there was a slightly worn gear that caused the chatter. After exchanging some kind emails with the people at Fertigungsmaschinenbau Kilgenstein, we arranged to get a spare part. It arrived some weeks ago. The spare gear was finely grinded and of perfect dimensions. However, it has a small chip in a pair of teeth as shown in the next image:
After touching them with a file, we tested the new gear: the noise has disappeared. Here you can see some pictures of the parts disassembled together with the special wrenches:
We assembled again the gears and the apron. We used some gasket sealer for the rod screw and the round nut. The apron is now ready to be mounted again!