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
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Thanks again for watching,
I'll catch you on the next video.
