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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The Gravity of the Village Clock

Some time ago I made mention of getting involved with the running of the village clock.   There was an added incentive as this was a Cooke of York movement dating back to 1869.  Anything associated with the  City of York is always of high interest as it was my birth place.  That is except the soccer team but I try not to get drawn into that discussion.

Back to the clock.  Myself and a colleague in the village have been working on the clock to bring it back to time.  We were getting pretty close.   Then it stopped.   Despite a few restarts it refused to run for any length of time.

We rolled up our sleeves and gave it a thorough visual inspection.   We were just about to leave it for the day when we spotted that the pins on the pin wheel on the gravity escapement arbor seemed very tight to the arbor supporting bracket.   Closer inspection revealed one pin of the five seemed to be slightly at an angle.  Even closer inspection showed a visible witness mark on the bracket where the pin or pins were rubbing on the bracket.   There was no mechanism to centre the arbor away from the bracket.   

We decided to remove the arbor complete with pin wheel, fly etc.   This was fairly straightforward.  A single screw at the opposite end to the escapement arms could be unfastened and the arbour came free.  Except it wasn’t that easy to unscrew as the screwdriver slot was very narrow.

Having got the arbor assembly back to my workshop I discovered two of the pins were lose, one of them to the point of falling out.   It seemed to have been held in place from coming out any further by the support bracket.   

I marked and numbered the escapement legs and removed each pin in turn, shortened each by 0.5mm, degreased the threads on each pin and its associated mounting hole and then refitted each one with a dab of Loctite to hold them in place. 

Spinning the arbor in my hand I could now see that all pins looked parallel and the length was much more consistent.  Hopefully the pins will now comfortably clear the mounting bracket.

The gravity escapement arbor showing the five escapement legs and the five pins that needed shortening

To make refitting easier I made a small hand tool to fit the slot on the mounting screw and used a hacksaw blade as a pseudo screwdriver blade to more easily turn and tighten the screw.

Since refitting the arbor the clock was running slightly slowly so we removed a penny from the weight tray and it now seems pretty much spot on.

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Automated Wheel Cutting GCode for the Tormach with the Sherline CNC Rotary Table

I have been putting this off as I thought it would be hassle and in fact it was very simple.

Here is the code which is the first time I have ever used a sub-routine.

The top section is my standard set up routine for the Tormach.

The middle section has some out of the way locations to try the idea so don’t get too fixated by these.   The important bit is the M98 call for the sub routine, the sub routine name (1001) and the number of repeats (the L5 for five repeats).

The last section is the sub routine indicated by the O1001.   The M64 command is specific to the Tormach USB Expansion board and it makes relay P0 in the box close its contact which in turn activates the Sherline CNC Rotary table to move one step.   The cutter than moves across to cut the tooth and then returns whereupon the relay drops out (M65), waits and then closes once more to increment the table.   Once five repeats have happened the M99 closes the sub routine and the program jumps back to the Z10 line in the middle section before stopping the spindle, homing and ending.

The joy of this method is that it is a simple edit of one line (the M98  instruction) to change the number of tooth cutting increments. I like it a lot.

Some fine tuning is still needed on the back and forth distances needed to clear the cutter through the wheel blank.

The normal test of the cut depth routine will still be needed before this could be run but once this is done it should be a sit and watch job.   Hopefully.

I have yet to run a wheel in anger so I will let you know how it goes. 

A compiled write up of all the related blog entries on this subject can be found on the downloads page.

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Cycloidal Cutters and Finding the Cutting Centre

It has been a thoughtful morning on the Tormach wheel cutting setup. 

In order to cut clock wheels the first step is that I need to be able to set the cycloidal cutter centre line accurately on the centre line of the blank brass diameter.  See the picture and description below.

Exaggerated mock up of the cutting setup showing a brass blank mounted on a super glue arbor and a cutter mounted in the Tormach Slitting Saw arbor. The centre line of the cycloidal cutter teeth sits on the centre line of the brass blank and cuts on the rear edge as seen above. After each cut the CNC rotary table increments the blank by one tooth ready for the next cut. (In practice the super glue arbor would need to be much larger in diameter in order to be more in keeping with the diameter of the brass blank and so ensure maximum support while the cutting was done).

From previous posts you will know I have got the chuck securely and centrally mounted on the CNC rotary table and this assembly is in turn rigidly fixed on the tooling table. The position of the centre line of the chuck is now fixed relative to the tooling plate on the bed.  The chuck and rotary table mounting bracket is sufficiently Woody over engineered to hopefully be repeatable.   Likewise the distance from the spindle to the chuck can be repeatably zeroed using the Haimer and its associated tool table entry (#90).

Expanding this a little, if I put my favourite piece of 11mm diameter silver steel in the chuck and bring the Haimer down to contact it, rock the Haimer back and forth in Y to get the steel diameter peak, I can get a Z zero reading to the top of the steel.   By creating a new entry in the Tormach tool table (#91) which is the Haimer length plus 5.5mm (the radius of the silver steel) I can use this virtual length stored as a new tool #91 to allow me to set the Haimer on the silver steel while actually giving me Z0 on the centre line of the chuck. So far so good.

As you might have read from an earlier post, the idea of using the Tormach Slitting Saw arbor to hold my cycloidal cutters would in theory create a repeatable tool length to the centre line of the cycloidal cutter teeth.   Having this as a tool table set up in the Tormach would simplify setting the cutter centre to the centre line of the chuck and therefore the  centre line of the wheel blank being cut. This is where the thinking drifted somewhat.   

I created a new tool table entry (#77) that was the length of the saw arbor to the shoulder that the cycloidal cutter fastens against.   I thought I could then follow the same routine as detailed above and add to this length the half thickness of the cutter and create a new tool table entry to match.  This would once again create a length which would give the centre line of the cycloidal cutter.

That was fine until I measured my tray of cycloidal cutters to see what the thickness of the cutters were …… sadly consistent they are not.   There seems to be no standard by manufacturer or diameter.   I have cutters with thicknesses from 3mm through to 7mm.   I could create a new tool table entry for each thickness but this is a recipe for a mistake when selecting the correct tool table entry for the cutter being used.

The simple solution I think is to use slitting saw arbor tool table length (#77) as the initial setting length to Z0 and then do a G0 Z-x.xx where x.xx is the half thickness of the cutter being used.   Once Z has dropped to this reading the Z axis can be re-zeroed to run the wheel in question with the cutter in question now sitting on its centre line on the centre line of the chuck.

Simple diagram showing the concept of using the Tormach tool table facility to allow easy setting of the centre line of a wheel blank and cycloidal cutter centre lines

I hope that all makes sense …. I could of course just eyeball it and not try to be so fussy but when you have the tools to make things easier you might as well use them.  I also need to look after my precious piece of 11mm diameter silver steel.

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Tormach USB Expansion Board boxed and mounted

Another piece of the clock wheel cutting hardware completed.

The Tormach USB expansion board is now boxed and the connectors wired to the board.   I milled a viewing window in the box with a matching piece of perspex.   This allows me to see the status LEDs on the pcb. Port #P0 is now dedicated to the Sherline CNC rotary table controller which requires a closure to increment the table stepper motor.   

The connectors are all 8 pin MiniDIN which matches the interface on the rotary table.

Tormach USB Expansion card mounted in an IP55 enclosure and on the blanking panel where the ATC would normally be fitted.
Internal view of the Tormach USB Expansion board mounted in its enclosure. It shows the MiniDIN connectors on the pcb milled using my vacuum table and also the lazy cable gland on the USB cable

Still got to do the GCode …

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