This week’s assignment was to make something big using the waterjet cutter and/or ShopBot.  I decided to build a big wooden clock.  I’ve never attempted to build a clock before.  To say the least, it was a learning experience.

By far, the thing that I learned the most about this week was gears.  I figured that before I attempted to actually design the clock, I should learn about how gears are designed.  This is not nearly as simple as it sounds.

Most gears in use in machines today are involute gears.  This means that the contour of their teeth is an involute curve.  An involute curve is the curve traced by the free end of a taut string as it is being wound onto a circle.  This tooth profile allows the teeth to be in constant contact while they are interacting and keeps the line of action between them on a straight line.

The desired involute profile is somewhat time consuming to draw by hand in a CAD program.  Fortunately, I was able to find a Python script on Thingiverse which would generate DXF files from a desired module and number of teeth.  Since the module is a function of the working diameter and number of teeth of a gear, I was able to choose the sizes of the gears fairly easily.

After generating the DXF files for the gears, I imported them into SolidWorks and extruded them into parts.  I also made parts for the escape gear, escape lever, and pendulum.  Next, I put all the parts into an assembly and arranged them in space, optimizing  for functionality as well as aesthetics.  This was a non-trivial task.

After this was done, I designed the four support frames around the holes in the parts.  I then created drawings of all the parts and frames, and exported those to DXF.  I imported the DXF files into two ShopBot sheets and cut the parts out of 4’ square 3/8” medium density fiberboard (MDF) with a 1/8” end mill.

After the parts were cut, I cut wooden dowels to be the axles and support members, and assembled the clock.  I was then dismayed to find a beautiful, yet quite nonfunctional wooden clock sitting in front of me.

The gear train is totally seized because the gears do not mesh fully, creating tons of friction on the axles due to the effective working diameters being bigger than the original design.  I am not sure if this is due to the Python script that I used to make the original gear outlines or because of imprecision in the milling process.  If the gear generation script is at fault, I should be able to make drop-in replacements with the same working diameters using a gear generating program that produces correct tooth profiles.  If the problem is due to imprecision in the milling process, I can just increase the size of the axle holes.

I will post the SolidWorks part files here if and when I get the gears right.

Resources:

Here is a very informative document explaining the module of a gear.

This instructable contains a very useful spreadsheet for calculating clock gear train ratios.

These step-by-step instructions detail how to draw a deadbeat escapement in any CAD program.