Workshop resources all coming together like clockwork

Another JSN Job ?

You know how I keep on going on about how solutions to problems are often solved by coming at them from different and often unconventional directions, by utilising and marrying available resources ? It was a philosophy that I encouraged in my team while running my business and it has carried over into my way of working in retirement.   A recent job brought his home to me.

A client had a very old clock that had had a new barrel wheel made and fitted but the clock would not run for more than a few minutes.   There appeared to be an incompatibility either between the modulus of the new wheel and its mating pinion or the shape of the original pinion did not match the shape of the new wheel. 

If you spun the barrel wheel you could feel the resistance build up as the synchronisation between the two profiles drifted out.   Adding extra weight to the barrel helped but did not solve the problem.

So what to do ?   

The barrel wheel was serious engineering and I did not fancy making a new one.   The existing mating pinion was a seven leaf format and its leaves were what you might call pear drop shaped rather than the expected profile.  The pinion arbor had a 72 tooth wheel driving the next part of the clock train but we did have a spare one of these to hand from the minute dial.

Calculations from the geometry of the barrel wheel resulted in a modulus        figure of 1.86.  A rather large value and not one that conventional cutters are readily available for.  The pinion was perhaps something that could be drawn in Fusion 360 and then made on my Tormach CNC PCNC440 milling machine.   The only snag was that the profile needed on the pinion would likely be weird and the world’s supply of brass could diminish rapidly while getting the profile correct.

Using Gearwheel Designer I created what would be the expected profile for a 7 leaf pinion with a modulus of 1.86.  This was exported as a DXF line drawing into Fusion 360.  This outline was extruded in Fusion into a 3D design and a boss was added to mount the 72 tooth wheel. 

The design was 3D printed on my Sindoh 3DWOX printer and was mounted on a 6mm silver steel arbor.   I added a driving disc that interlocked with the printed pinion and the crossings on the wheel to drive the assembly.  Surprise surprise it didn’t run but it did mirror the regular pattern of stiffness of the original pinion. 

Original arbor , pinion and wheel with the driving disc and a test profile
The original arbor, pinion and wheel together with the driving disc and a 3D printed pinion test profile. The driving disc has screws to lock it to the wheel and two protruding pins to lock into the 3D printed pinion profile under test. The 3D printed profile was a tight pressure fit onto the new 6mm arbor.

I now had the test bed for quickly making and testing different pinion profiles. There followed a number of hours watching the engagement progression of the profile of the pinion into the barrel wheel and then trying to conceive a profile for the pinion that might run. 

3D printed test profiles
Various trial profiles and the temporary driving disc to engage with the 72 tooth wheel

 

Test pinion in place on the new arbor
A test pinion in place showing the 72 tooth wheel and the driving disc

Fusion 360 made this process so easy and round 10 printed test profiles later I had success with a clock that now ran.    The driving weight on the barrel was around 11kg and it looked to be worthwhile wasting some brass making a ‘proper’ one. 

I took the 3D design and produced CAM code in Fusion.   This would cut the profile ‘on end’ using an adaptive first cut with a 4mm end mill followed by rest machining the remaining material with a 2mm end mill. 

Images of the Fusion 360 process of creating the new 7 leaf pinion
The Fusion 3D model of the pinion, the CAM simulation of the leaf cutting, first adaptive cut of the leaves and rest machining final pinion

The resulting brass pinion was mounted on the arbor and the clock ran with a strong beat.   As expected the brass pinion gave less surface to surface resistance than the 3D printed part and the barrel driving weight was now able to be reduced down to 6.25kg.

new pinion mounted in the clock
The finished pinion mounted in the clock on the new arbor

I ran my Microset Timer on the clock overnight and had a first off timing error of 5 minutes per day which was fixable with a pendulum tweak. The movement had an instability of a few seconds per day which was quite astonishing.

The conclusion of the experience is that at first glance this seemed like a conventional pinion cutting exercise …. but M1.86 cutters are not readily available.   If a cutter could have been found at less than a King’s Ransom it is likely that the resulting conventional profile would have been wrong to match the barrel wheel.   

The alternative route that was taken of Gearwheel Designer to Fusion to 3D print to Fusion CAM to CNC machining solved the problem albeit with a final weird profile.    The purists and traditionalists will groan.   There will be a gnashing of teeth and a pulling out of hair. 

Does it really matter if the result is new life for what could have become a heap of scrap metal ?

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Replacement Chuck Key for a Cowells ME Lathe

Broken Tooth and No Dentist ?

The Cowells Model Engineer miniature lathe is very popular in home workshops.   It is a well made machine and very accurate to use.

There appears to be one recurring problem with the design and that is the chuck key for the TMC3001 3 jaw chuck often ends up with broken teeth.   To understand this better you need to be aware that the Cowells chuck does not have a standard style chuck key.    It is more like a drill press chuck key as you will see from the image below.  It also has 12 teeth which is unusual compared with drill press chuck keys which usually have 11 teeth. Using too much strength trying to over tighten the chuck rotary mechanism could lead to severe machinist depression.

cowells 3 jaw chuck and chuck key
Cowells 3 jaw chuck and chuck key

I have to admit this is going to be another JSN job that slipped through the net while the sign had been left facing the wall from the last one …. a client wanted to know if I could make a replacement chuck key. 

It seems that these are not readily available as replacement parts.  So another little challenge was beginning to niggle at me. I thought about try to use Fusion  360 to create CAM for my Tormach PCNC440 CNC mill it but it didn’t feel like the right approach.  There had to be an easier way.   

While siting in the sunshine at lunch time (probably not paying attention to what my wife was telling me …. (again) …. ) I wondered if standard wheel cutting techniques could be used.   This would mean a custom made fly cutter which didn’t fill me with joy and suggested a lot of grief.  I then wondered if a standard clock wheel cutter might fit the same profile as the chuck key teeth.

With lunch over I dug out my treasure trove of PP Thornton wheel cutters and compared them with the profile of the chuck key.   The PP Thornton 0.95-7 modulus one looked a good bet as a match.   In its normal life this would be a 7 tooth pinion cutter. 

The idea looked like it might work.  I measured and sketched up the rough dimensions of the chuck key head profile which is shown below.   For ease of making a proof of concept prototype I decided to use aluminium.

First job was to profile the aluminium stock to the outline shape of the chuck key.  This completed I then mounted my Sherline CNC rotary table in the mill table vice and with some jiggery pokery managed to get the vice / table aligned at 14 degrees (90-76) to the X axis movement.   I set the centre line of the pinion cutter with the centre line of the aluminium profile.  I dialled in 12 steps on the Sherline and began cutting back and forth.   

To match the original teeth depth I had to go down to the full depth of what the pinion  cutter profile would allow.  On the prototype I didn’t bother finishing the shank of the key and below are some process images and the final prototype result.

process shots of making a cowells chuck key replacement and resulting piece
Images of the process and final result making a prototype Cowells lathe chuck key replacement.

The prototype worked.  I just have to make a fully finished steel version …… oh and remember to turn the JSN notice back over so I can’t miss seeing it next time an intriguing enquiry comes in.

Update : –  Silver steel ruined my cutter … they are really meant for brass. Looks like it will have to be a CNC method.

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Making a Brocot Escape Wheel using Fusion 360 and Tormach PCNC440 CNC milling machine

A Steep Learning Curve

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.

Brocot 30 tooth escape wheel
The original Brocot 30 tooth escape wheel that needed a new one making

As you can see it is not an ordinary escape wheel and I had to delve into one of my favourite books ‘Wheel and Pinion Cutting in Horology’ by J Malcolm Wild FBHI in order to learn about Brocot Escape wheels.   Malcolm is a great guy and his book should be on any clock experimenters bookcase.

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.

Read all about the adventure and see the result in this pdf download …….

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Silencing the Bill Smith Gearless Gravity Arm Clock

You will find details of my activity building the Bill Smith Hip Toggle Gearless Gravity Arm clock elsewhere on my site.   This was the first clock I ever made and it taught me a lot about techniques all of which were well documented by Bill in his write up and in his many other books and videos.

Let me state now that the clock design as intended by Bill works and works well.  It has one distinct disadvantage that every minute or so it gives a very loud ‘clunk’ as the pendulum amplitude diminishes, the hip toggle triggers and the solenoid resets.    Running it in the workshop was fine as I became immune to the noise but sadly the clock will never progress into the house given my wife’s sensitivity to noise.

This has been a frustration to me as the clock looks splendid and its motion work action is a fascination to behold.  It deserves to be on display in a more public arena than the workshop.

All of which lead to some head scratching and a compromise re-design.   If I accept that the clock is an electro-mechanical device then my conscience allows me to consider other electro-mechanical solutions that are significantly less noisy.   This is the fundamental premise to my re-design.

There are many clock designs that use magnetism attraction and repulsion as the driving force and my thoughts turned to this as a potential solution.

I 3D printed a magnet holder to fit on the pendulum rod.   This holds two magnets.   There is a large one facing the direction of swing and a small one perpendicular to the swing towards the backboard.  I mounted a Hall Effect Sensor (HES) on a prototype board onto the back board at the mid swing position.   The gist of my idea was to have the pendulum swinging back and forth across the HES with the HES being triggered by the small magnet.   I would count the number of swings detected by the HES and after a defined number of swings I would energize a solenoid to repel the large magnet.

There was a little bit of electronics involved.   I had a 4060 binary counter counting the swings and used the divide by 32 output to trigger a 555 in monostable mode.   This would create a delay period from the mid point to the end of swing before the solenoid was energized.   A second 555 would then define how long the solenoid repelling pulse would last.   I also added LED indicators to all key timing points so I can easy diagnose what was going on.   I also allowed selection of the 16,32 and 64 divisions from the 4060 until I established the optimum choice.

The pendulum period is 4800 beats per minute so one swing lasts for 750ms.  The first 555 must therefore provide a delay of 375ms before energizing the solenoid.   The second 555 delay would be a ‘suck it and see’ period to be determined.

The concept was lashed up and worked OK …. except that the pendulum amplitude just grew and grew until the large pendulum magnet attached itself to the solenoid core …. not a good idea .   What was needed was a maximum amplitude detector to act as feedback to inhibit the solenoid pulse action.   

A second HES was mounted at a position that represented the maximum swing position and the output from this, when triggered, would feed back to the 4060 RESET pin to stop the count until the amplitude diminished sufficiently.   This worked and the result was very repeatable.   There was one proviso that the pendulum must be started by triggering the over swing HES and releasing.   Without this the 4060 could be one count out of step and would energize the solenoid as the pendulum was swinging back from its furthest point.  This would cause a repelling and slowing of the swing.

Overview image of the new pendulum sustaining mechanism on the Bill Smith Gearless Gravity Arm clock
Overview image of the new pendulum sustaining mechanism. Top left is the timing board and lower right is the Hall Effect Sensor board. The solenoid is the original Bill Smith design. The white block on the pendulum is the magnets holder. You can see the two Hall Effect Sensors hanging below the blue prototyping board.
close up view of the sensor board
A close up view of the Hall Effect Sensor board where the two sensors can be more clearly seen along with diagnostic LEDs and interface converters to the timing board.  The pendulum magnet holder can just be seen coming into shot.
Oscilloscope display of the new timing sequence for the Bill Smith Gearless clock
Oscilloscope display showing the yellow regular negative going pulse from the centre swing HES detector, the blue delay pulse to allow the pendulum to travel to its extreme and commence its swing back and the purple pulse triggered from the back edge of the delay pulse which defines the solenoid ‘On’ time.

The prototype has proved the concept but I now need to engineer a clean solution.   First choice is perhaps an Arduino Mini but it could also be PIC based.   

More to follow.

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A Spanner in the Works – or simply a Stick

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 : –

church clock with stick jamming the mechanism

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.

Another bit of knowledge gained.

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