Friday, December 23, 2016

Clock case

The clock I made has been attached to a set of shelves in my workshop.  That's really the best spot to temporarily test a clock, where it can be held up with clamps.

But now that it is done, it really ought to move to a more permanent home both for its own sake and so that I can build another clock.  That requires the clock to either be mounted to a wall or to be free standing.  Since it's fairly heavy (10 lb weight plus a few pounds for the movement itself), wall hanging doesn't seem like a good idea.

So it needs a free standing, tall case.  Especially with the two-fall pulleys now in place, the clock should run for about a day if it's mounted appropriately.

  • The minute wheel makes 12 rotations per day (recall that the minute hand makes a rotation every two hours!)
  • The winding barrel is 1.8125 inches in diameter
  • A two-fall pulley is used
Given that, the weight drops 12 * 1.8125*3.14159 / 2 = 35 inches per day or about 1.4 inches per hour. Located where it is, there is about 46.5 inches available for drop, so the clock should run for about 32 hours.

Building a case for a clock is essentially an exercise in carpentry, and can be utilitarian or artistic or somewhere in between.  My style tends to be more utilitarian, mostly featuring straight lines and circular arcs.

I obtained wood for the case from Home Depot. I had hoped for birch to match the movement, which is birch plywood.  None of the birch plywood they had was in particularly good shape (nor was it high quality).  As a second option, I'd go with maple, since the color and grain is similar.  However, the selection of sizes wasn't sufficient.  So instead, I went with oak.  Overall, oak is not a bad choice: it's easy to work, sufficiently strong, and looks nice.  However, once finished, oak will be darker and will have a more prominent grain than birch.

I cut out the parts for the case with a jigsaw, held mostly freehand.  Though for the long straight cuts I used fences.  After rough cutting, I used a router to transfer the oval cutout from one of the sides to the other.

The most critical part is where the two sides (the longest parts) interface with the movement.  This needs to be a fairly tight friction fit, so that the movement stays where it should during running, but the can be removed easily as well.  I cut the slots small with the jigsaw and then filed to fit.  I first filed the sides separately until a scrap of birch plywood slid into place with some friction.  Once satisfied, I tried each side individually with the movement itself, filing that to fit.  Then I assembled the rest of the unfinished case.  Once assembled, only then did I try both sides together with the movement.  This ensured that I didn't overdo the filing and that everything was aligned properly.  Although filing to fit in this way is a time consuming process, it worked well enough.

To install the clock, it is lifted up and over the back of the case, which is open, and then slipped into the slots.  To remove the clock, the process is reversed.  The fit is tight enough so that the movement does not shift when the weight is applied or removed, or when the clock is wound.

The front of the case is a door.  This is mostly for show, since none of the oval holes cut in the front or sides is covered with windows.  The holes serve a dual purpose: to show the pendulum and to allow the pendulum to swing slightly out of the sides.

In order to operate the door, I made a small oak handle and a brass catch on the lathe.  The catch is threaded on the outside to fit the handle and on the inside to accept a screw for a wooden finger.  I'm dubious about the wooden finger.  Although it seems strong enough, the finger might wear out or break.  If this happens, I'll replace it with a metal one.

To hold the door shut, the finger on the catch engages a slot on the left side of the case.

After trying the fit of all parts, I assembled the case, drilling and countersinking all screw holes carefully.  The video above shows the assembled (but unfinished) case with the door removed.  I used brass screws with Phillips heads throughout, which may or may not be a good idea.  Oak can be a bit rough on brass screws.  Sadly, Home Depot was out of flat head brass screws, which would have matched the screws on the movement better.

Once confident that everything was in order, I disassembled the case, reattached the clock to the shelves temporarily, and spent the better part of a day sanding the case parts.  After all parts were smooth, I polyurethaned (three coats) all the parts. I left the insides of the slots where the clock fits into the side panels unfinished, so that the fit to the movement isn't disrupted.

And, here's the case completed!  Although the case looks quite nice (the picture doesn't do it justice), it now is clear that the movement needs some finishing as well.  Numerous wooden clock makers warn about finishing the wheels or anything that moves, it should be safe to re-sand and polyurethane the plates of the movement.  But that will need to be a task for another day...