G'day Chris here, and welcome back to Clickspring.
In this video, I make the gearing that drives the Metonic Calendar pointer,
as well as two small adjacent pointers:
One showing the Callipic cycle,
and the other indicating the 4 year cycle of the Pan Hellenic Games.
The Ancient Greek concept of a calendar was very different to our own.
In fact they used a system of several calendars,
that tied together the cycle of the sun and the moon,
annual seasonal events, and also recorded various intervals of solar years.
Remarkably, the Antikythera Mechanism provided a coherent display
of all of this data for the user to interpret.
At the heart of the calendar display is the gearing that represents the Metonic Cycle.
A fortunate astronomical coincidence where 235 Lunar months
almost exactly equals 19 solar years.
To provide more room for the engraved text,
the cycle is represented on the dial as 5 spirals of 47 months.
The pointer moves through 5 full turns to cover the 235 months in 19 years,
after which its reset to the inside of the spiral again to re-start the process.
So ignoring sign convention for the moment,
and viewed from the perspective of the pointers,
the gearing ensures that a full turn of the Metonic pointer occurs once every 3.8 years.
And you might notice that two of the gears cancel out in the train calculation,
so they appears to be redundant.
But as it happens a 53 tooth gear turns out to be essential for another part of the mechanism
thats driven by the first section of this train,
and I'll cover that in detail in a later video Now there's a small error in the Metonic cycle
thats corrected in the Callipic cycle,
by essentially multiplying the entire Metonic cycle by 4 to give a more accurate
76 year relationship.
And finally the Olympiad pointer indicates the passage of the 4 year Games cycle.
So there are 6 wheel assemblies to be made to complete this part of the mechanism,
as well as a number of supporting components.
And as I make a start on the wheels,
its worth commenting on one of the engineering techniques used in this part of the mechanism.
You'll have noticed that the assemblies are all rotating on either a single pivot,
or in the case of the M assembly, a through pivot with a bracket support.
This single pivot idea is also present in the wreckage of the Byzantine Sundial Calendar.
But generally, it has to be said that its not a practice that survived from antiquity
to be used in the modern age.
Somewhere along the line, the idea evolved.
Becoming more mechanically sound and efficient.
So that today a wheel assembly in a modern clock or watch for example
has 2 very slender, low friction pivots.
Now there are many engineering "firsts" like this within the mechanism,
that I'll cover cover the coming episodes.
Some were evolutionary dead ends, and simply didn't make it.
Some, like the single pivot idea were the origins of an idea that continued to evolve.
And others were evolutionary winners from the very beginning.
Travelling through time essentially unchanged, right up to the modern era.
Now one idea that of course definitely evolved is the tooth profile.
But that's not to say it wasn't largely effective as it was.
Because despite its mechanical inefficiency,
a triangular tooth profile brings a tremendous advantage when it comes to
certain aspects of the build.
Like for example, depthing.
For a traditional clock movement,
depthing involves using a specialised tool like the one I made in a previous video series.
Unlike modern cycloidal teeth,
the optimum depthing for triangular teeth is very close to full depth of engagement.
So by simply pushing the wheels together,
a very close approximation of the correct center distance is immediately achieved.
Then by adjusting the wheels out slightly,
a good working distance can be set, captured with dividers,
and subsequently marked on the plates.
The entire mechanism can then be depthed using this simple method,
with no requirement for a specialty tool.
And there's a further advantage to triangular teeth,
if the wheel engagement is found to be a little tight,
once the pivot positions have already been drilled.
In that instance the entire outside profile of one or even both of the wheels
can be very slightly filed back, much like when the teeth were originally formed.
So that the final stage of depthing essentially becomes
an extension of the tooth forming process,
and I'll show you some more detail on this later in the video.
I used a deburring tool to tidy up the perimeter of the pivot holes.
And then used a transfer punch to locate the three holes
that allow the pointers to pass through the dial plate.
OK, so that's both plates complete for the moment, next up I made the wheel assemblies.
Which follow a basic pattern of square holes threaded over square shafts.
Each of the assemblies presents different combinations of tapered arbors,
and interference fits, rivets and pins as fastening techniques
to keep the wheels in place on their respective arbors.
But the way that these ideas have been executed in each case
says a great deal about not only the state of the engineering technology of the day,
but also the objectives of the maker when constructing the device.
And the L assembly is a great example to show you what I mean.
This assembly sits directly beneath the main solar drive wheel B1.
The clearance between the top of this assembly and the underside of B1 is less than a millimetre,
so by design there's not enough room for a retaining pin to be used on its upper surface.
And, rather than provide a shoulder for the 2 wheels to face onto,
the pivot has been deliberately turned smaller than the square mounting boss.
Presumably to keep the pivot friction to a minimum.
Now in each case, it didn't necessarily need to be this way.
The main bearing could have been made to lift B1 a little higher,
and so provide enough room for a fastening pin.
And, the L pivot could have been made with a larger diameter
without that much of a friction penalty.
But in each case, they weren't.
Instead the much more challenging route was taken,
of fastening the wheels using a firm interference fit on a very small tapered square arbor.
And this leads to a couple of conclusions.
The first is that there can be no doubt that the Maker was consciously pursuing
miniaturisation of the mechanism.
Bypassing the easy options,
and instead choosing the much more difficult, but sleeker options.
Which then naturally leads to a second conclusion:
That this must surely have been a mature version of the design,
and that other versions must have come before it.
Now as tends to happen when pushing the limits,
things don't always go according to plan.
The scans show at least one small retaining rivet used to hold the two wheels together,
that strictly speaking isn't required.
We can never know for sure of course,
but maybe the wheels were tapped off their arbor for some reason,
perhaps to improve a depthing as mentioned earlier.
And when re-assembling, the fit perhaps wasn't quite as firm the second time,
requiring the rivet.
Regardless, the slim profile of a pinless assembly was achieved,
and the entire device was that much more compact because of it.
Next up are the O,P and Q assemblies.
The O assembly is present in the wreckage,
so there's a bit guidance about how this part of the mechanism was put together.
But the P and Q assemblies are absent,
so I've reconstructed them along similar lines to the O assembly.
I used the lathe and mill to form the basic arbor profile,
taking light cuts due to the extent of the work overhang.
And I then used needle files to bring them to final dimension.
In each case, I gave the arbors a gentle taper,
so that the wheels could be held in place with a light interference fit.
Each of the square holes was carefully opened up
until the wheels just threaded onto the end of the arbor.
From there, the holes were further opened until each wheel
could be easily pushed onto the arbor for staking.
And its worth pointing out that this process of opening up each of the squares
was incredibly delicate,
with barely a handful of file strokes between initial entry onto the arbor,
and getting the wheel into a good position for staking.
The wheels, pinions and a small retaining disc on the O assembly
were then gently hammered into position on their arbors.
Now again, much of the M assembly is also missing from the wreckage.
But from what remains, we know that the assembly passed through the main plate.
So it serves as a possible example of how close fitting parts,
and removable taper pins could have been used to enable the disassembly
of some parts of the mechanism.
It would have been necessary to provide the M assembly
with some degree of what clockmakers call "end shake":
A small clearance gap between the plate and the assembly that ensures free movement.
The main plate is a bit cumbersome to use for this job,
so I used a scrap of the same thickness instead.
It provided a clear profile view of the assembly as it will be when in position,
allowing good inspection of the end shake.
The last assembly to look at is the N assembly, which is the one that carries the Metonic
pointer.
And this assembly neatly ties together most of the ideas presented in the previous 5.
It has a wheel and pinion that are firmly staked into position.
But it also has the larger wheel at the pivot end as a press fit,
so that it can be pushed or tapped into place during the assembly process.
You'll see this happen later in the video.
OK thats the bulk of the work on the assemblies complete,
so next I moved on to the pointer components,
starting with the support frame for the metonic pointer.
And since I'll soon need a second one of these for the Saros pointer,
I figured I'd better make both of them at the same time.
The Metonic and Saros pointers, are a nice straight forward shape,
designed to be a loose sliding fit within the support frames.
There'll be a small stylus on the end of each of these pointers
that will follow the spiral groove of the dial plates,
but I'll save installing those until I cut the spirals in a later episode.
The Callippic and Olympiad pointers are not present in the wreckage,
but the rest of the surrounding structure requires that they be quite short.
So I found it convenient to form the squares first while they were still part of the parent stock,
and then once they were cut free I draw filed them to a smooth teardrop profile.
And that completes all of the parts for the assemblies,
so each one that requires a taper pin was then marked out and the cross holes drilled.
The holes were then lightly taper broached, and the retaining pins fitted into place.
Now most of the assemblies for this part of the gear train run directly in the main plate.
But both the P and Q assemblies require raised bushings,
to bring them to the correct height for meshing.
They're basically small cylinders that are intended to sit proud of the main plate,
held in place with a pin and washer on the other side.
And speaking of washers, I turned those next.
The largest of the three being the spacer that raises the O assembly up off the main plate,
to bring it to the correct meshing height with the N assembly.
The final part to be made for this part of the build is the small bracket supporting
the M arbor.
And this is one of those Ancient engineering ideas I mentioned earlier in the video,
that was most definitely an evolutionary winner.
Because we see essentially the same support structure
used throughout all forms of mechanical horology.
The wreckage scans shows that it was fastened to the main plate
using with 2 diagonally opposed rivets.
And the rivets are required to be filed flush to permit the motion of wheels
both directly above and below the fitting.
Ok, so thats enough of the hard work, time for my favourite part of the build: Assembly.
Now as that last assembly is inserted, notice how its just a little bit tighter than the others,
as its seated into place.
This is a perfect candidate for the depthing adjustment that I mentioned earlier in the video.
By cutting the teeth ever so slightly deeper into the wheel,
the outside diameter is slightly reduced, easing the depthing to a better engagement.
And on the second insertion, its much improved.
In the next episode, I'll continue to build the gearing
that existed beneath the rear dial of the mechanism.
Thanks for watching, I'll see you later.
Now if geared mechanisms like this are your thing,
and you'd like to help me make more of these videos,
then consider becoming a Clickspring Patron.
As a patron of the channel you get immediate access to the Patron Series of videos.
This includes the 5 videos of the Wedge Style Hand Vise build,
and the first 6 videos of the Byzantine Sundial Calendar build,
with more to come as that series progresses.
In the most recent episode,
I made 2 versions of the mechanism body,
one fabricated from sheet stock,
and the other by casting the part from scrap brass.
And don't forget that as a Patron you also get free access to the plans
for the Patron Series projects.
So you can follow along, and build them yourself if you wish.
Now as an added Patron Reward, for a limited time I'm also offering $10 off
on your purchase of the Clickspring Fire Piston.
Its a terrific little fire starting device,
based on the prototype that I made some time ago.
And it makes a great addition to any camping or hiking bag.
So be sure to visit patreon.com/clickspring to find out more.
Thanks again for watching,
I'll catch you on the next video.
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