Monday Lessons

***A quick Monday afternoon exercise turned out to be more of a challenge than originally anticipated.
 
While I was working on Valjoux 7750 I asked Tyler to quickly work out how many times the chronograph hand ticks per second. He recorded the motion then slowed down the video and quickly pronounced the chrono seconds hand ticks 4 times a second. I then asked him to figure out how many teeth the chrono wheel has.

He quickly concluded that the count is 240, "give or take 1 or 2", and this made perfect sense because if you divide the frequency of the oscillator by the number of ticks per second then 240 would be the right number.  But for some reason I really wanted him to physically count the number of teeth exactly. After all Valjoux 7750 is the movement he will be working on at the TAFE in his third year and we don’t want him embarrassing us.

As enthusiastic about watchmaking as he is, he found the task of counting a bit trivial and assigned the job to Laura, who to her credit, without being burdened by estimation did the proper count. To our surprise the centre wheel has not 240 but 200 teeth. Which means that the seconds hand actually ticks 3.333 times a second, not 4. The point is you can be deceived by what you see, and even what you recorded. Once again, watchmaking is all about being overly pedantic, even when you are “100%” sure that you are right.

The question remains, how are we going to cut a 200 teeth on a 7.2mm diameter wheel one day in our workshop? We have no idea yet, but we will do it. 

Happy collecting,
Nick