My wife has presented me with a sign that has just got JSN written on it. It is to remind me when I answer the phone to a ‘can you just do’ enquiry…… to Just Say No.
I try my best to live up to her expectations but sometimes something comes along that should really be a JSN job but which scratches an itch. You know what I mean. You think about it and you do all the right mental arithmetic in your head and the answer keeps coming back to the same – don’t even think about it. But the the other side of my brain is screaming at me … what a challenge, what a learning experience, what fun to have a go at it. Providing the asking party is aware of your thought process or lack of it and accepts that it might just go belly up and never come to fruition then why not ?
Back to the story – 10 days or so ago I had a call from David Pawley who is a turret clock expert extraordinaire to say someone he knew was after an escape wheel for a turret clock and was desperate. David passed on the details and a couple of days later the potential customer arrived on our driveway. After a suitably socially distant conversation and a rubber gloves inspection of the old damaged wheel …. I got sucked in and turned the JSN sign over to face the wall.
The Brocot is no ordinary escape wheel. In fact it is a real challenge. Not a simple fly cutter job. Traditionally it would be cut in an indexing device such as a lathe with two different cutters, one for the curve and one for the notch. I didn’t have these so I thought I would probably upset the traditionalists and try to use CNC.
This write up is not for the purists with years of experience but is an explanation of how I thought through how to machine something over size that would not fit into my Tormach PCNC440 milling footprint as a single operation. Hopefully it might help others to grasp the process.
The challenge began when a local turret clock expert came to me and asked if I could machine a new Hour and Minute Hand for a clock he was working on. The Hour Hand was around 14” long and the Minute Hand some 18” long.
Here is the Fusion 360 view of the minute Hand.
Clearly these lengths were way outside the 440 table X movement (10”) so a plan was needed. There then followed a lot of staring into the distance at mealtimes and also at bedtime accompanied by vocal “hmmm”s as I tried to mentally visualise what was needed. This idiosyncrasy is something my wife has come to terms with over the years…..
My conclusion from this mental preparation was that I needed to be able to accurately step the stock across the tooling table and then take two or three bites at the profile machining.
What follows would almost certainly benefit from a video but sadly I am not set up for this.
Click the link below to download the PDF document.
From previous posts you will be aware of my involvement maintaining the local church clock. Over the past months my colleague and I have been nibbling away at various little problemettes with the movement and things are now looking quite good. For the past two weeks it has run sweetly and maintained +/-1 second over that period.
Then last night it stopped.
This morning we wandered round to see what the problem might be. The first thing we do on arrival is look at the front dial to see at what time it had stopped. This time it had stopped at around 10.35 last night. We climbed the tower and inspected the movement.
There did not seem anything obviously wrong so we decided to swing the pendulum and get it working again. We had arrived at just before 10am and our inspection took us over the hour and the front the dial was showing 10.35. Because we were now ‘within the hour’ it was acceptable to wind the hands back to the correct time which was now just after 10am.
I pulled out the motion work locking pin and began to move the hands (which were now independent of the movement) in a backwards direction to set the time. Except the hands would not move backwards. There was resistance. Something bad had happened to the motion work.
We checked the mechanism to both the front and rear dial but there was nothing obviously wrong but the hands refused to go backwards under light pressure and I did not want to force anything at this stage.
We went outside again and this time checked the front and now also the rear dial and this is what we saw : –
Our feathered friends had built a nest on the belfry window ledge and a stick had fallen from the nest and jammed itself in the dial. The odds of this happening must be pretty thin.
A careful waggle of the hands back and forth broke the stick free and we then reset the time and hopefully all will now be well.
The interesting observation was that the stick was only brushing the hand in the forward direction but in reverse it was pushing against it. The forward resistance was still sufficient to reflect back through the motion work into the main mechanism to stop the escapement and therefore the clock.