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How To Make A Clock In The Home Machine Shop - Part 22 - Making The Stopwork Mechanism - YouTube
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G'day Chris here and welcome back to Clickspring.
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In this video I make the small assembly designed
to safeguard the clock from overwinding
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the stopwork mechanism.
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The stopwork design used in this clock is
essentially a simple counting device.
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Pinned to the side of the great wheel, it
consists of two small wheels,
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a pivot, and a small friction bow spring.
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As the barrel arbor rotates with each turn
of the winding key,
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it also turns the stopwork driving wheel.
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The count wheel records each pass of the driving
wheel tooth,
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until the maximum number of desired turns
has occurred.
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At that point the wheels become locked, and
no further winding is possible,
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preventing the clock from being damaged.
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The barrel arbor requires a slight modification
to accept the hexagon shape of the driving wheel.
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Which is so shaped to permit the wheel to
be positioned in up to six starting points.
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The wheels for this part of the mechanism
were cut back in episode 4,
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so the next step required to prepare them
was to trim off the waste stock on that driving wheel,
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and then bring it to an accurate circular
profile.
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I started by marking out the work using the
wheel crossing jig.
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I used the scroll saw and belt sander to bring
the work closer to the line,
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taking care to stay clear of that solitary
tooth,
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and then I finished off the perimeter with
needle files and abrasive paper.
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The driving wheel hole needs to be filed to
match the arbor hex,
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so that'll come later when I've made the required
modification to the barrel arbor.
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In the meantime, I need to determine the optimimum
depthing for these two wheels,
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and then mark it out on the great wheel.
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Which means disassembling the clock for the
first time since it was put on test a few weeks ago.
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And I have to say that the depthing was a
bit of a surprise.
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The interaction when gears are used like this
for single tooth movements
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is very different to a normal tooth engagement.
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To permit the driving wheel to lightly pass
into engagement,
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tick over the count wheel, and then carry
on for another full rotation,
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the wheels require a much shallower depthing
than normal.
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In fact I ended up using a value quite a bit
larger than the theoretical depthing figure,
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to guarantee a reliable count with no jamming.
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Once that depth was established, I transferred
it to the great wheel,
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making sure that the marked position was central
to the wheel spoke.
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The marked position was then drilled and tapped
to accept the shouldered pivot.
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A quick deburr with a hand held countersink
and that hole is ready to receive the pivot.
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The pivot itself is essentially a stud,
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designed to both locate the count wheel and
have sufficient length
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to be able to accommodate the small friction
spring.
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I turned the shouldered section to be close
fit with the count wheel,
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and then formed a short thread,
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making sure that it would be no longer than
the thickness of the great wheel.
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This is to ensure that the end of the thread
remains clear of the barrel when assembled.
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The screw was quench hardened and polished,
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and then heat blued on a bed of brass chips.
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OK so with the main components mostly formed,
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I can move on to making the hex shape on the
barrel arbor.
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The feature needs to be positioned just outside
the barrel cap,
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and have a small cylindrical section turned
just beyond that.
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I took care of the flats on the mill,
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using the hex shaped collet block for fast
and accurate indexing.
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With the flats formed, I located the work
between centers on the lathe,
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to turn down the small cylindrical section.
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That hexagon now becomes the shape to aim
for when making the hole in the driving wheel.
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Again marking out the work on the wheel crossing
jig
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this time using a couple of custom turned
spacing pins to give the required dimensions.
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I used some very careful filing to open up
that hex hole,
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and then I gave the wheel a light rub on some
abrasive paper to knock down the small filing burr.
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Importantly, the wheel now fits comfortably
in all positions around the arbor hex.
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Its a snug fit, but not so tight that it can't
be gently lifted off the arbor
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when setting the stopwork.
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The design calls for a clock pin to sit in
front of the wheel and hold it in place,
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so its back to the mill to drop in a small
hole.
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And that completes the driving wheel and barrel
arbor modification.
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The last item on the parts list is the friction
spring.
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And I've found that its a lot safer and easier
to make holes in thin stock using a punch.
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So I've added another size to the punch that
I used in the previous episode for the mainspring,
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to match the required hole in this little
spring.
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A few strokes on an India stone gets the perimeter
into shape,
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and then a quick bend and its done.
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And although not specified in the plans,
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I decided to make a thin washer to lift the
countwheel clear of the great wheel,
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and so avoid it marking the surface of the
wheel over time.
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OK, so all the bits are good to go, lets put it together and have a better look at how it all works.
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Once installed the count wheel sits just clear
of the great wheel surface,
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and although free to rotate, the friction
spring generates just enough force
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to keep the wheel from moving when its out
of mesh with the driving wheel.
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A secondary feature of this stopwork design
is that the initial position of the driving wheel
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can be carefully set to exclude both the first
and last few turns of the mainspring,
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where torque variability is at its worst.
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It effectively limits operation to the best
section of the mainspring,
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significantly improving the performance of
the clock.
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As the clock runs down, the count wheel engages
with the driving wheel tooth
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and reverses its rotation.
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Until it is once again back in the original
position, ready to be rewound.
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Which brings me to the subject of the next
and final episode of this build series.
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A custom winding key.
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Thanks for watching, I'll see you later.
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And if you've just made your way into this
clockmaking series,
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thanks for checking it out.
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This is just one episode of a longer series,
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where I show all of the steps to make a mechanical
clock from raw metal stock,
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so be sure to check out those other videos.
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If you enjoyed this video and would like to
help me bring you more project videos like this one,
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then consider becoming a Clickspring Patron.
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As a patron of the channel you get access
to exclusive Patron only video content,
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free plans for the patron projects,
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and the chance to win the actual project at
the end of each build.
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Find out more by visiting patreon.com/clickspring
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And finally, if you're looking for some new
projects for your lathe or mill,
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then take a moment to visit clickspringprojects.com
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where you'll find a range of plans available
for download,
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including plans for some of the tools I've
made to help me construct this clock.
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Thanks again for watching, I'll catch you on the next video.
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