Saturday, February 28, 2015

Wooden clock #1 lessons

  1. Make the pinions have more than 6 leaves.  Pinions with a small number of leaves are more difficult to cut, have more friction, and just cause more trouble.  Cutting extra teeth on a larger gear is actually easier
  2. Be a little more careful with computing gear ratios, especially involving the action of the escapement.  Although I got the minute hand wrong, I managed to make a second error in the motion work so the hour hand runs properly
  3. Make the pendulum longer than you'd expect to need -- you can always trim it back
  4. Although it seems unbalanced, consider putting the winding barrel at the top of the mechanism, to give more room for weight drop.
  5. Connecting the anchor without a crutch assembly works, although tolerances are probably improved by using one.
  6. Along those lines, mounting the pendulum in a pivot also seems to work but probably contributes excess friction
  7. Slipit!

Thursday, February 26, 2015

Frictional winding

Due to the fact that the clock seems to need about 10 lb of weight, it seems like the endless chain mechanism I had intended to use probably won't work.  In particular, it basically requires doubling the drive weight, which doesn't seem like a good idea.  So instead, I've opted to try a simpler mechanism for winding.  Merely winding a few turns of cord around the drive pulley seems to give quite a bit of friction -- enough to hold the drive weight with a small amount of force on the other end. 

Specifically, it looks like if the drive weight is the 9 lb 8 oz from before, a 7.5 oz counterweight seems to hold it in place.  In this case, since there are two weights that need to be held clear of the pendulum and each other, I've used a pair of pulleys to run the cord.

This seems to work well enough, when the drive weight runs down, you can lift the drive weight and gently take up the slack with the counterweight.

Wednesday, February 25, 2015

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...

Tuesday, February 17, 2015

Debugging (Part 4 -- it runs!)

After getting the third wheel re-depthed, I was worried that I would have to move the center wheel.  It turned out that this was unnecessary.  Based on my previous weight measurements later in the train, I need about 6-8 lb to get a reliable run.  Just to see if it worked, I grabbed a large piece of threaded steel rod that came with my lathe (and isn't actually part of it) that seemed to be heavy enough (4 lb 9 oz), attached it to the center wheel pulley, and let it run!

After getting the motion work installed, the whole clock is "complete."  To my surprise, however, it looks like I made a big design mistake.  The escape wheel is supposed to complete a revolution every 15 seconds.  But instead, it takes just shy of 30 seconds (roughly 27 seconds).  So the clock runs at "half speed."  I guess I'll need to notate 24 hours for the hour hand and 120 minutes for the minute hand! 
 Here is a video of the clock running.  (There are more detailed videos on my YouTube page.)

Every so often, the clock stops with the escape wheel mid-impulse.  At 4 lb 9 oz, the weight is probably a bit too light.  I've just updated the weight by bringing it to 7 lb 8 oz by adding another weight to the shaft.  If need be, I can add more weight in roughly 2 lb increments, but that probably won't be needed.

Motion work

The motion work is mounted on the front plate, and consists of four gears.  One is attached to the center wheel arbor, two on an offset arbor, and the hour wheel mounted coaxially with the center wheel arbor.  As described before, 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)
Here are the four gears:

The offset wheel and pinion are glued together.  To keep their centers aligned, I passed a steel rod through their common centers before clamping.

The center pinion hole was cut slightly smaller than the center arbor.  This allowed me to slowly enlarge it with a round needle file until it was a snug friction fit over the arbor.

The offset wheel has an arbor cut out of 1/8" steel rod with rounded ends.  To set the arbor location, I applied a small amount of chalk to the end, and manually depthed the wheel.

Since the offset wheel is solid (no cutouts), if I don't like the depthing, it's easy to move without making an obvious blemish.

Next, I marked out the hands on pieces of 1/4" birch plywood.  I opted for simple hands rather than something more elaborate.

Then, I cut the hands out with the scroll saw and finished shaping them with a file.  Notice that I predrilled the center holes slightly smaller than the arbors, but then made a cut on the rear of the hand to allow it to grip the arbor.

Here is the hour hand mounted on its hour wheel.  The minute hand is mounted on the end of the center shaft and holds the motion work in place.

Monday, February 16, 2015

Debugging (Part 3)

I spent a lot of time debugging the third/fourth wheel engagement.  I tried
  • Many different pinions
  • Flipping the fourth wheel around to use the other set of contact surfaces.  This helped considerably...
But, what really did it was the thing I wanted to avoid.  Specifically, moving the fourth wheel closer to the third wheel.  Experimenting with the depthing tool -- which I now understand how to use: with force on the fourth wheel and light backpressure on the third wheel -- I determined that I needed to move about 1/16".

So, moving the wheel is a pain.  First, I needed to extract the bearings.  That was messy, and involved digging out the bearing a bit with a knife and levering it out.  After it was removed, I used a milling bit on the end of my hand drill to extend the hole upward 1/16" of an inch.  Once the bearing was back in place, I slid a wooden shim below it to anchor it.

I realize I'll have to do this for the center wheel, too.  That will be especially annoying since the center bearing goes through the front plate.  I may need a tighter shim to anchor it.

At this point, the clock runs with 11.35 oz on the fourth wheel pinion.  A smaller weight (namely 7 oz) appears to almost run it, but the clock stops occasionally.

Pendulum bob

For the end of the pendulum...  Simple, with an adjustable screw.

The adjustable screw has a nut inset on the inner edge of the runner to ensure positive contact with the pendulum. 

Sunday, February 8, 2015

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.

Thursday, February 5, 2015

Debugging (Part 2)

A quick note: filing the anchor teeth helped considerably.  The escapement now runs well with 7.5 oz on the fourth wheel pinion.  After also filing the escape wheel teeth so they're very nearly all the same length and very carefully balancing the anchor, it runs with about 2.5 oz on the fourth wheel pinion.

Sunday, February 1, 2015

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...