Carriage clock with alarm quirks

I am working on a beautiful carriage clock or should I say timepiece because it does not strike. But it has an alarm. The alarm mechanism is extremely sensitive to the correct springiness in a thin rod-spring (see picture below) that releases the alarm.

It took me quite a while to figure out how to get the alarm right and where to focus and intervene to get it to work reliably. I must make a point to explain to the client that alarms are finicky and have a character of their own. The slightest change in the springiness of that rod-spring can silence the alarm forever or, worse, keep it running until the alarm spring is fully unwound; after which winding it may no longer be possible because the rod-spring will not lock the alarm arm any longer. I wonder if this rod-spring is so sensitive that a change in temperature or humidity might already cause it to malfunction? Or, could it be hysteresis? In other words the sheer fact of the spring being pushed repeatedly could make it less springy over time.

I was so pleased that I got the alarm to go off reliably after quite a few trial and error iterations. So, I put the carriage clock together again with a feeling of great satisfaction that another clock can go to the test bench for about ten days before being returned to the client. And, once that happens, getting paid!

So my frustration was hard to control when I had the clock all finished and in its case again to only notice then that if I set the minute hand to time the alarm hand would move along. Why on earth was this happening? It couldn't possibly be a feature of the carriage clock? So what caused this?

I took the whole clock apart again and started burnishing the stud on the alarm setting wheel that rubs against the alarm setting disc (see picture below: the alarm setting wheel is second from top left, the disc is just below it). I assumed there was too much friction between the disc and the stud.

After a while I noticed that the alarm setting arbor has a four armed friction washer (see picture below: the four armed friction washer is just below the number 390) pressing against the inside of the back plate.

This washer must be there to introduce sufficient friction for the alarm hand not to turn along with the minute hand when you are setting the minute hand to time. While, when you set the alarm, this washer must not have enough friction to prevent you from turning the alarm setting arbor. So again we seem to be working here with a part that is ever so finely balanced. I can already see more trial and error iterations coming!

I then noticed that this friction washer which looks like a starfish was doing a miserable job. It just hung there on the arbor flat as a pancake, while it should be pushing the alarm setting arbor away from the back plate. When it does that properly it will create minimal friction when the alarm setting wheel stud travels over the surface of the alarm setting disc.

So now, my next move will be to either bring life back into this washer by hammering it convex again, or make a new one and then see if the alarm hand will stay put when I turn the minute hand.

To be continued.

Drilling small holes with a tungsten carbide drill

I have been drilling a hole of 0.35 mm diameter to re-pivot one of the pivots of the contrate wheel arbor of a miniature carriage clock. The diameter of the arbor is 1 millimetre. It ended up in total failure. It was the first time I was drilling with a tungsten carbide drill. Or rather, tungsten carbide drills in plural. I went through about ten of the box of 50 before I gave up. They all broke in the arbor and I could not get them out anymore, let alone crush them to smithereens in the small hole. The hole was simply too small to do any crushing. Lesson learned: be ultra careful and ditto patient when drilling with tungsten carbide drills if they are that small.

I had to start again with a new arbor and pinion. I was determined to turn the pivots of 0.35 mm and 0.39 mm diameter myself. Again, I failed. I am sure it must be my limited skill in sharpening the tool I use on my watchmaker's lathe. I therefore turned it into an exercise of re-pivoting by drilling 0.45 mm (slightly larger this time) holes at either end of the arbor. I decided to make the pivot holes about 150% longer than what the length of the pivot should be. I made this 150% up as a good gut-feel number. I hope someone will come around and tell me whether this happens to be one of the golden rules of clock making, or, a violation of it. [Addendum: 30 May 2012: Today, an experienced clockmaker kindly explained to me that the generally accepted depth for a hole to repivot is two and a half times the pivot diameter, but that some clockmakers even go for three times.]

Just have the drill touch the arbor and just apply enough pressure to stay connected with it but do not push! Wait for some swarf to become visible then pull the drill slowly back out of what begins to be the pivot hole. Rub away the swarf, wipe your finger gently over the drill to wipe away any swarf sticking to the drill and start again. This way you advance about one tenth of a millimetre every 2 minutes. It does work. You get there in the end. I noticed that I needed complete silence around me because you want to hear the tip of the tungsten carbide drill make contact with the inside of the arbor. Once you hear that slight hissing sound you know that you can apply the minimum amount of pressure you can possibly put on. I was afraid to be disturbed by any unexpected sound around me. The carpenters at the neighbours seemed to have their day off, the children were still in bed, the wife had gone to work. Only the washing machine was spinning in the background which meant I could drill in between its centrifugal spurts.

I noticed that the deeper the hole the longer you will have to wait for swarf to come out. Do not become impatient if you do not see any swarf yet. It is easy, but fatal, to think that you are not applying enough pressure for the drill to do its work. "There is not any swarf, so I must press a bit harder!" Don't! Just sit patiently and wait. Swarf will come. And as soon as you see it, keep that same pressure going for a few more seconds and then pull the drill out again to clean away the swarf from pivot hole and drill and start again.

My turning speed was slow, the same speed at which I would turn steel. Perhaps the whole process can be sped up by increasing the lathe's speed. I did not dare to do this out of fear for breaking the drill which would undoubtedly (Sod's law) park itself in the arbor for good.