Clock 4 drive assembly

Previously, I haven't made key-wound mechanisms, so Clock 4 is to be driven by a roughly 1" barrel wound with a key.  The drive wheel rides loosely on the barrel arbor, which is (at least for now) friction-locked onto the barrel. 

The barrel arbor has a 1/8" plain pivot that fits a hole in the back plate.  To engage the key, there is a cross drilling for a steel pin.  The drive wheel is supported in a cock screwed onto the back plate.

The drive wheel meshes with a pinion that directly drives the escape wheel. 

The pinion rides on a small arbor that itself screws into the back plate.

I will cut away some of the cock to allow clearance for the escape wheel, which also will carry the minute hand (directly).  I also plan to try Aaron Dodd Crane's daisy wheel motion work to drive the hour hand coaxially.

Count wheels and pendulums

Philip Woodward invented many interesting clock mechanisms, which are based around intermittent escaping.  To time the intervals between impulses, his escapements use count wheels.  But he cautions that a poorly-designed count wheel can dramatically alter the Q of the pendulum, and cause reliability problems.  Since my next clock is planned to use Woodward's intermittent grasshopper, impulsing once per minute, I wanted to make sure that the count wheel worked well on its own.

Realizing that my previous clock #1 pendulum has a low unloaded Q because I chose to suspend it in a plain brass pivot, I tried a knife edge suspension.  To hang the pendulum, I took a piece of wood with a branch at a right angle and shaped it into a strong bracket.

The bracket has a slot cut into it to receive the pendulum's knife edge. The pendulum knife edge is crude at this point, and not very artistic.

Unloaded, the Q is around 300-500 with some steel weight I tied onto the bottom. 

For the count wheel, I cut a simple 30 tooth ratchet wheel from 1/8" plywood.  I tried to get the tooth spacing about what would correspond to a degree or two of pendulum amplitude about 6" from the suspension.

The count wheel is driven by two wire lever pallets.

The left pallet attaches to a hole in the pendulum and pulls the count wheel to advance it. The right pallet attaches to a separate anchor point, and serves as the backstop.

Here is the assembly, ready for testing.

Starting from a comfortable amplitude, which pushes the backstop about halfway back, the mechanism will run reliably for somewhat longer than 2 minutes.

Starting from just below an amplitude causing double counting, it will run for over 3 minutes.  This is heartening, because it indicates that impulsing every one minute is feasible, because there is plenty of extra energy available to let off the escapement (not built yet).

Clock 1 humidity redux

I think I have finally figured out what goes wrong with Clock 1 when the humidity changes.  After much watching and poking, I found that the third wheel appeared to be sticking slightly, at least whenever the clock stopped.  Carefully pulling the third wheel backwards once the clock stopped, I could feel some rubbing...

I think the third wheel pinion expanded a bit beyond where it was supposed to, or maybe the frame expanded, but in any event, it was fouling at the tip.  After ensuring that the pivot was rigidly planted where it was supposed to be, I took to very slightly filing the addenda of the pinion.

This makes the pinions closer to an ogive form.  I don't think this really matters, because the clock is now happily running even though the humidity has been around 60%.  Hopefully I won't run into an "opposite" problem when the humidity drops later in the year.

Drilling and sanding

The next logical step in the clock is to drill holes for all the arbors and the like.  For large-ish wheels, they can be comfortably held in the lathe chuck, like so.

For the pinions, I was concerned both with getting a hole that was truly axial and with not damaging the pinion leaves.  I remembered (somewhere) reading about cutting a recess in a wax chuck to receive the outer diameter of a wheel.  Once cut, the wheel would be set into this recess and the center could be drilled correctly with certainty.  This seemed like a good idea, so I chucked a piece of scrap pine in the lathe chuck and bored a recess to receive each pinion.  I made sure that the recess wasn't too deep, so that I would be able to grip the pinion with my fingers to remove it.  I cut aggressively with the boring tool first, and then when I got close, cut in 0.0025" increments until a snug fit.  Here is one pinion fit into the recess.

Here is another view as drilling is in progress.  I gave myself practice striking centers with the graver, which was much more efficient than using a center drill.

I used three drills, starting from 1/16" to 1/8" to ensure that I didn't have much tearout from any pinion or wheel.

The resulting pinion holes were indeed dead on center.  Since the arbors are 1/8" exactly, I broached the holes slightly larger to give a smooth, low-friction fit.

Emboldened by my success, I used a wobble stick and lathe faceplate to drill the precise holes on the escapement detent and hour rack.  This turned out to be less harrowing than drilling by hand, and ensured that no parts got damaged!

After this, I sanded off the paper, using grits up to 320 and removed the dust with a rag dampened with mineral spirits.  I think I really only needed to go to 220, though.

Trying all the parts on the depthing tool indicated that the gears run very smoothly.  All the time I spent sanding paid off!  But the hour rack had a design flaw.  The teeth of the ratchet pinion (right) foul on the rack teeth (left).

So to fix this, I cut and sanded the hour rack teeth to allow the pinion to run smoothly, which wasn't too hard.

New clock depthing tool

Picture says most of it.. I designed a new depthing tool in OpenSCAD that should be a bit more stable than the previous one.  It's also intended that the arbors are fixed and the wheels spin around them.  The design consists of two parallel bars that hold three I-shaped blocks.  The outer two I-shaped blocks are attached to the bars, while the middle one can slide.  The left two blocks have arbors through them.  The sliding block is locked by tightening a knurled nut holding everything together.

Stabilizing gear tooth length

My wooden clock has been acting tempermentally lately... It seemed to run for somewhere between 5-10 minutes before stopping.  I tried relubricating it in place, but to no avail.  That is bit less than one rotation of the third wheel.  Upon disassembly, I found that the fourth wheel pinion roots where pretty heavily plugged with dried-up slipit, which could be the cause.  If the tips of the third wheel foul slightly on the roots of the fourth pinion and there's a lot of sticky residue there, it might be a problem.  I also checked to see if the teeth of the third wheel varied much in length by clamping an engineer's square so that most teeth passed by with no clearance.

 It turned out that a few teeth were just a hair longer, and these would have been presented to the pinion about every five minutes.  So I filed these teeth so that they, like the others, just pass the square.  Then I cleaned and relubricated everything... At last check, the was still running after 10 minutes...

Much filing ensues

I decided that I should rough cut out parts for the timer, and the file them to their final shape.  This way, I should get finer control of the accuracy of the parts, and would avoid gouging them with the saw.

Anyhow, this is a time consuming proposition...  Many hours of filing later, I have this (almost complete) pile of parts:

The gravity arms have yet to be filed, but they're not too critical.  The cycloidal gear teeth took a long time... much longer than the involute teeth on my previous clock.  Also, I was very particular about the spokes.

I still have yet to cut the blocks that will hold the plates apart; I'm waiting until I finish assembling the drive pulley and ratchet since I'm a little unclear of how large that will be.

Pins and slip-it

Now that the clock "runs," I've been tuning it so that it runs well.  The largest apparent problem was that that the anchor slipped on its arbor.  This meant that sometimes the clock would run with an even beat, and then lose this...  So to fix the problem, I made a small pin, like the one shown below:

In the end, the pin shown here didn't grip well, so I turned a second one that was tapered.  Then, with the anchor in place, I drilled through the anchor into its arbor and gently hammered the pin in place.

This helped with keeping the beat even, but the clock still was stopping abruptly.  After talking with my father, we settled on lubricating the teeth.  Nearly all wooden clock enthusiasts argue strongly against such a move, most wood lubricants or finishes turn hopelessly gummy and become hard to remove.  However, one caught our attention: slipit.  They sell two kinds, a wood-working kind (without silicone) and a "mechanics" kind (with silicone).  The reviews indicate that the kind without silicone doesn't work well, but the kind WITH silicone seems to work better.

So we settled on the stuff with silicone, and I applied it to all gear teeth with a small paintbrush.  After putting on a frightfully large weight (9 lb 8 oz), the clock has worked absolutely smoothly since!  Amazing!

Now, I did notice something odd.  Although the minute hand seems to run at about half speed, the hour hand runs at the right speed.  Weird.  I guess I made TWO gear ratio calculation errors...

Gear depthing issues

By filing the anchor pallets and escapement wheel, I managed to get the escapement mechanism working well with the escapement wheel, anchor, and fourth wheel.  The next step is adding the third wheel.  This seems to cause problems.  It will run smoothly for a while and then suddenly the power transfer to the escapement stops abruptly.

After carefully tracing what was happening, (mostly by watching and poking at the mechanism when it stopped) I've concluded that the problem is with the meshing of the third wheel and the fourth wheel pinion.  Some of the third wheel teeth seem to jam on the pinion.

So I went through a very extensive (multiple hours) exercise of trying to file teeth on both the pinion and the third wheel.  Some of the third wheel teeth a bit narrower than they should be, and this seems to be where the most trouble occurs.

However, I may have overdone the filing, especially because I now think that the problem is that the gears are improperly depthed.  It seems like the centers are spaced too far apart -- probably no more than about 1/16" and possibly much less. I can reproduce the symptoms in the idealized drawings as shown below.

The trailing leaf of the pinion usually jams, as also shown in the diagram.  There should be (and usually is) a gap that allows the leaf to move into position.  Every so often, it fouls.

Probably, the right way to fix this is to remove the bearings, redrill the bearing holes (filling the inevitable gaps) to proceed.  This seems fraught with difficulty, so I'd like to avoid moving the bearings unless there's no other reasonable option.

So I'm going to try to cut a new fourth wheel pinion that has extended addenda.  This should improve the engagement as the diagram below shows:

Notice the addenda are extended by about the error in depthing -- around 1/16" -- though I can file it back later if needed.  This solution does have a drawback, in that the pinion engages before the centerline.  There's likely some increased sliding friction and power loss as the force is not tangential.  But it seems worth a try.

Debugging (Part 1)

With the completion of the pendulum, the clock at least runs for a while.  But it always seems to stop.  There are a number of possible points of failure:

1. Gear profiles not cut perfectly
2. Not enough or too much drive weight
3. Not enough or too much pendulum bob weight
4. Design flaw -- especially in the escapement.
5. Too much friction, somewhere in the mechanism

So with all of these possibilities, it is reasonable to try to break things down a bit.  I started by observing many, many runs.  It seems like there are three modes of failure:

1. The escapement stops getting driven.  In particular, one of the escape wheel teeth falls off the anchor and (usually) gently comes to a stop.
2. The anchor hangs up on an escape wheel tooth (which is clearly still pushing)
3. The anchor slips on its arbor, and therefore is out of alignment (less common)

The first of these seemed most serious.  Poking at things carefully, it looked like the third wheel arbor was binding on its pinion.  So, I removed the pinion and started filing teeth, to give it some more space (hint: this isn't a good idea)

After getting all six teeth, the profile changed like the one at the left in the picture below to the one on the right (arrow).

This didn't seem to fix anything, and indeed, made it worse.  So I just cut a new pinion from scratch.  That seemed to help solve the escape wheel losing power issue -- but not completely.  I'll come back to that later, I imagine.

So I decided to switch to the other major problem -- the point where the anchor gets hung up on the escape wheel.  This seemed to be a drive power issue, and possibly an anchor design flaw.  To test this, I removed the center, third, and ratchet wheels from the frame.  I attached a cord to the fourth wheel pinion with a plumb bob.

Still, the mechanism stops, so I suspected the problem was isolated to just this.  I've been concerned that the anchor requires too much lift, so I filed back the anchor teeth a bit.  Of course, I now realized that if I didn't like the result -- like the third wheel pinion -- I'd be making a new anchor.

But now, the mechanism seemed a bit closer to running.  And indeed, with quite a lot of weight it runs.  Specifically:

• With 7.5 oz on the fourth wheel pinion, the mechanism slows to a stop
• With 11.5 oz on the fourth wheel pinion, the mechanism runs
• With 1 lb 3 oz on the fourth wheel pinion, the mechanism runs, and can even tolerate the pendulum getting bumped a bit.

That's far more weight than I wanted to need at that point in the train.  Indeed, that would require some 35+ pounds on the actual drive pulley.  Really, I'd like only about 1-2 oz to be needed on the fourth wheel pinion to drive the mechanism.

Since there's not a whole lot of friction in the pivots, I suspect the right course of action is to try to reduce the lift on the anchor a bit more...

Clock #1 plans

Here is a plan for a simple clock mechanism with an anchor escapement and co-axial hands.

The drive train has the following teeth counts, which give a ratio of 240:

• Escape wheel: 15 teeth, rotates 4 times per minute, 1 second per tooth
• Escape pinion: 6 teeth
• Fourth wheel: 30 teeth
• Fourth pinion: 6 teeth
• Third wheel: 36 teeth
• Third pinion: 6 teeth
• Center wheel: 48 teeth (rotates once per hour)

Thus far, I've cut the center, third, and fourth wheels, along with all of the pinions.  The escape wheel remains to be cut, along with the motion work.  The motion work includes four gears giving a ratio of 1/12:

• Center pinion: 6 teeth (rotates once per hour)
• Offset wheel: 18 teeth
• Offset pinion: 8 teeth
• Hour wheel: 16 (rotates twice per day)

The third, fourth, and escape arbors are as described here.  The pendulum arbor is longer.  The center and hour arbors are more complex as they need to fit together coaxially as shown below.

The escapement is a deadbeat anchor escapement as shown below.

Brass wheel arbor

For the clock wheels I made, I need arbors to support both the wheel and the pinion.  I decided to make these out of brass, with pivots to match the bearings I've been making.  Here's the overall design:

The arbor is cut from 1/4" brass rod, starting from one side, and then turning it around for the other.  The steps are made either using the slide rest or by hand with a graver, but the pivots are made by hand.  First, the rod is inserted in the chuck, and then lengths marked.

Then, the steps are cut.

Once the step are cut, the end is parted off, and the pivot is shaped using the graver.  Below shows the completed pivot, and is in progress of being parting off.

After this, the arbor is turned around and rechucked, with the other pivot cut.  Here's the completed arbor.  (The extra groove on the small step is a mistake.)

Here's the arbor fitted with a wheel and pinion.  It takes a bit of fiddling to get the arbor diameter correct.  It seems like a few taps of the hammer are useful to set the arbor into the wheel.

Here are two wheels fitted together in the depthing tool.  The fit is fairly sensitive -- a millimeter one way or another is enough to mess up the meshing.  Once depthed correctly, the gears run very smoothly!

Cutting gear teeth

I cut teeth on the gear blanks in several stages.  First, I made cuts along one side of each tooth.

Then, I cut along the other side of each tooth.  Notice that I do not yet cut the roots.

To cut out each root, I took two cuts.  First, parallel to one face, removing a little triangle from each tooth.

Finally, the remaining fragment was cut from the root.

Here are three sizes of gears with all teeth cut.

The roots of each tooth were a rough from the blade marks.  So to smooth them out, I used a file.  This is a little subtle: the gear on top in the picture below is after filing and the one on the bottom is before filing.

Finally, I sanded the front and back of each gear to smooth (and remove the paper plans that were initially glued to the gear).

Depthing tool

The process of setting the distance between gear centers is called "depthing."  The pitch circles of two meshing gears should be tangent -- deviation from this ideal can cause problems.  Gary Mahony has some nice animations of what various problems can occur.

So how does one set the distance between a pair of actual gears, rather than on the plans?  George Daniels says (in his fabulous book, Watchmaking) that one should never rely on calculations to depth a pair of hand-made gears.  One should instead use a gear depthing tool.  Such things can be purchased, or made.  For instance, the tool made by Jesse Donaldson is pretty close to what I envisioned, though John Moran's tool is perhaps a bit more standard.

I decided to make a gear depthing tool, though I wanted one that can fit a pair of 8" diameter gears.  So it has to be big.  On the other hand, as a first pass, I was not too concerned with overall parallelism of the two axes.  We'll see how well this works, and will modify if needed.

In order to cut the slot between the two jaws, I set up a fence and used a router.  This worked OK, though it took a few passes to get the router aligned correctly.  I made each pass about 1/8" deeper than the previous one, until the bit finally just cut through.

I made both of the jaws on the same piece of wood, so it was easy to cut the second slot once the fence was in the right place.  This ensured that both slots were aligned at the same distance from the outer edges.  Once cutting the jaws apart, the inner edges weren't exactly even, so I planed both jaws so they were the same size, and then screwed them together.  Since the jaws are soft pine, I made sure to drill pilot holes first, and I sighted the opposite edges to ensure that the slots were exactly parallel.

Each of the four arbors is cut from 1/4" brass rod in the lathe.  I started by facing off the end, and then drilling a 45 degree bevel into the end with the tailstock drill.  After this, I drilled the center with a 5/64" bit to receive a pivot.

After that, I turned down the the shank of the arbor to the appropriate diameter to receive 8-32 threads.  This took a few tries to get right, since I cut the first arbor too narrow and the second one wasn't all a uniform diameter.

I tapered the non-bearing end to a point, and then threaded the entire length of the rod at SAE 8-32.  This took a few tries -- I messed up two rods by cutting them too narrow, but finally ended up getting the right diameter for my die.  That's somewhere between 0.148" to 0.156", which is a bit off from the specified outer diameter of the threads.

Threading on a pair of wingnuts and washers attaches the bearing to the frame.  Adding a second bearing allows a gear to be supported between the two bearings.  The picture below shows an escape wheel mounted in the frame. 

After two more bearings, the tool is completed.  Here it is holding an escape wheel and an anchor.